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Across the Universe is an interstellar radio message (IRM) consisting of the song "Across the Universe" by the Beatles that was transmitted on 4 February 2008, at 00:00 UTC by NASA in the direction of the star Polaris. This transmission was made using a 70-meter "DSS-63" dish in the NASA Deep Space Network's (DSN) Madrid Deep Space Communication Complex, located in Robledo, near Madrid, Spain. The transmission ran in the 4.2-cm band (around 7.14 GHz, C band) at a power of 18 kilowatt. The format was digital, transmitted at a rate of 128 kbps, lasting 3.6 minutes – the normal speed and data rate for a digital recording on Earth.
This action was done in order to celebrate the 40th anniversary of the song's recording, the 45th anniversary of the DSN, and the 50th anniversary of NASA. The idea was hatched by Beatles historian Martin Lewis, who encouraged all Beatles fans to play the track as it was beamed towards the distant star. The event marked the third time a song had ever been intentionally transmitted into deep space (the first being Russia's Teen Age Message in 2001, and the second being the 2003 Cosmic Call 2 message which included "Starman" by David Bowie and music from the Hungarian band KFT), and was approved by Paul McCartney, Yoko Ono, and Apple Records.
A. L. Zaitsev, part of the Teen Age Message project, argues that the NASA project is only a publicity stunt. The compressed digital format used makes the data more fragile to errors compared to TAM's analogue approach, not to mention aliens would not have knowledge on human audio compression algorithms. The transmission data rate is also too high to allow for a remote radio station to faithfully receive; a data rate 300,000 times lower would be required. Finally, the choice of Polaris also makes the message unlikely to reach any alien lifeform should they exist.
== See also ==
List of interstellar radio messages
== References ==

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Active SETI (Active Search for Extra-Terrestrial Intelligence) are attempts to send messages to intelligent extraterrestrial life. Active SETI messages are predominantly sent in the form of radio signals. Physical messages like that of the Pioneer plaque may also be considered an active SETI message. Active SETI is also known as METI (Messaging to Extra-Terrestrial Intelligence). Whether humans should engage in Active SETI is controversial due to concerns about potential impacts of extraterrestrial contact, sparking a vigorous policy debate.
== History ==
'Active SETI' was a term as early as 2005, though some decades after the term SETI. The term METI was coined in 2006 by Russian scientist Alexander Zaitsev, who proposed a subtle distinction between Active SETI and METI:
The science known as SETI deals with searching for messages from aliens. METI deals with the creation and transmission of messages to aliens. Thus, SETI and METI proponents have quite different perspectives. SETI scientists are in a position to address only the local question “does Active SETI make sense?” In other words, would it be reasonable, for SETI success, to transmit with the object of attracting ETI's attention? In contrast to Active SETI, METI pursues not a local, but a more global purpose – to overcome the Great Silence in the Universe, bringing to our extraterrestrial neighbors the long-expected annunciation “You are not alone!”
Concern over METI was raised by the science journal Nature in an editorial in October 2006, which commented on a recent meeting of the International Academy of Astronautics SETI study group. The editor said, "It is not obvious that all extraterrestrial civilizations will be benign, or that contact with even a benign one would not have serious repercussions". In the same year, astronomer and science fiction author David Brin expressed similar concerns. In 2013 Brin amended his initial article based on the recent developments in METI.
In 2010, Douglas A. Vakoch from SETI Institute, addressed concerns about the validity of Active SETI alone as an experimental science by proposing the integration of Active SETI and Passive SETI programs to engage in a clearly articulated, ongoing, and evolving set of experiments to test various versions of the Zoo hypothesis, including specific dates at which a first response to messages sent to particular stars could be expected.
On 13 February 2015, scientists including Douglas Vakoch, David Grinspoon, Seth Shostak, and David Brin at an annual meeting of the American Association for the Advancement of Science discussed Active SETI, and whether transmitting a message to possible intelligent extraterrestrials in the Cosmos was a good idea. That same week, a statement was released, signed by many in the SETI community including Berkeley SETI Research Center director Andrew Siemion, advocating that a "worldwide scientific, political and humanitarian discussion must occur before any message is sent".
In July 2015, the Breakthrough Message program was announced. This was an open competition to design a digital message that could be transmitted from Earth to an extraterrestrial civilization, with a US$1,000,000 prize pool. The message was to be "representative of humanity and planet Earth". The program pledged "not to transmit any message until there has been a wide-ranging debate at high levels of science and politics on the risks and rewards of contacting advanced civilizations".
== Rationale for METI ==
In the paper Rationale for METI, transmission of the information into the Cosmos is treated as one of the pressing needs of an advanced civilization. This view is not universally accepted, and it is not agreed with by those who are against the transmission of interstellar radio messages, but at the same time are not against SETI searching. Such duality is called the SETI Paradox.
== Radio message construction ==
The lack of an established communications protocol is a challenge for METI. While trying to synthesize and project an Interstellar Radio Message (IRM), the receiving extraterrestrials (ETs) will first encounter a physical phenomenon and, only after that, perceive the information. Initially, a receiving system will detect the radio signal; then the issue of the extraction of the received information and comprehension of the obtained message will arise. Therefore, above all, the constructor of an IRM should be concerned about the ease of signal determination. In other words, the signal should have maximum openness, which is understood here as an antonym of the term security. This branch of signal synthesis is termed anticryptography.
To this end, in 2010, Michael W. Busch created a general-purpose binary language, later used in the Lone Signal project to transmit crowdsourced messages to extraterrestrial intelligence. Busch developed the coding scheme
and provided Rachel M. Reddick with a test message, in a blind test of decryption. Reddick decoded the entire message after approximately twelve hours of work. This was followed by an attempt to extend the syntax used in the Lone Signal hailing message to communicate in a way that, while neither mathematical nor strictly logical, was nonetheless understandable given the prior definition of terms and concepts in the hailing message.
In addition, characteristics of the radio signal, such as wavelength, type of polarization, and modulation are considered. Over galactic distances, the interstellar medium induces some scintillation effects and artificial modulation of electromagnetic signals. This modulation is higher at lower frequencies and is a function of the sky direction. Over large distances, the depth of the modulation can exceed 100%, making any METI signal very difficult to decode.

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=== Error correction ===
In METI research, any message must have some redundancy, although the exact amount of redundancy and message formats are still in great dispute. Using ideograms, instead of binary sequence, already offers some improvement against noise resistance. In faxlike transmissions, ideograms are spread on many lines. This increases its resistance against short bursts of noise like radio frequency interference or
interstellar scintillation.
One format approach proposed for interstellar messages was to use the product of two prime numbers to construct an image. Unfortunately, this method works only if all the bits are present. As an example, the message sent by Frank Drake from the Arecibo Observatory in 1974 did not have any feature to support mechanisms to cope with the inevitable noise degradation of the interstellar medium.
Error correction tolerance rates for previous METI messages are 9% (one page) for the 1974 Arecibo Message, 44% (23 separate pages) for the 1999 Evpatoria Message, and 46% (one page, estimated) for the 2003 Evpatoria Message.
==== Examples ====
The 1999 Cosmic Call transmission was far from being optimal (from a terrestrial viewpoint) as it was essentially a monochromatic signal spiced with supplementary information. Additionally, the message had a very small modulation index overall, a condition not viewed as being optimal for interstellar communication. Over the 370,967 bits (46,371 bytes) sent, some 314,239 were "1" and 56,768 were "0"—5.54 times as many 1's as 0's. Since frequency-shift keying modulation scheme was used, most of the time the signal was on the "0" frequency. In addition, "0" tended to be sent in long stretches, which appeared as white lines in the message.
== Realized projects ==
The below projects have targeted stars between 17 and 69 light-years from the Earth. The exception is the Arecibo message, which targeted globular cluster M13, approximately 24,000 light-years away. The first interstellar message to reach its destination was the Altair (Morimoto–Hirabayashi) Message, which likely reached its target in 1999.
The Morse Message (1962)
Arecibo Message (1974)
Cosmic Call 1 (1999)
Teen Age Message (2001)
Cosmic Call 2 (2003)
Across the Universe (2008)
Hello from Earth (2009)
Wow! Reply (2012)
Lone Signal (2013)
A Simple Response to an Elemental Message (2016)
The choice of targets, transmission rate, and encoding may effect how likely a message will be received and interpreted by extraterrestrial intelligence. "Across the Universe" and "A Simple Response to an Elemental Message" were sent to Polaris, which is 431 light years distant from us and whose planetary system, even if it exists, may not be suited for life, because it is a supergiant star, spectral type F7Ib which is only 70 million years old. In addition, both transmission rates were very high, about 128 kbit/s, for such moderate transmitter power (about 18 kW). The main defect of the "Hello From Earth" is an insufficient scientific and technical justification, since no famous SETI scientist made statements with validation of HFE's design. As it follows from [1]: "After the final message was collected on Monday 24 August 2009, messages were exported as a text file and sent to NASA's Jet Propulsion Laboratory in California, where they were encoded into binary, packaged and tested before transmission", but nobody explained why he hopes that such encoded and packaged text will be understood and conceived by possible extraterrestrials.
== Transmissions ==
Below is a table of messages sent and target/destination stars, ordered chronologically by date of sending:

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== Controversy ==
Whether or not to conduct Active SETI, as well as the tone of any message, is a highly controversial topic. Active SETI has primarily been criticized due to the perceived risk of revealing the location of the Earth to alien civilizations, without some process of prior international consultation. That is, Active SETI does not meet the criteria for informed consent in a mass experiment involving human subjects and, potentially, nonhuman sentient subjects.
Active SETI is discussed in terms of the ethics of space policy. Issues include whether to send belligerent versus defensive messages, cosmopolitanism, communicative burden, consensus, messaging content, proscriptions on premature messaging, responsibility, and shared values, with concerns that even if successful, humanity could be reduced to a cargo cult. David Brin also urged for an extensive international consultation before any METI activities and has debunked key rationalizations for active SETI (METI), such as the "barn door" argument (unintentional "leaked signals" were millions-fold weaker than intentional METI signals), ignoring/dismissing the precautionary principle (that requires taking extreme precaution e.g. handling extraterrestrial samples even without any known example of risks), and treating METI as being prayer-like which disregards the issue of informed consent from other people. Notable among METI's critics was Stephen Hawking. Hawking, who in his book A Brief History of Time suggests that "alerting" extraterrestrial intelligences to our existence is foolhardy, citing humankind's history of treating its own kind harshly in meetings of civilizations with a significant technology gap, e.g., the extermination of Tasmanian aborigines. He suggested, in view of this history, that we "lay low". Scientist and science fiction author David Brin expressed similar concerns. Similarly, Liu Cixin's trilogy of novels The Remembrance of Earth’s Past Trilogy highlights the potential dangers of METI.
However, some scientists consider these fears about the dangers of METI as panic and irrational superstition; Russian and Soviet radio engineer and astronomer Alexander L. Zaitsev has argued against these concerns. Zaitsev argues that we should consider the risks of not attempting to contact extraterrestrial civilizations, since the knowledge and wisdom an ETI could impart to us would save us from humanity's self-destructive tendencies. Similarly, in a March 2015 essay astronomer Seth Shostak considered the risk and ended by stressing that any danger was hypothetical and that humanity would better off risk contact than "endlessly tremble at the sight of the stars".
Astronomer Jill Tarter also disagrees with Hawking, arguing that aliens developed and long-lived enough to communicate and travel across interstellar distances would have evolved a cooperative and less violent intelligence. She however thinks it is too soon for humans to attempt active SETI and that humans should be more advanced technologically first but keep listening in the meantime.
To lend a quantitative basis to discussions of the risks of transmitting deliberate messages from Earth, the SETI Permanent Study Group of the International Academy of Astronautics adopted in 2007 a new analytical tool, the San Marino Scale. Developed by Prof. Ivan Almar and Prof. H. Paul Shuch, the San Marino Scale evaluates the significance of transmissions from Earth as a function of signal intensity and information content. Its adoption suggests that not all such transmissions are created equal, thus each must be evaluated separately before establishing a blanket international policy regarding Active SETI.
In 2012, Jacob Haqq-Misra, Michael Busch, Sanjoy Som, and Seth Baum argued that while the benefits of radio communication on Earth likely outweigh the potential harms of detection by extraterrestrial watchers, the uncertainty regarding the outcome of contact with extraterrestrial beings creates difficulty in assessing whether or not to engage in long-term and large-scale METI.
In 2015, in the context of the Zoo Hypothesis, biologist João Pedro de Magalhães proposed transmitting an invitation message to any extraterrestrial intelligences watching us already and inviting them to respond, arguing this would not put us in any more danger than we are already if the Zoo Hypothesis is correct.
Douglas Vakoch, president of METI, argues that passive SETI itself is already an endorsement of active SETI, since "If we detect a signal from aliens through a SETI program, there's no way to prevent a cacophony of responses from Earth."
In the context of potentially detected extraterrestrial activity on Earth, physicist Mark Buchanan argued that humanity needs to determine whether it would be safe or wise to attempt to communicate with extraterrestrials and work on ways to handle such attempts in an organized manner.
== Beacon proposals ==
One proposal for a 10 billion watt interstellar SETI beacon was dismissed by Robert A. Freitas Jr. as being infeasible for a pre-Type I civilization, such as humanity, on the Kardashev scale. However, this 1980s technical argument assumes omni-directional beacons, which may not be the best way to proceed on many technical grounds. Advances in consumer electronics have made possible transmitters that simultaneously transmit many narrow beams, covering the million or so nearest stars but not the spaces between. This multibeam approach can reduce the power and cost to levels that are reasonable with early 21st century Earth technology.
Once civilizations have discovered each other's locations, the energy requirements for maintaining contact and exchanging information can be significantly reduced through the use of highly directional transmission technologies. To this end, a 2018 study estimated a 1–2 megawatt infrared laser focused through a 30-45 meter telescope could be seen from about 20,000 light years away.
== See also ==
Communication with extraterrestrial intelligence
Dark forest hypothesis, the idea that planetary civilizations remain silent to avoid the possibility of contact with potentially aggressive worlds
Detecting Earth from distant star-based systems
METI (Messaging Extraterrestrial Intelligence) – Organization conducting active SETIPages displaying short descriptions of redirect targets
SETIcon – Public conventions on the search for extraterrestrial intelligence
SETI@home
Wow! signal
== References ==
== External links ==
Interstellar Radio Messages
ActiveSETI.org Archived 2010-09-30 at the Wayback Machine
active-seti.info
Making a Case for METI
Self-Decoding Messages
Should We Shout Into the Darkness?
Error Correction Schemes In Active SETI
The Evpatoria Messages
Encounter 2001 Message
Zaitsev, Alexander L. (2011). "METI: Messaging to ExtraTerrestrial Intelligence". Searching for Extraterrestrial Intelligence. The Frontiers Collection. pp. 399–428. doi:10.1007/978-3-642-13196-7_21. ISBN 978-3-642-13195-0.
The Pros and Cons of METI from Centauri Dreams
Zaitsev, Alexander (2012). "Classification of interstellar radio messages". Acta Astronautica. 78: 16–19. Bibcode:2012AcAau..78...16Z. doi:10.1016/j.actaastro.2011.05.026.
Lone Signal

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The Allen Telescope Array (ATA), formerly known as the One Hectare Telescope (1hT), is a radio telescope array dedicated to astronomical observations and a simultaneous search for extraterrestrial intelligence (SETI). The array is situated at the Hat Creek Radio Observatory in Shasta County, 290 miles (470 km) northeast of San Francisco, California.
The project was originally developed as a joint effort between the SETI Institute and the Radio Astronomy Laboratory (RAL) at the University of California, Berkeley (UC Berkeley), with funds obtained from an initial US$12.5 million donation by the Paul G. Allen Family Foundation and Nathan Myhrvold. The first phase of construction was completed and the ATA finally became operational on 11 October 2007 with 42 antennas (ATA-42), after Paul Allen (co-founder of Microsoft) had pledged an additional $13.5 million to support the construction of the first and second phases.
Although overall Allen has contributed more than $30 million to the project, it has not succeeded in building the 350 6.1 m (20 ft) dishes originally conceived, and the project suffered an operational hiatus due to funding shortfalls between April and August 2011, after which observations resumed. Subsequently, UC Berkeley exited the project, completing divestment in April 2012. The facility is now managed by SRI International (formerly Stanford Research Institute), an independent, nonprofit research institute. As of 2016, the SETI Institute performs observations with the ATA between the hours of 6 pm and 6 am daily.
In August 2014, the installation was threatened by a forest fire in the area and was briefly forced to shut down, but ultimately emerged largely unscathed.

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== Overview ==
First conceived by SETI pioneer Frank Drake, the idea has been a dream of the SETI Institute for years. However, it was not until early 2001 that research and development began, after a donation of $11.5 million by the Paul G. Allen Family Foundation. In March 2004, following the successful completion of a three-year research and development phase, the SETI Institute unveiled a three-tier construction plan for the telescope. Construction began immediately, thanks to the pledge of $13.5 million by Paul Allen (co-founder of Microsoft) to support the construction of the first and second phases. The SETI Institute named the telescope in Allen's honor. Overall, Paul Allen contributed more than $30 million to the project.
The ATA is a centimeter-wave array which pioneers the Large-Number Small-Diameter concept of building radio telescopes. Compared to a large dish antenna, large numbers of smaller dishes are cheaper for the same collecting area. To get similar sensitivity, the signals from all telescopes must be combined. This requires high-performance electronics, which had been prohibitively expensive. Due to the declining cost of electronic components, the required electronics became practicable, resulting in a large cost-saving over telescopes of more conventional design. This is informally referred to as "replacing steel with silicon".
The ATA has four primary technical capabilities that make it well suited for a range of scientific investigations: a very wide field of view (2.45° at λ = 21 cm, the wavelength of the hydrogen line), complete instantaneous frequency coverage from 0.5 to 11.2 gigahertz (GHz), multiple simultaneous backends, and active interference mitigation. The area of sky which can be instantaneously imaged is 17 times that obtainable by the Very Large Array telescope. The instantaneous frequency coverage of more than four octaves is unprecedented in radio astronomy, and is the result of a unique feed, input amplifier and signal path design. Active interference mitigation will make it possible to observe even at the frequencies of many terrestrial radio emitters.
All-sky surveys are an important part of the science program, and the ATA will have increased efficiency through its ability to conduct extraterrestrial intelligence searches (SETI) and other radio astronomy observations simultaneously. The telescope can do this by splitting the recorded signals in the control room prior to final processing. Simultaneous observations are possible because for SETI, wherever the telescope is pointed, several target stars will lie within the large field of view afforded by the 6 m dishes. By agreement between the UC Berkeley Radio Astronomy Laboratory (RAL) and the SETI Institute, the needs of conventional radio astronomy determined the pointing of the array up until 2012.
The ATA is planned to comprise 350 6 m dishes and will make possible large, deep radio surveys that were not previously feasible. The telescope design incorporates many new features, including hydroformed antenna surfaces, a log-periodic feed covering the entire range of frequencies from 500 megahertz (MHz) to 11.2 GHz, and low-noise, wide-band amplifiers with a flat response over the entire band, thus making it possible to amplify the sky signal directly. This amplified signal, containing the entire received bandwidth, is brought from each antenna to the processing room via optical fiber cables. This means that as electronics improve and wider bandwidths are obtainable, only the central processor needs to change, and not the antennas or feeds.
The instrument was operated and maintained by RAL until development of the array was put on hold in 2011. RAL worked hand in hand with the SETI Institute during design and prototyping and was the primary designer of the feed, antenna surfaces, beamforming, correlator, and imaging system for radio astronomy observations.
The panel for the Astronomy and Astrophysics Decadal Survey in its fifth report, Astronomy and Astrophysics in the New Millennium (2001), endorsed SETI and recognized the ATA (then called the 1-Hectare Telescope) as an important stepping stone towards the building of the Square Kilometer Array telescope (SKA). The most recent Decadal report recommended ending the US's financial support of the SKA, although US participation in SKA precursors such as MeerKAT, the Hydrogen Epoch of Reionization Array and the Murchison Widefield Array.
Although cost estimates of unbuilt projects are always dubious, and the specifications are not identical (conventional telescopes have lower noise temperature, but the ATA has a larger field of view, for example), the ATA has potential promise as a much cheaper radio telescope technology for a given effective aperture. For example, the amount spent on the first ATA-42 phase, including technology development, is roughly one third of the cost of a new copy of a Deep Space Network 34 m antenna of similar collecting area. Similarly, the estimated total cost of building the remaining 308 dishes was estimated (as of October 2007) at about $41 million. This is about two times cheaper than the $85 million cost of the last large radio astronomy antenna built in the US, the Green Bank Telescope, of similar collecting area. The contractor filed for a $29 million overrun, but only $4 million of this was allowed.
The ATA aspires to be among the world's largest and fastest observing instruments, and to permit astronomers to search many different target stars simultaneously. If completed as originally envisioned, it will be one of the largest and most powerful telescopes in the world.

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== History ==
Since its inception, the ATA has been a development tool for astronomical interferometer technology (specifically, for the Square Kilometer Array).
The ATA was originally planned to be constructed in four stages, ATA-42, ATA-98, ATA-206 and ATA-350, each number representing the number of dishes in the array at a given time (See Table 1). The ATA is planned to comprise 350 dishes with a diameter of 6 m each.
Regular operations with 42 dishes started on 11 October 2007. Funding for building additional antennas is currently being sought by the SETI Institute from various sources, including the United States Navy, Defense Advanced Research Projects Agency (DARPA), National Science Foundation (NSF) and private donors.
Simultaneous astronomical and SETI observations are performed with two 32-input dual polarization imaging correlators. Numerous articles reporting conventional radio astronomy observations have been published.
Three phased array beamformers utilizing the Berkeley Emulation Engine 2 (BEE2) were deployed in June 2007 and have been integrated into the system to allow for simultaneous astronomical and SETI observations. As of April 2008, the first pulsar observations were conducted using the beamformer and a purpose-built pulsar spectrometer.
The workhorse SETI search system (SETI on ATA or SonATA) performs fully automated SETI observations. SonATA follows up on detected signals in real time and continues to track them until 1) the signal is shown to have been generated on Earth or rarely, 2) the source sets, which triggers follow up the next day. As of 2016, more than two hundred million signals have been followed up and classified using the ATA. Not one of these signals had all the characteristics expected for an ETI signal. The results of SETI Institute's observations are published in a number of papers.
In April 2011, the ATA was put into hibernation owing to funding shortfalls, meaning that it was no longer available for use. Operation of the ATA resumed on 5 December 2011. Efforts are now led by Andrew Siemion.
=== Status ===
In 2012, the ATA was funded by a $3.6 million philanthropic donation by Franklin Antonio, cofounder and Chief Scientist of Qualcomm Incorporated. This gift supports upgrades of all the receivers on the ATA dishes to have dramatically greater sensitivity (2 − 10× from 1–8 GHz) than before and support sensitive observations over a wider frequency range, from 1–15 GHz, when initially the radio frequency electronics went to only 11 GHz. By July 2016, the first ten of these receivers had been installed and proven. Full installation on all 42 antennas is planned as of June 2017.
In November 2015, the ATA studied the anomalous star KIC 8462852, and in autumn 2017 the Allen Telescope Array examined the interstellar asteroid ʻOumuamua for signs of technology, but detected no unusual radio emissions.
== Key science goals ==
The science goals listed below represent the most important projects to be conducted with the ATA. Each of these goals is associated with one of the four stages of development mentioned earlier. (See Table 1). Also listed is some of the science that it is hoped each will produce.
Determine the hydrogen line (HI) content of galaxies out to z ~ 0.2 over 3π steradians, in order to measure how much intergalactic gas external galaxies are accreting; to search for dark, starless galaxies; to lay the foundation for dark energy detection by the Square Kilometer Array.
Classify 250,000 extra-galactic radio sources as active galactic nuclei or starburst galaxies, in order to probe and quantify star formation in the Local Universe; to identify high redshift objects; to probe large-scale structure in the Universe; to identify gravitational lens candidates for dark matter and dark energy detection.
Explore the transient sky, in order to probe accretion onto black holes; to find orphan gamma ray burst afterglows; to discover new and unknown transient phenomena.
Survey 1,000,000 stars for SETI-related emission with enough sensitivity to detect an Arecibo radar out to 300 parsecs within the range of 1–10 GHz.
Survey the 4×1010 stars of the inner galactic plane from 1.42–1.72 GHz for very powerful transmitters.
Measure the magnetic fields in the Milky Way and other Local Group galaxies, in order to probe the role of magnetic fields in star formation and galaxy formation and evolution.
Detect the gravitational wave background from massive black holes through pulsar timing.
Measure molecular cloud and star formation properties using new molecular tracers, in order to map star formation conditions on the scale of entire giant molecular clouds (GMCs); to determine the metallicity gradient of the Milky Way.
== Opportunistic science ==
Since construction of the array began, a few science goals not specifically drawn up for it have been suggested.
For example, the Allen Telescope Array has offered to provide the mooncast data downlink for any contestants in the Google Lunar X Prize. This is practical, since the array, with no modifications, covers the main space communications bands (S-band and X-band). A telemetry decoder would be the only needed addition.
Also, the ATA was mentioned as a candidate for searching for a new type of radio transient. It is an excellent choice for this owing to its large field of view and wide instantaneous bandwidth. Following this suggestion, Andrew Siemion and an international team of astronomers and engineers developed an instrument called "Fly's Eye" that allowed the ATA to search for bright radio transients, and observations were carried out between February and April 2008.
== Instruments ==

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The ATA-42 configuration will provide a maximum baseline of 300 m (and ultimately for the ATA-350, 900 m). A cooled log-periodic feed on each antenna is designed to provide a system temperature of ~45K from 1–10 GHz, with reduced sensitivity in the ranges of 0.5–1.0 GHz and 10–11.2 GHz. Four separate frequency tunings (IFs) are available to produce 4 x 672 MHz intermediate frequency bands. Two IFs support correlators for imaging; two will support SETI observing. All tunings can produce four dual polarization phased array beams which can be independently pointed within the primary beam and can be used with a variety of detectors. The ATA can therefore synthesize up to 32 phased array beams.
The wide field of view of the ATA gives it an unparalleled capability for large surveys. The time required for mapping a large area to a given sensitivity is proportional to (ND)2, where N is the number of elements and D is the diameter of the dish. This leads to the surprising result that a large array of small dishes can outperform an array with a smaller number of elements but considerably greater collecting area in the task of large surveys. As a consequence, even the ATA-42 is competitive with much larger telescopes in its capability for both brightness temperature and point source surveys. For point source surveys, the ATA-42 is comparable in speed to Arecibo and the Green Bank Telescope (GBT), but three times slower than the Very Large Array (VLA). The ATA-350, on the other hand, will be one order of magnitude faster than the Very Large Array for point source surveys, and is comparable to the Expanded Very Large Array (EVLA) in survey speed. For surveys up to a specified brightness temperature sensitivity, the ATA-98 will exceed the survey speed of even the VLA-D configuration. The ATA-206 should match the brightness temperature sensitivity of Arecibo and the GBT. The ATA, however, provides better resolution than either of these single-dish telescopes.
The antennas for the ATA are 6.1 x 7.0 meters (20.0 ft x 23.0 ft) hydroformed offset Gregorian telescopes, each with a 2.4 meter sub-reflector with an effective focal length/diameter (f/D) ratio of 0.65. (See DeBoer, 2001). The offset geometry eliminates blockage, which increases efficiency and decreases the side lobes. It also allows for the large sub-reflector, providing good low frequency performance. The hydroforming technology used to make these surfaces is the same as that used by Andersen Manufacturing of Idaho Falls, Idaho to generate low-cost satellite reflectors. The unique interior frame rim-supported compact mount allows excellent performance at low cost. The drive system employs a spring-loaded passive anti-backlash azimuth drive train. Most components designed by Matthew Fleming and manufactured at Minex Engineering Corp. in Antioch, CA.
== Data management ==
As with other arrays, the huge amount of incoming sensory information requires real-time array processing capability in order to reduce data volume for storage. For ATA-256, the average data rates and total data volume for the correlator are estimated to be 100 Mbyte/s and 15 Pbytes for the five-year survey period. Experiments such as transient surveys will exceed this rate significantly. The beamformers produce data at a much higher rate (8 gigabytes per second (Gb/s)) but only a very small fraction of this data is archived. In 2009, the signal detection hardware and software was called Prelude, which was
composed of rack-mounted PCs augmented by two custom accelerator cards based on digital signal processing (DSP) and field-programmable gate array (FPGA) chips. Each Programmable Detection Module (one of 28 PCs) can analyze 2 MHz of dual-polarization input data to generate spectra with spectral resolution of 0.7 Hz and time samples of 1.4 seconds.
In 2009, the array had a 40 Mbit/s internet connection, adequate for remote access and transferring of data products for ATA-256. An upgrade to 40 Gbit/s was planned, which would enable direct distribution of raw data for offsite computing.
=== Computational complexity and requirement ===
Like other array systems the ATA has a computational complexity and cross-connect which scales as O(N2) with the number of antennas
N
{\displaystyle N}
.
The computation requirement, for example, for correlating the full ATA bandwidth (
B
{\displaystyle B}
= 11 GHz) for the proposed
N
{\displaystyle N}
= 350 dual-polarization antenna build-out, using an efficient frequency-multiply (FX) architecture and a modest 500 kHz channel width (with number of channels
F
{\displaystyle F}
= 2200), is given by:
2
B
⟨
N
log
2
⁡
(
F
)
(
10
O
P
s
)
+
(
N
N
+
1
2
)
×
4
(
8
O
P
s
)
⟩
{\displaystyle 2B\langle N\log _{2}(F)(10OPs)+(N{\frac {N+1}{2}})\times 4(8OPs)\rangle }
= 44 Peta-OPs per second
where
O
p
s
{\displaystyle Ops}
is an operation. Note that since each dish has a dual polarization antenna, each signal sample is actually a two data set, hence
2
B
{\displaystyle 2B}
.
== See also ==
Carl Sagan Institute – Institute for the search of habitable worlds
Exoplanet – Planet outside of the Solar System
List of radio telescopes
SETI Institute – Not-for-profit research organization
Search for extraterrestrial intelligence – Effort to find civilizations not from Earth
setiQuest
Deep Synoptic Array, a more recent radio telescope also based on an array of smaller antennas
== References ==
== External links ==
Official website
"Radio Astronomy Laboratory's ATA site". Archived from the original on 2006-09-02. Retrieved 2015-11-16.
The Search Continues with the Allen Telescope Array. Mountain View, CA: SETI Institute. March 25, 2004.
Radio Astronomy Laboratory, University of California, Berkeley: NSF proposal, June 15, 2005.
https://web.archive.org/web/20111006031806/https://setistars.org/
Minex Engineering Corporation in Antioch, CA

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Andrew Fraknoi (born 1948) is a Hungarian-born American retired professor of astronomy recognized for his lifetime of work using everyday language to make astronomy more accessible and popular for both students and the general public. In 2017 Fraknoi retired from his position as Chair of the Department of Astronomy at Foothill College. In retirement he continues to teach through the Fromm Institute for Lifelong Learning and the Osher Lifelong Learning Institute at San Francisco State University, to give public lectures, and to add to his body of written work. He is the recipient of numerous awards and honors in his field.
Fraknoi continues to serve on the Board of Directors of the Search for Extraterrestrial Intelligence (SETI) Institute, a scientific and educational organization. He is also an elected Fellow of the California Academy of Sciences, an Honorary Member of the Royal Astronomical Society of Canada, and a Fellow of the Committee for Skeptical Inquiry. He has a special interest in debunking astrology and other pseudosciences connected to astronomy.
== Early life and education ==
Fraknoi was born in Hungary in 1948. Eight years later, following the Hungarian Revolution of 1956, he and his family fled their home in Budapest. They spent almost a year in an Austrian refugee camp and finally resettled in New York City. He entered his first American school at age 11, unable to speak English. Comic books, with their pictures and simple language, became his preferred entry point to learning. His initial interest was superhero comic books, and then comics with outer space themes. "This isn't just comic books – this is real," he recalls thinking about space.
Fraknoi graduated from the Bronx High School of Science in 1966. He earned his A.B. in Astronomy (with a minor in Physics) from Harvard University in 1970, and his M.A. in Astronomy from University of California, Berkeley in 1972.
== Professional career ==
Fraknoi held the position of Chair of the Department of Astronomy at Foothill College from 1992 to 2017. He also taught astronomy and physics at other institutions including San Francisco State University, City College of San Francisco, Cañada College, and several campuses of the University of California Extension Division. Fraknoi served as the executive director of the Astronomical Society of the Pacific from 1978 to 1992, edited its popular magazine "Mercury", both expanding circulation and reaching out to lay people as well as teachers. In this role he also established the newsletter "The Universe in the Classroom" specifically for teachers. He is the founder and was director of "Project ASTRO", which sets up partnerships between volunteer astronomers and 4th-9th grade teachers; each astronomer "adopts" one classroom for a year, visits at least four times, and works with the teacher to do hands-on activities in astronomy. The program is still operating in sites around the country. Later he founded and directed "Family ASTRO", a project to design activities, kits and games to help families share the excitement of astronomical discovery.
Fraknoi is recognized for both his multi-dimensional approach, and his innovation, in making astronomy more accessible to all. His popular interdisciplinary course on Albert Einstein's life and work, Physics for Poets (nicknamed "Einstein Without Tears"), won the 2005 "Innovation of the Year" award from the League for Innovation. In this course students learn about areas of modern physics that Einstein had a role in creating or changing, and then read novels, stories, and poems, and hear music influenced by Einstein's work and ideas. According to Thuy Thi Nguyen, president of Foothill College at the time of Fraknoi's retirement, the college sent a memo to the student body to warn them that spring semester 2017 would be their last chance to attend this very popular course. Fraknoi also created and offered various other courses for non-science majors.
In 2007 Fraknoi was the narrator for Gustav Holst's "The Planets" for the California Symphony Orchestra, a role he repeated with the Peninsula Symphony in 2017. He holds a long-time interest in astronomically correct science fiction, which he also uses in his teaching and writing. He has compiled an extensive resource with examples of scientifically accurate science fiction. He also is a science fiction author in his own right with twelve published stories: for example, two in the magazine Sci Phi Journal, one in the online magazine Flash Fiction, and two in science fiction anthologies.
Since 1999, Fraknoi has organized and moderated the Silicon Valley Astronomy Lecture Series where noted astronomers from around California and the nation give nontechnical public talks on new developments in our exploration of the universe in the large Smithwick Theater at Foothill College. Cosponsored by the SETI Institute, and the Astronomical Society of the Pacific, the talks, attended by 400 to 900+ people each time, have featured Nobel laureates, members of the National Academy of Sciences, and many other distinguished scientists. Many lectures have been taped and are available on YouTube where the series has over 4 million views.
Fraknoi has served on the Board of Directors of the SETI Institute, a scientific and educational organization devoted to the search for life in the universe, since its inception in 1985. From 2010 to 2012, he was vice-chair of the Board and served on the program committee planning the first and second SETIcon, a national weekend public conferences devoted to the scientific quest for our counterparts among the stars. In 2013, he was elected to the board of trustees of the Friends of the Lick Observatory, later called the Lick Observatory Council.
In his retirement Fraknoi continues to teach classes at both the Fromm Institute for Lifelong Learning through University of San Francisco, and the Osher Lifelong Learning Institute at San Francisco State University.

