Menu
Home
People
Places
Arts
History
Plants & Animals
Science
Life & Culture
Technology
Reference.org
Compton Gamma Ray Observatory
open-in-new
<h2 id="instruments">Instruments</h2> <p>CGRO carried a complement of four instruments that covered an unprecedented six orders of the <a href="/facts/Electromagnetic_spectrum/24LARvFY">electromagnetic spectrum</a>, from 20 <a href="/facts/Electronvolt/YMT1fqX9">keV</a> to 30 GeV (from 0.02 MeV to 30000 MeV). Those are presented below in order of increasing spectral energy coverage: </p> <h3>BATSE</h3> <p>The Burst and Transient Source Experiment (BATSE) by NASA's <a href="/facts/Marshall_Space_Flight_Center/Q5XqseqT">Marshall Space Flight Center</a> searched the sky for <a href="/facts/Gamma-ray_burst/oRRU6Fhn">gamma-ray bursts</a> (20 to >600 keV) and conducted full-sky surveys for long-lived sources. It consisted of eight identical detector modules, one at each of the satellite's corners.<a class="footnote-ref" id="fnref:4" href="#fn:4"><sup>4</sup></a> Each module consisted of both a <a href="/facts/Gamma_spectroscopy/sSsWI23T">NaI(Tl)</a> Large Area Detector (LAD) covering the 20 keV to ~2 MeV range, 50.48 cm in dia by 1.27 cm thick, and a 12.7 cm dia by 7.62 cm thick NaI Spectroscopy Detector, which extended the upper energy range to 8 MeV, all surrounded by a plastic scintillator in active anti-coincidence to veto the large background rates due to cosmic rays and trapped radiation. Sudden increases in the LAD rates triggered a high-speed data storage mode, the details of the burst being read out to <a href="/facts/Telemetry/Mz9czXTl">telemetry</a> later. Bursts were typically detected at rates of roughly one per day over the 9-year CGRO mission. A strong burst could result in the observation of many thousands of gamma-rays within a time interval ranging from ~0.1 s up to about 100 s. </p> <h3>OSSE</h3> <p>The Oriented Scintillation Spectrometer Experiment (OSSE) by the <a href="/facts/Naval_Research_Laboratory/brXxYx34">Naval Research Laboratory</a> detected gamma rays entering the field of view of any of four detector modules, which could be pointed individually, and were effective in the 0.05 to 10 MeV range. Each detector had a central scintillation spectrometer crystal of <a href="/facts/Gamma_spectroscopy/sSsWI23T">NaI(Tl)</a> 12 in (303 mm) in diameter, by 4 in (102 mm) thick, optically coupled at the rear to a 3 in (76.2 mm) thick <a href="/facts/Caesium_iodide/GX2FrsVy">CsI</a>(Na) crystal of similar diameter, viewed by seven <a href="/facts/Photomultiplier_tube/bjbB9fBE">photomultiplier tubes</a>, operated as a <a href="/facts/Phoswich_Detector/vp7GrVSS">phoswich</a>: i.e., particle and gamma-ray events from the rear produced slow-rise time (~1 μs) pulses, which could be electronically distinguished from pure NaI events from the front, which produced faster (~0.25 μs) pulses. Thus the CsI backing crystal acted as an active <a href="/facts/Electronic_anticoincidence/l0RgBSYy">anticoincidence</a> shield, vetoing events from the rear. A further barrel-shaped CsI shield, also in electronic anticoincidence, surrounded the central detector on the sides and provided coarse collimation, rejecting gamma rays and charged particles from the sides or most of the forward field-of-view (FOV). A finer level of angular collimation was provided by a tungsten slat collimator grid within the outer CsI barrel, which collimated the response to a 3.8° x 11.4° FWHM rectangular FOV. A plastic scintillator across the front of each module vetoed charged particles entering from the front. The four detectors were typically operated in pairs of two. During a gamma-ray source observation, one detector would take observations of the source, while the other would slew slightly off source to measure the background levels. The two detectors would routinely switch roles, allowing for more accurate measurements of both the source