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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Comet | 9/10 | https://en.wikipedia.org/wiki/Comet | reference | science, encyclopedia | 2026-05-05T13:31:54.670973+00:00 | kb-cron |
The Halley Armada describes the collection of spacecraft missions that visited and/or made observations of Halley's Comet 1980s perihelion. The Space Shuttle Challenger was intended to do a study of Halley's Comet in 1986, but exploded shortly after being launched. Deep Impact. Debate continues about how much ice is in a comet. In 2001, the Deep Space 1 spacecraft obtained high-resolution images of the surface of Comet Borrelly. It was found that the surface of comet Borrelly is hot and dry, with a temperature of between 26 and 71 °C (79 and 160 °F), and extremely dark, suggesting that the ice has been removed by solar heating and maturation, or is hidden by the soot-like material that covers Borrelly. In July 2005, the Deep Impact probe blasted a crater on Comet Tempel 1 to study its interior. The mission yielded results suggesting that the majority of a comet's water ice is below the surface and that these reservoirs feed the jets of vaporized water that form the coma of Tempel 1. Renamed EPOXI, it made a flyby of Comet Hartley 2 on 4 November 2010. Ulysses. In 2007, the Ulysses probe passed through the tail of the comet C/2006 P1 (McNaught) and recorded data on the interaction of the tail and solar winds over a period of five days. Stardust. Data from the Stardust mission show that materials retrieved from the tail of Wild 2 were crystalline and could only have been "born in fire", at extremely high temperatures of over 1,000 °C (1,830 °F). Although comets formed in the outer Solar System, radial mixing of material during the early formation of the Solar System is thought to have redistributed material throughout the proto-planetary disk. As a result, comets contain crystalline grains that formed in the early, hot inner Solar System. This is seen in comet spectra as well as in sample return missions. More recent still, the materials retrieved demonstrate that the "comet dust resembles asteroid materials". These new results have forced scientists to rethink the nature of comets and their distinction from asteroids. Rosetta. The Rosetta probe orbited Comet Churyumov–Gerasimenko. On 12 November 2014, its lander Philae successfully landed on the comet's surface, the first time a spacecraft has ever landed on such an object in history.
== Classification == Comets can be classified by astronomers using the planets that control their perihelion and aphelion, Tisserand orbital parameters. Other more qualitative and casual classifications are also possible.
=== Great comets ===
Approximately once a decade, a comet becomes bright enough to be noticed by a casual observer, leading such comets to be designated as great comets. Predicting whether a comet will become a great comet is notoriously difficult, as many factors may cause a comet's brightness to depart drastically from predictions. Broadly speaking, if a comet has a large and active nucleus, will pass close to the Sun, and is not obscured by the Sun as seen from Earth when at its brightest, it has a chance of becoming a great comet. However, Comet Kohoutek in 1973 fulfilled all the criteria and was expected to become spectacular but failed to do so. Comet West, which appeared three years later, had much lower expectations but became an extremely impressive comet. The Great Comet of 1577 is a well-known example of a great comet. It passed near Earth as a non-periodic comet and was seen by many, including well-known astronomers Tycho Brahe and Taqi ad-Din. Observations of this comet, together with the nova of 1572, cast doubt on the Aristotlean view of a constant universe. The late 20th century saw a lengthy gap without the appearance of any great comets, followed by the arrival of two in quick succession—Comet Hyakutake in 1996, followed by Hale–Bopp, which reached maximum brightness in 1997 having been discovered two years earlier. The first great comet of the 21st century was C/2006 P1 (McNaught), which became visible to naked eye observers in January 2007. It was the brightest in over 40 years.
=== Sungrazing comets ===
A sungrazing comet is a comet that passes extremely close to the Sun at perihelion, generally within a few million kilometers. Although small sungrazers can be completely evaporated during such a close approach to the Sun, larger sungrazers can survive many perihelion passages. However, the strong tidal forces they experience often lead to their fragmentation. About 90% of the sungrazers observed with SOHO are members of the Kreutz group, which all originate from one giant comet that broke up into many smaller comets during its first passage through the inner Solar System. The remainder contains some sporadic sungrazers, but four other related groups of comets have been identified among them: the Kracht, Kracht 2a, Marsden, and Meyer groups. The Marsden and Kracht groups both appear to be related to Comet 96P/Machholz, which is the parent of two meteor streams, the Quadrantids and the Arietids.
=== Unusual comets ===
Of the thousands of known comets, some exhibit unusual properties. Comet Encke (2P/Encke) orbits from outside the asteroid belt to just inside the orbit of the planet Mercury whereas the Comet 29P/Schwassmann–Wachmann currently travels in a nearly circular orbit entirely between the orbits of Jupiter and Saturn. 2060 Chiron, whose unstable orbit is between Saturn and Uranus, was originally classified as an asteroid until a faint coma was noticed. Similarly, Comet Shoemaker–Levy 2 was originally designated asteroid 1990 UL3.
=== Centaurs ===
Centaurs typically behave with characteristics of both asteroids and comets. Centaurs can be classified as comets such as 60558 Echeclus, and 166P/NEAT. 166P/NEAT was discovered while it exhibited a coma, and so is classified as a comet despite its orbit, and 60558 Echeclus was discovered without a coma but later became active, and was then classified as both a comet and an asteroid (174P/Echeclus). One plan for Cassini involved sending it to a centaur, but NASA decided to destroy it instead.