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== Written works ==
Fraknoi is the author or co-author of 14 books in the field of astronomy. He was the lead author of Voyages through the Universe, an introductory college astronomy textbook published by Brooks-Cole, which went through three editions. In the 1980s, he co-edited with Byron Preiss two collections of science articles and science fiction stories, "The Universe" and "The Planets." With Sidney Wolff, Fraknoi founded and was co-editor of the first on-line journal devoted to astronomy education, "Astronomy Education Review". He edited two collections of resources for K-12 teachers, The Universe at Your Fingertips and More Universe at Your Fingertips published through the Astronomical Society of the Pacific. Additionally, he is the lead author of the 2016 college textbook "Astronomy", published by OpenStax as a free book for college students around the world, part of a project at Rice University (supported by the Bill and Melinda Gates Foundation and the William and Flora Hewlett Foundation) to make college more affordable.
He also authored multiple resources for young people. He is the co-author of the richly illustrated 2017 children's book about eclipses "When the Sun Goes Dark", that came out just ahead of the North American solar eclipse in August 2017. In 2007, his first children's book "Wonderful World of Space" was published by Disney. When asked about the book in a 2008 podcast interview, Fraknoi explained, "This has been a fun project. My son, who was 13 at the time, and I got a chance to write a picture book on astronomy for Disney and the challenge was how can we convey some of these modern ideas, including the Big Bang, to kids who are in 4th or 5th grade." His 2015 book, "Solar Science", published by the National Science Teachers Association, is full of 45 hands-on activities about the Sun, the seasons, the Moon, eclipses, and more.
Fraknoi frequently writes articles on interdisciplinary topics, such as using music, poetry, or science fiction to teach science. He has published a list of science fiction stories using good astronomy, as well as a resource guide to music inspired by astronomy. Fraknoi himself has had twelve science-fiction stories published. two of them in anthologies. The story "Cave in Arsia Mons" is in the book "Building Red: Mission Mars," edited by Janet Cannon, and the story "Supernova Rhythm" is published in the book "Science Fiction by Scientists," edited by Mike Brotherton and published by Springer. Two of his stories appeared in Sci Phi Journal, while others can be found on-line in Flash Fiction, Wyldblood, The Worlds Within, and Theme of Absence magazines; see External Links below.
== Media appearances ==
Fraknoi has been a frequent radio, television and podcast guest explaining astronomical developments in everyday language. According to his published biography at Wonderfest, the science education organization that awarded him their 2002 Carl Sagan Prize for Science Popularization, Fraknoi "appeared for over 20 years on the Jim Eason Show on KGO or KSFO radio and [as] a regular guest on both the Pete Wilson Show (later the "Gil Gross Show") on KGO and Michael Krasny's Forum program on KQED. Nationally, he has been heard regularly on Science Friday and Weekend All Things Considered on National Public Radio. He has given over 400 public lectures on topics ranging from the death of stars to the origin of the universe." His television appearances include The Today Show, CBS Morning News, and Larry King Live. He also posts frequently on his own blog Exploring the Universe.
== Awards and recognition ==
Fraknoi has been the recipient of many notable awards throughout his career. In 1994 he was awarded the American Astronomical Society's Annenberg Foundation Award -- then the highest honor in the field of astronomy education, as well as the Klumpke-Roberts Award of The Astronomical Society of the Pacific (given for a lifetime of contributions to popularizing astronomy). In 2002 he received the Carl Sagan Prize for Science Popularization. In 2007 he was named California Professor of the Year by the Carnegie Foundation for the Advancement of Teaching and the Council for Advancement and Support of Education. Additionally he was conferred the Astronomical Society of the Pacific's 2007 Richard H. Emmons award and the American Institute of Physics' 2007 Andrew Gemant Award (given for a lifetime of contributions to the intersection of physics and culture). In 2011 he was elected an Honorary Member by the Royal Astronomical Society of Canada and in 2013 was conferred the Faraday Science Communicator Award. Asteroid 4859 was named Asteroid Fraknoi by the International Astronomical Union "to honor his work in sharing the excitement of modern astronomy with students, teachers and the public".
In March 2019, Fraknoi was awarded the 2019 Space Educator: Lifetime Achievement Award from The National Space Club, a prestigious award that recognizes people for significant contributions to space-science education. He was presented with the award at the 62nd Annual Goddard Memorial Dinner on March 22, 2019.
He was elected a Legacy Fellow of the American Astronomical Society in 2020.

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== Personal philosophy ==
Fraknoi has demonstrated, through a lifetime of work, his commitment to advancing public understanding of astronomy and science using everyday language. He said in an interview after being named 2007 California Professor of the Year: "I believe that an understanding of our place in the wider universe and the methods of science are part of the birthright of everyone living on our planet.... Yet, the way science is taught in this country can often discourage non-science majors from taking a life-long interest, or even a course-long interest, in science. My philosophy is to show students that science is engaging, human, and part of our cultural heritage." In 2013 Fraknoi received the Faraday Science Communicator Award. Michael Faraday, for whom the award was named, was an influential 19th century scientist and a great advocate for rigorous, skeptical thinking and recognizing how easy it is for the mind to deceive itself. Fraknoi was quoted on receipt of the award, "I too try to encourage students and the public to examine claims at the fringes of science with skepticism and fact-based thinking".
== Personal life ==
Fraknoi and his wife live in San Francisco. They have one son.
== Works ==
Fraknoi, Andrew; Morrison, David; Wolff, Sidney (March 9, 2022). Astronomy. Houston, Texas: OpenStax and 12th Media Services. ISBN 9781680920383.
Fraknoi, Andrew; Morrison, David; Wolff, Sidney (March 11, 2005). Voyages Through the Universe (3 ed.). Belmont, CA: Brooks Cole. ISBN 978-0495017899.
Fraknoi, Andrew; Schatz, Dennis (2017). When the Sun Goes Dark. National Science Teachers Association. ISBN 978-1-68140-011-2.
Schatz, Dennis; Fraknoi, Andrew (2015). Solar Science: Exploring Sunspots, Seasons, Eclipses, and More. National Science Teachers Association Press. ISBN 978-1-941316-07-8.
Fraknoi, Andrew (3 July 2007). Space. Disney Press. ISBN 978-0-7868-4969-7.
Fraknoi, Andrew (2007). Wonderful World of Space. Disney Learning.
Fraknoi, Andrew (1985). Universe in the classroom : a resource guide for teaching astronomy and instructor's manual for Universe by William J. Kaufmann, III. New York: W.H. Freeman. ISBN 978-0716716921.
Some articles on the web by Fraknoi
Fraknoi's 2019 humorous science fiction story on a message from an extra-terrestrial civilization
Fraknoi's 2021 short science fiction story 'I Swallowed a Martian'
Fraknoi's 2022 short science fiction story 'Slow-time Station'
Fraknoi's short science fiction story 'No One Bet on Canis Major'
Fraknoi's 2025 short science fiction story 'The Lurker'
Fraknoi's 2026 short science fiction story 'Who Speaks for Earth'
Fraknoi's catalog of over 250 pieces of music inspired by astronomy
Fraknoi's annotated subject index of science fiction stories with good astronomy
== References ==
== External links ==
Fraknoi's website
Fraknoi Facebook page
Video
Fraknoi explaining black holes in everyday terms YouTube
Fraknoi's talk on "The 'All-American' Eclipse of the Sun" YouTube
SVAstronomyLectures YouTube channel

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Archaeology, Anthropology, and Interstellar Communication is a 2014 collection of essays edited by Douglas Vakoch and published by NASA. The book is focused on the role that the humanities and social sciences, in particular anthropology and archaeology, play in the search for extraterrestrial intelligence (SETI). The seventeen essays are gathered into four sections, which respectively explore the history of SETI as a field; archaeological comparisons for human-alien communication, such as the difficulties of translating ancient languages; the inferential gap between humans and aliens, and the consequences this would have for communication and trade; and the potential nature of alien intelligences.
Originally scheduled for publication in June 2014, a PDF of Archaeology, Anthropology, and Interstellar Communication was accidentally released a month before the intended date and reviewed by Gizmodo. The positive response to the review inspired NASA to bring forward its release as an e-book, making it available on their website from May of that year.
The book gained widespread media coverage upon release. As well as receiving generally positive reviews, it was at the center of controversy regarding misinterpretation of one of its essays. A quote about ancient terrestrial stone carvings, rhetorically stating that they "might have been made by aliens" for all that they were understood by modern anthropologists, was misreported by publications such as TheBlaze, The Huffington Post, and Artnet.
== Synopsis ==
Historically, research into extraterrestrial intelligence has fallen within natural science and focused primarily on the technological obstacles to alien communication, such as processing the data encoded in signals that could be received from extraterrestrial civilizations. Archaeology, Anthropology, and Interstellar Communication was written as part of an expansion of the field to humanities and social sciences, focusing on the role archaeologists and anthropologists play in extraterrestrial intelligence research. The problems of studying ancient societies on Earth, editor Douglas Vakoch argues, are applicable to those of studying potential societies outside Earth.
Archaeology, Anthropology, and Interstellar Communication is a collection of essays exploring these roles, focusing on both historical and modern perspectives. The book consists of seventeen essays, with an introduction and epilogue by Vakoch and fifteen chapters by researchers in the relevant fields; contributing authors include John Traphagan, Albert Harrison, Ben Finney, Steven J. Dick, John Billingham, and Dominique Lestel. Issues discussed in the essays include the evolutionary and cultural prerequisites for interstellar communication, the challenges for semiotics in decoding alien signs and symbols, and the complexities of cross-cultural communication with aliens by analogy to anthropological first contact experiences.
=== Essays ===
Archaeology, Anthropology, and Interstellar Communication is subdivided into four sections, each with several essays. "Historical Perspectives on SETI" is a historiography of NASA's SETI (search for extraterrestrial intelligence) program, which ran for much of the late twentieth century before being dissolved due to lack of funding, and its humanities and social sciences representation. "Archaeological Analogues" draws comparisons between archaeology on Earth, where archaeologists frequently need to research societies they have little understanding of or shared context with, and communication with extraterrestrials. "Anthropology, Culture, and Communication" examines the role of anthropology in studying alien cultures and societies, such as the assumptions and challenges involved in cross-cultural communication and contact. "The Evolution and Embodiment of Extraterrestrials" deals with topics such as the appearance, diversity, and message design of alien intelligence.
==== "Historical Perspectives on SETI" ====
The essays in this section summarize the history of SETI at NASA, the circumstances that led to the government cutting public funding for SETI, and the role the social sciences have historically played in the search. The first essay in this section, "SETI: The NASA Years", is a synopsis of NASA's involvement with SETI by John Billingham, who was involved with the project from its genesis in 1969 to its closure in 1995. He discusses the 1960s origins of the search for alien life in the universe, the project's struggle to receive popular respect and government funding, and its ultimate cessation at the agency due to funding cuts, after which it was absorbed by the privately funded SETI Institute.
"A Political History of NASA's SETI Program" by Stephen J. Garber analyzes the circumstances that led to the end of public funding for the program. NASA's SETI program was small and provided few jobs that would make cutting it politically complex; widespread skepticism about the existence of intelligent alien life also made the project inherently controversial. NASA's funding also suffered through the 1990s due to publicized issues with the Hubble Space Telescope, weakening its ability to defend a marginal program such as SETI. "The Role of Anthropology in SETI: A Historical View" by Steven J. Dick discusses the history of SETI, anthropology, and their intersection. Representation of the social sciences in SETI research began during the field's early days in the 1960s and 1970s, but was, according to the essay, often tokenistic; Dick traces significant interdisciplinary work as beginning in the 1980s, with particular focus on the publication of Interstellar Migration and the Human Experience in 1986.
==== "Archaeological Analogues" ====

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The essays in this section focus on the relevance of archaeological comparisons for discussing the anticipated difficulties with communication between humans and aliens. "A Tale of Two Analogues" by Ben Finney and Jerry Bentley draws on Finney's studies of Mayan culture. They draw comparisons between the protracted process of translating Mayan works and the difficulty of translating an alien work, and cast doubt on the views of some mathematicians and natural scientists that an extraterrestrial civilization would communicate with humanity solely through the "universal language" of mathematics and science. The second essay, "Beyond Linear B" by Richard Saint-Gelais, analyzes potential alien communication through a semiotic lens, commenting on the issues involved in interpreting the signs and symbols of a fundamentally different culture. He notes that the issues faced in semiotic challenges such as decoding an unknown human language may be even greater for an alien language. For instance, he describes how all known writing systems for human languages are either alphabetic, syllabic, or ideographic, and anthropologists are able to estimate which type an unknown system is by its number of characters, which may not be a shared assumption for an extraterrestrial writing system.
"Learning to Read" focuses on the hypothetical alien translation of interstellar messages transmitted by humanity. Its author Kathryn E. Denning deems the task of writing alien-translatable messages "neither trivial nor impossible", considering it a difficult task but one worthy of study; she discusses the need for interdisciplinary study to produce such messages, with important work from fields such as cryptography and anthropology. She also discusses the polarized views of natural and social sciences on the issue of alien translation, with natural scientists tending to take far more optimistic perspectives of the ease of translation than social scientists. "Inferring Intelligence" by Paul K. Wason describes the difficulty of understanding the work of prehistoric cultures and compares this difficulty to that of understanding the work of extraterrestrial cultures. He refers to the controversy regarding the meaning of Paleolithic cave art, as well as the relative recency of identifying stone tools as the intentional productions of intelligent beings.
==== "Anthropology, Culture, and Communication" ====
John W. Traphagan has two essays in this section, "Anthropology at a Distance" and "Culture and Communication with Extraterrestrial Intelligence". In the former, he draws comparison between the "anthropology at a distance" practice of anthropologists in the early nineteenth century, who often lacked the resources to perform fieldwork with the societies they studied, and the practice of SETI in discussing and studying uncontacted extraterrestrials. The latter focuses on the concept of hypothetical "universal languages", such as music or mathematics, and the differences human and alien cultures may have in their interpretation of these languages.
"Contact Considerations" by Douglas Raybeck considers human-alien interaction through comparison to terrestrial colonial interactions. He gives the specific examples of European contact with Aztec, Japanese, Chinese, Iroquois, and Māori cultures, all five cultures being politically and technologically complex at the time of first European contact. He discusses the likely significance of trade to human-alien interactions, both for goods and services exchanged in trade between human cultures, but potentially also for things such as music that may not exist in an alien culture. In "Speaking for Earth", Albert A. Harrison discusses the development, longevity, and potential consequences of projecting interstellar messages. He takes an optimistic position of the benevolence of extraterrestrial civilizations, referring to his own anthropological research that shows societies that endure for long periods tend to be more peaceful and less aggressive. Harrison supports Active SETI, the process of actively transmitting messages from Earth to potential interstellar societies, and discusses planned and actual attempts at it.
==== "The Evolution and Embodiment of Extraterrestrials" ====
Vakoch's chapter, "The Evolution of Extraterrestrials", focuses on hypotheses of what an alien intelligence would look like, such as whether it would be humanoid or nonhumanoid. He discusses how as early as The Celestial Worlds Discover'd, published by Christiaan Huygens in 1698 and one of the first works to consider the lives of extraterrestrial beings, the possibility was raised that aliens would have similar body plans to humans (such as walking upright) but look radically different within such confines. Both arguments in favour of convergent evolution to a functionally humanoid form and divergent evolution to a radically inhuman form are summarized and considered.
"Biocultural Prerequisites for the Development of Interstellar Communication" by Garry Chick discusses the Drake equation, a means by which to estimate the number of extraterrestrial civilizations in the Milky Way galaxy capable of communicating with humans. Referring to statements by figures such as the author Michael Crichton that the parameters in the Drake equation are unknowable, and that this casts foundational doubt on the validity of SETI, Chick aims to narrow the range of possible estimates for these parameters. In "Ethology, Ethnology, and Communication with Extraterrestrial Intelligence", Lestel considers the philosophical definition of 'communication' in the context of human-alien contact. He holds that contact between terrestrial and extraterrestrial societies would have traits of both ethnology, the study of other human cultures, and ethology, the study of animal behavior. In the book's final essay, "Constraints on Message Construction for Communication with Extraterrestrial Intelligence", William H. Edmondson summarizes the issue of designing messages to be understood by extraterrestrial societies. He notes the assumptions involved in constructing interstellar messages, such as that aliens will have senses and that aspects of cognitive function (e.g. intentional behavior) will be shared between all intelligent organisms.

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== Publication history ==
Vakoch is a professor emeritus of clinical psychology at the California Institute of Integral Studies and a self-described exo-semiotician whose research interests include psychology, comparative religion, and the philosophy of science. He is the Director of Interstellar Message Composition at the SETI Institute. In a 2002 interview with Dennis Overbye for The New York Times, he discussed his criticism of the natural sciences focus of SETI research and his work to view the subject through a humanities-focused lens, including the comparison of interstellar communication to cross-cultural interactions between terrestrial societies. One of Vakoch's goals in compiling and editing Archaeology, Anthropology, and Interstellar Communication was to highlight less optimistic perspectives on interstellar communications from such fields, addressing concerns about significant inferential gaps that had been neglected by the physical sciences.
NASA intended to publish Archaeology, Anthropology, and Interstellar Communication in both print and e-book form on 10 June 2014. On 21 May, a PDF file of the book was accidentally published on NASA's website and picked up by Gizmodo. The PDF was taken down rapidly after Gizmodo published a review, with the intention of re-releasing it on the originally intended date, but demand for copies was so high that the publication was accelerated; MOBI, EPUB, and PDF versions were officially released on 22 May and made freely available online. A paperback edition was published in September 2014 and a hardcover edition was published that December. The collection was published by NASA's History Program Office, part of the Public Outreach Division of its Office of Communications, under the NASA History Series imprint.
== Cultural impact and reception ==
Gizmodo described Archaeology, Anthropology, and Interstellar Communication as "truly fascinating stuff" that managed to be both complex and accessible. The review began with an out-of-context quote from William Edmondson's essay on how mysterious stone carvings "might have been made by aliens" as a metaphor for the difficulties in researching long-lost ancient societies. Though it went on to note that this should not be interpreted as a literal statement, the quote was picked up by publications such as Artnet, The Blaze, and The Huffington Post as a clickbait headline. Some of these articles noted that the statement was not representative of the essay's content; others took it at face value. The aerospace analyst Jeff Foust decried the phenomenon in his review, but noted its role in highlighting how difficult even communication between human beings of similar cultures can be.
Upon the book's official release, it received mostly positive reviews. Emily Gertz, writing for Popular Science, found it "refreshing" and compared the issues it raised to those explored by science fiction works such as The Sparrow, a novel about a Jesuit priest making contact with an alien civilization. Michael Franco of CNET lauded its comprehensiveness, and Jolene Creighton, co-founder of the science news site From Quarks to Quasars, called it "a fantastic text to save for a rainy day". Writing for The Daily Dot, Aja Romano commented that the book came from a thoroughly optimistic point of view about both the existence and benevolence of extraterrestrial intelligence, but that it provided thorough investigation into SETI and had a strong understanding of the subject it investigated. Mark Anderson, Chair of the Notable Documents Panel of the American Library Association's Government Documents Round Table and research librarian at the University of Northern Colorado, reviewed Archaeology, Anthropology, and Interstellar Communication for Library Journal alongside other books published by United States government offices. He highlighted the depth of the book's scholarship and its nonetheless accessible writing.
In June 2014, weeks after the book's official release, Joshua Rothman interviewed Vakoch for The New Yorker about the struggles of extraterrestrial communication. Vakoch explained the book's purpose, discussing the integral role archaeologists and anthropologists play in extraterrestrial research. He referred to the conclusions reached by the essayists, such as Lestel's discussion of the implications involved in being unable to understand or decode potential alien messages. Vakoch described the humanities perspective on extraterrestrial communication as increasingly "skeptical and critical", but "a criticism that engages, as opposed to a criticism that dismisses". He noted that although bridging the communication gap with an extraterrestrial civilization would be a difficult ask, the rapid discovery of exoplanets in the past decades increased the likelihood extraterrestrial intelligence would be identified, making the issue more relevant.
== References ==
== External links ==
PDF available for free download through NASA

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The Arecibo Observatory, also known as the National Astronomy and Ionosphere Center (NAIC) and formerly known as the Arecibo Ionosphere Observatory, is an observatory in Barrio Esperanza, Arecibo, Puerto Rico, owned by the US National Science Foundation (NSF).
The observatory's main instrument was the Arecibo Telescope, a 305 m (1,000 ft) spherical reflector dish built into a natural sinkhole, with a cable-mount steerable receiver and several radar transmitters for emitting signals mounted 150 m (492 ft) above the dish. Completed in 1963, it was the world's largest single-aperture telescope for 53 years, surpassed in July 2016 by the Five-hundred-meter Aperture Spherical Telescope (FAST) in China. On August 10 and November 6, 2020, two of the receiver's support cables broke and the NSF announced that it would decommission the telescope. The telescope collapsed on December 1, 2020. In 2022, the NSF announced the telescope would not be rebuilt, with an educational facility to be established on the site.
The observatory also includes a smaller radio telescope, a LIDAR facility, and a visitor center, which remained operational after the telescope's collapse. The asteroid 4337 Arecibo was named after the observatory by Steven J. Ostro, in recognition of the observatory's contributions to the characterization of Solar System bodies.
== History ==
In the 1950s, the United States Department of Defense (DoD) Advanced Research Projects Agency (ARPA) was seeking a means to detect missiles in Earth's ionosphere. On November 6, 1959, Cornell University entered into a contract with ARPA to carry out development studies for a large-scale ionospheric radar probe. The Arecibo Telescope was consequently built to study the ionosphere as well as to serve as a general-purpose radio telescope. Construction of the telescope was started in September 1960. The telescope and supporting observatory were formally opened as the Arecibo Ionospheric Observatory on November 1, 1963.
DoD transferred the observatory to the National Science Foundation on October 1, 1969. NSF appointed Cornell University to manage the observatory. By September 1971, NSF had renamed the observatory the National Astronomy and Ionosphere Center (NAIC) and had made it a federally funded research and development center (FFRDC). NASA began contributing funds to the observatory alongside NSF for its planetary radar mission.
In the early 2000s, NASA eliminated funding for the Arecibo Observatory. In 2006, NSF indicated that it would reduce funding for the observatory, and decommission it if other funding could not be found. Academics and politicians lobbied to stave off its closure, and NASA recommitted funding in 2011 for study of near-Earth objects. In 2011, NSF delisted Arecibo as an FFRDC, which allowed the observatory to seek funding from a wider variety of sources; the agency also replaced Cornell as the site operator with a team led by SRI International.
Damage to the telescope by 2017's Hurricane Maria led NSF again to suggest closing the observatory. A consortium led by the University of Central Florida (UCF) proposed to manage the observatory and cover much of the operations and maintenance costs, and in 2018, NSF made UCF's consortium the new site operators, though no specific actions or funding were announced.
On August 6, 2020, an auxiliary cable broke on the telescope, followed by a main cable on November 7. The NSF announced that they would decommission the telescope through controlled demolition, but that the other facilities on the observatory would remain operational. Before demolition could occur, remaining support cables from one tower rapidly failed in the morning of December 1, 2020, causing the instrument platform to crash through the dish, shearing off the tops of the support towers, and partially damaging some of the other buildings, though with no injuries. NSF officials said in 2020 that they aimed to have the other observatory facilities operational as soon as possible and were considering rebuilding a new telescope instrument in its place. However, in 2022, the NSF announced the telescope would not be rebuilt but an educational facility would be established on the site. The following year, NSF picked a consortium of universities—Cold Spring Harbor Laboratory in New York; the University of Maryland, Baltimore County; the University of Puerto Rico, Río Piedras Campus, in San Juan; and the University of the Sacred Heart, also in San Juan—to set up and run an education center called Arecibo C3 (Arecibo Center for Culturally Relevant and Inclusive Science Education, Computational Skills, and Community Engagement).
== Facilities ==
=== Arecibo Telescope ===

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The observatory's main feature was its large radio telescope, the main collecting dish of which was an inverted spherical dome 1,000 feet (305 m) in diameter with an 869-foot (265 m) radius of curvature, constructed inside a karst sinkhole. The dish's surface was made of 38,778 perforated aluminum panels, each about 3 by 7 feet (1 by 2 m), supported by a mesh of steel cables. The ground beneath supported shade-tolerant vegetation.
Since its completion in November 1963, the Telescope had been used for radar astronomy and radio astronomy, and had been part of the Search for extraterrestrial intelligence (SETI) program. It was also used by NASA for Near-Earth object detection. Since around 2006, NSF funding support for the telescope had waned as the Foundation directed funds to newer instruments, though academics petitioned to the NSF and Congress to continue support for the telescope. Numerous hurricanes, including Hurricane Maria, had damaged parts of the telescope, straining the reduced budget.
Two cable breaks, one in August 2020 and a second in November 2020, threatened the structural integrity of the support structure for the suspended platform and damaged the dish. The NSF determined in November 2020 that it was safer to decommission the telescope rather than to try to repair it, but the telescope collapsed before a controlled demolition could be carried out. The remaining support cables from one tower failed around 7:56 a.m. local time on December 1, 2020, causing the receiver platform to fall into the dish and collapsing the telescope.
NASA led an extensive failure investigation and reported the findings, along with a technical bulletin with industry recommendations. The investigation concluded that "a combination of low socket design margin and a high percentage of sustained loading revealed an unexpected vulnerability to zinc creep and environments, resulting in long-term cumulative damage and progressive zinc/wire failure".
In 2024, the National Academies of Science, Engineering, and Medicine published their definitive report, "Failure Analysis of the Arecibo Observatory 305-Meter Telescope Collapse". The report cited many of the previous reports and findings, including the role of the high-energy output interacting with the zinc wire rope "brooming". It also raised the issue of the original design standards available in the 1980s versus the later advances in wind load engineering.
=== Additional telescopes ===
The Arecibo Observatory also has other facilities beyond the main telescope, including a 12-meter (39 ft) radio telescope intended for very-long-baseline interferometry (VLBI) with the main telescope; and a LIDAR facility whose research has continued since the main telescope's collapse.
=== Ángel Ramos Foundation Visitor Center ===
Opened in 1997, the Ángel Ramos Foundation Visitor Center features interactive exhibits and displays about the operations of the radio telescope, astronomy and atmospheric sciences. The center is named after the financial foundation that honors Ángel Ramos, owner of the El Mundo newspaper and founder of Telemundo. The Foundation provided half of the funds to build the Visitor Center, with the remainder received from private donations and Cornell University.
The center, in collaboration with the Caribbean Astronomical Society, hosts a series of Astronomical Nights throughout the year, which feature diverse discussions regarding exoplanets, astronomical phenomena, and discoveries (such as Comet ISON). The purposes of the center are to increase public interest in astronomy, the observatory's research successes, and space endeavors.
== List of directors ==
Source(s):
== See also ==
== References ==
== Further reading ==
== External links ==
Official website

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The Arecibo Telescope was a 305 m (1,000 ft) spherical reflector radio telescope built into a natural sinkhole at the Arecibo Observatory located near Arecibo, Puerto Rico. A cable-mounted, steerable receiver and several radar transmitters for emitting signals were mounted 150 m (492 ft) above the dish. Completed in November 1963, the Arecibo Telescope was the world's largest single-aperture telescope for 53 years, until it was surpassed in July 2016 by the Five-hundred-meter Aperture Spherical Telescope (FAST) in Guizhou, China. Decommissioning the Arecibo Telescope was announced in November 2020, and the telescope collapsed in December 2020.
The Arecibo Telescope was primarily used for research in radio astronomy, atmospheric science, and radar astronomy, as well as for programs that search for extraterrestrial intelligence (SETI). Scientists wanting to use the observatory submitted proposals that were evaluated by independent scientific referees. NASA also used the telescope for near-Earth object detection programs. The observatory, funded primarily by the National Science Foundation (NSF) with partial support from NASA, was managed by Cornell University from its completion in 1963 until 2011, after which it was transferred to a partnership led by SRI International. In 2018, a consortium led by the University of Central Florida assumed operation of the facility.
The telescope's unique and futuristic design led to several appearances in film, gaming and television productions, such as for the climactic fight scene in the James Bond film GoldenEye (1995). It is one of the 116 pictures included in the Voyager Golden Record. It has been listed on the US National Register of Historic Places since 2008. The telescope was named an IEEE Milestone in 2001.
The NSF reduced its funding commitment to the observatory from 2006, leading academics to push for additional funding support to continue its programs. The telescope was damaged by Hurricane Maria in 2017 and was affected by earthquakes in 2019 and 2020. Two cable breaks, one in August 2020 and a second in November 2020, threatened the structural integrity of the support structure for the suspended platform and damaged the dish. Due to uncertainty over the remaining strength of the other cables supporting the suspended structure, and the risk of collapse owing to further failures making repairs dangerous, the NSF announced on November 19, 2020, that the telescope would be decommissioned and dismantled, with the LIDAR facility remaining operational. Before it could be decommissioned, several of the remaining support cables suffered a critical failure and the support structure, antenna, and dome assembly all fell into the dish at 7:55 a.m. local time on December 1, 2020, destroying the telescope. The NSF decided in October 2022 that it would not rebuild the telescope or build a similar observatory at the site.
== General information ==
The telescope's main collecting dish had the shape of a spherical cap 305 metres (1,000 ft) in diameter with an 265-metre (870 ft) radius of curvature, and was constructed inside a karst sinkhole. The dish surface was made of 38,778 perforated aluminum panels, each about 1 by 2 metres (3 by 7 ft), supported by a mesh of steel cables. The ground beneath supported shade-tolerant vegetation.
The telescope had three radar transmitters, with effective isotropic radiated powers (EIRPs) of 22 TW (continuous) at 2380 MHz, 3.2 TW (pulse peak) at 430 MHz, and 200 MW at 47 MHz, as well as an ionospheric modification facility operating at 5.1 and 8.175 MHz.
The dish remained stationary, while receivers and transmitters were moved to the proper focal point of the telescope to aim at the desired target. As a spherical mirror, the reflector's focus was along a line rather than at one point. As a result, complex line feeds were implemented to carry out observations, with each line feed covering a narrow frequency band measuring 10–45 MHz. A limited number of line feeds could be used at any one time, limiting the telescope's flexibility.
The receiver was on an 820-tonne (900-short-ton) platform suspended 150 m (492 ft) above the dish by 18 main cables running from three reinforced concrete towers (six cables per tower), one 111 m (365 ft) high and the other two 81 m (265 ft) high, placing their tops at the same elevation. Each main cable was a 8 cm (3.1 in) diameter bundle containing 160 wires, with the bundle painted over and dry air continuously blown through to prevent corrosion due to the humid tropic climate. The platform had a rotating, bow-shaped track 93 m (305 ft) long, called the azimuth arm, carrying the receiving antennas and secondary and tertiary reflectors. This allowed the telescope to observe any region of the sky in a forty-degree cone of visibility about the local zenith (between −1 and 38 degrees of declination). Puerto Rico's location near the Northern Tropic allowed the Arecibo telescope to view the planets in the Solar System over the northern half of their orbit. The round trip light time to objects beyond Saturn is longer than the 2.6-hour time that the telescope could track a celestial position, preventing radar observations of more distant objects.
== History ==
=== Design and construction ===