and background. The instruments could <a href="/facts/Slew_(spacecraft)/8FvjQFUJ">slew</a> with a speed of approximately 2 degrees per second. </p> <h3>COMPTEL</h3> <p>The Imaging Compton Telescope (COMPTEL) by the <a href="/facts/Max_Planck_Institute_for_Extraterrestrial_Physics/WjJjmpg4">Max Planck Institute for Extraterrestrial Physics</a>, the <a href="/facts/University_of_New_Hampshire/RDqRkwTA">University of New Hampshire</a>, <a href="/facts/Netherlands_Institute_for_Space_Research/WuC1eKsu">Netherlands Institute for Space Research</a>, 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 <a href="/facts/Steradian/PRCqT4n1">steradian</a>. For cosmic gamma-ray events, the experiment required two nearly simultaneous interactions, in a set of front and rear scintillators. Gamma rays would <a href="/facts/Compton_scattering/Q98Ty8rz">Compton scatter</a> in a forward detector module, where the interaction energy <i>E1</i>, 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, <i>E2</i>, would be measured. From these two energies, <i>E1</i> and <i>E2</i>, the Compton scattering angle, angle θ, can be determined, along with the total energy, <i>E1 + E2</i>, of the incident photon. The positions of the interactions, in both the front and rear scintillators, was also measured. The <a href="/facts/Euclidean_vector/bCLrdksy">vector</a>, 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. </p> <h3>EGRET</h3> <table><tbody><tr><td colspan="8">Instruments</td></tr><tr><th>Instrument</th><th>Observing</th></tr><tr><td>BATSE</td><td>0.02 – 8 MeV</td></tr><tr><td>OSSE</td><td>0.05 – 10 MeV</td></tr><tr><td>COMPTEL</td><td>0.75 – 30 MeV</td></tr><tr><td>EGRET</td><td>20 – 30 000 MeV</td></tr></tbody></table> <p class="note">Main article: <a href="/facts/Energetic_Gamma_Ray_Experiment_Telescope/LxrNrRUB">Energetic Gamma Ray Experiment Telescope</a></p> <p>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 <a href="/facts/Goddard_Space_Flight_Center/WfoEKknM">Goddard Space Flight Center</a>, the <a href="/facts/Max_Planck_Institute_for_Extraterrestrial_Physics/WjJjmpg4">Max Planck Institute for Extraterrestrial Physics</a>, and <a href="/facts/Stanford_University/JEQeF9z2">Stanford University</a>. Its detector operated on the principle of electron-<a href="/facts/Positron/HgV4zBzA">positron</a> <a href="/facts/Pair_production/MmN9EUG0">pair production</a> 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 <i>V</i> of the two emerging particles projected to the sky. Finally, their total energy was measured in a large <a href="/facts/Calorimeter_(particle_physics)/FCA2evfS">calorimeter</a> <a href="/facts/Scintillation_detector/vhxA1Ufo">scintillation detector</a> at the rear of the instrument. </p> <h2 id="results">Results</h2> <h3>Basic results</h3> <ul><li>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.</li> <li>The COMPTEL instrument completed an all sky map of 26Al (a radioactive <a href="/facts/Isotopes_of_aluminium/8nsVdoC5">isotope of aluminum</a>).</li> <li>The OSSE instrument completed the most comprehensive survey of the galactic center, and discovered a possible <a href="/facts/Antimatter/NvpGDLof">antimatter</a> "cloud" above the center.</li> <li>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 <a href="/facts/Milky_Way/ulF6lccN">Milky Way</a>, and therefore must be enormously energetic.</li> <li>The discovery of the first four <a href="/facts/Soft_gamma_repeater/0VryRaB8">soft gamma ray repeaters</a>; these sources were relatively weak, mostly below 100 keV and had unpredictable periods of activity and inactivity</li> <li>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.