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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Compton Gamma Ray Observatory | 2/2 | https://en.wikipedia.org/wiki/Compton_Gamma_Ray_Observatory | reference | science, encyclopedia | 2026-05-05T12:57:50.048251+00:00 | kb-cron |
=== COMPTEL === The Imaging Compton Telescope (COMPTEL) by the Max Planck Institute for Extraterrestrial Physics, the University of New Hampshire, Netherlands Institute for Space Research, and ESA's Astrophysics Division was tuned to the 0.75-30 MeV energy range and determined the angle of arrival of photons to within a degree and the energy to within five percent at higher energies. The instrument had a field of view of one steradian. For cosmic gamma-ray events, the experiment required two nearly simultaneous interactions, in a set of front and rear scintillators. Gamma rays would Compton scatter in a forward detector module, where the interaction energy E1, given to the recoil electron was measured, while the Compton scattered photon would then be caught in one of the second layers of scintillators to the rear, where its total energy, E2, would be measured. From these two energies, E1 and E2, the Compton scattering angle, angle θ, can be determined, along with the total energy, E1 + E2, of the incident photon. The positions of the interactions, in both the front and rear scintillators, was also measured. The vector, V, connecting the two interaction points determined a direction to the sky, and the angle θ about this direction, defined a cone about V on which the source of the photon must lie, and a corresponding "event circle" on the sky. Because of the requirement for a near coincidence between the two interactions, with the correct delay of a few nanoseconds, most modes of background production were strongly suppressed. From the collection of many event energies and event circles, a map of the positions of sources, along with their photon fluxes and spectra, could be determined.
=== EGRET ===
The Energetic Gamma Ray Experiment Telescope (EGRET) measured high energy (20 MeV to 30 GeV) gamma-ray source positions to a fraction of a degree and photon energy to within 15 percent. EGRET was developed by NASA Goddard Space Flight Center, the Max Planck Institute for Extraterrestrial Physics, and Stanford University. Its detector operated on the principle of electron-positron pair production from high energy photons interacting in the detector. The tracks of the high-energy electron and positron created were measured within the detector volume, and the axis of the V of the two emerging particles projected to the sky. Finally, their total energy was measured in a large calorimeter scintillation detector at the rear of the instrument.
== Results ==
=== Basic results === The EGRET instrument conducted the first all sky survey above 100 MeV. Using four years of data it discovered 271 sources, 170 of which were unidentified. The COMPTEL instrument completed an all sky map of 26Al (a radioactive isotope of aluminum). The OSSE instrument completed the most comprehensive survey of the galactic center, and discovered a possible antimatter "cloud" above the center. The BATSE instrument averaged one gamma ray burst event detection per day for a total of approximately 2700 detections. It definitively showed that the majority of gamma-ray bursts must originate in distant galaxies, not nearby in our own Milky Way, and therefore must be enormously energetic. The discovery of the first four soft gamma ray repeaters; these sources were relatively weak, mostly below 100 keV and had unpredictable periods of activity and inactivity The separation of GRBs into two time profiles: short duration GRBs that last less than 2 seconds, and long duration GRBs that last longer than this.
=== GRB 990123 ===
Gamma ray burst 990123 (23 January 1999) was one of the brightest bursts recorded at the time, and was the first GRB with an optical afterglow observed during the prompt gamma ray emission (a reverse shock flash). This allowed astronomers to measure a redshift of 1.6 and a distance of 3.2 Gpc. Combining the measured energy of the burst in gamma-rays and the distance, the total emitted energy assuming an isotropic explosion could be deduced and resulted in the direct conversion of approximately two solar masses into energy. This finally convinced the community that GRB afterglows resulted from highly collimated explosions, which strongly reduced the needed energy budget.
=== Miscellaneous results === The completion of both a pulsar survey and a supernova remnant survey The discovery of terrestrial gamma ray sources in 1994 that came from thunderclouds
== History == Proposal Work started in 1977. Funding and Development CGRO was designed for in-orbit refuelling/servicing. Construction and test
Launch and Commissioning Launched 7 April 1991. Fuel line problems were found soon after launch which discouraged frequent orbital reboosts. Communications
Loss of data tape recorder, and mitigation Onboard data recorders failed in 1992 which reduced the amount of data that could be downlinked. Another TDRS ground station was built to reduce the gaps in data collection.
=== Orbital re-boost ===
It was deployed to an altitude of 450 km on April 7, 1991, when it was first launched. Over time the orbit decayed and needed re-boosting to prevent atmospheric entry sooner than desired. It was reboosted twice using onboard propellant: in October 1993 from 340 km to 450 km altitude, and in June 1997 from 440 km to 515 km altitude, to potentially extend operation to 2007.
== De-orbit == After one of its three gyroscopes failed in December 1999, the observatory was deliberately de-orbited. At the time, the observatory was still operational; however the failure of another gyroscope would have made de-orbiting much more difficult and dangerous. With some controversy, NASA decided in the interest of public safety that a controlled crash into an ocean was preferable to letting the craft come down on its own at random. It entered the Earth's atmosphere on 4 June 2000, with the debris that did not burn up ("six 1,800-pound aluminum I-beams and parts made of titanium, including more than 5,000 bolts") falling into the Pacific Ocean. This de-orbit was NASA's first intentional controlled de-orbit of a satellite.
== See also ==
Gamma-ray astronomy NASA – Great Observatories program List of heaviest spacecraft
== References ==
== External links ==
NASA Compton Gamma Ray Observatory site NASA CGRO images Archived 2016-03-04 at the Wayback Machine Mapping of BATSE GRB detections NASA's GRO Remote Terminal System Installed at Canberra Deep Space Communication Complex