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The origins of the observatory trace to late 1950s efforts to develop anti-ballistic missile (ABM) defenses as part of the newly formed United States Department of Defense (DoD) Advanced Research Projects Agency (ARPA) ABM umbrella-effort, Project Defender. Even at this early stage it was clear that the use of radar decoys would be a serious problem at the long ranges needed to successfully attack a warhead, ranges on the order of 1,600 km (1,000 miles).
Among the many Defender projects were several studies based on the concept that a re-entering nuclear warhead would cause unique physical signatures while still in the upper atmosphere. It was known that hot, high-speed objects caused ionization of the atmosphere that reflects radar waves, and it appeared that a warhead's signature would be different enough from decoys that a detector could pick out the warhead directly, or alternately, provide added information that would allow operators to focus a conventional tracking radar on the single return from the warhead.
Although the concept appeared to offer a solution to the tracking problem, there was almost no information on either the physics of re-entry or a strong understanding of the normal composition of the upper layers of the ionosphere. ARPA began to address both simultaneously. To better understand the radar returns from a warhead, several radars were built on Kwajalein Atoll, while Arecibo started with the dual purpose of understanding the ionosphere's F-layer while also producing a general-purpose scientific radio observatory.
On November 6, 1959, Cornell University entered into a contract with ARPA to carry out development studies for a large-scale ionospheric radar probe, exploring how this instrument could also be utilized in radio astronomy and other scientific areas. The observatory was built between mid-1960 and November 1963. William E. Gordon and George Peter of Cornell University oversaw its design for study of the Earth's ionosphere. He was attracted to the sinkholes in the karst regions of Puerto Rico that offered perfect cavities for a very large dish. Originally, a fixed parabolic reflector was envisioned, pointing in a fixed direction with a 150 m (492 ft) tower to hold equipment at the focus. This design would have limited its use in other research areas, such as radar astronomy, radio astronomy and atmospheric science, which require the ability to point at different positions in the sky and track those positions for an extended time as the Earth rotates.
Ward Low of the ARPA pointed out this flaw and put Gordon in touch with the Air Force Cambridge Research Laboratory (AFCRL) in Boston, Massachusetts, where one group headed by Phil Blacksmith was working on spherical reflectors and another group was studying the propagation of radio waves in and through the upper atmosphere. Cornell University proposed the project to ARPA in mid-1958 and a contract was signed between the AFCRL and the University in November 1959. Cornell University and Zachary Sears published a request for proposals (RFP) asking for a design to support a feed moving along a spherical surface 133 metres (435 ft) above the stationary reflector. The RFP suggested a tripod or a tower in the center to support the feed. On the day the project for the design and construction of the antenna was announced at Cornell University, Gordon had also envisioned a 133 m (435 ft) tower centered in the 305 m (1,000 ft) reflector to support the feed.
George Doundoulakis, who directed research at the General Bronze Corporation in Garden City, New York, along with Zachary Sears, who directed Internal Design at Digital B & E Corporation, New York, received the RFP from Cornell University for the antenna design and studied the idea of suspending the feed with his brother, Helias Doundoulakis, a civil engineer. George Doundoulakis identified the problem that a tower or tripod would have presented around the center, (the most important area of the reflector), and devised a better design by suspending the feed. He presented his proposal to Cornell University for a doughnut or torus-type truss suspended by four cables from four towers above the reflector, having along its edge a rail track for the azimuthal truss positioning. This second truss, in the form of an arc, or arch, was to be suspended below, which would rotate on the rails through 360 degrees. The arc also had rails on which the unit supporting the feed would move for the feed's elevational positioning. A counterweight would move symmetrically opposite to the feed for stability and, if a hurricane struck, the whole feed could be raised and lowered. Helias Doundoulakis designed the cable suspension system which was finally adopted. The final configuration was substantially the same as in the original drawings by George and Helias Doundoulakis, although with three towers, instead of the four drawn in the patent, which was granted to Helias Doundoulakis by the U.S. Patent office.
The suspended structure was designed by Dr. Thomas C. Kavanagh, Fred Severud, and Dr. Hans Bandel, who were selected after the 1959 RFP issued by Cornell University. A proposal by the General Bronze Corporation was not selected as it did not meet specifications, according to an editorial response by Donald Cooke (Cornell's spokesperson) to Helias Doundoulakis in a newsletter of the Institute of Electrical and Electronics Engineers (IEEE). Cooke stated that Doundoulakis used an incorrect feed/paraxial surface measurement. However, the measurement Cooke used was from Doundoulakis’ patent issued in 1966, and not from the 1959 RFP meetings which predated the patent by seven years. Furthermore, proposal measurements presented by George Doundoulakis and Helias Doundoulakis at the RFP meeting on December 10, 1959, were not referenced in Cooke's editorial response. The originators of this proposal subsequently filed a dispute, originally for $1.2 million but was settled for $10,000 because "the defense in a court trial would cost far more than the $10,000 for which the case was settled," and accordingly, on April 11, 1975, Doundoulakis v. U.S. (Case 412-72) had been ruled in plaintiff's favor by the United States Court of Federal Claims, that “(a) a judgment has been entered in favor of the plaintiffs (Helias Doundoulakis, William J. Casey, and Constantine Michalos) against the United States and (b) in consideration of the sum of $10,000 to be paid by the United States Government to the plaintiff, the plaintiffs grants to the United States Government an irrevocable, fully-paid, non-exclusive license under the aforesaid U.S. Patent No. 3, 273, 156 to Cornell University.”
The idea of a spherical reflecting mirror with a steerable secondary has since been used in optical telescopes, in particular, the Hobby–Eberly Telescope
Construction began in mid-1960, with the telescope operational about three years later. The telescope's and the supporting observatory's official opening as the Arecibo Ionospheric Observatory (AIO) was held on November 1, 1963.

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=== Upgrades ===
Since its construction, the telescope was upgraded several times, following the facility's oversight from the DoD to the National Science Foundation on October 1, 1969, and subsequent renaming of the AIO to the National Astronomy and Ionosphere Center (NAIC) in September 1971.
Initially, when the maximum expected operating frequency was about 500 MHz, the surface consisted of half-inch galvanized wire mesh laid directly on the support cables. In 1973, a high-precision surface consisting of 38,000 individually adjustable aluminum panels replaced the old wire mesh, and the highest usable frequency rose to about 5000 MHz.
A Gregorian reflector system was installed in 1997, incorporating secondary and tertiary reflectors to focus radio waves to one point. This allowed installing a suite of receivers, covering the full 1–10 GHz range, that could be easily moved to the focal point, giving Arecibo more flexibility. The additional instrumentation added 270 tonnes (300 short tons) to the platform, so six additional support cables were added, two for each tower. A metal mesh screen was also installed around the perimeter to block the ground's thermal radiation from reaching the feed antennas. As part of this upgrade, the power of the 2380 MHz transmitter was doubled to 1 MW by adding a second Klystron tube and improving the design.
Finally, in 2013 with a grant of US$2.5 million, work for adding the ionospheric modification HF facility began, which was completed in 2015. The HF facility consisted, on the sender side, of six foldable 100 kW crossed dipoles inside the main dish, and a hanging 100m-wide subreflector mesh between the dish and the platform.
=== Funding reductions ===
The Astronomical Sciences and Atmospheric Sciences divisions of the NSF had financially supported Arecibo since its completion in the 1970s, with incremental support by NASA, for operating the planetary radar. In 2001 NASA announced a rampdown and elimination of its support of the planetary radar by 2005.
In 2002, after several years of discussion, the US Congress passed a bill to double the NSF budget, and instructed the NSF to begin new projects. As a result, the NSF began committing to major projects. However, the funding increase never arrived and the NSF was left with the new commitments. In 2005 the Astronomical Sciences division commissioned a "Senior Review" of its facilities to deal with its increasingly constrained budget. The Senior Review report released in November 2006 "regretfully" recommended substantially decreased astronomy funding for the Arecibo Observatory, beginning with a cut to US$10.5 million in 2007 and continuing to decrease to US$4.0 million in 2011. The report further stated that if other sources of funding could not be found, closure of the Observatory was recommended.
Academics and researchers responded by organizing to protect and advocate for the observatory. They established the Arecibo Science Advocacy Partnership (ASAP) in 2008, to advance the scientific excellence of Arecibo Observatory research and to publicize its accomplishments in astronomy, aeronomy and planetary radar as to seek additional funding support for the observatory. An additional US$3 million in bonds were issued by the government of Puerto Rico to fund the Observatory, which were used to modernize power generation and improve other aging infrastructure. Academics, media and influential politicians pressured the United States Congress on the importance of the work of the observatory. led to additional US$3.1 million in funding to support Arecibo in the American Recovery and Reinvestment Act of 2009. This was used for basic maintenance and for a second, much smaller, antenna to be used for very long baseline interferometry, new Klystron amplifiers for the planetary radar system and student training.
Arecibo's budget from NSF continued to wane in the following years. Starting in FY2010, NASA restored its historical support by contributing $2.0 million per year for planetary science, particularly the study of near-Earth objects, at Arecibo. NASA implemented this funding through its Near Earth Object Observations program. NASA increased its support to $3.5 million per year in 2012.
In 2011, NSF removed Cornell University, which had managed the National Astronomy and Ionosphere Center (NAIC) since the 1970s, as the operator and transferred these responsibilities to SRI International, along with two other managing partners, Universities Space Research Association and Universidad Metropolitana de Puerto Rico, with a number of other collaborators. NSF also decertified NAIC as a Federally Funded Research and Development Center (FFRDC), which the NSF said would give NAIC greater freedom to establish broader scientific partnerships and pursue funding opportunities for activities beyond the scope of those supported by NSF, but which would also remove the FFRDC's promise of stability intended to retain the very best technical staff.
While the Observatory continued to operate under the reduced NSF budget and NASA funds, NSF signaled in 2015 and 2016 that it was looking towards potential decommissioning of the Observatory by initiating environmental impact statements on the effect of disassembling the unit. The NSF continued to indicate it would like to reduce funding to the Observatory in the short term. As in 2008, academics expressed their concern over the loss of scientific discoveries that could occur should the Observatory be shut down.
=== 2020 damage, decommissioning plans, and collapse ===

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Several hurricanes and storms over the 2010s had raised the concerns of structural engineers over the stability of the observatory. On September 21, 2017, high winds associated with Hurricane Maria caused the 430 MHz line feed to break and fall onto the primary dish, damaging roughly 30 of the 38,000 aluminum panels. Most Arecibo observations did not use the line feed but instead relied on the feeds and receivers located in the dome. Overall, the damage inflicted by Maria was minimal, but it further clouded the observatory's future. Restoring all the previous capabilities required more than the observatory's already-threatened operating budget, and users feared that the decision would be made to decommission it instead.
A consortium consisting of the University of Central Florida (UCF), Yang Enterprises and UMET came forward to supply funding in February 2018 to allow the NSF to reduce its contribution towards Arecibo's operating costs from $8 million to $2 million from the fiscal year 2022–2023, thus securing the observatory's future. With this, the UCF consortium were named the new operators of the observatory in 2018.
On August 10, 2020, an auxiliary platform support cable separated from Tower 4, causing damage to the telescope, including a 30 meters (100 ft) gash in the reflector dish. Damage included six to eight panels in the Gregorian dome, and to the platform used to access the dome. No one was reported to have been hurt by the partial collapse. The facility was closed as damage assessments were made.
The facility had recently reopened following the passing of Tropical Storm Isaias. It was unclear if the cable failure was caused by Isaias. Former Arecibo Observatory director Robert Kerr stated that prior to the 1997 installation of the Gregorian dome, the main support cables and support towers had been engineered with a safety factor of two, as to be able to sustain twice the weight of the platform. When the dome was added in 1997, the auxiliary cables were intended to retain the safety factor of two once all design factors were considered, but Kerr believed that that was never the case, as evenly distributing the loads following that install would be difficult to do. Kerr also stated that there had been periods of neglect at the Observatory, during which the fans that were used to blow dry air along the wire bundles were not operating. The earlier storms would have brought seawater to the cables, which could accelerate the rate of corrosion as well, according to Kerr. Engineering firms hired by UCF inspected the socket area where the cable had failed, and found a similar problem that had been observed in the 1980s during a routine cable replacement, in which the use of molten zinc to affix the cable to the socket mount at the tower was not complete, allowing moisture to get into the wire bundle and cause corrosion, and leading to the cable slipping from its socket. The firms had developed models of the telescope that showed that the safety factor for Tower 4 had dropped to 1.67, believing that the structure was still safe while repairs could be effected, even if another cable collapsed. Plans were made to replace all six auxiliary cables since their socket welds were all considered suspect, at a cost of US$10.5 million.
Before repairs could be started, on November 7, 2020, one of the two main support cables from Tower 4 snapped, shattering part of the dish itself as it fell. The UCF engineering staff, which had been monitoring the cables with support from the U.S. Army Corps of Engineers, and the engineering firms they had hired previously evaluated the remaining cables from Tower 4. One engineering firm proposed stabilization efforts, while another suggested that they try to sever parts of the instrument platform such as the Gregorian dome to reduce the load. The third firm made the determination that there was no way to safely repair the damage at that point, as the remaining cables could be suspect, and furthermore that a controlled decommissioning of the telescope was the only effective means to avoid catastrophic failure which would threaten the other buildings on campus. The NSF took this advice and made the announcement on November 19, 2020, that they would decommission Arecibo over the following few weeks after determining the safest route to do so, with a safety exclusion zone immediately put in place. NSF's Sean Jones stated, "This decision is not an easy one for NSF to make, but safety of people is our number one priority." The lidar facility was to remain operational.
While waiting for NSF to make the decommissioning plans, steps had been taken to try to reduce the load that each of the towers was carrying, including reducing the strain on the backstay support cables for the individual towers. Other plans, such as having helicopters hoisting part of the load while hovering above the telescope, were proposed but deemed too risky. Engineers from UCF had been monitoring the telescope and observed that wires in the backstay cables for the support towers had been breaking at a rate of one or two a day, and estimated that the telescope would soon collapse. In the weekend prior to December 1, 2020, wire strands in the receiver's supporting cables had also been snapping apart at a rapid rate, according to Ángel Vázquez, the director of operations. This culminated in the collapse of the receiver platform at around 6:55 a.m. AST (10:55 UTC) on December 1, 2020, as the second main cable from Tower 4 failed, with the other two remaining support cables failing moments later. The collapse of the receiver structure and cables onto the dish caused extensive additional damage. As the receiver fell, it also sheared the tips of the towers which the support cables ran through. Once the main cables from Tower 4 released, the backstay cables, which normally balanced the horizontal component of force from the main cables, pulled the tower outwards and broke off the top. The other two towers, once the force of supporting the platform was released, also had their tips sheared off due to the backstay cable tension. The top of Tower 12 caused some structural damage to other buildings on the observatory as it fell. No injuries from the collapse were reported.

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=== Post-collapse ===
In the weeks following Arecibo's collapse, the administration of the Five-hundred-metre Aperture Spherical Telescope (FAST) in China, which had drawn some design principles from Arecibo, stated that they would start taking applications for international researchers to use the telescope starting in 2021.
In late December 2020, Wanda Vázquez Garced, then governor of Puerto Rico signed an executive order for $8 million for the removal of debris and for the design of a new observatory to be built in its place. The governor stated reconstruction of the observatory is a "matter of public policy". The executive order also designated the area as a history site.
As required by the Consolidated Appropriations Act, 2021, the NSF sent a report to Congress in March 2022 "on the causes and extent of the damage, the plan to remove debris in a safe and environmentally sound way, the preservation of the associated [Arecibo Observatory] facilities and surrounding areas, and the process for determining whether to establish comparable technology at the site, along with any associated cost estimates". On March 25, 2022, a survey salvage committee formed by UCF and the NSF issued a final report, identifying materials from the site that may be salvaged for their "historic importance or scientific utility."
A team from the University of Texas at Austin was able to completely recover and back up the 3 petabytes of data that the telescope had captured since opening in the 1960s by May 2021 before further harm could come to the storage equipment. The data was relocated to the school's servers at the Texas Advanced Computing Center to be made available for continued research.
An early plan developed by NSF scientists suggest one possible replacement called the Next Generation Arecibo Telescope, using 1000 closely-packed 9-meter (30 ft) telescopes mounted on one or more flat plate(s) that would cover the 300-meter (980 ft) width of the Arecibo sinkhole. While the telescopes themselves would be fixed, the plate(s) would be able to be rotated more than 45° off the horizontal in any direction. This would allow the new instrument to have 500 times the field of view of the original Arecibo Telescope, and be twice as sensitive with four times the radar power. It was expected this would cost roughly US$450 million to build. This would enable better study of the supermassive black hole at the center of the Milky Way as a prime target.
NSF decided in October 2022 that the Arecibo site would not be used for a new telescope, instead converting the site to be a STEM educational center.
The Arecibo Salvage Survey committee preserved some parts of the telescope, including parts of the zenith and azimuth tracks, a corner of the platform, the rotary joint, and the cable car.
In 2024 the National Academies of Sciences, Engineering and Medicine issued a report on the collapse. The report cited many of the previous reports and findings, including the role of the high-energy output interacting with the zinc wire rope "booming". Issues of redundancy, a typical engineering principle, was raised since there was no provision for major overhauls. The report also discussed the organizational issues of transfer of knowledge over time regarding various long term maintenance issues. It also raised the issue of the original engineering safety standards available in the 1980s versus the later advances in wind load engineering.
== Research and discoveries ==
Many scientific discoveries were made with the observatory. On April 7, 1964, soon after it began operating, Gordon Pettengill's team used it to determine that the rotation period of Mercury was not 88 days, as formerly thought, but only 59 days. In 1968, the discovery of the periodicity of the Crab Pulsar (33 milliseconds) by Richard V. E. Lovelace and others provided the first solid evidence that neutron stars exist. In 1974, Hulse and Taylor discovered the first binary pulsar PSR B1913+16, an accomplishment for which they later received the Nobel Prize in Physics. In 1982, the first millisecond pulsar, PSR B1937+21, was discovered by Donald C. Backer, Shrinivas Kulkarni, Carl Heiles, Michael Davis, and Miller Goss. This object spins 642 times per second and, until the discovery of PSR J1748-2446ad in 2005, was identified as the fastest-spinning pulsar.
In 1980, Arecibo made the first radar observation of a comet when it successfully detected Comet Encke. In July 1985, the observatory obtained the first two-dimensional image of an asteroid, 1627 Ivar.
In August 1989, radar observations of near-Earth asteroid 4769 Castalia yielded a sequence of delay–Doppler images showing two lobes in contact, providing the first observation of a contact binary. The following year, Polish astronomer Aleksander Wolszczan made the discovery of pulsar PSR B1257+12 (Lich), which later led him to discover its three orbiting planets. These were the first extrasolar planets discovered. In 1994, John Harmon used the Arecibo Radio Telescope to map the distribution of ice in the polar regions of Mercury.
In January 2008, detection of prebiotic molecules methanimine and hydrogen cyanide were reported from the observatory's radio spectroscopy measurements of the distant starburst galaxy Arp 220.
From January 2010 to February 2011, astronomers Matthew Route and Aleksander Wolszczan detected bursts of radio emission from the T6.5 brown dwarf 2MASS J10475385+2124234. This was the first time that radio emission had been detected from a T dwarf, which has methane absorption lines in its atmosphere. It is also the coolest brown dwarf (at a temperature of ~900K) from which radio emission has been observed. The highly polarized and highly energetic radio bursts indicated that the object has a >1.7 kG-strength magnetic field and magnetic activity similar to both the planet Jupiter and the Sun.
=== The Arecibo message ===

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In 1974, the Arecibo message, an attempt to communicate with potential extraterrestrial life, was transmitted from the radio telescope toward the globular cluster Messier 13, about 25,000 light-years away. The 1,679 bit pattern of 1s and 0s defined a 23 by 73 pixel bitmap image that included numbers, stick figures, chemical formulas and a crude image of the telescope.
=== SETI and METI projects ===
Search for extraterrestrial intelligence (SETI) is the search for extraterrestrial life or advanced technologies. SETI aims to answer the question "Are we alone in the Universe?" by scanning the skies for transmissions from intelligent civilizations elsewhere in our galaxy.
In comparison, METI (messaging to extraterrestrial intelligence) refers to the active search by transmitting messages.
Arecibo was the source of data for the SETI@home and Astropulse distributed computing projects put forward by the Space Sciences Laboratory at the University of California, Berkeley, and was used for the SETI Institute's Project Phoenix observations. The Einstein@Home distributed computing project has found more than 20 pulsars in Arecibo data.
=== Other uses ===
Terrestrial aeronomy experiments at Arecibo included the Coqui 2 experiment, supported by NASA. The telescope also originally had military intelligence uses, including locating Soviet radar installations by detecting their signals bouncing off the Moon.
Limited amateur radio operations were carried out, using Moon bounce or Earth–Moon–Earth communication, in which radio signals aimed at the Moon are reflected back to Earth. The first of these operations was on June 13–14, 1964, using the call sign KP4BPZ. A dozen or so two-way contacts were made on 144 and 432 MHz. On July 3 and 24, 1965, KP4BPZ was again activated on 432 MHz, making approximately 30 contacts on 432 MHz during the limited time slots available. For these tests, a very wide-band instrumentation recorder captured a large segment of the receiving bandwidth, enabling later verification of other amateur station call signs. These were not two-way contacts. From April 16–18, 2010, the Arecibo Amateur Radio Club KP4AO again conducted Moon-bounce activity using the antenna. On November 10, 2013, the KP4AO Arecibo Amateur Radio Club conducted a Fifty-Year Commemoration Activation, lasting seven hours on 14.250 MHz SSB, without using the main dish antenna.
== Cultural significance ==
Due to its unique shape and concept, the telescope had been featured in many contemporary works. It serves as one of the central locations in The Sparrow, a science fiction novel written by Mary Doria Russell. It was used as a filming location in the films GoldenEye (1995), Species (1995), and Contact (1997) (based on Carl Sagan's novel of the same name, which also featured the observatory), The Losers (2010), , as the climatic battleground against Ultron in Season 3 of ’’Avengers Assemble’’and in The X-Files television episode "Little Green Men". One map in the 2013 video game Battlefield 4, while set in China, is based on the distinctive layout of the Arecibo Telescope. In 2014, a video art installation piece titled The Great Silence by artists Jennifer Allora and Guillermo Calzadilla in collaboration with science fiction writer Ted Chiang featured the radio telescope at Arecibo Observatory to represent the search for extraterrestrial life. The next year, Chiang published a novelette also called The Great Silence. The juxtaposed text was later published as a short story with the same title in a special issue of the art journal e-flux in 2015 and was included in the author's short story collection Exhalation: Stories in 2019.
The asteroid 4337 Arecibo is named after the observatory by Steven J. Ostro, in recognition of the observatory's contributions to the characterization of Solar System bodies.
== See also ==
List of radio telescopes
Lunar Crater Radio Telescope - a proposed project by NIAC to place a radio telescope on the far side of the Moon
Orgov Radio-Optical Telescope
== References ==
== Further reading ==
Friedlander, Blaine (November 14, 1997). "Research rockets, including an experiment from Cornell, are scheduled for launch into the ionosphere next year from Puerto Rico". Cornell University.
Ruiz, Carmelo (March 3, 1998). "Activists protest US Navy radar project". Global Network Against Weapons and Nuclear Power in Space. Archived from the original on May 1, 2001.
Amir Alexander (July 3, 2008). "Budget Cuts Threaten Arecibo Observatory". The Planetary Society. Archived from the original on July 21, 2008.
Blaine Friedlander (June 10, 2008). "Arecibo joins global network to create 6,000-mile (9,700 km) telescope". EurekAlert.
Lauren Gold (June 5, 2008). "Clintons (minus Hillary) visit Arecibo; former president urges more federal funding for basic sciences". Cornell university.
Henry Fountain (December 25, 2007). "Arecibo Radio Telescope Is Back in Business After 6-Month Spruce-Up". New York Times.
Entry into the National Register of Historic Places
Cohen, Marshall H. (2009). "Genesis of the 1000-foot Arecibo Dish". Journal of Astronomical History and Heritage. 12 (2): 141–152. Bibcode:2009JAHH...12..141C. doi:10.3724/SP.J.1440-2807.2009.02.06. S2CID 18990068. PDF
Altschuler, Daniel R.; Salter, Christopher J. (2013). "The Arecibo Observatory: Fifty astronomical years". Physics Today. 66 (11): 43. Bibcode:2013PhT....66k..43A. doi:10.1063/PT.3.2179.
== External links ==
Official website
Youtube video of collapse 12.56 mins
The wondrous life—and dramatic death—of Puerto Rico's Arecibo Observatory

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The Arecibo message is an interstellar radio message carrying basic information about humanity and Earth that was sent to the globular cluster Messier 13 (M13) in 1974. It was meant as a demonstration of human technological achievement rather than a real attempt to enter into a conversation with extraterrestrials.
The message was broadcast into space a single time via frequency-modulated radio waves at a ceremony to mark the remodeling of the Arecibo Telescope in Puerto Rico on 16 November 1974. The message was aimed at the current location of M13, about 25,000 light years from Earth, because M13 was a large and relatively close collection of stars that was available in the sky at the time and place of the ceremony. When correctly translated into graphics, characters, and spaces, the 1,679 bits of data contained within the message form the image shown here.
== Description ==
The content of the Arecibo message was designed by a group of Cornell University and Arecibo scientists: Frank Drake, formulator of the Drake equation, James C. G. Walker, Linda M. French, and Richard Isaacman. Carl Sagan and others also contributed. The message was meant more as a demonstration of human technological achievement than a serious attempt to enter into a conversation with possible extraterrestrials. As globular cluster M13, at which the message was aimed, is more than 25,000 light-years from Earth, the message, traveling at the speed of light, will take at least 25,000 years to arrive there. By that time, the core of M13 will no longer be in precisely the same location because of the orbit of the star cluster around the Galactic Center. Even so, the proper motion of M13 is small, so the message will still arrive near the center of the cluster.
The message consists of seven parts that encode the following (from the top down in the image):
The numbers one to ten (white; left to right)
The atomic numbers of the elements hydrogen, carbon, nitrogen, oxygen, and phosphorus, which make up deoxyribonucleic acid (DNA) (purple)
The formulas for the chemical compounds that make up the nucleotides of DNA (green)
The estimated number of DNA nucleotides in the human genome, and a graphic of the double helix structure of DNA (white and blue, respectively)
The dimension (physical height, 5'9") of an average man (blue/white), a graphic figure of a human being (red), and the human population of Earth which was about 4 billion at the time (white)
A graphic of the Solar System (including Pluto), indicating which of the planets the message is coming from (yellow). The Sun is on the left and the third planet, Earth, raised toward the human figure
A graphic of the Arecibo radio telescope and the dimension (the physical diameter) of the transmitting antenna dish (purple, white, and blue)
The entire message consisted of 1,679 binary digits, approximately 210 bytes, transmitted at a frequency of 2,380 MHz and modulated by shifting the frequency by 10 Hz, with a power of 450 kW. The "ones" and "zeros" were transmitted by frequency shifting at the rate of 10 bits per second. The total broadcast was less than three minutes.
The number 1,679 was chosen because it is a semiprime (the product of two prime numbers), to be arranged rectangularly as 73 rows by 23 columns. The alternative arrangement, 23 rows by 73 columns, produces an unintelligible set of characters.
=== Message as binary string ===
The message as a binary string is included below. Note that the choice of 1 representing higher frequency and 0 representing lower frequency is entirely arbitrary and the line breaks after every 23 bits are only included to allow for some ease in human readability.
=== Numbers ===
1 2 3 4 5 6 7 8 9 10
----------------------
0 0 0 1 1 1 1 00 00 00
0 1 1 0 0 1 1 00 00 10
1 0 1 0 1 0 1 01 11 01
X X X X X X X X X X <-Least-significant-digit marker.
The numbers from 1 to 10 appear in binary format, to be read from the top down. The bottom row contains markers which indicate the column from which the binary code for each number is intended to begin.
Even assuming that any extraterrestrial recipients would recognize binary, the encoding of the numbers may not be immediately obvious because of the way they have been written. To read the first seven digits, ignore the bottom row, and read them as three binary digits from top to bottom, with the top digit being the most significant. The readings for 8, 9, and 10 are a little different, as their binary code has been distributed across an additional column next to the first (to the right in the image). This is intended to show that numbers too large to fit in a single column can be written in several contiguous ones (a scheme which is used elsewhere in the message). The additional columns are not marked by the least-significant-digit marker.
=== DNA elements ===
H C N O P
1 6 7 8 15
----------
0 0 0 1 1
0 1 1 0 1
0 1 1 0 1
1 0 1 0 1
X X X X X
The numbers 1, 6, 7, 8, and 15 appear, denoting the atomic numbers of hydrogen (H), carbon (C), nitrogen (N), oxygen (O), and phosphorus (P), the elements from which DNA is composed.
=== Nucleotides ===

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The chemical groups from which the nucleotides of polymeric DNA sequences are built – the sugar deoxyribose, phosphate, and the four canonical nucleobases used in DNA – are then described as sequences of the five elements that appear on the preceding line. Each sequence represents the molecular formula of the chemical as it exists when incorporated into DNA (as opposed to the free form).
For example, the compound in the top left in the image is deoxyribose (C5H7O in DNA, C5H10O4 when free), whose formula is read as:
11000
10000
11010
XXXXX
-----
75010
i.e., 7 atoms of hydrogen, 5 atoms of carbon, 0 atoms of nitrogen, 1 atom of oxygen, and 0 atoms of phosphorus.
It is displayed in this order because the DNA Elements in the previous section (Purple image as reference) describe hydrogen (H), carbon (C), nitrogen (N), oxygen (O), and phosphorus (P) in that order as well.
=== Double helix ===
11
11
11
11
11
01
11
11
01
11
01
11
10
11
11
01
X
11111111 11110111 11111011 01011110 (binary) [Using the double vertical columns above, read from top to bottom starting from the right column first, and then top to bottom from the left column.]
= 4,294,441,822 (decimal)
A graphic of the approximate shape of the double helix in which double-stranded DNA polymers naturally exist; the vertical bar in the middle is a binary representation of the number of nucleotide base pairs in the human genome. The value depicted is around 4.3 billion, which was believed to be the case when the message was transmitted in 1974; it is now known that there are only approximately 3.2 billion base pairs in the human genome.
=== Humanity ===
The graphic in the center is a simple illustration of a human being. The element on the left (in the image) indicates the average height of an adult male in the US: 1.764 m (5 ft 9.4 in). This value is indicated by a horizontally written binary representation of the number 14, which is intended to be multiplied by the wavelength of the message (126 mm); 14 × 126 = 1,764 millimeters.
The element on the right of the image indicates the size of the global human population in 1974, approximately 4.3 billion (which, coincidentally, is within 0.1% of the number of DNA base pairs suggested for the size of the human genome earlier in the message). In this case, the number is oriented in the data horizontally rather than vertically. The least-significant-digit marker is in the upper left in the image, with bits going to the right and more significant digits below.
=== Planets ===
Earth
Sun Mercury Venus Mars Jupiter Saturn Uranus Neptune Pluto
A graphic depicting the Solar System, showing the Sun and nine planets in the order of their distance from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto (Pluto was reclassified in 2006 as a dwarf planet by the International Astronomical Union). Earth is the third planet from the Sun; its graphic is shifted up to identify it as the planet from which the signal was sent. Additionally, the human figure is shown just above the Earth graphic.
In addition to showing position, the graphic provides a general, not-to-scale size reference of each planet and the Sun.
=== Telescope ===
bottom middle two rows shown in White as reference in the image:
100101
<--- 111110X --->100101 111110 (binary) = 2,430 (decimal)
The last part is a graphic representing the Arecibo radio telescope and indicating its diameter with a binary representation of the number 2,430; multiplying by the wavelength of 126 mm gives 306.18 m (1,004 ft 6 in). In this case, the number is oriented horizontally, with the least-significant-digit marker to the lower right in the image.
The part of the image that resembles a letter "M" is there to demonstrate that the curved line is a concave curved mirror.
== Arecibo Answer crop circle hoax ==
The "Hampshire pattern" or "Chilbolton Code formation" or "Arecibo answer" was a crop circle that appeared in 2001 near the Chilbolton radio telescope in Hampshire, UK, which echoed the visual representation and most of the information from the original Arecibo message with some significant differences including location/origin, DNA configuration, and appearance.
The SETI Institute Online rebutted the idea that this was a genuine extraterrestrial response, by saying, "This is highly improbable. There is no evidence to suggest an other-than-earthly origin for these graphics." The crop circle is a near replica of the Arecibo message, with the same 23 × 73 grid. Most of the chemical data remains the same, with the exception that in the section detailing important chemical elements, silicon has been added, and the diagram of DNA has been rewritten. At the bottom, the pictogram of a human is replaced with a figure with a large, bulbous head. A solar system with 9 planets is also depicted, with emphasis placed on the 3rd, 4th, and 5th planets of the system. The Arecibo telescope is replaced by a replica of a crop circle that appeared in the same field one year before, and the binary representation of the transmitter's diameter is altered.