</li></ul> <h3>GRB 990123</h3> <p class="note">Main article: <a href="/facts/GRB_990123/N8cPRaB1">GRB 990123</a></p> <p><a href="/facts/GRB_990123/N8cPRaB1">Gamma ray burst 990123</a> (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 <a href="/facts/Redshift/Qfl4VjDF">redshift</a> 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. </p> <h3>Miscellaneous results</h3> <ul><li>The completion of both a <a href="/facts/Pulsar/f8c7S5LQ">pulsar</a> survey and a <a href="/facts/Supernova_remnant/tYboe2n4">supernova remnant</a> survey</li> <li>The discovery of <a href="/facts/Terrestrial_gamma-ray_flash/6WykoU8y">terrestrial gamma ray sources</a> in 1994 that came from <a href="/facts/Cumulonimbus_cloud/DPWNqTDE">thunderclouds</a></li></ul> <h2 id="history">History</h2> Proposal Work started in 1977. Funding and Development CGRO was designed for in-orbit refuelling/servicing.<a class="footnote-ref" id="fnref:5" href="#fn:5"><sup>5</sup></a> 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.<a class="footnote-ref" id="fnref:6" href="#fn:6"><sup>6</sup></a> <h3>Orbital re-boost</h3> <p>It was deployed to an altitude of 450 km on April 7, 1991, when it was first launched.<a class="footnote-ref" id="fnref:7" href="#fn:7"><sup>7</sup></a> Over time the orbit decayed and needed re-boosting to prevent atmospheric entry sooner than desired.<a class="footnote-ref" id="fnref:8" href="#fn:8"><sup>8</sup></a> 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.<a class="footnote-ref" id="fnref:9" href="#fn:9"><sup>9</sup></a> </p> <h2 id="de-orbit">De-orbit</h2> <p>After one of its three <a href="/facts/Gyroscope/UjXGsfMq">gyroscopes</a> 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.<a class="footnote-ref" id="fnref:10" href="#fn:10"><sup>10</sup></a> 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.<a class="footnote-ref" id="fnref:11" href="#fn:11"><sup>11</sup></a> </p><p>This de-orbit was NASA's first intentional controlled de-orbit of a satellite. <a class="footnote-ref" id="fnref:12" href="#fn:12"><sup>12</sup></a> </p> <h2 id="see-also">See also</h2> <ul> <li>Spaceflight portal</li></ul> <ul><li><a href="/facts/Gamma-ray_astronomy/s59YdwaX">Gamma-ray astronomy</a></li> <li><a href="/facts/Great_Observatories_program/KNFpk4V8">NASA – Great Observatories program</a></li> <li><a href="/facts/List_of_heaviest_spacecraft/PGkCWGVb">List of heaviest spacecraft</a></li></ul> <h2 id="external-links">External links</h2> Wikimedia Commons has media related to Compton Gamma Ray Observatory. <ul><li><a href="https://web.archive.org/web/19961209190738/http://cossc.gsfc.nasa.gov/">NASA Compton Gamma Ray Observatory site</a></li> <li><a href="https://heasarc.gsfc.nasa.gov/docs/cgro/images/epo/gallery/cgro/">NASA CGRO images</a></li> <li><a href="https://heasarc.gsfc.nasa.gov/docs/cgro/batse/">Mapping of BATSE GRB detections</a></li> <li><a href="http://researchandideas.com/index.php?title=NASA%27s_GRO_Remote_Terminal_System_(GRTS)">NASA's GRO Remote Terminal System Installed at Canberra Deep Space Communication Complex</a></li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>"Gamma-Ray Astronomy in the Compton Era: The Instruments". Gamma-Ray Astronomy in the Compton Era. NASA/ GSFC. Archived from the original on 2009-02-24. Retrieved 2007-12-07. <a href="https://web.archive.org/web/20090224125500/http://heasarc.gsfc.nasa.gov/docs/cgro/epo/brochures/compton/bro5.html" target="_blank">https://web.archive.org/web/20090224125500/http://heasarc.gsfc.nasa.gov/docs/cgro/epo/brochures/compton/bro5.html</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>"Spaceflight Now | CGRO Deorbit | NASA space telescope heads for fiery crash into Pacific". spaceflightnow.com. <a href="https://spaceflightnow.com/cgrodeorbit/index.html" target="_blank">https://spaceflightnow.com/cgrodeorbit/index.html</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>Barry Logan : MSFC, Kathy Forsythe : MSFC. "NASA – NASA's Great Observatories". www.nasa.gov. Archived from the original on 2011-08-20. Retrieved 2020-11-02. <a href="https://web.archive.org/web/20110820072915/http://www.nasa.gov/audience/forstudents/postsecondary/features/F_NASA_Great_Observatories_PS.html" target="_blank">https://web.archive.org/web/20110820072915/http://www.nasa.gov/audience/forstudents/postsecondary/features/F_NASA_Great_Observatories_PS.html</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li> <li id="fn:4"><p>BATSE GUEST INVESTIGATOR PROGRAM <a href="https://heasarc.gsfc.nasa.gov/docs/cgro/nra/appendix_g.html#V." target="_blank">https://heasarc.gsfc.nasa.gov/docs/cgro/nra/appendix_g.html#V.