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== Computational analysis ==
Some researchers have analysed whether the Arecibo signal could be identified as a carrying message by some information theoretic measure that was agnostic in the sense that it does not know the particular shapes of a human or double-helix etc. Presumably such an agnostic measure would be all that any potential ETI could apply to this signal. It is also a useful testbed for humanity's own ability to detect potential extraterrestrial signals, as we would presumably be similarly agnostic about any shapes particularly special to the broadcasting ETI.
Mahon (2025) described an information theoretic measure of complexity, the LCC score, that gives a high score to human language and meaningful images (such as photographs or drawings), and to the Arecibo message, while also giving a low score to random noise signals and simple repetitive signals. This measure also identified the correct aspect ratio of 73 by 23. Other attempts have been made with varying degrees of success. Zenil et al. (2023) also identified the correct aspect ratio and distinguished it from random noise signals, but not from uniform or repetitive signals. McCowan et al. (1999) described a measure called entropic slope, which could differentiate between random noise or repetitive signals, and meaningful vocalisations by multiple species including humans, but did not detect a message in the Arecibo signal.
== See also ==
Active SETI
Communication with extraterrestrial intelligence (CETI)
Cosmos, a 2019 science fiction film featuring a response to the Arecibo message
List of interstellar radio messages
METI (Messaging Extraterrestrial Intelligence) (organization)
Pioneer plaque
Voyager Golden Record
Wow! signal
== References ==
== External links ==
Media related to Arecibo message at Wikimedia Commons

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Sir Arthur Charles Clarke (16 December 1917 – 19 March 2008) was an English science fiction writer, science writer, futurist, inventor, undersea explorer, and television series host.
Clarke was a science fiction writer, an avid populariser of space travel, and a futurist of distinguished ability. He wrote many books and many essays for popular magazines. In 1961, he received the Kalinga Prize, a UNESCO award for popularising science. Clarke's science and science fiction writings earned him the moniker "Prophet of the Space Age". His science fiction writings in particular earned him a number of Hugo and Nebula awards, which, along with a large readership, made him one of the towering figures of the genre. For many years, science fiction had a "Big Three", comprising Clarke and American writers Robert Heinlein and Isaac Asimov. Clarke co-wrote the screenplay for the 1968 film 2001: A Space Odyssey, widely regarded as one of the most influential films of all time.
Clarke was a lifelong proponent of space travel. In 1934, while still a teenager, he joined the British Interplanetary Society (BIS). In 1945, he proposed a satellite communication system using geostationary orbits. He was the chairman of the BIS from 1946 to 1947 and again in 1951–1953.
Clarke emigrated to Ceylon (now Sri Lanka) in 1956, to pursue his interest in scuba diving. That year, he discovered the underwater ruins of the ancient original Koneswaram Temple in Trincomalee. Clarke augmented his popularity in the 1980s, as the host of television shows such as Arthur C. Clarke's Mysterious World. He lived in Sri Lanka until his death.
Clarke was appointed Commander of the Order of the British Empire (CBE) in 1989 "for services to British cultural interests in Sri Lanka". He was knighted in 1998 and was awarded Sri Lanka's highest civil honour, Sri Lankabhimanya, in 2005.
== Biography ==
=== Early years ===
Clarke was born in Minehead, Somerset, England, and grew up in nearby Bishops Lydeard. As a boy, he lived on a farm, where he enjoyed stargazing, fossil collecting, and reading American science fiction pulp magazines. He received his secondary education at Huish's Grammar School in Taunton. Some of his early influences included dinosaur cigarette cards, which led to an enthusiasm for fossils starting about 1925. Clarke attributed his interest in science fiction to reading three items: the November 1928 issue of Amazing Stories in 1929; Last and First Men by Olaf Stapledon in 1930; and The Conquest of Space by David Lasser in 1931.
In his teens, he joined the Junior Astronomical Association and contributed to Urania, the society's journal, which was edited in Glasgow by Marion Eadie. At Clarke's request, she added an "Astronautics" section, which featured a series of articles written by him on spacecraft and space travel. Clarke also contributed pieces to the "Debates and Discussions Corner", a counterpoint to a Urania article offering the case against space travel, and also his recollections of the Walt Disney film Fantasia. He moved to London in 1936 and joined the Board of Education as a pensions auditor. He and some fellow science fiction writers shared a flat in Gray's Inn Road, where he got the nickname "Ego" because of his absorption in subjects that interested him, and later named his office filled with memorabilia as his "ego chamber".
=== World War II ===
During the Second World War from 1941 to 1946, he served in the Royal Air Force as a radar specialist and was involved in the early-warning radar defence system, which contributed to the RAF's success during the Battle of Britain. Clarke spent most of his wartime service working on ground-controlled approach (GCA) radar, as documented in the semiautobiographical Glide Path, his only non-science fiction novel. Although GCA did not see much practical use during the war, after several years of development it proved vital to the Berlin Airlift of 1948–1949. Clarke initially served in the ranks and was a corporal instructor on radar at No. 2 Radio School, RAF Yatesbury in Wiltshire. He was commissioned as a pilot officer (technical branch) on 27 May 1943. He was promoted to flying officer on 27 November 1943. He was appointed chief training instructor at RAF Honiley in Warwickshire and was demobilised with the rank of flight lieutenant.
=== Post-war ===
After the war, he attained a first-class degree in mathematics and physics from King's College London. After this, he worked as assistant editor at Physics Abstracts. Clarke served as president of the British Interplanetary Society from 1946 to 1947 and again from 1951 to 1953.
Although he was not the originator of the concept of geostationary satellites, one of his most important contributions in this field was his idea that they would be ideal telecommunications relays. He advanced this idea in a paper privately circulated among the core technical members of the British Interplanetary Society in 1945. The concept was published in Wireless World in October of that year. Clarke also wrote a number of nonfiction books describing the technical details and societal implications of rocketry and space flight. The most notable of these may be Interplanetary Flight: An Introduction to Astronautics (1950), The Exploration of Space (1951), and The Promise of Space (1968). In recognition of these contributions, the geostationary orbit 36,000 kilometres (22,000 mi) above the equator is officially recognised by the International Astronomical Union as the Clarke Orbit.
His 1951 book, The Exploration of Space, was used by the rocket pioneer Wernher von Braun to convince President John F. Kennedy that it was possible to go to the Moon.
Following the 1968 release of 2001, Clarke became much in demand as a commentator on science and technology, especially at the time of the Apollo space program. On 20 July 1969, Clarke appeared as a commentator for the CBS News broadcast of the Apollo 11 Moon landing.

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=== Sri Lanka and diving ===
Clarke lived in Sri Lanka from 1956 until his death in 2008, first in Unawatuna on the south coast, and then in Colombo. Initially, he and his friend Mike Wilson travelled around Sri Lanka, diving in the coral waters around the coast with the Beachcombers Club. In 1957, during a dive trip off Trincomalee, Clarke discovered the underwater ruins of a temple, which subsequently made the region popular with divers. He described it in his 1957 book The Reefs of Taprobane. This was his second diving book after the 1956 The Coast of Coral. Though Clarke lived mostly in Colombo, he set up a small dive school and a simple dive shop near Trincomalee. He dived often at Hikkaduwa, Trincomalee, and Nilaveli.
The Sri Lankan government offered Clarke resident guest status in 1975. He was held in such high esteem that when fellow science fiction writer Robert A. Heinlein came to visit, the Sri Lanka Air Force provided a helicopter to take them around the country.
In the early 1970s, Clarke signed a three-book publishing deal, a record for a science fiction writer at the time. The first of the three was Rendezvous with Rama in 1973, which won all the main genre awards and spawned sequels that along with the 2001 series formed the backbone of his later career.
In 1986, Clarke was named a Grand Master by the Science Fiction Writers of America.
In 1988, he was diagnosed with post-polio syndrome, having originally contracted polio in 1962, and needed to use a wheelchair most of the time thereafter. Clarke was for many years a vice-patron of the British Polio Fellowship.
In the 1989 Queen's Birthday Honours, Clarke was appointed Commander of the Order of the British Empire (CBE) "for services to British cultural interests in Sri Lanka". The same year, he became the first chancellor of the International Space University, serving from 1989 to 2004. He also served as chancellor of Moratuwa University in Sri Lanka from 1979 to 2002.
In 1994, Clarke appeared in a science fiction film; he portrayed himself in the film Without Warning, an American production about an apocalyptic alien first-contact scenario presented in the form of a faux newscast.
Clarke also became active in promoting the protection of gorillas and became a patron of the Gorilla Organization, which fights for the preservation of gorillas. When tantalum mining for mobile phone manufacture threatened the gorillas in 2001, he lent his voice to their cause. The dive shop that he set up continues to operate from Trincomalee through the Arthur C Clarke Foundation.
=== Television series host ===
In the 1980s and early 1990s, Clarke presented his television programmes Arthur C. Clarke's Mysterious World, Arthur C. Clarke's World of Strange Powers, and Arthur C. Clarke's Mysterious Universe.
=== Personal life ===
On a trip to Florida in 1953, Clarke met and quickly married Marilyn Mayfield, a 22-year-old American divorcee with a young son. They separated permanently after six months, although the divorce was not finalised until 1964. "The marriage was incompatible from the beginning", said Clarke. Marilyn never remarried and died in 1991.
Clarke also never remarried, but was close to a Sri Lankan man, Leslie Ekanayake (13 July 1947 – 4 July 1977), whom Clarke called his "only perfect friend of a lifetime" in the dedication to his novel The Fountains of Paradise. Clarke is buried with Ekanayake, who predeceased him by three decades, in Kanatte Cemetery, Colombo's main burial ground and crematorium. In his biography of Stanley Kubrick, John Baxter cites Clarke's homosexuality as a reason why he relocated, due to more tolerant laws with regard to homosexuality in Sri Lanka. Journalists who enquired of Clarke whether he was gay were told, "No, merely mildly cheerful." However, Michael Moorcock wrote:
Everyone knew he was gay. In the 1950s, I'd go out drinking with his boyfriend. We met his protégés, western and eastern, and their families, people who had only the most generous praise for his kindness. Self-absorbed he might be and a teetotaller, but an impeccable gent through and through.
In an interview in the July 1986 issue of Playboy magazine, when asked if he had had a bisexual experience, Clarke stated, "Of course. Who hasn't?". In his obituary, Clarke's friend Kerry O'Quinn wrote: "Yes, Arthur was gay ... As Isaac Asimov once told me, 'I think he simply found he preferred men.' Arthur didn't publicise his sexuality – that wasn't the focus of his life – but if asked, he was open and honest."
Clarke accumulated a vast collection of manuscripts and personal memoirs, maintained by his brother Fred Clarke in Taunton, Somerset, England, and referred to as the "Clarkives". Clarke said some of his private diaries will not be published until 30 years after his death. When asked why they were sealed, he answered, "Well, there might be all sorts of embarrassing things in them."
=== Knighthood ===
On 26 May 2000, he was made a Knight Bachelor "for services to literature" at a ceremony in Colombo. The knighthood had been awarded in the 1998 New Year Honours list, but investiture with the award had been delayed, at Clarke's request, because of an accusation by the tabloid the Sunday Mirror of paying boys for sex. The charge was subsequently found to be baseless by the Sri Lankan police. According to The Daily Telegraph, the Sunday Mirror subsequently published an apology, and Clarke chose not to sue for defamation. The Independent alleged that a similar story was not published because Clarke was a friend of newspaper tycoon Rupert Murdoch. Clarke himself said, "I take an extremely dim view of people mucking about with boys", and Rupert Murdoch allegedly promised him the reporters responsible would never work in Fleet Street again.
=== Later years ===

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Although he and his home were unharmed by the 2004 Indian Ocean earthquake tsunami, his "Arthur C. Clarke Diving School" (now called "Underwater Safaris") at Hikkaduwa near Galle was destroyed. He made humanitarian appeals, and the Arthur C. Clarke Foundation worked towards better disaster notification systems.
Because of his post-polio deficits, which limited his ability to travel and gave him halting speech, most of Clarke's communications in his last years were in the form of recorded addresses. In July 2007, he provided a video address for the Robert A. Heinlein Centennial in which he closed his comments with a goodbye to his fans. In September 2007, he provided a video greeting for NASA's Cassini probe's flyby of Iapetus (which plays an important role in the book of 2001: A Space Odyssey). In December 2007 on his 90th birthday, Clarke recorded a video message to his friends and fans bidding them good-bye.
Clarke died in Colombo on 19 March 2008, at the age of 90. His aide described the cause as respiratory complications and heart failure stemming from post-polio syndrome.
Just hours before Clarke's death, a major gamma-ray burst (GRB) reached Earth. Known as GRB 080319B, the burst set a new record as the farthest object that can be seen from Earth with the naked eye. It occurred about 7.5 billion years ago, the light taking that long to reach Earth. Larry Sessions, a science writer for Sky and Telescope magazine blogging on earthsky.org, suggested that the burst be named the "Clarke Event". American Atheist Magazine wrote of the idea: "It would be a fitting tribute to a man who contributed so much, and helped lift our eyes and our minds to a cosmos once thought to be province only of gods."
A few days before he died, he had reviewed the manuscript of his final work, The Last Theorem, on which he had collaborated by e-mail with contemporary Frederik Pohl. The book was published after Clarke's death. Clarke was buried in Colombo in traditional Sri Lankan fashion on 22 March. His younger brother, Fred Clarke, and his Sri Lankan adoptive family were among the thousands in attendance.
Clarke's papers were donated to the American National Air and Space Museum in 2014.
On 8 January 2024, a sample of Clarke's DNA was launched on the Peregrine Mission One to the Moon. The Peregrine spacecraft failed to land on the Moon, and the spacecraft disintegrated in the Earth's atmosphere on 19 January 2024.
== Science fiction writer ==
=== Beginnings ===
While Clarke had a few stories published in fanzines, between 1937 and 1945, his first professional sale appeared in Astounding Science Fiction in 1946: "Loophole" was published in April, while "Rescue Party", his first sale, was published in May. Along with his writing, Clarke briefly worked as assistant editor of Science Abstracts (1949) before devoting himself in 1951 to full-time writing.
Clarke began carving out his reputation as a "scientific" science fiction writer with his first science fiction novel, Against the Fall of Night, published as a novella in 1948. It was very popular and considered ground-breaking work for some of the concepts it contained. Clarke revised and expanded the novella into a full novel, which was published in 1953. Clarke later rewrote and expanded this work a third time to become The City and the Stars in 1956, which rapidly became a definitive must-read in the field. His third science fiction novel, Childhood's End, was also published in 1953, cementing his popularity. Clarke capped the first phase of his writing career with his sixth novel, A Fall of Moondust, in 1961, which is also an acknowledged classic of the period.
During this time, Clarke corresponded with C. S. Lewis in the 1940s and 1950s and they once met in an Oxford pub, the Eastgate, to discuss science fiction and space travel. Clarke voiced great praise for Lewis upon his death, saying The Ransom Trilogy was one of the few works of science fiction that should be considered literature.
=== "The Sentinel" ===
In 1948, he wrote "The Sentinel" for a BBC competition. Though the story was rejected, it changed the course of Clarke's career. Not only was it the basis for 2001: A Space Odyssey, but "The Sentinel" also introduced a more cosmic element to Clarke's work. Many of Clarke's later works feature a technologically advanced but still-prejudiced mankind being confronted by a superior alien intelligence. In the cases of Childhood's End, and the 2001 series, this encounter produces a conceptual breakthrough that accelerates humanity into the next stage of its evolution. This also applies in the far-distant past (but our future) in The City and the Stars (and its original version, Against the Fall of Night).
In Clarke's authorised biography, Neil McAleer writes: "many readers and critics still consider Childhood's End Arthur C. Clarke's best novel." But Clarke did not use ESP in any of his later stories, saying, "I've always been interested in ESP, and of course, Childhood's End was about that. But I've grown disillusioned, partly because after all this time, they're still arguing about whether these things happen. I suspect that telepathy does happen."
A collection of early essays was published in The View from Serendip (1977), which also included one short piece of fiction, "When the Twerms Came". Clarke also wrote short stories under the pseudonyms of E. G. O'Brien and Charles Willis. Almost all of his short stories can be found in the book The Collected Stories of Arthur C. Clarke (2001).
=== "Big Three" ===

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For much of the later 20th century, Clarke, Isaac Asimov, and Robert A. Heinlein were informally known as the "Big Three" of science fiction writers. Clarke and Heinlein began writing to each other after The Exploration of Space was published in 1951, and first met in person the following year. They remained on cordial terms for many years, including during visits to the United States and Sri Lanka.
Clarke and Asimov first met in New York City in 1953, and they traded friendly insults and gibes for decades. They established an oral agreement, the "Clarke–Asimov Treaty", that when asked who was better, the two would say Clarke was the better science fiction writer and Asimov was the better science writer. In 1972, Clarke put the "treaty" on paper in his dedication to Report on Planet Three and Other Speculations.
In 1984, Clarke testified before Congress against the Strategic Defense Initiative (SDI). Later, at the home of Larry Niven in California, a concerned Heinlein attacked Clarke's views on United States foreign and space policy (especially the SDI), vigorously advocating a strong defence posture. Although the two later reconciled formally, they remained distant until Heinlein's death in 1988.
=== Space Odyssey series ===
2001: A Space Odyssey, Clarke's most famous work, was extended well beyond the original 1968 film as the Space Odyssey series. In 1982, Clarke wrote a sequel to 2001 titled 2010: Odyssey Two, which was made into a film in 1984. Clarke wrote two further sequels which have not been adapted into motion pictures: 2061: Odyssey Three (published in 1987) and 3001: The Final Odyssey (published in 1997).
2061: Odyssey Three involves a visit to Halley's Comet on its next plunge through the Inner Solar System and a spaceship crash on the Jovian moon Europa. The whereabouts of astronaut Dave Bowman (the "Star Child"), the artificial intelligence HAL 9000, and the development of native life on Europa, protected by the alien Monolith, are revealed.
Finally, in 3001: The Final Odyssey, astronaut Frank Poole's freeze-dried body, found by a spaceship beyond the orbit of Neptune, is revived by advanced medical science. The novel details the threat posed to humanity by the alien monoliths, whose actions are not always as their builders had intended.
==== 2001: A Space Odyssey ====
Clarke's first venture into film was 2001: A Space Odyssey, directed by Stanley Kubrick. Kubrick and Clarke had met in New York City in 1964 to discuss the possibility of a collaborative film project. As the idea developed, they decided to loosely base the story on Clarke's short story, "The Sentinel", written in 1948 as an entry in a BBC short-story competition. Originally, Clarke was going to write the screenplay for the film, but Kubrick suggested during one of their brainstorming meetings that before beginning on the actual script, they should let their imaginations soar free by writing a novel first, on which they would base the film. "This is more or less the way it worked out, though toward the end, novel and screenplay were being written simultaneously, with feedback in both directions. Thus, I rewrote some sections after seeing the movie rushes – a rather expensive method of literary creation, which few other authors can have enjoyed." The novel ended up being published a few months after the release of the movie.
Due to the hectic schedule of the film's production, Kubrick and Clarke had difficulty collaborating on the book. Clarke completed a draft of the novel at the end of 1964 with the plan to publish in 1965 in advance of the film's release in 1966. After many delays, the film was released in the spring of 1968, before the book was completed. The book was credited to Clarke alone. Clarke later complained that this had the effect of making the book into a novelisation, and that Kubrick had manipulated circumstances to play down Clarke's authorship. For these and other reasons, the details of the story differ slightly from the book to the movie. The film contains little explanation for the events taking place. Clarke, though, wrote thorough explanations of "cause and effect" for the events in the novel. James Randi later recounted that upon seeing the premiere of 2001, Clarke left the theatre at the intermission in tears, after having watched an eleven-minute scene (which did not make it into general release) where an astronaut is doing nothing more than jogging inside the spaceship, which was Kubrick's idea of showing the audience how boring space travels could be.
In 1972, Clarke published The Lost Worlds of 2001, which included his accounts of the production, and alternative versions of key scenes. The "special edition" of the novel A Space Odyssey (released in 1999) contains an introduction by Clarke in which he documents the events leading to the release of the novel and film.

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==== 2010: Odyssey Two ====
In 1982, Clarke continued the 2001 epic with a sequel, 2010: Odyssey Two. This novel was also made into a film, 2010, directed by Peter Hyams for release in 1984. Because of the political environment in America in the 1980s, the film presents a Cold War theme, with the looming tensions of nuclear warfare not featured in the novel. The film was not considered to be as revolutionary or artistic as 2001, but the reviews were still positive.
Clarke's email correspondence with Hyams was published in 1984. Titled The Odyssey File: The Making of 2010, and co-authored with Hyams, it illustrates his fascination with the then-pioneering medium of email and its use for them to communicate on an almost daily basis at the time of planning and production of the film while living on opposite sides of the world. The book also included Clarke's personal list of the best science fiction films ever made.
Clarke appeared in the film, first as the man feeding the pigeons while Dr. Heywood Floyd is engaged in a conversation in front of the White House. Later, in the hospital scene with David Bowman's mother, an image of the cover of Time portrays Clarke as the American President and Kubrick as the Soviet Premier.
=== Rendezvous with Rama ===
In 1996, Sierra Entertainment created Rama as a point and click adventure game in the style of Myst. Along with highly detailed graphics, Arthur C. Clarke also appeared in the game as the guide for the player. This game featured details from Rendezvous with Rama and characters from the Rama II novel.
Rendezvous with Rama was optioned for filmmaking in the early 21st century but this motion picture has remained in "development hell". After a drawn-out development process, which actor Morgan Freeman attributed to difficulties in getting financing, it appeared in 2003 that this project might be proceeding. The film was to be produced by Freeman's production company, Revelations Entertainment, with David Fincher being touted as the film's director. After years of no progress, in late 2008, Fincher stated the movie is unlikely to be made, given Morgan Freeman's health. In 2010, though, the film was still planned for future production and both Freeman and Fincher mentioned it as still needing a worthy script. In late 2021, Denis Villeneuve was introduced as director.
== Science writer ==
Clarke published a number of nonfiction books with essays, speeches, addresses, etc. Several of his nonfiction books are composed of chapters that can stand on their own as separate essays.
=== Space travel ===
In particular, Clarke was a populariser of the concept of space travel. In 1950, he wrote Interplanetary Flight, a book outlining the basics of space flight for laymen. Later books about space travel included The Exploration of Space (1951), The Challenge of the Spaceship (1959), Voices from the Sky (1965), The Promise of Space (1968, rev. ed. 1970), and Report on Planet Three (1972) along with many others.
=== Futurism ===
His books on space travel usually included chapters about other aspects of science and technology, such as computers and bioengineering. He predicted telecommunication satellites (albeit serviced by astronauts in space suits, who would replace the satellite's vacuum tubes as they burned out).
His many predictions culminated in 1958 when he began a series of magazine essays which eventually became Profiles of the Future, published in book form in 1962. A timetable up to the year 2100 describes inventions and ideas including such things as a "global library" for 2005. The same work also contained "Clarke's First Law" and text that became Clarke's three laws in later editions.

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In a 1959 essay, Clarke predicted global satellite TV broadcasts that would cross national boundaries indiscriminately and would bring hundreds of channels available anywhere in the world. He also envisioned a "personal transceiver, so small and compact that every man carries one". He wrote: "the time will come when we will be able to call a person anywhere on Earth merely by dialing a number." Such a device would also, in Clarke's vision, include means for global positioning so "no one need ever again be lost". Later, in Profiles of the Future, he predicted the advent of such a device taking place in the mid-1980s. Clarke described a global computer network similar to the modern World Wide Web in a 1964 presentation for the BBC's Horizon programme, predicting that, by the 21st century, access to information and even physical tasks such as surgery could be accomplished remotely and instantaneously from anywhere in the world using internet and satellite communication.
In a 1974 interview with the Australian Broadcasting Corporation, the interviewer asked Clarke how he believed the computer would change the future for the everyday person, and what life would be like in the year 2001. Clarke accurately predicted many things that became reality, including online banking, online shopping, and other now commonplace things. Responding to a question about how the interviewer's son's life would be different, Clarke responded: "He will have, in his own house, not a computer as big as this, [points to nearby computer], but at least, a console through which he can talk, through his friendly local computer and get all the information he needs, for his everyday life, like his bank statements, his theatre reservations, all the information you need in the course of living in our complex modern society, this will be in a compact form in his own house ... and he will take it as much for granted as we take the telephone."
An extensive selection of Clarke's essays and book chapters (from 1934 to 1998; 110 pieces, 63 of them previously uncollected in his books) can be found in the book Greetings, Carbon-Based Bipeds! (2000), together with a new introduction and many prefatory notes. Another collection of essays, all previously collected, is By Space Possessed (1993). Clarke's technical papers, together with several essays and extensive autobiographical material, are collected in Ascent to Orbit: A Scientific Autobiography (1984).
== Geostationary communications satellite ==
Clarke contributed to the popularity of the idea that geostationary satellites would be ideal telecommunications relays. He first described this in a letter to the editor of Wireless World in February 1945 and elaborated on the concept in a paper titled Extra-Terrestrial Relays – Can Rocket Stations Give Worldwide Radio Coverage?, published in Wireless World in October 1945. The geostationary orbit is sometimes known as the Clarke orbit or the Clarke belt in his honour.
It is not clear that this article was actually the inspiration for the modern telecommunications satellite. According to John R. Pierce, of Bell Labs, who was involved in the Echo satellite and Telstar projects, he gave a talk upon the subject in 1954 (published in 1955), using ideas that were "in the air", but was not aware of Clarke's article at the time. In an interview given shortly before his death, Clarke was asked whether he had ever suspected that one day communications satellites would become so important; he replied: "I'm often asked why I didn't try to patent the idea of a communications satellite. My answer is always, 'A patent is really a licence to be sued.'"
Though different from Clarke's idea of telecom relay, the idea of communicating via satellites in geostationary orbit itself had been described earlier. For example, the concept of geostationary satellites was described in Hermann Oberth's 1923 book Die Rakete zu den Planetenräumen (The Rocket into Interplanetary Space), and then the idea of radio communication by means of those satellites in Herman Potočnik's (written under the pseudonym Hermann Noordung) 1928 book Das Problem der Befahrung des Weltraums), sections: Providing for Long Distance Communications and Safety, and (possibly referring to the idea of relaying messages via satellite, but not that three would be optimal) Observing and Researching the Earth's Surface, published in Berlin. Clarke acknowledged the earlier concept in his book Profiles of the Future.
== Undersea explorer ==
Clarke was an avid scuba diver and a member of the Underwater Explorers Club. In addition to writing, Clarke set up several diving-related ventures with his business partner Mike Wilson. In 1956, while scuba diving, Wilson and Clarke uncovered ruined masonry, architecture, and idol images of the sunken original Koneswaram temple – including carved columns with flower insignia, and stones in the form of elephant heads – spread on the shallow surrounding seabed. Other discoveries included Chola bronzes from the original shrine, and these discoveries were described in Clarke's 1957 book The Reefs of Taprobane.
In 1961, while filming off Great Basses Reef, Wilson found a wreck and retrieved silver coins. Plans to dive on the wreck the following year were stopped when Clarke developed paralysis, ultimately diagnosed as polio. A year later, Clarke observed the salvage from the shore and the surface. The ship, ultimately identified as belonging to the Mughal Emperor, Aurangzeb, yielded fused bags of silver rupees, cannon, and other artefacts, carefully documented, became the basis for The Treasure of the Great Reef. Living in Sri Lanka and learning its history also inspired the backdrop for his novel The Fountains of Paradise in which he described a space elevator. This, he believed, would make rocket-based access to space obsolete, and more than geostationary satellites, would ultimately be his scientific legacy. In 2008, he said in an interview with IEEE Spectrum, "maybe in a generation or so the space elevator will be considered equally important" as the geostationary satellite, which was his most important technological contribution.
== Views ==

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=== Religion ===
Themes of religion and spirituality appear in much of Clarke's writing. He said: "Any path to knowledge is a path to God – or Reality, whichever word one prefers to use." He described himself as "fascinated by the concept of God". J. B. S. Haldane, near the end of his life, suggested in a personal letter to Clarke that Clarke should receive a prize in theology for being one of the few people to write anything new on the subject, and went on to say that if Clarke's writings had not contained multiple contradictory theological views, he might have been a menace. When he entered the Royal Air Force, Clarke insisted that his dog tags be marked "pantheist" rather than the default, Church of England, and in a 1991 essay entitled "Credo", described himself as a logical positivist from the age of 10. In 2000, Clarke told the Sri Lankan newspaper, The Island, "I don't believe in God or an afterlife", and he identified himself as an atheist. He was honoured as a Humanist Laureate in the International Academy of Humanism. He has also described himself as a "crypto-Buddhist", insisting Buddhism is not a religion. He displayed little interest about religion early in his life, for example, only discovering a few months after marrying that his wife had strong Presbyterian beliefs.
Later in his life, Clarke began to hold a more hostile view of religion. A famous quotation of Clarke's is often cited: "One of the great tragedies of mankind is that morality has been hijacked by religion." He was quoted in Popular Science in 2004 as saying of religion: "Most malevolent and persistent of all mind viruses. We should get rid of it as quick as we can." In a three-day "dialogue on man and his world" with Alan Watts, Clarke said he was biased against religion and could not forgive religions for what he perceived as their inability to prevent atrocities and wars over time. In his introduction to the penultimate episode of Mysterious World, entitled "Strange Skies", Clarke said, "I sometimes think that the universe is a machine designed for the perpetual astonishment of astronomers", reflecting the dialogue of the episode, in which he stated this concept more broadly, referring to "mankind". Near the very end of that same episode, the last segment of which covered the Star of Bethlehem, he said his favourite theory was that it might be a pulsar. Given that pulsars were discovered in the interval between his writing the short story, "The Star" (1955), and making Mysterious World (1980), and given the more recent discovery of pulsar PSR B1913+16, he said: "How romantic, if even now, we can hear the dying voice of a star, which heralded the Christian era."
Despite his atheism, themes of deism are a common feature within Clarke's work. Clarke left written instructions for a funeral: "Absolutely no religious rites of any kind, relating to any religious faith, should be associated with my funeral."
=== Politics ===
Regarding freedom of information Clarke believed, "In the struggle for freedom of information, technology, not politics, will be the ultimate decider." Clarke also wrote, "It is not easy to see how the more extreme forms of nationalism can
long survive when men have seen the Earth in its true perspective as a single small
globe against the stars." Clarke opposed claims of sovereignty over space stating "There is hopeful symbolism in the fact that flags do not wave in a vacuum." Clarke was an anti-capitalist, stating that he did not fear automation because, "the goal of the future is full unemployment, so we can play. That's why we have to destroy the present politico-economic system."
=== Technology ===
Regarding human jobs being replaced by robots, Clarke said: "Any teacher that can be replaced by a machine should be!" Clarke supported the use of renewable energy, saying: "I would like to see us kick our current addiction to oil, and adopt clean energy sources ... Climate change has now added a new sense of urgency. Our civilisation depends on energy, but we can't allow oil and coal to slowly bake our planet."
=== Intelligent life ===
About intelligent life and the Fermi paradox, Clarke stated: The best proof that there's intelligent life in outer space is the fact that it hasn't come here ... the fact that we have not yet found the slightest evidence for life—much less intelligence—beyond this Earth does not surprise or disappoint me in the least. Our technology must still be laughably primitive; we may well be like jungle savages listening for the throbbing of tom-toms, while the ether around them carries more words per second than they could utter in a lifetime. Two possibilities exist: either we are alone in the Universe or we are not... Both are equally terrifying.

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=== Paranormal phenomena ===
Early in his career, Clarke had a fascination with the paranormal and said it was part of the inspiration for his novel Childhood's End. Citing the numerous promising paranormal claims that were later shown to be fraudulent, Clarke described his earlier openness to the paranormal having turned to being "an almost total sceptic" by the time of his 1992 biography. Similarly, in the prologue to the 1990 Del Rey edition of Childhood's End, he writes: "after ... researching my Mysterious World and Strange Powers programmes, I am an almost total skeptic. I have seen far too many claims dissolve into thin air, far too many demonstrations exposed as fakes. It has been a long, and sometimes embarrassing, learning process." During interviews, both in 1993 and 2004–2005, he stated that he did not believe in reincarnation, saying there was no mechanism to make it possible, though "I'm always paraphrasing J. B. S. Haldane: 'The universe is not only stranger than we imagine, it's stranger than we can imagine.'" He described the idea of reincarnation as fascinating, but favoured a finite existence.
Clarke was known for hosting several television series investigating the unusual: Arthur C. Clarke's Mysterious World (1980), Arthur C. Clarke's World of Strange Power (1985), and Arthur C. Clarke's Mysterious Universe (1994). Topics examined ranged from ancient, man-made artefacts with obscure origins (e.g., the Nazca lines or Stonehenge), to cryptids (purported animals unknown to science), or obsolete scientific theories that came to have alternate explanations (e.g., Martian canals).
In Arthur C. Clarke's Mysterious World, he describes three kinds of "mysteries":
Mysteries of the First Kind: Something that was once utterly baffling but is now completely understood, e.g. a rainbow.
Mysteries of the Second Kind: Something that is currently not fully understood and can be in the future.
Mysteries of the Third Kind: Something of which we have no understanding.
Clarke's programmes on unusual phenomena were parodied in a 1982 episode of the comedy series The Goodies, in which his show is cancelled after it is claimed that he does not exist.
== Themes, style, and influences ==
Clarke's work is marked by an optimistic view of science empowering mankind's exploration of the Solar System and the world's oceans. His images of the future often feature a Utopian setting with highly developed technology, ecology, and society, based on the author's ideals. His early published stories usually featured the extrapolation of a technological innovation or scientific breakthrough into the underlying decadence of his own society.
A recurring theme in Clarke's works is the notion that the evolution of an intelligent species would eventually make them something close to gods. This was explored in his 1953 novel Childhood's End and briefly touched upon in his novel Imperial Earth. This idea of transcendence through evolution seems to have been influenced by Olaf Stapledon, who wrote a number of books dealing with this theme. Clarke has said of Stapledon's 1930 book Last and First Men that "No other book had a greater influence on my life ... [It] and its successor Star Maker (1937) are the twin summits of [Stapledon's] literary career."
Clarke was well known as an admirer of Irish fantasy writer Lord Dunsany, also having corresponded with him until Dunsany's death in 1957. He described Dunsany as "one of the greatest writers of the century". He also listed H. G. Wells, Jules Verne, and Edgar Rice Burroughs as influences.
== Awards, honours, and other recognition ==
Clarke won the 1963 Stuart Ballantine Medal from the Franklin Institute for the concept of satellite communications, and other honours. He won more than a dozen annual literary awards for particular works of science fiction.

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In 1952, Clarke won the International Fantasy Award's Non-Fiction category for The Exploration of Space.
In 1956, Clarke won a Hugo Award for his short story, "The Star".
Clarke won the UNESCO–Kalinga Prize for the Popularization of Science in 1961.
He won the Stuart Ballantine Medal in 1963.
He shared a 1969 Academy Award nomination with Stanley Kubrick in the category Best Writing, Story and Screenplay – Written Directly for the Screen for 2001: A Space Odyssey.
The fame of 2001 was enough for the Command Module of the Apollo 13 spacecraft to be named "Odyssey".
Clarke won the Nebula (1973) for his novella, A Meeting with Medusa.
Clarke won both the Nebula (1973) and Hugo (1974) awards and the 1974 Jupiter Award for his novel, Rendezvous with Rama.
Clarke won both the Nebula (1979) and Hugo (1980) awards for his novel, The Fountains of Paradise.
In 1982, he won the Marconi Prize for innovation in communications and remote sensing in space.
In 1985 the Science Fiction Writers of America named him its 7th SFWA Grand Master.
In 1986, he was elected to the American National Academy of Engineering "For conception of geosynchronous communications satellites, and for other contributions to the use and understanding of space".
In 1988, he was awarded an honorary degree (Doctor of Letters) by the University of Bath.
Readers of the British monthly Interzone voted him the all-time second best science fiction author in 1988–1989.
He received a CBE in 1989, and was knighted in 2000. Clarke's health did not allow him to travel to London to receive the latter honour personally from the Queen, so the United Kingdom's High Commissioner to Sri Lanka invested him as a Knight Bachelor at a ceremony in Colombo.
In 1994, Clarke was nominated for a Nobel Peace Prize by law professor Glenn Reynolds.
The Science Fiction and Fantasy Hall of Fame inducted Clarke in 1997, its second class of two deceased and two living persons. Among the living, Clarke and Andre Norton followed A. E. van Vogt and Jack Williamson.
In 2000, he was named a Distinguished Supporter of the British Humanist Association.
The 2001 Mars Odyssey orbiter is named in honour of Clarke's works.
In 2003, Clarke was awarded the Telluride Tech Festival Award of Technology, where he appeared on stage via a 3-D hologram with a group of old friends including Jill Tarter, Neil Armstrong, Lewis Branscomb, Charles Townes, Freeman Dyson, Bruce Murray, and Scott Brown.
In 2004, Clarke won the Heinlein Award for outstanding achievement in hard or science-oriented science fiction.
On 14 November 2005 Sri Lanka awarded Clarke its highest civilian award, the Sri Lankabhimanya (The Pride of Sri Lanka), for his contributions to science and technology and his commitment to his adopted country.
Clarke was the Honorary Board Chair of the Institute for Cooperation in Space, founded by Carol Rosin, and served on the Board of Governors of the American National Space Society, a space advocacy organisation founded by Wernher von Braun.
=== Named after Clarke ===
==== Awards ====
Arthur C. Clarke Award for science fiction writing, awarded annually in the United Kingdom.
In 1986, Clarke provided a grant to fund the prize money (initially £1,000) for the Arthur C. Clarke Award for the best science fiction novel published in the United Kingdom in the previous year. In 2001 the prize was increased to £2001, and its value now matches the year (e.g., £2005 in 2005).
In 2005 he lent his name to the inaugural Sir Arthur Clarke Award, for achievements in space, dubbed the "Space Oscars", awarded annually in the United Kingdom. His brother attended the awards ceremony, and presented an award specially chosen by Arthur (and not by the panel of judges who chose the other awards) to the British Interplanetary Society.
Arthur C. Clarke Foundation awards: "Arthur C. Clarke Innovator's Award" and "Arthur C. Clarke Lifetime Achievement Award"
The Sir Arthur C. Clarke Memorial Trophy Inter School Astronomy Quiz Competition, held in Sri Lanka every year and organised by the Astronomical Association of Ananda College, Colombo. The competition started in 2001 as "The Sir Arthur C. Clarke Trophy Inter School Astronomy Quiz Competition" and was renamed after his death.
Arthur C. Clarke Award for Imagination in Service to Society
==== Other ====
An asteroid was named in Clarke's honour, 4923 Clarke (the number was assigned prior to, and independently of, the name – 2001, however appropriate, was unavailable, having previously been assigned to Albert Einstein).
A species of ceratopsian dinosaur, discovered in Inverloch in Australia, was named after Clarke, Serendipaceratops arthurcclarkei. The genus name may also be an allusion to his adopted country, Sri Lanka, one of whose former names is Serendib.
The Learning Resource Centre at Richard Huish College, Taunton, which Clarke attended when it was Huish Grammar School, is named after him.
Clarke was a distinguished vice-president of the H. G. Wells Society, being strongly influenced by Wells as a science fiction writer.
Arthur C. Clarke Institute for Modern Technologies, one of the major research institutes in Sri Lanka, is named after him.
The main protagonist of the Dead Space series of video games, Isaac Clarke, takes his surname from Arthur C. Clarke, and his given name from Clarke's friendly rival and associate Isaac Asimov.
An outer-circular orbital beltway in Colombo, Sri Lanka, was named Arthur C. Clarke Expressway in honour of Clarke.
'The Clarke Event' is a proposed name for GRB 080319B, a gamma-ray burst detected just hours before Clarke's death which set a new record for the most intrinsically bright object ever observed by humans in the universe.
Clarke Montes, a mountain on Pluto's moon Charon, is named after Clarke.
== Selected works ==

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=== Novels ===
Against the Fall of Night (1948, 1953), original version of The City and the Stars
Prelude to Space (1951)
The Sands of Mars (1951)
Islands in the Sky (1952)
Childhood's End (1953)
Earthlight (1955)
The City and the Stars (1956)
The Deep Range (1957)
A Fall of Moondust (1961)
Dolphin Island: A Story of the People of the Sea (1963)
Glide Path (1963)
2001: A Space Odyssey (1968), produced alongside the film version with Stanley Kubrick
Rendezvous with Rama (1973)
Imperial Earth (1975)
The Fountains of Paradise (1979)
2010: Odyssey Two (1982)
The Songs of Distant Earth (1986)
2061: Odyssey Three (1987)
Cradle (1988) (with Gentry Lee)
The Ghost from the Grand Banks (1990)
The Hammer of God (1993)
Richter 10 (1996) (with Mike McQuay)
3001: The Final Odyssey (1997)
The Trigger (1999) (with Michael P. Kube-McDowell)
The Light of Other Days (2000) (with Stephen Baxter)
The Last Theorem (2008) (with Frederik Pohl)
=== Short stories and short story collections ===
Travel by Wire! (1937)
How We Went to Mars (1938)
The Awakening (1942)
Loophole (1946)
Technical Error (1950)
Expedition to Earth (1953)
Reach for Tomorrow (1956)
Tales from the White Hart (1957)
The Other Side of the Sky (1958)
Tales of Ten Worlds (1962)
The Nine Billion Names of God (1967)
Of Time and Stars (1972)
The Wind from the Sun (1972)
The Best of Arthur C. Clarke (1973)
The Sentinel (1983)
Tales From Planet Earth (1990)
More Than One Universe (1991)
The Collected Stories of Arthur C. Clarke (2001)
=== Non-fiction ===
Interplanetary Flight: an introduction to astronautics (1950), London: Temple Press, ISBN 0-425-06448-4
The Exploration of Space (1951), New York: Harper & Brothers
The Exploration of the Moon (1954), with R. A. Smith, New York: Harper Brothers
The Coast of Coral (1955), London: Frederick Muller
Boy Beneath the Sea (1958), New York: Harper, ISBN 0060212667
Voice Across the Sea (1958), New York: Harper
Profiles of the Future: An Inquiry into the Limits of the Possible (1962), New York: Harper & Row
The Treasure of the Great Reef (1964), with Mike Wilson, New York: Harper & Row
Man and Space (1964), Life Science Library, New York: Time Life
Voices from the Sky: Previews of the Coming Space Age (1965), New York: Harper & Row
The Promise of Space (1968), New York: Harper & Row
Mars and the Mind of Man (1971), New York: Harper & Row ISBN 978-0-06-010443-6
Report on Planet Three And Other Speculations (1972), New York: Berkley, ISBN 0-425-07592-3
The View from Serendip (1977), New York: Random House, ISBN 0-394-41796-8
1984: Spring / A Choice of Futures (1984), collected non-fiction writings, New York: Del Rey / Ballantine, ISBN 0-345-31357-7
Astounding Days: A Science Fictional Autobiography (1989), London: Gollancz, ISBN 0-575-04446-2
How the World Was One: Beyond the Global Village (1992), London: Gollancz, ISBN 0-575-05226-0
Greetings, Carbon-Based Bipeds! : Collected Essays, 1934–1998 (1999), New York: St. Martin's Press, and London: Voyager
== Media appearances ==
The City in the Image of Man: Ideas and Work of Paolo Soleri (1972)
2010: The Odyssey Continues (1984)
The Day of Five Billion (1987)
Without Warning (1994)
Fractals: The colors of infinity (1995), narrated documentary
Future Fantastic (BBC, 1996)
Arthur C. Clarke: The Man Who Saw the Future (1997)
Odyssey of Survival (1999)
2001: HAL's Legacy (2001)
Stanley Kubrick: A Life in Pictures (2001)
To Mars by A-Bomb: The Secret History of Project Orion (BBC, 2003)
The Martians and Us (2006)
Planetary Defense (2007)
Vision of a Future Passed: The Prophecy of 2001 (2007)
== Notes ==
== References ==
== External links ==
Official website of Arthur C. Clarke
Official website of the Arthur C. Clarke Foundation
"Sir Arthur C. Clarke biography". Science Fiction and Fantasy Hall of Fame.
Arthur C. Clarke (1917–2008) – International Astronautical Federation
Arthur C. Clarke at the Internet Speculative Fiction Database
Arthur C. Clarke at the Internet Book List
Arthur C. Clarke at IMDb
Sir Arthur C Clarke: 90th Birthday Reflections on YouTube
Works by or about Arthur C. Clarke at the Internet Archive
Works by Arthur C. Clarke at LibriVox (public domain audiobooks)
Works by Arthur C. Clarke at Open Library
"Arthur C. Clarke and Gentry Lee". Official transcript, Sci Fi Channel chat. 1 November 1996. Archived from the original on 1 December 2002.
Arthur C. Clarke Papers, Special Collections at the University of Southern Mississippi (de Grummond Children's Literature Collection)

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Proposed by Ronald N. Bracewell in 1960, a Bracewell probe is defined as a hypothetical autonomous interstellar space probe designed for communication with alien civilizations. It offers a potential solution to the inherent challenges of interstellar radio communication, such as signal delay, synchronization, and detection over vast distances.
== Description ==
A Bracewell probe is defined as an autonomous robotic interstellar space probe with advanced AI, pre-loaded with information or data its creators wish to convey. It would seek out existing technological civilizations or monitor worlds where such civilizations are likely to arise, establishing contact, making its presence known, conducting a dialogue over short distances (compared to interstellar distances), and transmitting the results of this interaction back to its origin. In essence, such probes would act as autonomous local representatives of their home civilization and serve as the point of contact between the cultures.
In contrast to radio communication across interstellar distances, a Bracewell probe offers key advantages: sustained presence in a target star system, active search capabilities, high-bandwidth local communication, and direct observation. Its physical presence serves as an unambiguous message. However, the probe cannot communicate information beyond its pre-loaded memory or update its contact protocols remotely. This inflexibility risks obsolescence and limits responses to unforeseen situations. Additionally, designing a Bracewell probe requires anticipating diverse alien biologies, psychologies, and technological levels — an inherently challenging task.
While a Bracewell probe does not need to be a von Neumann probe as well, the two concepts are compatible, and a self-replicating device as proposed by von Neumann would greatly speed up a Bracewell probe's search for alien civilizations.
It is also possible that such a probe (or system of probes if launched as a von Neumann–Bracewell probe) may outlive the civilization that created and launched it.
The search for Bracewell probes falls under SETA (Search for Extraterrestrial Artifacts) and SETV (Search for Extraterrestrial Visitation), encompassing efforts to detect evidence of extraterrestrial activity within the Solar System or nearby space. Detection methods might include searching for anomalous objects or emissions, analyzing long-delayed radio echoes (LDEs), and observing gravitational microlensing events. The LDE connection, however, remains highly speculative. These efforts often overlap with broader SETI initiatives, such as Breakthrough Listen.
The near-Earth object 1991 VG was initially considered a possible Bracewell probe due to its unusual rotation and orbit. However, subsequent observations identified it as a natural asteroid, with its characteristics attributed to the Yarkovsky effect and other non-gravitational forces.
== Variations ==
Bracewell's original 1960 paper proposed an autonomous probe for interstellar communication, but later researchers and science fiction writers have expanded on this idea.
=== Messenger probes ===
Following Bracewell’s original vision, these autonomous explorers carry pre-loaded messages and are designed to establish contact with technological civilizations.
=== Fly-through and rendezvous ===
Some are built for high-speed travel, gathering data during brief encounters before transmitting their findings home. Others are designed to decelerate and enter orbit, allowing for prolonged observation and near-real-time communication.
=== Non-replicating and self-replicating ===
While some remain singular, self-contained explorers, others incorporate von Neumann's concept of replication, enabling them to multiply and extend their search range exponentially.
=== Communicative and berserker ===
Certain probes act as peaceful electronic ambassadors, fostering interstellar dialogue. Speculative berserker machines, rooted in science fiction, are envisioned as autonomous weapons capable of eradicating alien civilizations.
== Fictional examples ==
In Arthur C. Clarke's 1979 novel The Fountains of Paradise the extraterrestrial Starglider probe is an example of a Bracewell probe. In Clarke's 1951 story "The Sentinel", later adapted into the 1968 film 2001: A Space Odyssey, the 'Monolith' appears to be a Bracewell probe placed on the Moon to ensure that only a civilization capable of spaceflight would be able to discover it.
Alien Planet is a 2005 94-minute Discovery Channel special about two internationally-built robot probes, and their mothership, searching for alien life on the fictional planet Darwin IV.
Bracewell probes are featured in David Brin's 2012 novel Existence.
In Star Trek: The Motion Picture (1979), a Voyager-class probe was upgraded and repurposed by an alien civilization. It acquired so much information, it became conscious and returned "home" to share what it learned
In the 1992 Star Trek: The Next Generation episode "The Inner Light", a Bracewell probe transmits details of an extinct civilization to Captain Picard.
In the 2001 Star Trek: Voyager episode "Friendship One", the crew is tasked with recovering a Bracewell probe that humans launched and find it on a planet whose inhabitants used the knowledge they obtained from it with disastrous results.
An alien probe contacts the space station Babylon 5 in the season 3 episode "A Day in the Strife". The probe disguises itself as a Bracewell probe, asking a series of questions and offering new technologies, medicine and science in return for answers to said questions. However, it's discovered that the probe was actually a berserker probe that would destroy any civilization that gave it correct answers by detonating an internal 500 megaton warhead.
The Snark, an alien probe that visits Earth in the novel In the Ocean of Night (1977) by Gregory Benford.
Bracewell probes in the role-playing game Eclipse Phase infect the seed AIs created by humanity with a deadly computer virus.
In the 2010 Doctor Who episode "Victory of the Daleks", the Daleks create a Bracewell probe in the form of a convincingly human android (appropriately named Bracewell) and covertly send it to Earth during World War II in order to communicate a false cover story to humans that disguises the Daleks' own arrival.
== See also ==
Astrobiology – Science concerned with life in the universe
Breakthrough Initiatives – Science-based program founded in 2015
Drake equation – Estimate of extraterrestrial civilizations
Extraterrestrial life – Life that does not originate on Earth
Fermi paradox – Discrepancy of the lack of evidence for alien life despite its apparent likelihood
First contact (science fiction) – Science fiction theme about the first meeting between humans and extraterrestrial life
Great Filter – Hypothesis of barriers to forming interstellar civilizations
Interstellar communication – Communication between planetary systems
Interstellar probe – Space probe that can travel out of the Solar System
Kardashev scale – Measure of a civilization's evolution
Project Daedalus – 1970s proposal for an interstellar probe
Self-replicating spacecraft – Space exploration concept
SETI Institute – Not-for-profit research organization
Technosignature – Property that provides scientific evidence for the presence of technology
== References ==
Bracewell, R. N. (1973). "The Opening Message from an Extraterrestrial Probe". Astronautics & Aeronautics. 11: 58–60.
McCollum, M. (1983). LifeProbe. New York: Ballantine Books.
Freitas Jr., R.A.; Valdes, F. (1985). "The Search for Extraterrestrial Artifacts". Acta Astronautica. 12 (12): 1027–1034. Bibcode:1985AcAau..12.1027F. CiteSeerX 10.1.1.118.4668. doi:10.1016/0094-5765(85)90031-1.
== External links ==
Biographical entry for Ronald Bracewell at The Encyclopedia of Astrobiology, Astronomy, and Spaceflight
Bracewell probes, also at The Encyclopedia of Astrobiology, Astronomy, and Spaceflight
SETV projects

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Breakthrough Initiatives is a science-based program founded in 2015 and funded by Julia and Yuri Milner, also of Breakthrough Prize, to search for extraterrestrial intelligence over a span of at least 10 years. The program is divided into multiple projects. Breakthrough Listen will comprise an effort to search over 1,000,000 stars for artificial radio or laser signals. A parallel project called Breakthrough Message is an effort to create a message "representative of humanity and planet Earth". The project Breakthrough Starshot, co-founded with Mark Zuckerberg, aims to send a swarm of probes to the nearest star at about 20% the speed of light. The project Breakthrough Watch aims to identify and characterize Earth-sized, rocky planets around Alpha Centauri and other stars within 20 light years of Earth. Breakthrough plans to send a mission to Saturn's moon Enceladus, in search for life in its warm ocean, and in 2018 signed a partnership agreement with NASA for the project.
== History ==
The Breakthrough Initiatives were announced to the public on 20 July 2015, at London's Royal Society by physicist Stephen Hawking. Russian tycoon Yuri Milner created the Initiatives to search for intelligent extraterrestrial life in the Universe and consider a plan for possibly transmitting messages out into space.
The announcement included an open letter co-signed by multiple scientists, including Hawking, expressing support for an intensified search for alien radio communications. During the public launch, Hawking said: "In an infinite Universe, there must be other life. There is no bigger question. It is time to commit to finding the answer."
The US$100 million cash infusion is projected to mark up the pace of SETI research over the early 2000s rate, and will nearly double the rate NASA was spending on SETI research annually in approximately 1973–1993.
== Projects ==
=== Breakthrough Listen ===
Breakthrough Listen is a program to search for intelligent extraterrestrial communications in the Universe. With $100 million in funding and thousands of hours of dedicated telescope time on state-of-the-art facilities, it is the most comprehensive search for alien communications to date. The project began in January 2016, and is expected to continue for 10 years.
The project uses radio wave observations from the Green Bank Observatory and the Parkes Observatory, and visible light observations from the Automated Planet Finder. Targets for the project include one million nearby stars and the centers of 100 galaxies. All data generated from the project are available to the public, and SETI@Home is used for some of the data analysis. The first results were published in April 2017, with further updates expected every 6 months.
=== Breakthrough Message ===
The Breakthrough Message program is to study the ethics of sending messages into deep space. It also launched an open competition with a US$1 million prize pool, to design a digital message that could be transmitted from Earth to an extraterrestrial civilization. The message should be "representative of humanity and planet Earth". The program pledges "not to transmit any message until there has been a global debate at high levels of science and politics on the risks and rewards of contacting advanced civilizations".
=== Breakthrough Starshot ===
Breakthrough Starshot, announced 12 April 2016, is a US$100 million program to develop a proof-of-concept light sail spacecraft fleet capable of making the journey to Alpha Centauri at 20% the speed of light (60,000 km/s or 215 million km/h) taking about 20 years to get there, and about 4 years to notify Earth of a successful arrival.
The interstellar journey may include a flyby of Proxima Centauri b, an Earth-sized exoplanet that is in the habitable zone of its host star in the Alpha Centauri system. From a distance of 1 Astronomical Unit (150 million kilometers or 93 million miles), the four cameras on each of the spacecraft could potentially capture an image of high enough quality to resolve surface features. The spacecraft fleet would have 1000 craft, and each craft, named StarChip, would be a very small centimeter-sized craft weighing several grams. They would be propelled by several ground-based lasers of up to 100 gigawatts. Each tiny spacecraft would transmit data back to Earth using a compact on-board laser communications system. Pete Worden is the head of this project. The conceptual principles to enable this interstellar travel project were described in "A Roadmap to Interstellar Flight", by Philip Lubin of UC Santa Barbara. METI president Douglas Vakoch summarized the significance of the project, saying that "by sending hundreds or thousands of space probes the size of postage stamps, Breakthrough Starshot gets around the hazards of spaceflight that could easily end a mission relying on a single spacecraft. Only one nanocraft needs to make its way to Alpha Centauri and send back a signal for the mission to be successful. When that happens, Starshot will make history."
In July 2017, scientists announced that precursors to StarChip, named Sprites, were successfully launched and flown.
=== Breakthrough Watch ===
Breakthrough Watch is a multimillion-dollar astronomical program to develop Earth- and space-based technologies that can find Earth-like planets in our cosmic neighborhood – and try to establish whether they host life. The project aims to identify and characterize Earth-sized, rocky planets around Alpha Centauri and other stars within 20 light years of Earth, in search of oxygen and other "biosignatures."
=== Breakthrough Enceladus ===
Breakthrough Enceladus is an astrobiology space probe mission concept to explore the possibility of life on Saturn's moon, Enceladus. In September 2018, NASA signed a collaboration agreement with Breakthrough to jointly create the mission concept. This mission would be the first privately funded deep space mission. It would study the content of the plumes ejecting from Enceladus's warm ocean through its southern ice crust. Enceladus's ice crust is thought to be around two to five kilometers thick, and a probe could use an ice-penetrating radar to constrain its structure.
== See also ==
== References ==
== External links ==
Breakthrough Initiatives web site
Breakthrough Listen
Breakthrough Message
Yuri Milner and Stephen Hawking announce $100 million Breakthrough Initiative to dramatically accelerate search for intelligent life in the Universe / Breakthrough Initiatives, London, 20 July 2015
Breakthrough Listen, Breakthrough Initiatives website
Breakthrough Initiatives' official website
Launching a StarChip – concept on YouTube
Going interstellar (NASA) on YouTube
Will Starshot's Insterstellar Journey Succeed? (PBS Digital Studios) on YouTube
Official website
Creation of Stephen Hawking's Universe with Nanotechnology

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Breakthrough Listen is an astronomy project to search for intelligent extraterrestrial communications. With $100 million in funding and thousands of hours of dedicated telescope time on state-of-the-art facilities, it is the most comprehensive search for alien communications to date. The project began in January 2016, and is expected to continue for 10 years. It is a component of Yuri Milner's Breakthrough Initiatives program. The science program for Breakthrough Listen is based at Berkeley SETI Research Center, located in the Astronomy Department at the University of California, Berkeley.
The project uses radio wave observations from the Green Bank Observatory and the Parkes Observatory, and visible light observations from the Automated Planet Finder. Targets for the project include one million nearby stars and the centers of 100 galaxies. All data generated from the project are available to the public, and SETI@Home (BOINC) is used for some of the data analysis. The first results were published in April 2017, with further updates expected every 6 months.
== Overview ==
The project aims to discover signs of extraterrestrial civilizations by searching stars and galaxies for radio signals and laser transmissions. The search for radio signals is carried out on the Green Bank Telescope in the Northern Hemisphere and the Parkes Telescope in the Southern Hemisphere. The Green Bank Telescope is the world's largest steerable radio telescope, and the Parkes Telescope is the second-largest steerable radio telescope in the Southern Hemisphere.
Together, the radio telescopes will cover ten times more sky than previous searches and scan the entire 1-to-10 GHz range, the so-called "quiet zone" in the spectrum where radio waves are unobscured by cosmic sources or Earth's atmosphere.
The radio telescopes are sensitive enough to detect "Earth-leakage" levels of radio transmission from stars within 5 parsecs, and can detect a transmitter of the same power as a common aircraft radar from the 1,000 nearest stars. The Green Bank Telescope began operations in January 2016, and the Parkes Telescope from October 2016. The FAST radiotelescope in China also joined forces in October 2016 with the Breakthrough Initiatives to launch a coordinated search, including the rapid sharing of promising new signals for additional observation and analysis.
The search for optical laser transmissions is carried out by the Automated Planet Finder of Lick Observatory. The telescope has the sensitivity to detect a 100 watt laser from a star 25 trillion miles (4.25 light years) away.
== Announcement ==
Breakthrough Listen was announced to the public on July 20, 2015 (the anniversary of the Apollo 11 Moon landing) by Milner at London's Royal Society. The event was flanked by scientists such as Frank Drake, who is known for the Drake equation that estimates the number of detectable alien civilizations, and Geoff Marcy, an astronomer who has helped find hundreds of exoplanets. The announcement included an open letter co-signed by multiple scientists, including physicist Stephen Hawking, expressing support for an intensified search for alien life. During the public launch, Hawking said:
In an infinite Universe, there must be other life. There is no bigger question. It is time to commit to finding the answer.
== Significance ==
The project is the most comprehensive search for alien communications to date. It is estimated that the project will generate as much data in one day as previous SETI projects generated in one year. Compared to previous programs, the radio surveys cover 10 times more of the sky, at least 5 times more of the radio spectrum, and work 100 times faster. The optical laser survey is also the deepest and broadest search in history.
Andrew Siemion, director of the Berkeley SETI Research Center at the University of California, Berkeley, describes that "We would typically get 24–36 hours on a telescope per year, but now we'll have thousands of hours per year on the best instruments...It's difficult to overstate how big this is. It's a revolution."
== Targets ==
As of April 2016, the targets for the radio search with the Green Bank Radio Telescope in the Northern Hemisphere include the following:
All 43 stars within 5 parsec, or 16.3 light-years, or 1.03 million astronomical units, or 1.545×1014 kilometres
1000 stars of all spectral-types within 50 pc, or 163 ly, or 10.3 million au, or 1.545×1015 km
One million nearby stars
Center regions of at least 100 nearby galaxies, including spiral galaxies, elliptical galaxies, dwarf galaxies and irregular galaxies
Exotic stars: 20 white dwarfs, 20 neutron stars, 20 black holes
The Parkes Radio Telescope will cover similar targets in the Southern Hemisphere from 1–4 GHz, and also the galactic plane and center.
The targets for the Automated Planet Finder will closely match those of the Green Bank radio search, with small adjustments due to the telescope's much smaller field of view.
While the telescopes are observing, the current targets of the Green Bank Radio Telescope and the Automated Planet Finder can be viewed live at the Berkeley Seti Research Center.
In January 2017, the project published its initial targets, which are the 60 nearest stars and a further 1649 stars which are the closest representatives of each spectral type. The initial targets also include 123 galaxies which cover all morphological types of galaxies.
In October 2019 it was announced that Breakthrough Listen will collaborate with scientists from NASA's Transiting Exoplanet Survey Satellite (TESS) team. Over a thousand new planets found by TESS will be scanned for technosignatures. The search will use Listen's primary facilities (Green Bank and Parkes Telescopes, MeerKAT, and the Automated Planet Finder) as well as partner facilities (including VERITAS, NenuFAR, FAST, the Murchison Widefield Array, LOFAR stations in Ireland and Sweden, Jodrell Bank Observatory, e-MERLIN, Keck Observatory, Sardinia Radio Telescope, along with the Allen Telescope Array). In addition to targeting of TESS planets with Listen facilities, the TESS lightcurves themselves will be searched for anomalies, for example caused by megastructures.

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== Breakthrough Listen Exotica Catalog ==
Breakthrough Listen Exotica Catalog is a list of 700 targets that were chosen "to include "one of everything" in the observed Universe – ranging from comets to galaxies, from mundane objects to the most rare and violent celestial phenomena".
There are four types of targets in the catalog:
"Prototypes: a list containing at least one example of every known kind of celestial object (apart from those too transient to present realistic observation targets). Planets and moons, stars at every point of their life cycle, galaxies big and small, serene star clusters and blazing quasars, and more are all included in the list."
"Superlatives: objects with the most extreme properties. These include examples like the hottest planet, stars with unusually high or low metal content, the most distant quasar and fastest-spinning pulsar, and the densest galaxy."
"Anomalies: enigmatic targets whose behavior is currently not satisfactorily explained. For instance, the famous "Tabby's Star" with its bizarre dimming behavior; ʻOumuamua – the interstellar object that passed near Earth in 2017; unexplained optical pulses that last mere nanoseconds; and stars with excess infrared radiation that could conceivably be explained as waste heat from alien megastructures."
A control sample of sources not expected to produce positive results.
== Data processing ==
Analyzing radio observations for possible signals requires intensive data analysis to cover all of the possible signal types. To carry out an in-depth search, the data recorder at the Green Bank telescope has been significantly upgraded. The system records 6 GHz of bandwidth at 24GB of data per second, making it among the highest data rate recording systems in radio astronomy, and there is a plan to double its capabilities in the near future. Once this data has been recorded, it is analysed for signals using a computing cluster with 64 GTX 1080 GPUs. The raw data is reduced to a lower resolution to allow long-term storage, but even this reduced data totals approximately 1 petabyte per year.
All data generated from Breakthrough Listen project will be open to the public. The data is uploaded on the initiative's Open Data Archive, where any user can download it for software analysis. Breakthrough Initiatives are developing open source software to assist users in understanding and analyzing the data, which are available on GitHub under UCBerkeleySETI.
The data is also processed by the SETI@home (BOINC) volunteer computer network, with the first batch of data being made available to SETI@home in April 2016.
== Funding ==
The project is funded with $100 million from the foundation co-founded by Yuri Milner. One third of this funding will be used to purchase telescope time. So far, the project has signed contracts for around 20 percent of the time on the Green Bank Telescope for the next five years, and 25 percent of the time on the Parkes Telescope. Another third will be used for the development of new equipment to receive and process potential signals, and the final third will be used to hire astronomy staff.
== Project leadership ==
Among the projects leaders are:
Frank Drake, chairman emeritus, SETI Institute; professor emeritus of astronomy and astrophysics, University of California, Santa Cruz; founding director, National Astronomy and Ionosphere Center; former Goldwin Smith Professor of Astronomy, Cornell University.
Ann Druyan, creative director of the Voyager Interstellar Message, NASA Voyager; co-founder and CEO, Cosmos Studios; Emmy Award- and Peabody Award-winning writer and producer.
Martin Rees, Astronomer Royal, Fellow of Trinity College; emeritus professor of cosmology and astrophysics, University of Cambridge.
Andrew Siemion, director, Berkeley SETI Research Center.
Dan Werthimer, co-founder and chief scientist of the SETI@home project; director of SERENDIP; principal investigator for CASPER.
Pete Worden, chairman, Breakthrough Prize Foundation.
== Results ==
In April 2017, the project released its first set of results, covering the observations of 692 nearby stars at frequencies from 1.1–1.9 GHz (the L-band). These observations included 11 events which passed the threshold for significance, but it was concluded that they were all consistent with radio frequency interference. A summary of the observations and the raw data relating to them has been published online. The project plans to continue publishing updated results approximately every 6 months.
The project has begun at lower frequencies as these have a lower frequency range which is easier to record and process, and plans eventually to observe in a wide range of frequencies from 1.15 GHz to 93 GHz.