</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li> <li id="fn:5"><p>Cowing, Keith (January 14, 2000). "NASA Preparing Plans for Destructive Reentry to End Compton Gamma Ray Observatory's Mission". SpaceRef. Archived from the original on December 28, 2023. <a href="https://spaceref.com/uncategorized/nasa-preparing-plans-for-destructive-reentry-to-end-compton-gamma-ray-observatorys-mission/" target="_blank">https://spaceref.com/uncategorized/nasa-preparing-plans-for-destructive-reentry-to-end-compton-gamma-ray-observatorys-mission/</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li> <li id="fn:6"><p>"March 1994 – Gamma Ray Observatory Remote Terminal System (GRTS) Declared Operational". NASA. March 1994. Archived from the original on December 28, 2023. <a href="https://www.nasa.gov/image-article/march-1994-gamma-ray-observatory-remote-terminal-system-grts-declared-operational/" target="_blank">https://www.nasa.gov/image-article/march-1994-gamma-ray-observatory-remote-terminal-system-grts-declared-operational/</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li> <li id="fn:7"><p>"CGRO SSC >> Successful Reboost of Compton Gamma Ray Observatory". heasarc.gsfc.nasa.gov. NASA. August 1, 2005. Archived from the original on November 27, 2023. <a href="https://heasarc.gsfc.nasa.gov/docs/cgro/epo/news/reboost.html" target="_blank">https://heasarc.gsfc.nasa.gov/docs/cgro/epo/news/reboost.html</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></p></li> <li id="fn:8"><p>"CGRO SSC >> Successful Reboost of Compton Gamma Ray Observatory". heasarc.gsfc.nasa.gov. NASA. August 1, 2005. Archived from the original on November 27, 2023. <a href="https://heasarc.gsfc.nasa.gov/docs/cgro/epo/news/reboost.html" target="_blank">https://heasarc.gsfc.nasa.gov/docs/cgro/epo/news/reboost.html</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></p></li> <li id="fn:9"><p>"CGRO SSC >> Successful Reboost of Compton Gamma Ray Observatory". heasarc.gsfc.nasa.gov. NASA. August 1, 2005. Archived from the original on November 27, 2023. <a href="https://heasarc.gsfc.nasa.gov/docs/cgro/epo/news/reboost.html" target="_blank">https://heasarc.gsfc.nasa.gov/docs/cgro/epo/news/reboost.html</a> <a href="#fnref:9" class="footnote-back-ref">↩</a></p></li> <li id="fn:10"><p>"Spaceflight Now | CGRO Deorbit | NASA space telescope heads for fiery crash into Pacific". spaceflightnow.com. <a href="https://spaceflightnow.com/cgrodeorbit/index.html" target="_blank">https://spaceflightnow.com/cgrodeorbit/index.html</a> <a href="#fnref:10" class="footnote-back-ref">↩</a></p></li> <li id="fn:11"><p>"Satellite Marked for Extinction Plunges Into the Sea, on Target (Published 2000)". The New York Times. Associated Press. June 5, 2000. <a href="https://www.nytimes.com/2000/06/05/us/satellite-marked-for-extinction-plunges-into-the-sea-on-target.html" target="_blank">https://www.nytimes.com/2000/06/05/us/satellite-marked-for-extinction-plunges-into-the-sea-on-target.html</a> <a href="#fnref:11" class="footnote-back-ref">↩</a></p></li> <li id="fn:12"><p>Mrozinski, Richard B. (June 2001). "Entry Debris Field estimation methods and application to Compton Gamma Ray Observatory". 2001 Flight Mechanics Symposium. Mission Operations Directorate Nasa Johnson Space Center. <a href="https://ntrs.nasa.gov/citations/20010084992" target="_blank">https://ntrs.nasa.gov/citations/20010084992</a> <a href="#fnref:12" class="footnote-back-ref">↩</a></p></li> </ol>