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On August 30, 2017, Breakthrough Listen said it picked a series of 15 radio bursts coming from a dwarf galaxy about 3 billion light years away. Breakthrough Listen researchers said the possibility of the source being extraterrestrial life cannot yet be ruled out. The radio emissions were detected by the Green Bank Telescope in West Virginia. The source is FRB 121102 which was already known but the activity was vastly different in the latest findings.
In December 2017, Breakthrough Listen observed ʻOumuamua, an interstellar asteroid with an unusually elongated shape, for any signs of radio emissions. Over eight hours of observing over a range of frequencies from 1.1–11.6 GHz, no emissions were detected.
In December 2018, a search for laser light emissions from Boyajian's Star was carried out using the Automated Planet Finder, which is sensitive enough to detect a 24 MW laser at this distance. Although a number of candidates were identified, further analysis showed that they are coming from the Earth and not from the star.
In January 2020, a preliminary results for the nearby (<150 parsecs away) stars were announced, with no positive detections of artificial transmitters comparable to the terrestrial Arecibo Observatory in the 3.95-8.00 GHz band. Also, it was concluded that at least 8% of 252 nearby stars in a zone allowing detection of Earth by occultation method do not have the 100%-duty (artificial) transmitters of the sort sought by the survey.
In December 2020, it was reported that in April and May 2019, a narrowband signal at 982.002 MHz was intercepted that showed shifts in its frequency consistent with the movement of a planet. No modulation was detected. The signal appears to have originated from the direction of Proxima Centauri. It has been given the name Breakthrough Listen Candidate 1 (BLC1). As of December 2020, the researchers were still working to rule out terrestrial interference, which they considered the most likely cause. One researcher called it "on par" with the Wow! signal.
In May 2022, Breakthrough Listen conducted the first targeted search for the Wow! Signal. It was its first collaboration between the Green Bank Telescope and the SETI Institute's Allen Telescope Array. The observations lasted 1 hour from Greenbank, 35 minutes from ATA, and 10 minutes simultaneously. No technosignature candidates were found.
== See also ==
Communication with extraterrestrial intelligence
Detecting Earth from distant stars – Detecting Earth as an exoplanetPages displaying short descriptions of redirect targets
List of nearest stars and brown dwarfs within 20 light years
Nexus for Exoplanet System Science
Ohio State University Radio Observatory
Open data, Open-source software
Search for extraterrestrial intelligence
SETI Institute
== References ==
== External links ==
Breakthrough Listen, Breakthrough Initiatives website
Berkeley SETI Research Center, Berkeley SETI Research Center website

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William Brian Binnie (April 26, 1953 – September 15, 2022) was a United States Navy officer and one of the test pilots for SpaceShipOne, the experimental spaceplane developed by Scaled Composites and flown from 2003 to 2004.
== Early life ==
Binnie was born in West Lafayette, Indiana, on April 26, 1953, where his Scottish father William P. Binnie was a professor of physics at Purdue University. The family returned to Scotland when Binnie was five, and lived in Aberdeen (his father taught at Aberdeen University) and later in Stirling. When Binnie was a teenager the family moved to Boston.
Binnie earned a bachelor's degree in aerospace engineering from Brown University. He earned a master's degree from Brown in fluid mechanics and thermodynamics. Binnie was rejected by the United States Air Force, and enrolled at Princeton University, where he earned a master's degree in mechanical and aerospace engineering He served for 21 years in the United States Navy as a naval aviator, reaching the rank of commander. He flew the LTV A-7 Corsair II, Grumman A-6 Intruder, McDonnell Douglas F/A-18 Hornet, and McDonnell Douglas AV-8B Harrier II. He graduated from the United States Naval Test Pilot School in 1988. Binnie also copiloted the Atmospheric Test Vehicle of the Rotary Rocket. In 2006, he received an honorary degree from the University of Aberdeen.
== SpaceShipOne and spaceflight ==
On December 17, 2003, the 100th anniversary of the Wright brothers' first powered flight, Binnie piloted the first powered test flight of SpaceShipOne, flight 11P, which reached a top speed of Mach 1.2 and a height of 12.9 miles (20.7 km). On October 4, 2004, he piloted SpaceShipOne's second Ansari X Prize flight, flight 17P, winning the X Prize and becoming the 436th person to go into space. His flight, which peaked at 367,442 feet (69.6 mi; 112.0 km), set a winged aircraft altitude record for suborbital flights, breaking the old record set by the North American X-15 in 1963. It also earned him the second Astronaut Badge to be given by the Federal Aviation Administration for a flight aboard a privately operated commercial spacecraft.
== Later career ==
In 2014 Binnie joined XCOR Aerospace as senior engineer and test pilot, after working as a test pilot and program business manager for Scaled Composites for many years.
== Personal life ==
Binnie and his wife, Bub, had three children.
Binnie died on September 15, 2022, at age 69.
== References ==
== External links ==
Biography at Scaled Composites website
Biography at SpaceFacts.de

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CTA 102, also known by its B1950 coordinates as 2230+114 (QSR B2230+114) and its J2000 coordinates as J2232+1143 (QSO J2232+1143), is a blazar-type quasar discovered in the early 1960s by a radio survey carried out by the California Institute of Technology. It has been observed by a large range of instruments since its discovery, including WMAP, EGRET, GALEX, VSOP and Parkes, and has been regularly imaged by the Very Long Baseline Array since 1995. It has also been detected in gamma rays, and a gamma-ray flare has been detected from it.
In 1963 Nikolai Kardashev proposed that the then-unidentified radio source could be evidence of a Type II or III extraterrestrial civilization on the Kardashev scale. Follow-up observations were announced in 1965 by Gennady Sholomitskii, who found that the object's radio emission was varying; a public announcement of these results on April 12, 1965, caused a worldwide sensation. The idea that the emission was caused by a civilization was rejected when the radio source was later identified as one of the many varieties of a quasar.
The American folk rock band The Byrds whimsically reflected the original view that CTA-102 was a sign of extraterrestrial intelligence in their song "C.T.A.-102" from their 1967 album Younger Than Yesterday.
In late 2016 CTA 102, usually glowing around magnitude +17, had a bright outburst in visible light to magnitude +11 (~250 times brighter than usual). This likely was the most luminous blazar state ever observed, with an absolute magnitude in excess of -32.
A new outburst began in December 2017, with increased gamma-ray and optical activity. As of 22 December 2017, it has reached magnitude +14.
CTA 102 displays a radio structure mainly made of a radio core and two other components. There is also a double knot feature. Additionally, it also has two radio lobes described having flux densities of 170 and 75 mJy, with a jet found as curved according to high resolution imaging by Very Long Baseline interferometry at 15 GHz. This jet contains jet components moving with apparent velocities of 15.4 ± 0.9c.
The quasar is also classified to be highly polarized with a flat radio spectrum, and such belongs to a classification of optically violent variable quasars.
== See also ==
PSR B1919+21, the first pulsar discovered, mistaken for an alien radio signal
HD 164595
== References ==

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Charles Stuart Bowyer (August 2, 1934 – September 23, 2020) was an American astronomer and academic. He was a professor at the University of California.
== Early life and education ==
Bowyer was born in Toledo, Ohio, to Howard and Elizabeth Bowyer. His father was a pilot. As a boy, he attended a one-room grade school near his father’s farm in Orland Park, Ill., before being valedictorian at Orland Park High School. He graduated from Miami University of Ohio with a degree in physics. He received his Ph.D. in physics from Catholic University in 1965.
== Career ==
Bowyer was a professor at University of California at Berkeley. He was also affiliated with the United States Naval Research Laboratory. He worked in a group directed by Herbert Friedman. He is generally given credit for starting the field of extreme ultraviolet astronomy. Bowyer’s pursuit of studying ultraviolet rays was met with resistance at first - astronomers argued that even outside of the Earth's atmosphere most ultraviolet light would get absorbed and be undetectable. However, in 1975 when Bowyer and his team mounted a sensor on Apollo–Soyuz, they were able to detect ultraviolet radiation from white dwarfs and a nova.
He is credited with shepherding the launch of the EUVE satellite and subsequent research activities. The EUVE launched in 1992 and circled Earth for nine years, cataloging about 800 EUV sources in the Milky Way galaxy, before losing operating funds and shutting down in 2001.
Bowyer was also active in the search for extraterrestrial intelligence, or SETI. In 1977, Bowyer started SERENDIP (the Search for Extraterrestrial Radio Emissions From Nearby Developed Intelligent Populations) using an 85-foot telescope at Hat Creek Radio Observatory near Lassen Peak in Northern California. The project was inspired by Project Cyclops, a 1971 NASA report that proposed an international network of radio telescopes to search for intelligent life in the universe. SERENDIP was designed to operate in the background of other astronomers' radio observations. While those observations were happening, it would scan 100 radio frequencies simultaneously in search of extraterrestrial emissions. The project was funded primarily by private donors, including writer and futurist Arthur C. Clarke.
== Death ==
Bowyer died from complications of COVID-19 at an Orinda, California, hospital on September 23, 2020. He was 86.
== Selected works ==
In a statistical overview derived from writings by and about Stuart Bowyer, OCLC/WorldCat encompasses roughly 30+ works in 40+ publications in 3 languages and 1,000+ library holdings.
X-Ray Astronomy Observational Results, Theoretical Aspects and Experimental Procedures (1970)
Research in extreme ultraviolet and far ultraviolet astronomy semi-annual status report December 1, 1984 – May 31, 1985 (1985)
Research in extreme ultraviolet and far ultraviolet astronomy semi-annual status report June 1, 1986 – December 31, 1986 (1986)
== Honors ==
Bowyer received the Humboldt Foundation of Germany Senior Scientist Award in 1982. Other honors and distinctions include the NASA Exceptional Technical Achievement Medal, the NASA Exceptional Scientific Achievement Medal, the Alexander von Humboldt Foundation’s Humboldt Prize, an honorary doctor of science degree from Miami University of Ohio, an honorary doctor of philosophy degree from the Catholic University of America, the NASA Distinguished Public Service Medal, the Computerworld Smithsonian Award and the COSPAR Massey Award.
== Notes ==
== References ==
Margenau, Henry and Roy Abraham Varghese. (1992). Cosmos, Bios, Theos: Scientists Reflect on Science, God, and the Origins of the Universe, Life, and Homo Sapiens. LaSalle, Illinois: Open Court. ISBN 9780812691856; OCLC 231415341
== External links ==
Oral History interview transcript with Charles Stuart Bowyer on 28 July 1976, American Institute of Physics, Niels Bohr Library and Archives - Session I
Oral History interview transcript with Charles Stuart Bowyer on 10 March 1978, American Institute of Physics, Niels Bohr Library and Archives - Session II
Publication listings Archived 2012-02-16 at the Wayback Machine

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Charles Thomas Coleman (1962 or 1963 – October 20, 2024) was an American aviator, aerospace engineer and airshow & test pilot. He worked as a design and performance engineer for several aircraft corporations such as McDonnell Aircraft Corporation, Bede Jet Corporation and Scaled Composites. Coleman was a member of the Society of Experimental Test Pilots SETP as an Associate Fellow, and served on the board of directors for the Mojave Air and Space Port. As a commercial, test, and instructor pilot, he logged more than 10,800 hours of flight time.
== Early life and education ==
Coleman was raised in St. Johns, Michigan, by Thomas Coleman and JoAnn (Benedict) Smith. He graduated from the University of Michigan in 1985 with a Bachelor's of Science in Aerospace/Mechanical Engineering.
== Career ==
Coleman was a design engineer for 6 years at McDonnell Aircraft Corporation in St. Louis, Missouri. He was involved in military jet projects including serving as the Senior Design Engineer on the High Alpha Research Vehicle, a modified F/A-18 Hornet, utilized by NASA to investigate controlled flight at high angles of attack by way of thrust vectoring.
Coleman also served as a Senior Engineer on the F/A-18 conversion from combat-ready aircraft into performance planes for the United States Navy Blue Angels flight demonstration squadron.
He was a project engineer at the Bede Jet Corporation in Chesterfield, Missouri at the Spirit of St. Louis Airport. He served as the test pilot on the BD-10, a kit-built experimental jet aircraft, and BD-12, a two-seat experimental plane with a pusher configuration.
Coleman joined Scaled Composites in Mojave, California as a performance engineer, test pilot, and chase pilot in 2002. He was on a team of five engineers that designed, constructed, and flight tested the Virgin Atlantic GlobalFlyer, which was the first jet powered aircraft to fly around the world non-stop un-refueled.
Coleman was also a test pilot for the Proteus high altitude jet and tested the Tier One Navigation System for SpaceShipOne, as well as conducting high-G astronaut training for SpaceShipOne astronauts and serving as chase pilot for SpaceShipOne flights. Coleman also served as a test pilot for the ICON A5, an American amphibious light-sport aircraft.
== Performance ==
Coleman performed at numerous airshows and flew aerobatic planes for Patty Wagstaff, Gene Soucy, Ian Groom, Tim Weber, Sean D. Tucker, Discovery Channel, Toyota Airsports, and Paramount Pictures. In 2018, Coleman trained the lead actors starring in Top Gun: Maverick featuring Tom Cruise, Val Kilmer, and Jennifer Connelly. Coleman conducted 140 G tolerance training flights in an Extra EA-300 with actors Glen Powell, Miles Teller, Monica Barbaro, Jay Ellis, Lewis Pullman, and Danny Ramirez, in a flight training regime designed by Cruise. These aerobatic flights were conducted in order to prepare the actors for flight in F/A-18F Super Hornets during actual filming.
== Death ==
On October 20, 2024, Coleman died in a crash during the Las Cruces Air & Space Expo at Las Cruces International Airport. He was 61 years old. The crash occurred around 2:30 pm. Coleman was performing aerobatics when his Extra EA-300 plane crashed half a mile west of the airport.
== Awards ==
Coleman won two Collier Trophies for his involvement in the development of the McDonnell Douglas C-17 Globemaster (1994) and Scaled Composites’ SpaceShipOne (2004). Coleman was also part of the Scaled Composites team that won the Ansari X Prize, a space competition in which the X Prize Foundation offered a cash prize for the first non-government organization to launch a reusable crewed spacecraft into space twice within two weeks.
== References ==
== External links ==
Official website
Chuck Coleman at IMDb

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Contact is a 1985 hard science fiction novel by American scientist Carl Sagan. The plot concerns contact between humanity and a more technologically advanced extraterrestrial life form.
Contact ranked number seven on Publishers Weekly's 1985 bestseller list. The only full work of fiction published by Sagan, the novel originated as a screenplay by Sagan and Ann Druyan (whom he later married) in 1979; when development of the film stalled, Sagan converted it into a novel. It was adapted in 1997 as the film Contact, starring Jodie Foster.
== Plot ==
As a child, Eleanor "Ellie" Arroway shows a strong aptitude for science and mathematics. Dissatisfied with a school lesson, she confirms in a library that pi is transcendental. In sixth grade, her father, Theodore ("Ted"), dies. Her new stepfather, John Staughton, does not support her interests. Ellie resents him and believes her mother remarried out of weakness.
After graduating from Harvard, Ellie earns a doctorate from Caltech under radio astronomer David Drumlin. She becomes director of "Project Argus," a New Mexico radio telescope array searching for extraterrestrial intelligence (SETI). This puts her at odds with much of the scientific community, including Drumlin, who pushes to defund SETI. Eventually, the project detects a signal from Vega, 26 light-years away, transmitting prime numbers. Further analysis reveals a retransmission of Adolf Hitler's 1936 Olympic speech, the first TV signal to escape Earth's ionosphere.
President Helen Lasker meets with Ellie to discuss first contact. Ellie begins a relationship with Presidential Science Advisor Ken der Heer. With Soviet colleague Vaygay Lunacharsky, she ensures continuous monitoring of the signal. A third message contains plans for an advanced machine, but decoding its 30,000 pages proves impossible without a missing primer.
At the President's insistence, Ellie meets religious leaders Billy Jo Rankin and Palmer Joss. A skeptic, she demonstrates her faith in science by trusting a Foucault pendulum. Dismissing Rankin's views, she finds Joss's perspective intriguing. In Paris, experts confirm the Machine is a five-seat dodecahedron. There, Ellie meets Devi Sukhavati, a doctor who lost her husband after leaving India for love. The final message piece is found when billionaire S. R. Hadden suggests checking for phase modulation, revealing the primer.
A U.S.–Soviet race to build the Machine ensues, but Soviet design flaws leave the American version as the only option. Ellie applies as a passenger but loses her spot to Drumlin. Extremists plant a bomb in the Wyoming facility, which detonates during testing, killing Drumlin and delaying the project indefinitely. Meanwhile, Ellie's mother suffers a stroke, leaving her paralyzed. Staughton accuses Ellie of neglecting her family.
Ellie later learns Hadden has moved to a private space station. There, he reveals his company has secretly built a third Machine in Hokkaido, Japan, set for launch on December 31, 1999. Ellie, Vaygay, and Devi secure seats, joined by Nigerian physicist Abonnema Eda and Chinese archaeologist Xi Qiaomu. Before departure, Joss gives Ellie a medallion, which she takes aboard.
The activated Machine transports the group through wormholes to a station near the Milky Way's center, where each meets an extraterrestrial in the form of a loved one. Ellie's visitor, appearing as Ted, explains their species' motives and a project to alter the universe's properties using mass in Cygnus A. The wormhole network was built by unknown precursors, and hidden messages exist in transcendental numbers like pi. Reunited, the travelers record evidence before the dodecahedron returns them to Earth.
Back home, their journey—seeming more than a day—took no time at all on Earth. Their video recordings are erased, likely by wormhole magnetic fields. With Hadden seemingly dead and the transmission halted, officials suspect a hoax. Pressured, the travelers stay silent, though Joss believes Ellie, who now relies on faith. Hadden faked his death, secretly launching himself into space using cryogenics, but this remains unknown to the characters.
Following "Ted's" suggestion, Ellie runs a program computing pi to unprecedented lengths. Before results emerge, her mother dies, leaving a final letter revealing Staughton—not Ted—is Ellie's biological father. When Ellie examines the program's output, she finds a circle formed from 0s and 1s after 1020 digits in pi's base-11 representation—evidence of her journey.
== Publication history ==
Reading science fiction and fantasy as a child inspired Sagan to become an astronomer. As an adult, he preferred realistic stories that helped readers understand real science and history, like Robert Heinlein's "—And He Built a Crooked House—" and L. Sprague de Camp's Lest Darkness Fall. In 1978, Sagan predicted that because of science fiction, "I know many young people who would, of course, be interested, but in no way astounded, were we to receive a message tomorrow from an extraterrestrial civilization". In 1981, Simon & Schuster gave Sagan a $2 million advance on the novel. At the time, "the advance was the largest ever made for a book that had not yet been written." The first printing was 265,000 copies. In the first two years it sold 1,700,000 copies. It was a main selection of Book-of-the-Month-Club.
Sagan's friend, physicist Kip Thorne, gave Sagan ideas on the nature of wormholes when Sagan was developing the outline of the novel.
Sagan named the novel's protagonist, Eleanor Arroway, after two people: Eleanor Roosevelt, a "personal hero" of Sagan's wife, Ann Druyan, and Voltaire, whose last name was Arouet. The character is based on the real-life SETI researcher Jill Tarter.
In 1986, the novel won the Locus Award for Best First Novel, and placed #15 in the poll for the Locus Award for Best Science Fiction Novel.
== See also ==
Communication with extraterrestrial intelligence
Fermi paradox
His Master's Voice
== Notes ==
== References ==
== Sources ==
Davidson, Keay. Carl Sagan: A Life. New York: John Wiley & Sons, 1999.
Sagan, Carl. Contact. New York: Simon and Schuster, 1985.
Spence, Jennifer. "20th-Century American Bestsellers: Contact". LIS 590AB/ENGL 564: 20th-Century American Bestsellers (Spring 2006). Graduate School of Library and Information Science, University of Illinois, Urbana-Champaign. Archived from the original on July 19, 2011. Retrieved August 18, 2010.
== External links ==
Larry Klaes' in-depth analysis of the film and novel

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Cosmic Call was the name of two sets of interstellar radio messages that were sent from RT-70 in Yevpatoria, Ukraine in 1999 (Cosmic Call 1) and 2003 (Cosmic Call 2) to various nearby stars. The messages were designed with noise-resistant format and characters.
The project was funded by Team Encounter, a Texas-based startup, which went out of business in 2004.
Both transmissions were at ~150 kW, 5.01 GHz (FSK +/-24 kHz).
== Message structure ==
Each Cosmic Call 1 session had the following structure. The Scientific Part (DDM, BM, AM, and ESM) was sent three times (at 100 bit/s), and the Public Part (PP) was sent once (at 2000 bit/s), according to the following arrangement:
DDM → BM → AM → ESM → DDM → BM → AM → ESM → DDM → BM → AM → ESM → PP,
where DDM is the Dutil-Dumas Message, created by Canadian scientists Yvan Dutil and Stéphane Dumas, BM is the Braastad Message, AM is the Arecibo Message, and ESM is the Encounter 2001 Staff Message.
Each Cosmic Call 2 session in 2003 had the following structure:
DDM2 → DDM2 → DDM2 → AM → AM → AM → BIG → BIG → BIG → BM → ESM → PP,
where DDM2 is modernized DDM (aka Interstellar Rosetta Stone, ISR), BIG is Bilingual Image Glossary. All but the PP were transmitted at 400 bit/s
The ISR was 263,906 bits; BM, 88,687 bits, AM, 1,679 bits; BIG was 12 binary images 121,301 bits; ESM 24,899 bits. Total = 500,472 bits for 53 minutes. PP was 220 megabytes and sent at a rate of 100,000 bit/s for 11 hours total.
== Error in Cosmic Call 1 ==
The DDM incorrectly gave the neutron mass as 1.67392, instead of the known value of 1.67492. This error was corrected in DDM2.
== Stars targeted ==
The messages were sent to the following stars:
== See also ==
Arecibo message
Active SETI
Communication with extraterrestrial intelligence
Interstellar messages
== References ==
== External links ==
Self-Decoding Messages
Discussion of the Call's "Rosetta Stone" and how it was developed
Report on Cosmic Call
Evpatoria 2003 discussion with bitmap and image

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David Pope Anderson (born 1955) is an American research scientist at the Space Sciences Laboratory, at the University of California, Berkeley, and an adjunct professor of computer science at the University of Houston. Anderson leads the SETI@home, BOINC, Bossa, and Bolt software projects.
== Education ==
Anderson received a BA in mathematics from Wesleyan University, and MS and PhD degrees in mathematics and computer science from the University of Wisconsin–Madison. While in graduate school he published four research papers in computer graphics. His PhD research involved using enhanced attribute grammars to specify and implement communication protocols.
== Career ==
From 1985 to 1992 he was an assistant professor in the UC Berkeley Computer Science Department, where he received the NSF Presidential Young Investigator and IBM Faculty Development awards. During this period he conducted several research projects:
FORMULA (Forth Music Language), a parallel programming language and runtime system for computer music based on Forth.
MOOD (Musical Object-Oriented Dialect), a parallel programming language and runtime system for computer music based on C++. A port for MS-DOS also exists.
DASH, a distributed operating system with support for digital audio and video.
Continuous Media File System (CMFS), a file system for digital audio and video
Comet, an I/O server for digital audio and video.
From 1992 to 1994 he worked at Sonic Solutions, where he developed Sonic System, the first distributed system for professional digital audio editing.
From 1995 to 1998 he was chief technical officer of Tunes.com, where he developed web-based systems for music discovery based on collaborative filtering, acoustics, and other models.
In 1995 he joined David Gedye and Dan Werthimer in creating SETI@home, an early volunteer computing project. Anderson continues to direct SETI@home.
From 2000 to 2002, he served as CTO of United Devices, a company that developed software for distributed computing.
In 2002 he created the Berkeley Open Infrastructure for Network Computing (BOINC) project, which develops an open-source software platform for volunteer computing. The project is funded by NSF and is based at the UC Berkeley Space Sciences Laboratory. BOINC has been used by about 100 projects, including SETI@home, Einstein@home, Rosetta@home, Climateprediction.net, and the IBM World Community Grid, in areas as diverse as mathematics, medicine, molecular biology, climatology, and astrophysics.
Anderson was involved in Stardust@home, which used 23,000 volunteers to identify interstellar dust particles via the Web – an approach called distributed thinking. In 2007 Anderson developed BOSSA a software framework for distributed thinking, using volunteers on the Internet to perform tasks that require human intelligence, knowledge, or cognitive skills.
He also developed BOLT, a framework for web-based training and education in the context of volunteer computing and distributed thinking.
=== Music ===
Since 2020 Anderson has been involved in software projects related to classical music:
Music Match is a social site where performers and composers can meet and communicate with each other.
Classical Music Index is an offshoot of IMSLP in which users can rate and review compositions, and can discover compositions likely to appeal to their taste.
Numula is a Python library for computer rendition of music with complex variations in dynamics, timing, articulation and pedaling, as are typical in human performance.
=== Inventions ===
In 1994 Anderson invented "Virtual Reality Television", a television system allowing viewers to control their virtual position and orientation. He was awarded a patent for this invention in 1996.
In 1994 he developed one of the first systems for collaborative filtering, and developed a web site, rare.com, that provided movie recommendations based on the user's movie ratings.
== References ==
== External links ==
Profile of David Anderson

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Douglas Bennett Shane is President of The Spaceship Company, as well as an American test pilot who has trained as a commercial astronaut. He was a member of the Scaled Composites astronaut team and one of the test pilots for SpaceShipOne, the experimental spaceplane developed by Scaled Composites.
Shane worked as the operations director on the SpaceShipOne project in addition to being one of the program pilots,
and later served as President of Scaled Composites from 2008 through early 2013.
== Biography ==
In 1982, Shane received a Bachelor of Science degree in aerospace engineering from the University of Kansas.
Shane built a Long-Ez he called the "Shane Runabout".
=== Scaled Composites ===
In 1982, Shane joined Scaled Composites, in Mojave, California, as a test pilot and founding member. After 1989, he was responsible for business development, contracts, and proposals, as well as the company's flight test operations.
In 1997, Shane received the Iven C. Kincheloe Award from the Society of Experimental Test Pilots for his flight test work on the Williams V-Jet II and Vision Aire Vantage. He is a past President of the Society of Experimental Test Pilots.
In 2004, as director of flight operations, Shane supervised the Ansari X Prize qualification flights from the control room. Although he was one of the four qualified pilots for SpaceShipOne, Shane did not fly any of the flights himself.
In June 2008, Shane was appointed president of Scaled Composites, taking over this role from previous president and company founder Burt Rutan. He stayed in this position for five years, from 2008 through early 2013.
=== The Spaceship Company ===
In June 2013, Shane joined The Spaceship Company (TSC) to become executive VP and general manager, ending a 31-year career with Scaled Composites, including five as president.
He was promoted to president of TSC in July 2014.
== See also ==
Scaled Composites
SpaceShipOne
The Spaceship Company
== References ==
== External links ==
Scaled Composites biography
Biography on Encyclopedia Astronautica
List of American Astronauts
Auction entry for Shane signature under category "Astronaut Autographs"
Name

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The Drake equation is a probabilistic argument used to estimate the number of active, communicative extraterrestrial civilizations in the Milky Way Galaxy.
The equation was formulated in 1961 by Frank Drake, not for purposes of quantifying the number of civilizations, but as a way to stimulate scientific dialogue at the first scientific meeting on the search for extraterrestrial intelligence (SETI). The equation summarizes the main concepts which scientists must contemplate when considering the question of other radio-communicative life. It is more properly thought of as an approximation than as a serious attempt to determine a precise number.
Criticism related to the Drake equation focuses not on the equation itself, but on the fact that the estimated values for several of its factors are highly conjectural, the combined multiplicative effect being that the uncertainty associated with any derived value is so large that the equation cannot be used to draw firm conclusions.
== Equation ==
The Drake equation is:
N
=
R
∗
⋅
f
p
⋅
n
e
⋅
f
l
⋅
f
i
⋅
f
c
⋅
L
{\displaystyle N=R_{*}\cdot f_{\mathrm {p} }\cdot n_{\mathrm {e} }\cdot f_{\mathrm {l} }\cdot f_{\mathrm {i} }\cdot f_{\mathrm {c} }\cdot L}
where
N = the number of civilizations in the Milky Way galaxy with which communication might be possible (i.e. which are on the current past light cone);
and
R∗ = the average rate of star formation in our galaxy.
fp = the fraction of those stars that have planets.
ne = the average number of planets that can potentially support life per star that has planets.
fl = the fraction of planets that could support life that actually develop life at some point.
fi = the fraction of planets with life that go on to develop intelligent life (civilizations).
fc = the fraction of civilizations that develop a technology that releases detectable signs of their existence into space.
L = the length of time for which such civilizations release detectable signals into space.
This form of the equation first appeared in Drake's 1965 paper.
== History ==
In September 1959, physicists Giuseppe Cocconi and Philip Morrison published an article in the journal Nature with the provocative title "Searching for Interstellar Communications". Cocconi and Morrison argued that radio telescopes had become sensitive enough to pick up transmissions that might be broadcast into space by civilizations orbiting other stars. Such messages, they suggested, might be transmitted at a wavelength of 21 cm (1,420.4 MHz). This is the wavelength of radio emission by neutral hydrogen, the most common element in the universe, and they reasoned that other intelligences might see this as a logical landmark in the radio spectrum.
Two months later, Harvard University astronomy professor Harlow Shapley speculated on the number of inhabited planets in the universe, saying "The universe has 10 million, million, million suns (10 followed by 18 zeros) similar to our own. One in a million has planets around it. Only one in a million million has the right combination of chemicals, temperature, water, days and nights to support planetary life as we know it. This calculation arrives at the estimated figure of 100 million worlds where life has been forged by evolution."
Seven months after Cocconi and Morrison published their article, Drake began searching for extraterrestrial intelligence in an experiment called Project Ozma. It was the first systematic search for signals from communicative extraterrestrial civilizations. Using the 85 ft (26 m) dish of the National Radio Astronomy Observatory, Green Bank in Green Bank, West Virginia, Drake monitored two nearby Sun-like stars: Epsilon Eridani and Tau Ceti, slowly scanning frequencies close to the 21 cm wavelength for six hours per day from April to July 1960. The project was well designed, inexpensive, and simple by today's standards. It detected no signals.
Soon thereafter, Drake hosted the first search for extraterrestrial intelligence conference on detecting their radio signals. The meeting was held at the Green Bank facility in 1961. The equation that bears Drake's name arose out of his preparations for the meeting.
As I planned the meeting, I realized a few day[s] ahead of time we needed an agenda. And so I wrote down all the things you needed to know to predict how hard it's going to be to detect extraterrestrial life. And looking at them it became pretty evident that if you multiplied all these together, you got a number, N, which is the number of detectable civilizations in our galaxy. This was aimed at the radio search, and not to search for primordial or primitive life forms.
The ten attendees were conference organizer J. Peter Pearman, Frank Drake, Philip Morrison, businessman and radio amateur Dana Atchley, chemist Melvin Calvin, astronomer Su-Shu Huang, neuroscientist John C. Lilly, inventor Barney Oliver, astronomer Carl Sagan, and radio-astronomer Otto Struve. These participants called themselves "The Order of the Dolphin" (because of Lilly's work on dolphin communication), and commemorated their first meeting with a plaque at the observatory hall.
== Usefulness ==

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The Drake equation results in a summary of the factors affecting the likelihood that we might detect radio-communication from intelligent extraterrestrial life. The last three parameters, fi, fc, and L, are not known and are very difficult to estimate, with values ranging over many orders of magnitude (see § Criticism). Therefore, the usefulness of the Drake equation is not in the solving, but rather in the contemplation of all the various concepts which scientists must incorporate when considering the question of life elsewhere, and gives the question of life elsewhere a basis for scientific analysis. The equation has helped draw attention to some particular scientific problems related to life in the universe, for example abiogenesis, the development of multi-cellular life, and the development of intelligence itself.
Within the limits of existing human technology, any practical search for distant intelligent life must necessarily be a search for some manifestation of a distant technology. After about 50 years, the Drake equation is still of seminal importance because it is a 'road map' of what we need to learn in order to solve this fundamental existential question. It also formed the backbone of astrobiology as a science; although speculation is entertained to give context, astrobiology concerns itself primarily with hypotheses that fit firmly into existing scientific theories. Some 50 years of SETI have failed to find anything, even though radio telescopes, receiver techniques, and computational abilities have improved significantly since the early 1960s. SETI efforts since 1961 have conclusively ruled out widespread alien emissions near the 21 cm wavelength of the hydrogen frequency.
== Estimates ==
=== Original estimates ===
There is considerable disagreement on the values of these parameters, but the 'educated guesses' used by Drake and his colleagues in 1961 were:
R∗ = 1 yr−1 (1 star formed per year, on the average over the life of the galaxy; this was regarded as conservative)
fp = 0.2 to 0.5 (one fifth to one half of all stars formed will have planets)
ne = 1 to 5 (stars with planets will have between 1 and 5 planets capable of developing life)
fl = 1 (100% of these planets will develop life)
fi = 1 (100% of which will develop intelligent life)
fc = 0.1 to 0.2 (10–20% of which will be able to communicate)
L = somewhere between 1000 and 100,000,000 years
Inserting the above minimum numbers into the equation gives a minimum N of 20 (see: Range of results). Inserting the maximum numbers gives a maximum of 50,000,000. Drake states that given the uncertainties, the original meeting concluded that N ≈ L, and there were probably between 1000 and 100,000,000 planets with civilizations in the Milky Way Galaxy.
=== Current estimates ===
This section discusses and attempts to list the best current estimates for the parameters of the Drake equation.
==== Rate of star creation in this Galaxy, R∗ ====
Calculations in 2010, from NASA and the European Space Agency indicate that the rate of star formation in this Galaxy is about 0.68–1.45 solar masses (M☉; 1.35×1030–2.88×1030 kg) of material per year. To get the number of stars per year, we divide this by the initial mass function (IMF) for stars, where the average new star's mass is about 0.5 M☉. This gives a star formation rate of about 1–3 stars per year.
==== Fraction of those stars that have planets, fp ====
Analysis of microlensing surveys, in 2012, has found that fp may approach 1—that is, stars are orbited by planets as a rule, rather than the exception; and that there are one or more bound planets per Milky Way star.

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==== Average number of planets that might support life per star that has planets, ne ====
In November 2013, astronomers reported, based on Kepler space telescope data, that there could be as many as 40 billion Earth-sized planets orbiting in the habitable zones of sun-like stars and red dwarf stars within the Milky Way Galaxy. 11 billion of these estimated planets may be orbiting sun-like stars. Since there are about 100 billion stars in the galaxy, this implies fp · ne is roughly 0.4. The nearest planet in the habitable zone is Proxima Centauri b, which is as close as about 4.2 light-years away.
The consensus at the Green Bank meeting was that ne had a minimum value between 3 and 5. Dutch science journalist Govert Schilling has opined that this is optimistic. Even if planets are in the habitable zone, the number of planets with the right proportion of elements is difficult to estimate. Brad Gibson, Yeshe Fenner, and Charley Lineweaver determined that about 10% of star systems in the Milky Way Galaxy are hospitable to life, by having heavy elements, being far from supernovae and being stable for a sufficient time.
The discovery of numerous gas giants in close orbit with their stars has introduced doubt that life-supporting planets commonly survive the formation of their stellar systems. So-called hot Jupiters may migrate from distant orbits to near orbits, in the process disrupting the orbits of habitable planets.
On the other hand, the variety of star systems that might have habitable zones is not just limited to solar-type stars and Earth-sized planets. It is now estimated that even tidally locked planets close to red dwarf stars might have habitable zones, although the flaring behavior of these stars might speak against this. The possibility of life on moons of gas giants (such as Jupiter's moon Europa, or Saturn's moons Titan and Enceladus) adds further uncertainty to this figure.
The authors of the rare Earth hypothesis propose a number of additional constraints on habitability for planets, including being in galactic zones with suitably low radiation, high star metallicity, and low enough density to avoid excessive asteroid bombardment. They also propose that it is necessary to have a planetary system with large gas giants which provide bombardment protection without a hot Jupiter; and a planet with plate tectonics, a large moon that creates tidal pools, and moderate axial tilt to generate seasonal variation.
==== Fraction of the above that actually go on to develop life, fl ====
Geological evidence from the Earth suggests that fl may be high; life on Earth appears to have begun around the same time as favorable conditions arose, suggesting that abiogenesis may be relatively common once conditions are right. However, this evidence only looks at the Earth (a single model planet), and contains anthropic bias, as the planet of study was not chosen randomly, but by the living organisms that already inhabit it (ourselves). From a classical hypothesis testing standpoint, without assuming that the underlying distribution of fl is the same for all planets in the Milky Way, there are zero degrees of freedom, permitting no valid estimates to be made. If life (or evidence of past life) were to be found on Mars, Europa, Enceladus or Titan that developed independently from life on Earth it would imply a value for fl close to 1. While this would raise the number of degrees of freedom from zero to one, there would remain a great deal of uncertainty on any estimate due to the small sample size, and the chance they are not really independent.
Countering this argument is that there is no evidence for abiogenesis occurring more than once on the Earth—that is, all terrestrial life stems from a common origin. If abiogenesis were more common it would be speculated to have occurred more than once on the Earth. Scientists have searched for this by looking for bacteria that are unrelated to other life on Earth, but none have been found yet. It is also possible that life arose more than once, but that other branches were out-competed, or died in mass extinctions, or were lost in other ways. Biochemists Francis Crick and Leslie Orgel laid special emphasis on this uncertainty: "At the moment we have no means at all of knowing" whether we are "likely to be alone in the galaxy (Universe)" or whether "the galaxy may be pullulating with life of many different forms." As an alternative to abiogenesis on Earth, they proposed the hypothesis of directed panspermia, which states that Earth life began with "microorganisms sent here deliberately by a technological society on another planet, by means of a special long-range unmanned spaceship".
In 2020, a paper by scholars at the University of Nottingham proposed an "Astrobiological Copernican" principle, based on the Principle of Mediocrity, and speculated that "intelligent life would form on other [Earth-like] planets like it has on Earth, so within a few billion years life would automatically form as a natural part of evolution". In the authors' framework, fl, fi, and fc are all set to a probability of 1 (certainty). Their resultant calculation concludes there are more than thirty current technological civilizations in the galaxy (disregarding error bars).

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==== Fraction of the above that develops intelligent life, fi ====
This value remains particularly controversial. Those who favor a low value, such as the biologist Ernst Mayr, point out that of the billions of species that have existed on Earth, only one has become intelligent and from this, infer a tiny value for fi. Likewise, the Rare Earth hypothesis, notwithstanding their low value for ne above, also think a low value for fi dominates the analysis. Those who favor higher values note the generally increasing complexity of life over time, concluding that the appearance of intelligence is almost inevitable, implying an fi approaching 1. Skeptics point out that the large spread of values in this factor and others make all estimates unreliable. (See Criticism).
In addition, while it appears that life developed soon after the formation of Earth, the transition from single-celled prokaryotes to eukaryotes, and the resulting increase in complexity, including a change in life's "operating system", required a considerable amount of time as the complexity of living organisms grew. Subsequently, the Cambrian explosion, in which a large variety of multicellular life forms came into being, occurred a considerable amount of time after this event, which suggests the possibility that special conditions for complex life were necessary. Some scenarios such as the snowball Earth or research into extinction events have raised the possibility that life on Earth is relatively fragile. Research on any past life on Mars is relevant since a discovery that life did form on Mars but ceased to exist might raise the estimate of fl but would indicate that in half the known cases, intelligent life did not develop.
Estimates of fi have been affected by discoveries that the Solar System's orbit is circular in the galaxy, at such a distance that it remains out of the spiral arms for tens of millions of years (evading radiation from novae). Also, Earth's large moon may aid the evolution of life by stabilizing the planet's axis of rotation.
There has been quantitative work to begin to define
f
l
⋅
f
i
{\displaystyle f_{\mathrm {l} }\cdot f_{\mathrm {i} }}
. One example is a Bayesian analysis published in 2020. In the conclusion, the author cautions that this study applies to Earth's conditions. In Bayesian terms, the study favors the formation of intelligence on a planet with identical conditions to Earth but does not do so with high confidence.
Planetary scientist Pascal Lee of the SETI Institute proposes that this fraction is very low (0.0002). He based this estimate on how long it took Earth to develop intelligent life (1 million years since Homo erectus evolved, compared to 4.6 billion years since Earth formed).
==== Fraction of the above revealing their existence via signal release into space, fc ====
For deliberate communication, the one example we have (the Earth) does not do much explicit communication, though there are some efforts covering only a tiny fraction of the stars that might look for human presence. (See Arecibo message, for example). There is considerable speculation why an extraterrestrial civilization might exist but choose not to communicate. However, deliberate communication is not required, and calculations indicate that current or near-future Earth-level technology might well be detectable to civilizations not too much more advanced than present day humans. By this standard, the Earth is a communicating civilization.
Another question is what percentage of civilizations in the galaxy are close enough for us to detect, assuming that they send out signals. For example, existing Earth radio telescopes could only detect Earth radio transmissions from roughly a light year away.

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==== Lifetime of such a civilization wherein it communicates its signals into space, L ====
Michael Shermer estimated L as 420 years, based on the duration of sixty historical Earthly civilizations. Using 28 civilizations more recent than the Roman Empire, he calculates a figure of 304 years for "modern" civilizations. It could also be argued from Michael Shermer's results that the fall of most of these civilizations was followed by later civilizations that carried on the technologies, so it is doubtful that they are separate civilizations in the context of the Drake equation. In the expanded version, including reappearance number, this lack of specificity in defining single civilizations does not matter for the result, since such a civilization turnover could be described as an increase in the reappearance number rather than increase in L, stating that a civilization reappears in the form of the succeeding cultures. Furthermore, since none could communicate over interstellar space, the method of comparing with historical civilizations could be regarded as invalid.
David Grinspoon has argued that once a civilization has developed enough, it might overcome all threats to its survival. It will then last for an indefinite period of time, making the value for L potentially billions of years. If this is the case, then he proposes that the Milky Way Galaxy may have been steadily accumulating advanced civilizations since it formed. He proposes that the last factor L be replaced with fIC · T, where fIC is the fraction of communicating civilizations that become "immortal" (in the sense that they simply do not die out), and T representing the length of time during which this process has been going on. This has the advantage that T would be a relatively easy-to-discover number, as it would simply be some fraction of the age of the universe.
It has also been hypothesized that once a civilization has learned of a more advanced one, its longevity could increase because it can learn from the experiences of the other.
The astronomer Carl Sagan speculated that all of the terms, except for the lifetime of a civilization, are relatively high and the determining factor in whether there are large or small numbers of civilizations in the universe is the civilization lifetime, or in other words, the ability of technological civilizations to avoid self-destruction. In Sagan's case, the Drake equation was a strong motivating factor for his interest in environmental issues and his efforts to warn against the dangers of nuclear warfare. Paleobiologist Olev Vinn suggests that the lifetime of most technological civilizations is brief due to inherited behavior patterns present in all intelligent organisms. These behaviors, incompatible with civilized conditions, inevitably lead to self-destruction soon after the emergence of advanced technologies.
An intelligent civilization might not be organic, as some have suggested that artificial general intelligence may replace humanity.
=== Range of results ===
As many skeptics have pointed out, the Drake equation can give a very wide range of values, depending on the assumptions, as the values used in portions of the Drake equation are not well established. In particular, the result can be N ≪ 1, meaning we are likely alone in the galaxy, or N ≫ 1, implying there are many civilizations we might contact. One of the few points of wide agreement is that the presence of humanity demonstrates that the probability of intelligence arising is greater than zero.
As an example of a low estimate, combining NASA's star formation rates, the rare Earth hypothesis value of fp · ne · fl = 10−5, Mayr's view on intelligence arising, Drake's view of communication, and Shermer's estimate of lifetime:
R∗ = 1.5–3 yr−1, fp · ne · fl = 10−5, fi = 10−9, fc = 0.2[Drake, above], and L = 304 years
gives:
N = 1.5 × 10−5 × 10−9 × 0.2 × 304 = 9.1 × 10−13 (0.00000000000091 in non scientific notation)
i.e., suggesting that we are probably alone in this galaxy, and possibly in the observable universe.
On the other hand, with larger values for each of the parameters above, values of N can be derived that are greater than 1. The following higher values that have been proposed for each of the parameters:
R∗ = 1.5–3 yr−1, fp = 1, ne = 0.2, fl = 0.13, fi = 1, fc = 0.2[Drake, above], and L = 109 years
Use of these parameters gives:
N = 3 × 1 × 0.2 × 0.13 × 1 × 0.2 × 109 = 15,600,000 civilizations in the Milky Way with which communication might be possible
Monte Carlo simulations of estimates of the Drake equation factors based on a stellar and planetary model of the Milky Way have resulted in the number of civilizations varying by a factor of 100.

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=== Possible former technological civilizations ===
In 2016, Adam Frank and Woodruff Sullivan modified the Drake equation to determine just how unlikely the event of a technological species arising on a given habitable planet must be, to give the result that Earth hosts the only technological species that has ever arisen, for two cases: (a) this Galaxy, and (b) the universe as a whole. By asking this different question, one removes the lifetime and simultaneous communication uncertainties. Since the numbers of habitable planets per star can today be reasonably estimated, the only remaining unknown in the Drake equation is the probability that a habitable planet ever develops a technological species over its lifetime. For Earth to have the only technological species that has ever occurred in the universe, they calculate the probability of any given habitable planet ever developing a technological species must be less than 2.5×10−24. Similarly, for Earth to have been the only case of hosting a technological species over the history of this Galaxy, the odds of a habitable zone planet ever hosting a technological species must be less than 1.7×10−11 (about 1 in 60 billion). The figure for the universe implies that it is extremely unlikely that Earth hosts the only technological species that has ever occurred. On the other hand, for this Galaxy one must think that fewer than 1 in 60 billion habitable planets develop a technological species for there not to have been at least a second case of such a species over the past history of this Galaxy.
== Modifications ==
As many observers have pointed out, the Drake equation is a very simple model that omits potentially relevant parameters, and many changes and modifications to the equation have been proposed. One line of modification, for example, attempts to account for the uncertainty inherent in many of the terms.
Combining the estimates of the original six factors by major researchers via a Monte Carlo procedure leads to a best value for the non-longevity factors of 0.85 1/years. This result differs insignificantly from the estimate of unity given both by Drake and the Cyclops report.
Others note that the Drake equation ignores many concepts that might be relevant to the odds of contacting other civilizations. For example, Glen David Brin states: "The Drake equation merely speaks of the number of sites at which ETIs spontaneously arise. The equation says nothing directly about the contact cross-section between an ETIS and contemporary human society". Because it is the contact cross-section that is of interest to the SETI community, many additional factors and modifications of the Drake equation have been proposed.
Colonization
Brin proposed to generalize the Drake equation to include additional effects of alien civilizations colonizing other star systems. Each original site expands with an expansion velocity v, and establishes additional sites that survive for a lifetime L. The result is a more complex set of 3 equations.
Reappearance factor
The Drake equation may furthermore be multiplied by how many times an intelligent civilization may occur on planets where it has happened once. Even if an intelligent civilization reaches the end of its lifetime, life may still prevail on the planet for billions of years, permitting the next civilization to evolve. Thus, several civilizations may come and go during the lifespan of one and the same planet. Thus, if nr is the average number of times a new civilization reappears on the same planet where a previous civilization once has appeared and ended, then the total number of civilizations on such a planet would be 1 + nr, which is the actual reappearance factor added to the equation.
METI factor
Alexander Zaitsev said that to be in a communicative phase and emit dedicated messages are not the same. For example, humans are in a communicative phase, but are not a communicative civilization; there are no purposeful and regular transmission of interstellar messages. For this reason, he suggested introducing the METI factor (messaging to extraterrestrial intelligence) to the classical Drake equation. He defined the factor as "fm = The fraction of communicative civilizations with clear and non-paranoid planetary consciousness (that is, those which actually engage in deliberate interstellar transmission)".
Biogenic gases
Astronomer Sara Seager proposed a revised equation that focuses on the search for planets with biosignature gases. These gases are produced by living organisms that can accumulate in a planet atmosphere to levels that can be detected with remote space telescopes.
The Seager equation looks like:
N
=
N
∗
⋅
F
Q
⋅
F
H
Z
⋅
F
O
⋅
F
L
⋅
F
S
{\displaystyle N=N_{*}\cdot F_{\mathrm {Q} }\cdot F_{\mathrm {HZ} }\cdot F_{\mathrm {O} }\cdot F_{\mathrm {L} }\cdot F_{\mathrm {S} }}
where:
N = the number of planets with detectable signs of life
N∗ = the number of stars observed
FQ = the fraction of stars that are quiet
FHZ = the fraction of stars with rocky planets in the habitable zone
FO = the fraction of those planets that can be observed
FL = the fraction that have life
FS = the fraction on which life produces a detectable signature gas
Carl Sagan's version of the Drake equation
American astronomer Carl Sagan made some modifications in the Drake equation and presented it in the 1980 program Cosmos: A Personal Voyage. The modified equation is:
N
=
N
∗
⋅
f
p
⋅
n
e
⋅
f
l
⋅
f
i
⋅
f
c
⋅
f
L
{\displaystyle N=N_{\mathrm {*} }\cdot f_{\mathrm {p} }\cdot n_{\mathrm {e} }\cdot f_{\mathrm {l} }\cdot f_{\mathrm {i} }\cdot f_{\mathrm {c} }\cdot f_{\mathrm {L} }}

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where:
N = the number of civilizations in the Milky Way galaxy with which communication might be possible (i.e. which are on the current past light cone);
N∗ = Number of stars in the Milky Way Galaxy
fp = the fraction of those stars that have planets.
ne = the average number of planets that can potentially support life per star that has planets.
fl = the fraction of planets that could support life that actually develop life at some point.
fi = the fraction of planets with life that go on to develop intelligent life (civilizations).
fc = the fraction of civilizations that develop a technology that releases detectable signs of their existence into space.
fL = fraction of a planetary lifetime graced by a technological civilization
Plate tectonics factor
Robert J. Stern and Taras V. Gerya proposed to add plate tectonics factors in the 2024 paper:
We resolve the Fermi Paradox (1) by adding two additional terms to the Drake Equation: foc (the fraction of habitable exoplanets with significant continents and oceans) and fpt (the fraction of habitable exoplanets with significant continents and oceans that have had plate tectonics operating for at least 0.5 Ga); and (2) by demonstrating that the product of foc and fpt is very small (< 0.00003–0.002). We propose that the lack of evidence for ACCs [active, communicative civilizations] reflects the scarcity of long-lived plate tectonics and/or continents and oceans on exoplanets with primitive life.
== Criticism ==
Criticism of the Drake equation is varied. Firstly, many of the terms in the equation are largely or entirely based on conjecture. Star formation rates are well-known, and the incidence of planets has a sound theoretical and observational basis, but the other terms in the equation become very speculative. The uncertainties revolve around the present day understanding of the evolution of life, intelligence, and civilization, not physics. No statistical estimates are possible for some of the parameters, where only one example is known. The net result is that the equation cannot be used to draw firm conclusions of any kind, and the resulting margin of error is huge, far beyond what some consider acceptable or meaningful.
Others point out that the equation was formulated before our understanding of the universe had matured. Astrophysicist Ethan Siegel, said:
The Drake equation, when it was put forth, made an assumption about the Universe that we now know is untrue: It assumed that the Universe was eternal and static in time. As we learned only a few years after Frank Drake first proposed his equation, the Universe doesn’t exist in a steady state, where it’s unchanging in time, but rather has evolved from a hot, dense, energetic, and rapidly expanding state: a hot Big Bang that occurred over a finite duration in our cosmic past.
One reply to such criticisms is that even though the Drake equation currently involves speculation about unmeasured parameters, it was intended as a way to stimulate dialogue on these topics. Then the focus becomes how to proceed experimentally. Indeed, Drake originally formulated the equation merely as an agenda for discussion at the Green Bank conference.
=== Fermi paradox ===
A civilization lasting for tens of millions of years could be able to spread throughout the galaxy, even at the slow speeds foreseeable with present-day technology. However, no confirmed signs of civilizations or intelligent life elsewhere have been found, either in this Galaxy or in the observable universe of 2 trillion galaxies. According to this line of thinking, the tendency to fill (or at least explore) all available territory seems to be a universal trait of living things, so the Earth should have already been colonized, or at least visited, but no evidence of this exists. Hence Fermi's question "Where is everybody?".
A large number of explanations have been proposed to explain this lack of contact; a book published in 2015 elaborated on 75 different explanations. In terms of the Drake Equation, the explanations can be divided into three classes:
Few intelligent civilizations ever arise. This is an argument that at least one of the first few terms, R∗ · fp · ne · fl · fi, has a low value. The most common suspect is fi, but explanations such as the rare Earth hypothesis argue that ne is the small term.
Intelligent civilizations exist, but we see no evidence, meaning fc is small. Typical arguments include that civilizations are too far apart, it is too expensive to spread throughout the galaxy, civilizations broadcast signals for only a brief period of time, communication is dangerous, and many others.
The lifetime of intelligent, communicative civilizations is short, meaning the value of L is small. Drake suggested that a large number of extraterrestrial civilizations would form, and he further speculated that the lack of evidence of such civilizations may be because technological civilizations tend to disappear rather quickly. Typical explanations include it is the nature of intelligent life to destroy itself, it is the nature of intelligent life to destroy others, they tend to be destroyed by natural events, and others.
These lines of reasoning lead to the Great Filter hypothesis, which states that since there are no observed extraterrestrial civilizations despite the vast number of stars, at least one step in the process must be acting as a filter to reduce the final value. According to this view, either it is very difficult for intelligent life to arise, or the lifetime of technologically advanced civilizations, or the period of time they reveal their existence must be relatively short.
An analysis by Anders Sandberg, Eric Drexler and Toby Ord suggests "a substantial ex ante (predicted) probability of there being no other intelligent life in our observable universe".
== In popular culture ==

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The equation was cited by Gene Roddenberry as supporting the multiplicity of inhabited planets shown on Star Trek, the television series he created. However, Roddenberry did not have the equation with him, and he was forced to "invent" it for his original proposal. The invented equation created by Roddenberry is:
F
f
2
(
M
g
E
)
−
C
1
R
i
1
⋅
M
=
L
/
S
o
{\displaystyle Ff^{2}(MgE)-C^{1}Ri^{1}\cdot M=L/So}
Regarding Roddenberry's fictional version of the equation, Drake himself commented that a number raised to the first power is just the number itself.
A commemorative plate on NASA's Europa Clipper mission, which launched October 14, 2024, features a poem by the U.S. Poet Laureate Ada Limón, waveforms of the word 'water' in 103 languages, a schematic of the water hole, the Drake equation, and a portrait of planetary scientist Ron Greeley on it.
== See also ==
Astrobiology – Science concerned with life in the universe
Goldilocks principle – Analogy for optimal conditions
Kardashev scale – Measure of a civilization's evolution
Planetary habitability – Known extent to which a planet is suitable for life
Ufology – Study of UFOs
Lincoln index – Statistical measure
The Search for Life: The Drake Equation, BBC documentary
== Notes ==
== References ==
== Further reading ==
Morton, Oliver (2002). "A Mirror in the Sky". In Graham Formelo (ed.). It Must Be Beautiful. Granta Books. ISBN 1-86207-555-7.
Rood, Robert T.; James S. Trefil (1981). Are We Alone? The Possibility of Extraterrestrial Civilizations. New York: Scribner. ISBN 0684178427.
Vakoch, Douglas A.; Dowd, Matthew F., eds. (2015). The Drake Equation: Estimating the Prevalence of Extraterrestrial Life Through the Ages. Cambridge, UK: Cambridge University Press. ISBN 978-1-10-707365-4.
== External links ==
Interactive Drake Equation Calculator
Frank Drake's 2010 article on "The Origin of the Drake Equation"
"Only a matter of time, says Frank Drake". A Q&A with Frank Drake in February 2010
Drake, Frank (December 2004). "The E.T. Equation, Recalculated". Wired.
Macromedia Flash page allowing the user to modify Drake's values from PBS's Nova
"The Drake Equation", Astronomy Cast episode #23; includes full transcript
Animated simulation of the Drake equation. (Archived 8 December 2015 at the Wayback Machine)
"The Alien Equation", BBC Radio program Discovery (22 September 2010)
"Reflections on the Equation" (PDF), by Frank Drake, 2013

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A Dyson–Harrop satellite is a hypothetical megastructure intended for power generation using the solar wind. It is inspired by the Dyson sphere, but much harder to detect from another star system. The satellite develops a useful voltage potential by capturing positive ions against a solar sail for a net positive voltage, while draining off electrons on a long wire, and guiding flux electrons along a short wire into a charge receiver for a net negative voltage. The voltage difference between the charge receiver and the solar sail is used to power a laser or microwave transmitter for power transfer off-board the satellite.
The concept for the so-called Dyson–Harrop satellite begins with a long metal wire loop pointed at the Sun. This wire is charged to generate a cylindrical magnetic field that snags the electrons that make up half the solar wind. These electrons get funneled into a metal spherical receiver to produce a current, which generates the wire's magnetic field – making the system self-sustaining. Any current not needed for the magnetic field powers an infrared laser trained on satellite dishes back on Earth, designed to collect the energy. Earth's air does not absorb infra-red light, so the system would be highly efficient. Back on the satellite, the current has been drained of its electrical energy by the laser – the electrons fall onto a ring-shaped sail, where incoming sunlight can excite them enough to keep the satellite in orbit around the Sun.
A relatively small Dyson–Harrop satellite using a 1-centimetre-wide copper wire 300 metres long, a receiver 2 metres wide and a sail 10 metres in diameter, sitting at roughly the same distance from the Sun as the Earth, could generate 1.7 megawatts of power – enough for about 1000 family homes in the US. Larger sizes could produce far greater amounts of power, even exceeding the current usage of Earth. Satellites could be placed anywhere in the Solar System, and networks of satellites could combine to generate terawatts of power.
== References ==

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An exoplanet or extrasolar planet is a planet outside the Solar System. The first confirmed detection of an exoplanet was in 1992 around a pulsar, and the first detection around a main-sequence star was in 1995. A different planet, first detected in 1988, was confirmed in 2003. In 2016, it was recognized that the first possible evidence of an exoplanet had been noted in 1917. As of 23 April 2026, there are 6,273 confirmed exoplanets in 4,694 planetary systems, with 1,049 systems having more than one planet.
There are many methods of detecting exoplanets. Transit photometry and Doppler spectroscopy have found the most, but these methods suffer from a clear observational bias favoring the detection of large planets close to the star. About 1 in 5 Sun-like stars are estimated to have an "Earth-sized" planet in the habitable zone. Assuming there are 200 billion stars in the Milky Way, it can be hypothesized that there are 11 billion potentially habitable Earth-sized planets in the Milky Way, rising to 40 billion if planets orbiting the numerous red dwarfs are included.
The least massive exoplanet known is Draugr, which is about twice the mass of the Moon. The most massive exoplanet listed on the NASA Exoplanet Archive is HR 2562 b, about 30 times the mass of Jupiter. However, according to some definitions of a planet (based on the nuclear fusion of deuterium), it is too massive to be a planet and might be a brown dwarf. Known orbital times for exoplanets vary from less than an hour (for those closest to their star) to thousands of years. Some exoplanets are so far away from the star that it is difficult to tell whether they are gravitationally bound to it.
The nearest exoplanets are located 4.2 light-years (1.3 parsecs) from Earth and orbit Proxima Centauri, the closest star to the Sun. At the other extreme, there is evidence for extragalactic planets – exoplanets located in other galaxies.
The discovery of exoplanets has intensified interest in the search for extraterrestrial life. There is special interest in planets that orbit in a star's habitable zone (sometimes called "goldilocks zone"), where it is possible for liquid water, a prerequisite for life as we know it, to exist on the surface. However, the study of planetary habitability also considers a wide range of other factors in determining the suitability of a planet for hosting life.
In collaboration with ground-based and other space-based observatories, the James Webb Space Telescope (JWST) is expected to give more insight into exoplanet traits, such as their composition, environmental conditions, and habitability.
Rogue planets are those that are not in planetary systems. Such objects are generally considered in a separate category from planets, especially if they are gas giants, often counted as sub-brown dwarfs. The number of rogue planets in the Milky Way is possibly in the billions.
== Definition ==
=== IAU ===
The official definition of the term planet used by the International Astronomical Union (IAU) only covers the Solar System and thus does not apply to exoplanets. The IAU Working Group on Extrasolar Planets issued a position statement containing a working definition of "planet" in 2001 and which was modified in 2003. An exoplanet was defined by the following criteria:
Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solar metallicity) that orbit stars or stellar remnants are "planets" (no matter how they formed). The minimum mass/size required for an extrasolar object to be considered a planet should be the same as that used in the Solar System.
Substellar objects with true masses above the limiting mass for thermonuclear fusion of deuterium are "brown dwarfs", no matter how they formed or where they are located.
Free-floating objects in young star clusters with masses below the limiting mass for thermonuclear fusion of deuterium are not "planets", but are "sub-brown dwarfs" (or whatever name is most appropriate).
This working definition was amended by the IAU's Commission F2: Exoplanets and the Solar System in August 2018. The official working definition of an exoplanet is now as follows:
Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solar metallicity) that orbit stars, brown dwarfs or stellar remnants and that have a mass ratio with the central object below the L4/L5 instability (M/Mcentral < 2/(25+√621)) are "planets" (no matter how they formed).
The minimum mass/size required for an extrasolar object to be considered a planet should be the same as that used in our Solar System.

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=== Alternatives ===
The IAU's working definition is not always used. One alternate suggestion is that planets should be distinguished from brown dwarfs on the basis of their formation. It is widely thought that giant planets form through core accretion, which may sometimes produce planets with masses above the deuterium fusion threshold; massive planets of that sort may have already been observed. Brown dwarfs form like stars from the direct gravitational collapse of clouds of gas, and this formation mechanism also produces objects that are below the 13 MJup limit and can be as low as 1 MJup. Objects in this mass range that orbit their stars with wide separations of hundreds or thousands of astronomical units (AU) and have large star/object mass ratios likely formed as brown dwarfs; their atmospheres would likely have a composition more similar to their host star than accretion-formed planets, which would contain increased abundances of heavier elements. Most directly imaged planets as of April 2014 are massive and have wide orbits so probably represent the low-mass end of a brown dwarf formation. One study suggests that objects above 10 MJup formed through gravitational instability and should not be thought of as planets.
Also, the 13-Jupiter-mass cutoff does not have a precise physical significance. Deuterium fusion can occur in some objects with a mass below that cutoff. The amount of deuterium fused depends to some extent on the composition of the object. In 2011, the Extrasolar Planets Encyclopaedia included objects up to 25 Jupiter masses, saying, "The fact that there is no special feature around 13 MJup in the observed mass spectrum reinforces the choice to forget this mass limit". As of 2016, this limit was increased to 60 Jupiter masses based on a study of mass–density relationships. The Exoplanet Data Explorer includes objects up to 24 Jupiter masses with the advisory: "The 13 Jupiter-mass distinction by the IAU Working Group is physically unmotivated for planets with rocky cores, and observationally problematic due to the sin i ambiguity." The NASA Exoplanet Archive includes objects with a mass (or minimum mass) equal to or less than 30 Jupiter masses. Another criterion for separating planets and brown dwarfs, rather than deuterium fusion, formation process or location, is whether the core pressure is dominated by Coulomb pressure or electron degeneracy pressure with the dividing line at around 5 Jupiter masses.
=== Confirmation ===
An exoplanet is confirmed for NASA's Exoplanet Archive either when "different observation techniques reveal features that can only be explained by a planet" or by analytical techniques. For the Extrasolar Planets Encyclopedia, "A planet is considered as Confirmed if it is claimed unambiguously in an accepted paper or a professional conference."
== Nomenclature ==
The convention for naming exoplanets is an extension of the system used for designating multiple-star systems as adopted by the International Astronomical Union (IAU). For exoplanets orbiting a single star, the IAU designation is formed by taking the designated or proper name of its parent star, and adding a lower case letter. Letters are given in order of each planet's discovery around the parent star, so that the first planet discovered in a system is designated "b" (the parent star is considered "a") and later planets are given subsequent letters. If several planets in the same system are discovered at the same time, the closest one to the star gets the next letter, followed by the other planets in order of orbital size. A provisional IAU-sanctioned standard exists to accommodate the designation of circumbinary planets. A limited number of exoplanets have IAU-sanctioned proper names. Other naming systems exist.
== History of detection ==
For centuries scientists, philosophers, and science fiction writers suspected that extrasolar planets existed, but there was no way of knowing whether they were real in fact, how common they were, or how similar they might be to the planets of the Solar System. Various detection claims made in the nineteenth century were rejected by astronomers.
The first evidence of a possible exoplanet, orbiting Van Maanen 2, was recorded in 1917, but was not recognized as such until 2016. The astronomer Walter Sydney Adams produced a spectrum of the star using Mount Wilson's 60-inch telescope which he interpreted the spectrum to be of an F-type main-sequence star. This spectrum was reexamined during studies of white dwarf stars with unpredicted compositions. It is now thought that such a spectrum could be caused by the residue of a nearby exoplanet that had been pulverized by the gravity of the star, the resulting dust then falling onto the star.
Numerous other claims of discovery took place in the mid 20th century, involving 61 Cygnus, Lalande 21185, and Barnard's Star, which were not discredited until the mid to late 1970s (see Discredited claims below). Another suspected scientific detection of an exoplanet occurred in 1988. Shortly afterwards, the first detection that is currently accepted came in 1992 when Aleksander Wolszczan and Dale Frail announced the discovery of two terrestrial-mass planets orbiting the millisecond pulsar PSR B1257+12. The first confirmation of an exoplanet orbiting a main-sequence star was made in 1995, when a giant planet was found in a four-day orbit around the nearby star 51 Pegasi. Some exoplanets have been imaged directly by telescopes, but the vast majority have been detected through indirect methods, such as the transit method and the radial-velocity method.
In February 2018, researchers using the Chandra X-ray Observatory, combined with a planet detection technique called microlensing, found evidence of planets in a distant galaxy, stating, "Some of these exoplanets are as (relatively) small as the moon, while others are as massive as Jupiter. Unlike Earth, most of the exoplanets are not tightly bound to stars, so they're actually wandering through space or loosely orbiting between stars. We can estimate that the number of planets in this [faraway] galaxy is more than a trillion."
=== Early speculations ===

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In the sixteenth century, the Italian philosopher Giordano Bruno, an early supporter of the Copernican theory that Earth and other planets orbit the Sun (heliocentrism), put forward the view that fixed stars are similar to the Sun and are likewise accompanied by planets.
In the eighteenth century, the same possibility was mentioned by Isaac Newton in the "General Scholium" that concludes his Principia. Making a comparison to the Sun's planets, he wrote "And if the fixed stars are the centres of similar systems, they will all be constructed according to a similar design and subject to the dominion of One."
In 1938, D.Belorizky demonstrated that it was realistic to search for exo-Jupiters by using transit photometry.
In 1952, more than 40 years before the first hot Jupiter was discovered, Otto Struve wrote that there is no compelling reason that planets could not be much closer to their parent star than is the case in the Solar System, and proposed that Doppler spectroscopy and the transit method could detect super-Jupiters in short orbits.
=== Discredited claims ===
Claims of exoplanet detections have been made since the nineteenth century. Some of the earliest involve the binary star 70 Ophiuchi. In 1855, William Stephen Jacob at the East India Company's Madras Observatory reported that orbital anomalies made it "highly probable" that there was a "planetary body" in this system. In the 1890s, Thomas J. J. See of the University of Chicago and the United States Naval Observatory stated that the orbital anomalies proved the existence of a dark body in the 70 Ophiuchi system with a 36-year period around one of the stars. However, Forest Ray Moulton published a paper proving that a three-body system with those orbital parameters would be highly unstable.
Multiple claims have been made that 61 Cygni might have a planetary system. Kaj Strand of the Sproul Observatory in 1942 observed tiny but systematic variations in the orbital motions of 61 Cygni A and B, suggesting that a third body of about 16 Jupiter masses must be orbiting 61 Cygni A. Multiple further claims were made, but more recent observations have yet to find confirmation. More information at 61 Cygni § Claims of a planetary system.
Around the same time that 61 Cygni was being investigated, similar claims about the presence of exoplanets were made about Lalande 21185: Lalande 21185 § Past claims of planets.
During the 1950s and 1960s, Peter van de Kamp of Swarthmore College made another prominent series of detection claims, this time for planets orbiting Barnard's Star. Astronomers now generally regard all early reports of detection as erroneous.
In 1991, Andrew Lyne, M. Bailes and S. L. Shemar claimed to have discovered a pulsar planet in orbit around PSR 1829-10, using pulsar timing variations. The claim briefly received intense attention, but Lyne and his team soon retracted it.
=== Confirmed discoveries ===

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As of 23 April 2026, a total of 6,273 confirmed exoplanets are listed in the NASA Exoplanet Archive, including a few that were confirmations of controversial claims from the late 1980s. The first published discovery to receive subsequent confirmation was made in 1988 by the Canadian astronomers Bruce Campbell, G. A. H. Walker, and Stephenson Yang of the University of Victoria and the University of British Columbia. Although they were cautious about claiming a planetary detection, their radial-velocity observations suggested that a planet orbits the star Gamma Cephei. Partly because the observations were at the very limits of instrumental capabilities at the time, astronomers remained skeptical for several years about this and other similar observations. It was thought some of the apparent planets might instead have been brown dwarfs, objects intermediate in mass between planets and stars. In 1990, additional observations were published that supported the existence of the planet orbiting Gamma Cephei, but subsequent work in 1992 again raised serious doubts. Finally, in 2003, improved techniques allowed the planet's existence to be confirmed.
On 9 January 1992, radio astronomers Aleksander Wolszczan and Dale Frail announced the discovery of two planets orbiting the millisecond pulsar PSR 1257+12 based on the variability of timing of the pulses. This discovery was confirmed, and is generally considered to be the first definitive detection of exoplanets. Follow-up observations solidified these results, and confirmation of a third planet in 1994 revived the topic in the popular press. These pulsar planets are thought to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation, or else to be the remaining rocky cores of gas giants that somehow survived the supernova and then decayed into their current orbits. As pulsars are aggressive stars, it was considered unlikely at the time that a planet could form in their orbit.
In the early 1990s, a group of astronomers led by Donald Backer, who were studying what they thought was a binary pulsar (PSR B1620−26 b), determined that a third object was needed to explain the observed Doppler shifts. Within a few years, the gravitational effects of the planet on the orbit of the pulsar and white dwarf had been measured, giving an estimate of the mass of the third object that was too small to be a star. The conclusion that the third object was a planet was announced by Stephen Thorsett and his collaborators in 1993.
On 6 October 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting a main-sequence star, nearby G-type star 51 Pegasi. This discovery, made at the Observatoire de Haute-Provence, ushered in the modern era of exoplanetary discovery, and was recognized by a share of the 2019 Nobel Prize in Physics. Technological advances, most notably in high-resolution spectroscopy, led to the rapid detection of many new exoplanets: astronomers could detect exoplanets indirectly by measuring their gravitational influence on the motion of their host stars. More extrasolar planets were later detected by observing the variation in a star's apparent luminosity as an orbiting planet transited in front of it.
Initially, most known exoplanets were massive planets that orbited very close to their parent stars. Astronomers were surprised by these "hot Jupiters", because theories of planetary formation had indicated that giant planets should only form at large distances from stars. But eventually more planets of other sorts were found, and it is now clear that hot Jupiters make up a minority of exoplanets. In 1999, Upsilon Andromedae became the first main-sequence star known to have multiple planets. Kepler-16 contains the first discovered planet that orbits a binary main-sequence star system.
On 26 February 2014, NASA announced the discovery of 715 newly verified exoplanets around 305 stars by the Kepler Space Telescope. These exoplanets were checked using a statistical technique called "verification by multiplicity". Before these results, most confirmed planets were gas giants comparable in size to Jupiter or larger because they were more easily detected, but the Kepler planets are mostly between the size of Neptune and the size of Earth.
On 23 July 2015, NASA announced Kepler-452b, a near-Earth-size planet orbiting the habitable zone of a G2-type star.
On 6 September 2018, NASA discovered an exoplanet about 145 light years away from Earth in the constellation Virgo. This exoplanet, Wolf 503b, is twice the size of Earth and was discovered orbiting a type of star known as an "Orange Dwarf". Wolf 503b completes one orbit in as few as six days because it is very close to the star. Wolf 503b is the only exoplanet that large that can be found near the so-called small planet radius gap. The gap, sometimes called the Fulton gap, is the observation that it is unusual to find exoplanets with sizes between 1.5 and 2 times the radius of the Earth.
=== Candidate discoveries ===
As of January 2020, NASA's Kepler and TESS missions had identified 4374 planetary candidates yet to be confirmed, several of them being nearly Earth-sized and located in the habitable zone, some around Sun-like stars.
In September 2020, astronomers reported evidence, for the first time, of an extragalactic planet, M51-ULS-1b, detected by eclipsing a bright X-ray source (XRS), in the Whirlpool Galaxy (M51a).
== Detection methods ==
=== Direct imaging ===

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Planets are extremely faint compared to their parent stars. For example, a Sun-like star is about a billion times brighter than the reflected light from any exoplanet orbiting it. It is difficult to detect such a faint light source, and furthermore, the parent star causes a glare that tends to wash it out. It is necessary to block the light from the parent star to reduce the glare while leaving the light from the planet detectable; doing so is a major technical challenge which requires extreme optothermal stability. Almost all exoplanets that have been directly imaged are both large (more massive than Jupiter) and widely separated from their parent stars.
Specially designed direct-imaging instruments such as Gemini Planet Imager, VLT-SPHERE, and SCExAO will image dozens of gas giants, but the vast majority of known extrasolar planets have only been detected through indirect methods.
=== Indirect methods ===
Transit method
If a planet crosses (or transits) in front of its parent star's disk, then the observed brightness of the star drops by a small amount. The amount by which the star dims depends on its size and on the size of the planet, among other factors. Because the transit method requires that the planet's orbit intersect a line-of-sight between the host star and Earth, the probability that an exoplanet in a randomly oriented orbit will be observed to transit the star is somewhat small. The Kepler telescope used this method.
Radial velocity or Doppler method
As a planet orbits a star, the star also moves in its own small orbit around the system's center of mass. Variations in the star's radial velocity—that is, the speed with which it moves towards or away from Earth—can be detected from displacements in the star's spectral lines due to the Doppler effect. Extremely small radial-velocity variations can be observed, of 1 m/s or even somewhat less.
Transit timing variation (TTV)
When multiple planets are present, each one slightly perturbs the others' orbits. Small variations in the times of transit for one planet can thus indicate the presence of another planet, which itself may or may not transit. For example, variations in the transits of the planet Kepler-19b suggest the existence of a second planet in the system, the non-transiting Kepler-19c.
Transit duration variation (TDV)
When a planet orbits multiple stars or if the planet has moons, its transit time can significantly vary per transit. Although no new planets or moons have been discovered with this method, it is used to successfully confirm many transiting circumbinary planets.
Gravitational microlensing
Microlensing occurs when the gravitational field of a star acts like a lens, magnifying the light of a distant background star. Planets orbiting the lensing star can cause detectable anomalies in magnification as it varies over time. Unlike most other methods which have a detection bias towards planets with small (or for resolved imaging, large) orbits, the microlensing method is most sensitive to detecting planets around 1–10 AU away from Sun-like stars.
Astrometry
Astrometry consists of precisely measuring a star's position in the sky and observing the changes in that position over time. The motion of a star due to the gravitational influence of a planet may be observable. Because the motion is so small, however, this method was not very productive until the 2020s. It has produced only a few confirmed discoveries, though it has been successfully used to investigate the properties of planets found in other ways.
Pulsar timing
A pulsar, a small, dense remnant of a star that has exploded as a supernova, emits radio waves regularly as it rotates. If planets orbit the pulsar, the motion of the pulsar around the system's center of mass alters the pulsar's distance to Earth over time. As a result, the radio pulses from the pulsar arrive on Earth at a later or earlier time. This light travel delay due to the pulsar being physically closer or farther from Earth is known as a Roemer time delay. The first confirmed discovery of an extrasolar planet was made using this method. But as of 2011, it has not been very productive; five planets have been detected in this way, around three different pulsars.
Variable star timing (pulsation frequency)
Like pulsars, there are some other types of stars which exhibit periodic activity. Deviations from periodicity can sometimes be caused by a planet orbiting it. As of 2013, a few planets have been discovered with this method.
Reflection/emission modulations
When a planet orbits very close to a star, it catches a considerable amount of starlight. As the planet orbits the star, the amount of light changes due to planets having phases from Earth's viewpoint or planets glowing more from one side than the other due to temperature differences.
Relativistic beaming
Relativistic beaming measures the observed flux from the star due to its motion. The brightness of the star changes as the planet moves closer or further away from its host star.
Ellipsoidal variations
Massive planets close to their host stars can slightly deform the shape of the star. This causes the brightness of the star to slightly deviate depending on how it is rotated relative to Earth.
Polarimetry
With the polarimetry method, a polarized light reflected off the planet is separated from unpolarized light emitted from the star. No new planets have been discovered with this method, although a few already discovered planets have been detected with this method.
Circumstellar disks
Disks of space dust surround many stars, thought to originate from collisions among asteroids and comets. The dust can be detected because it absorbs starlight and re-emits it as infrared radiation. Features on the disks may suggest the presence of planets, though this is not considered a definitive detection method.
== Formation and evolution ==

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Planets may form within a few to tens (or more) of millions of years of their star forming.
The planets of the Solar System can only be observed in their current state, but observations of different planetary systems of varying ages allows us to observe planets at different stages of evolution. Available observations range from young protoplanetary disks where planets are still forming to planetary systems of over 10 Gyr old. When planets form in a gaseous protoplanetary disk, they accrete hydrogen/helium envelopes. These envelopes cool and contract over time and, depending on the mass of the planet, some or all of the hydrogen/helium is eventually lost to space. This means that even terrestrial planets may start off with large radii if they form early enough. An example is Kepler-51b which has only about twice the mass of Earth but is almost the size of Saturn, which is a hundred times the mass of Earth. Kepler-51b is quite young at a few hundred million years old.
== Planet-hosting stars ==
There is at least one planet on average per star. About 1 in 5 Sun-like stars have an "Earth-sized" planet in the habitable zone.
Most known exoplanets orbit stars roughly similar to the Sun, i.e. main-sequence stars of spectral categories F, G, or K. Lower-mass stars (red dwarfs, of spectral category M) are less likely to have planets massive enough to be detected by the radial-velocity method. Despite this, several tens of planets around red dwarfs have been discovered by the Kepler space telescope, which uses the transit method to detect smaller planets.
Using data from Kepler, a correlation has been found between the metallicity of a star and the probability that the star hosts a giant planet, similar to the size of Jupiter. Stars with higher metallicity are more likely to have planets, especially giant planets, than stars with lower metallicity.
Some planets orbit one member of a binary star system, and several circumbinary planets have been discovered which orbit both members of a binary star. A few planets in triple star systems are known and one in the quadruple system Kepler-64.
== Orbital and physical parameters ==
== General features ==
=== Color and brightness ===
The apparent brightness (apparent magnitude) of a planet depends on how far away the observer is, how reflective the planet is (albedo), and how much light the planet receives from its star, which depends on how far the planet is from the star and how bright the star is. So, a planet with a low albedo that is close to its star can appear brighter than a planet with a high albedo that is far from the star.
In 2013, the color of an exoplanet was determined for the first time. The best-fit albedo measurements of HD 189733b suggest that it is deep dark blue. Later that same year, the colors of several other exoplanets were determined, including GJ 504 b which visually has a magenta color, and Kappa Andromedae b, which if seen up close would appear reddish in color. Helium planets are expected to be white or grey in appearance.
The darkest known planet in terms of geometric albedo is TrES-2b, a hot Jupiter that reflects less than 1% of the light from its star, making it less reflective than coal or black acrylic paint. Hot Jupiters are expected to be quite dark due to sodium and potassium in their atmospheres, but it is not known why TrES-2b is so dark—it could be due to an unknown chemical compound.
For gas giants, geometric albedo generally decreases with increasing metallicity or atmospheric temperature unless there are clouds to modify this effect. Increased cloud-column depth increases the albedo at optical wavelengths, but decreases it at some infrared wavelengths. Optical albedo increases with age, because older planets have higher cloud-column depths. Optical albedo decreases with increasing mass, because higher-mass giant planets have higher surface gravities, which produces lower cloud-column depths. Also, elliptical orbits can cause major fluctuations in atmospheric composition, which can have a significant effect.
There is more thermal emission than reflection at some near-infrared wavelengths for massive and/or young gas giants. So, although optical brightness is fully phase-dependent, this is not always the case in the near infrared.
Temperatures of gas giants reduce over time and with distance from their stars. Lowering the temperature increases optical albedo even without clouds. At a sufficiently low temperature, water clouds form, which further increase optical albedo. At even lower temperatures, ammonia clouds form, resulting in the highest albedos at most optical and near-infrared wavelengths.

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=== Magnetic field ===
In 2014, a magnetic field around HD 209458 b was inferred from the way hydrogen was evaporating from the planet. It is the first (indirect) detection of a magnetic field on an exoplanet. The magnetic field is estimated to be about one-tenth as strong as Jupiter's.
The magnetic fields of exoplanets are thought to be detectable by their auroral radio emissions with sensitive low-frequency radio telescopes such as LOFAR, although they have yet to be found. The radio emissions could measure the rotation rate of the interior of an exoplanet, and may yield a more accurate way to measure exoplanet rotation than by examining the motion of clouds. However, the most sensitive radio search for auroral emissions, thus far, from nine exoplanets with Arecibo also did not result in any discoveries.
Earth's magnetic field results from its flowing liquid metallic core, but on massive super-Earths with high pressure, different compounds may form which do not match those created under terrestrial conditions. Compounds may form with greater viscosities and high melting temperatures, which could prevent the interiors from separating into different layers and so result in undifferentiated coreless mantles. Forms of magnesium oxide such as MgSi3O12 could be a liquid metal at the pressures and temperatures found in super-Earths and could generate a magnetic field in the mantles of super-Earths.
Hot Jupiters have been observed to have a larger radius than expected. This could be caused by the interaction between the stellar wind and the planet's magnetosphere creating an electric current through the planet that heats it up (Joule heating) causing it to expand. The more magnetically active a star is, the greater the stellar wind and the larger the electric current leading to more heating and expansion of the planet. This theory matches the observation that stellar activity is correlated with inflated planetary radii.
In August 2018, scientists announced the transformation of gaseous deuterium into a liquid metallic hydrogen form. This may help researchers better understand giant gas planets, such as Jupiter, Saturn and related exoplanets, since such planets are thought to contain a lot of liquid metallic hydrogen, which may be responsible for their observed powerful magnetic fields.
Although scientists previously announced that the magnetic fields of close-in exoplanets may cause increased stellar flares and starspots on their host stars, in 2019 this claim was demonstrated to be false in the HD 189733 system. The failure to detect "star-planet interactions" in the well-studied HD 189733 system calls other related claims of the effect into question. A later search for radio emissions from eight exoplanets that orbit within 0.1 astronomical units of their host stars, conducted by the Arecibo radio telescope also failed to find signs of these magnetic star-planet interactions.
In 2019, the strength of the surface magnetic fields of 4 hot Jupiters were estimated and ranged between 20 and 120 gauss compared to Jupiter's surface magnetic field of 4.3 gauss.
In 2023 astronomers detected what might be the first sign of magnetosphere around a rocky exoplanet in the YZ Ceti system.
=== Plate tectonics ===
In 2007, two independent teams of researchers came to opposing conclusions about the likelihood of plate tectonics on larger super-Earths with one team saying that plate tectonics would be episodic or stagnant and the other team saying that plate tectonics is very likely on super-Earths even if the planet is dry.
If super-Earths have more than 80 times as much water as Earth, then they become ocean planets with all land completely submerged. However, if there is less water than this limit, then the deep water cycle would move enough water between the oceans and mantle to allow continents to exist.
=== Volcanism ===
Large surface temperature variations on 55 Cancri e have been attributed to possible volcanic activity releasing large clouds of dust which blanket the planet and block thermal emissions.
Tidal heating caused by gravitational tug of neighboring planets might lead to the emergence of volcanic activities on a terrestrial exoplanet.
=== Rings ===
In 2007, the star V1400 Centauri was occulted by an object (either a planet or brown dwarf) surrounded by an extensive disc of debris. The object, designated J1407b, was long believed to host a vast planetary ring system much larger than Saturn's rings. Follow-up observations found the supposed ring system could instead be a circumplanetary disk.
There is strong evidence of a ring system around HIP 41378 f, given the planet's measured radius is too large for its mass, the radius measurement might have been affected by a ring system around the planet.
The rings of the Solar System's gas giants are aligned with their planet's equator. However, for exoplanets that orbit close to their star, tidal forces from the star would lead to the outermost rings of a planet being aligned with the planet's orbital plane around the star. A planet's innermost rings would still be aligned with the planet's equator so that if the planet has a tilted rotational axis, then the different alignments between the inner and outer rings would create a warped ring system.

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=== Moons ===
There is evidence that moons around other planets, commonly referred to exomoons, may exist. None has been confirmed so far.
In 2012 a candidate exomoon was detected around WASP-12b via periodic light variations in the planet's light curve. Subsequent observations found this object might actually be a trojan planet.
In December 2013, a candidate exomoon was detected in the microlensing event MOA-2011-BLG-262, it was believed to be either a 0.5 M🜨 exomoon around a Jupiter-sized free-floating planet or a Neptune-mass planet around a red dwarf, but follow-up observations confirmed the latter scenario.
On 3 October 2018, evidence suggesting a large exomoon orbiting Kepler-1625b was reported, and in 2021 evidence of an exomoon around Kepler-1708b was also reported. Their existence, however, remain doubtful, but follow-up observations may confirm these exomoons.
The detection of sodium in hot Jupiters such as WASP-76b, HD 189733 b or WASP-49b is likely due to a Io-like exomoon around these planets.
=== Atmospheres ===
Atmospheres have been detected around several exoplanets. The first to be observed was HD 209458 b in 2001.
As of February 2014, more than fifty transiting and five directly imaged exoplanet atmospheres have been observed, resulting in detection of molecular spectral features; observation of day–night temperature gradients; and constraints on vertical atmospheric structure. Also, an atmosphere has been detected on the non-transiting hot Jupiter Tau Boötis b.
In May 2017, glints of light from Earth, seen as twinkling from an orbiting satellite a million miles away, were found to be reflected light from ice crystals in the atmosphere. The technology used to determine this may be useful in studying the atmospheres of distant worlds, including those of exoplanets.
==== Comet-like tails ====
Kepler-1520b is a small rocky planet, very close to its star, that is evaporating and leaving a trailing tail of cloud and dust like a comet. The dust could be ash erupting from volcanos and escaping due to the small planet's low surface-gravity, or it could be from metals that are vaporized by the high temperatures of being so close to the star with the metal vapor then condensing into dust.
In June 2015, scientists reported that the atmosphere of GJ 436 b was evaporating, resulting in a giant cloud around the planet and, due to radiation from the host star, a long trailing tail 14 million km (9 million mi) long.
=== Insolation pattern ===
Tidally locked planets in a 1:1 spin-orbit resonance would have their star always shining directly overhead on one spot, which would be hot with the opposite hemisphere receiving no light and being freezing cold. Such a planet could resemble an eyeball, with the hotspot being the pupil. Planets with an eccentric orbit could be locked in other resonances. 3:2 and 5:2 resonances would result in a double-eyeball pattern with hotspots in both eastern and western hemispheres. Planets with both an eccentric orbit and a tilted axis of rotation would have more complicated insolation patterns.
== Surface ==
=== Surface composition ===
Surface features can be distinguished from atmospheric features by comparing emission and reflection spectroscopy with transmission spectroscopy. Mid-infrared spectroscopy of exoplanets may detect rocky surfaces, and near-infrared may identify magma oceans or high-temperature lavas, hydrated silicate surfaces and water ice, giving an unambiguous method to distinguish between rocky and gaseous exoplanets.
=== Surface temperature ===
Measuring the intensity of the light it receives from its parent star can estimate the temperature of an exoplanet. For example, the planet OGLE-2005-BLG-390Lb is estimated to have a surface temperature of roughly −220 °C (50 K). However, such estimates may be substantially in error because they depend on the planet's usually unknown albedo, and because factors such as the greenhouse effect may introduce unknown complications. A few planets have had their temperature measured by observing the variation in infrared radiation as the planet moves around in its orbit and is eclipsed by its parent star. For example, the planet HD 189733b has been estimated to have an average temperature of 1,205 K (932 °C) on its dayside and 973 K (700 °C) on its nightside.
== Habitability ==
As more planets are discovered, the field of exoplanetology continues to grow into a deeper study of extrasolar worlds, and will ultimately tackle the prospect of life on planets beyond the Solar System. At cosmic distances, life can only be detected if it is developed at a planetary scale and strongly modified the planetary environment, in such a way that the modifications cannot be explained by classical physico-chemical processes (out of equilibrium processes). For example, molecular oxygen (O2) in the atmosphere of Earth is a result of photosynthesis by living plants and many kinds of microorganisms, so it can be used as an indication of life on exoplanets, although small amounts of oxygen could also be produced by non-biological means. If water is a requirement for life, a habitable planet must sustain water. It must orbit a stable star at a distance within which planetary-mass objects with sufficient atmospheric pressure can support liquid water at their surfaces.
=== Habitable zone ===

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The habitable zone around a star is the region where the temperature is just right to allow liquid water to exist on the surface of a planet under a suitable atmosphere; that is, not too close to the star for the water to evaporate and not too far away from the star for the water to freeze. The heat produced by stars varies depending on the size and age of the star, so that the habitable zone can be at different distances for different stars. Also, the atmospheric conditions on the planet influence the planet's ability to retain heat so that the location of the habitable zone is also specific to each type of planet: desert planets (also known as dry planets), with very little water, will have less water vapor in the atmosphere than Earth and so have a reduced greenhouse effect, meaning that a desert planet could maintain oases of water closer to its star than Earth is to the Sun. The lack of water also means there is less ice to reflect heat into space, so the outer edge of desert-planet habitable zones is further out. Rocky planets with a thick hydrogen atmosphere could maintain surface water much further out than the Earth–Sun distance. Planets with larger mass have wider habitable zones because gravity reduces the water cloud column depth which reduces the greenhouse effect of water vapor, thus moving the inner edge of the habitable zone closer to the star.
Planetary rotation rate is one of the major factors determining the circulation of the atmosphere and hence the pattern of clouds: slowly rotating planets create thick clouds that reflect more and so can be habitable much closer to their star. Earth with its current atmosphere would be habitable in Venus's orbit, if it had Venus's slow rotation. If Venus lost its water ocean due to a runaway greenhouse effect, it is likely to have had a higher rotation rate in the past. Alternatively, Venus never had an ocean because water vapor was lost to space during its formation and could have had its slow rotation throughout its history.
Tidally locked planets (a.k.a. "eyeball" planets) can be habitable closer to their star than previously thought due to the effect of clouds: at high stellar flux, strong convection produces thick water clouds near the substellar point that greatly increase the planetary albedo and reduce surface temperatures.
Planets in the habitable zones of stars with low metallicity are more habitable for complex life on land than high metallicity stars because the stellar spectrum of high metallicity stars is less likely to cause the formation of ozone thus enabling more ultraviolet rays to reach the planet's surface.
Habitable zones have usually been defined in terms of surface temperature, however over half of Earth's biomass is from subsurface microbes, and the temperature increases with depth, so the subsurface can be conducive for microbial life when the surface is frozen and if this is considered, the habitable zone extends much further from the star, even rogue planets could have liquid water at sufficient depths underground. In an earlier era of the universe the temperature of the cosmic microwave background would have allowed any rocky planets that existed to have liquid water on their surface regardless of their distance from a star. Jupiter-like planets might not be habitable, but they could have habitable moons.
=== Ice ages and snowball states ===
The outer edge of the habitable zone is where planets are completely frozen, but planets well inside the habitable zone can periodically become frozen. If orbital fluctuations or other causes produce cooling, then this creates more ice, but ice reflects sunlight causing even more cooling, creating a feedback loop until the planet is completely or nearly completely frozen. When the surface is frozen, this stops carbon dioxide weathering, resulting in a build-up of carbon dioxide in the atmosphere from volcanic emissions. This creates a greenhouse effect which thaws the planet again. Planets with a large axial tilt are less likely to enter snowball states and can retain liquid water further from their star. Large fluctuations of axial tilt can have even more of a warming effect than a fixed large tilt. Counterintuitively, planets orbiting cooler stars, such as red dwarfs, are less likely to enter snowball states - because the infrared radiation emitted by cooler stars is mostly at longer wavelengths that are absorbed by ice, which heats it up.
=== Tidal heating ===
If a planet has an eccentric orbit, then tidal heating can provide another source of energy besides stellar radiation. This means that eccentric planets in the radiative habitable zone can be too hot for liquid water. Tides also circularize orbits over time, so there could be planets in the habitable zone with circular orbits that have no water because they used to have eccentric orbits. Eccentric planets further out than the habitable zone would still have frozen surfaces, but the tidal heating could create a subsurface ocean similar to Europa's. In some planetary systems, such as in the Upsilon Andromedae system, the eccentricity of orbits is maintained or even periodically varied by perturbations from other planets in the system. Tidal heating can cause outgassing from the mantle, contributing to the formation and replenishment of an atmosphere.
=== Potentially habitable planets ===
A review in 2015 identified exoplanets Kepler-62f, Kepler-186f and Kepler-442b as the best candidates for being potentially habitable. These are at a distance of 1000, 490 and 1,120 light-years away, respectively. Of these, Kepler-186f is in similar size to Earth with its 1.2-Earth-radius measure, and it is located towards the outer edge of the habitable zone around its red dwarf star.
When looking at the nearest terrestrial exoplanet candidates, Proxima Centauri b is about 4.2 light-years away. Its equilibrium temperature is estimated to be −39 °C (234 K).
numberradius (earth radii)03006009001200150018002100r < 1r < 2r < 15numberExoplanets by radius.mw-chart-896a74a4b0f34b6a8842401bf585e3332d8b922e888c8c2658274e56223bc9d1ba872a197b5563dd794ac6c0f58a6e1753c361b8127bec60d8e3d790f2b22b57__zr634712-cls-3335393:hover{pointer-events:none}.mw-chart-896a74a4b0f34b6a8842401bf585e3332d8b922e888c8c2658274e56223bc9d1ba872a197b5563dd794ac6c0f58a6e1753c361b8127bec60d8e3d790f2b22b57__zr634712-cls-3335394:hover{cursor:pointer;fill:rgba(82,130,235,1)}.mw-chart-896a74a4b0f34b6a8842401bf585e3332d8b922e888c8c2658274e56223bc9d1ba872a197b5563dd794ac6c0f58a6e1753c361b8127bec60d8e3d790f2b22b57__zr634712-cls-3335395:hover{cursor:pointer;fill:rgba(0,0,0,0)}
==== Earth-size planets ====

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In November 2013, it was estimated that 22±8% of Sun-like stars in the Milky Way galaxy may have an Earth-sized planet in the habitable zone. Assuming 200 billion stars in the Milky Way, that would be 11 billion potentially habitable Earths, rising to 40 billion if red dwarfs are included.
Kepler-186f, a 1.2-Earth-radius planet in the habitable zone of a red dwarf, was reported in April 2014.
Proxima Centauri b, a planet in the habitable zone of Proxima Centauri, the nearest known star to the solar system with an estimated minimum mass of 1.27 times the mass of the Earth.
In February 2013, researchers speculated that up to 6% of small red dwarfs may have Earth-size planets. This suggests that the closest one to the Solar System could be 13 light-years away. The estimated distance increases to 21 light-years when a 95% confidence interval is used. In March 2013, a revised estimate gave an occurrence rate of 50% for Earth-size planets in the habitable zone of red dwarfs.
At 1.63 times Earth's radius Kepler-452b is the first discovered near-Earth-size planet in the "habitable zone" around a G2-type Sun-like star (July 2015).
In January 2020 researchers reported the discovery of TOI-700 d, the first Earth-size planet in the habitable zone to be detected by TESS.
== Planetary system ==
Exoplanets are often members of planetary systems of multiple planets around a star. The planets interact with each other gravitationally and sometimes form resonant systems where the orbital periods of the planets are in integer ratios. The Kepler-223 system contains four planets in an 8:6:4:3 orbital resonance.
Some hot Jupiters orbit their stars in the opposite direction to their stars' rotation. One proposed explanation is that hot Jupiters tend to form in dense clusters, where perturbations are more common and gravitational capture of planets by neighboring stars is possible.
== Search projects ==
ANDES – The ArmazoNes High Dispersion Echelle Spectrograph, a planet finding and planet characterisation spectrograph, is expected to be fitted onto ESO's ELT 39.3m telescope. ANDES was formally known as HIRES, which itself was created after a merger of the consortia behind the earlier CODEX (optical high-resolution) and SIMPLE (near-infrared high-resolution) spectrograph concepts.
CoRoT – Space telescope that found the first transiting rocky planet.
ESPRESSO – A rocky planet-finding, and stable spectroscopic observing, spectrograph mounted on ESO's 4 × 8.2 m VLT telescope, sited on the levelled summit of Cerro Paranal in the Atacama Desert of northern Chile.
HARPS – High-precision echelle planet-finding spectrograph installed on the ESO's 3.6m telescope at La Silla Observatory in Chile.
Kepler – Mission to look for large numbers of exoplanets using the transit method.
TESS – Mission to search for new exoplanets, active from 2018 to 2020 and rotating to observe stars from all over the sky. As of 22 March 2025, TESS had identified 7,525 candidate exoplanets, of which 618 had been confirmed.
== See also ==
Detecting Earth from distant star-based systems
Extrasolar Planets Encyclopedia
Extrasolar planets in fiction
Habitable zone for complex life
Lists of exoplanets
NASA Exoplanet Archive
Planetary capture
List of planet types
Desert planet
Carbon planet
Lava planet
Eyeball planet
Ocean world
Ice planet
Hycean planet
Chthonian planet
== Notes ==
== References ==
== Further reading ==
Boss, Alan (2009). The Crowded Universe: The Search for Living Planets. Basic Books. Bibcode:2009cusl.book.....B. ISBN 978-0-465-00936-7 (Hardback); ISBN 978-0-465-02039-3 (Paperback).
Dorminey, Bruce (2001). Distant Wanderers. Springer-Verlag. ISBN 978-0-387-95074-7 (Hardback); ISBN 978-1-4419-2872-6 (Paperback).
Jayawardhana, Ray (2011). Strange New Worlds: The Search for Alien Planets and Life Beyond Our Solar System. Princeton, NJ: Princeton University Press. ISBN 978-0-691-14254-8. (Hardcover.)
van Dishoeck, Ewine F.; Bergin, Edwin A.; Lis, Dariusz C.; Lunine, Jonathan I. (2014). "Water: From Clouds to Planets". Protostars and Planets VI. p. 835. arXiv:1401.8103. Bibcode:2014prpl.conf..835V. doi:10.2458/azu_uapress_9780816531240-ch036. ISBN 978-0-8165-3124-0. S2CID 55875067.
Villard, Ray; Cook, Lynette R. (2005). Infinite Worlds: An Illustrated Voyage to Planets Beyond Our Sun. University of California Press. ISBN 978-0-520-23710-0
Yaqoob, Tahir (2011). Exoplanets and Alien Solar Systems. New Earth Labs (Education and Outreach). ISBN 978-0-9741689-2-0. Paperback.
== External links ==
The Extrasolar Planets Encyclopaedia (Paris Observatory)
NASA Exoplanet Archive

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