View source for Compton Gamma Ray Observatory

{{Short description|NASA space observatory designed to detect X-rays and gamma rays (1991–2000)}}
{{Use American English|date=January 2014}}
{{Infobox spaceflight 
| name                  = Compton Gamma Ray Observatory
| image                 = [[File:CGRO s37-96-010.jpg|300px]]
| image_caption         = CGRO deployed in 1991
| insignia              =

| mission_type          = Astronomy 
| operator              = [[NASA]] 
| website               = {{URL|heasarc.gsfc.nasa.gov/docs/cgro/cossc/}}
| COSPAR_ID             = 1991-027B
| SATCAT                = 21225
| mission_duration      = 9&nbsp;years, 2&nbsp;months

| spacecraft_bus        = 
| manufacturer          = [[TRW Inc.]]
| dry_mass              = 
| launch_mass           = {{convert|16329|kg}}
| power                 = 2000.0 Watts<ref>{{Cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1991-027B|title=NASA – NSSDCA – Spacecraft – Details|website=nssdc.gsfc.nasa.gov|access-date=2018-04-30}}</ref>

| launch_date           = {{start date text|5 April 1991, 14:22:45|timezone=yes}}&nbsp;UTC
| launch_rocket         = {{OV|104}}<br />[[STS-37]]
| launch_site           = [[Kennedy Space Center|Kennedy]] [[Kennedy Space Center Launch Complex 39|LC-39B]]
| decay_date            = {{end date text|4 June 2000, 23:29:55}}&nbsp;UTC

| orbit_epoch           = 7 April 1991, 18:37:00&nbsp;UTC<ref>{{Cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1991-027B|title=NASA – NSSDCA – Spacecraft – Trajectory Details|website=nssdc.gsfc.nasa.gov|access-date=2018-04-30}}</ref>
| orbit_reference       = [[geocentric orbit|Geocentric]]
| orbit_regime          = [[low Earth orbit|Low Earth]]
| orbit_periapsis       = {{convert|362|km}}
| orbit_apoapsis        = {{convert|457|km}}
| orbit_eccentricity    = 0.006998
| orbit_inclination     = 28.4610&nbsp;degrees
| orbit_RAAN            = 68.6827&nbsp;degrees
| orbit_period          = 91.59&nbsp;minutes
| apsis                 = gee

| instrument_type       = Telescopes (Four)
| telescope_name        = <!--name, if different to the satellite-->
| telescope_type        = Scintillation detectors
| telescope_diameter    = <!--diameter of telescope-->
| telescope_focal_length= Varied by instrument
| telescope_area        = Varied by instrument
| telescope_wavelength  = [[X-ray]] to [[Gamma ray|γ-ray]], 20&nbsp;[[kiloelectronvolt|keV]] – 30&nbsp;GeV (40&nbsp;[[picometre|pm]] – 60&nbsp;[[atto-|am]])
| telescope_resolution  = <!--resolution of telescope-->

| instruments_list   = {{Infobox spaceflight/Instruments
  |acronym1 = BATSE   |name1 = Burst and Transient Source Experiment
  |acronym2 = OSSE    |name2 = Oriented Scintillation Spectrometer Experiment
  |acronym3 = COMPTEL |name3 = Imaging Compton Telescope
  |acronym4 = EGRET   |name4 = [[Energetic Gamma Ray Experiment Telescope]]
 }}

| programme             = '''[[Great Observatories program|NASA Great Observatories]]'''
| previous_mission      = [[Hubble Space Telescope|Hubble]]
| next_mission          = [[Chandra X-ray Observatory|Chandra]]
| programme2            = '''[[Large Strategic Science Missions]]'''<br><small>''Astrophysics Division''</small>
}}

[[Image:STS-37 Launch.jpg|thumb|right|240px|Launch of  [[Space Shuttle Atlantis|Space Shuttle ''Atlantis'']] carrying the observatory to Earth orbit ([[STS-37]])]]
[[File:NASA image STS37-051-021 Jay Apt on the first EVA of STS-37 with CGRO.jpg|thumb|Astronaut Jay Apt in the Space Shuttle bay with the observatory partially deployed but still attached to the Shuttle's robotic arm]]

The '''Compton Gamma Ray Observatory''' ('''CGRO''') was a [[space observatory]] detecting [[photons]] with [[photon energy|energies]] from 20&nbsp;k[[Electronvolt#Properties|eV]] to 30&nbsp;GeV, in Earth orbit from 1991 to 2000. The observatory featured four main telescopes in one spacecraft, covering [[X-ray]]s and [[gamma ray]]s, including various specialized sub-instruments and detectors. Following 14 years of effort, the observatory was launched from [[Space Shuttle Atlantis|Space Shuttle ''Atlantis'']] during STS-37 on April 5, 1991, and operated until its [[Atmospheric entry#Deorbit disposal|deorbit]] on June 4, 2000.<ref>{{cite web |url=http://heasarc.gsfc.nasa.gov/docs/cgro/epo/brochures/compton/bro5.html |title=Gamma-Ray Astronomy in the Compton Era: The Instruments |access-date=2007-12-07 |work=Gamma-Ray Astronomy in the Compton Era |publisher=NASA/ GSFC |archive-url=https://web.archive.org/web/20090224125500/http://heasarc.gsfc.nasa.gov/docs/cgro/epo/brochures/compton/bro5.html |archive-date=2009-02-24 |url-status=dead }}</ref> It was deployed in [[low Earth orbit]] at {{convert|450|km|mi|abbr=on}} to avoid the [[Van Allen radiation belt]]. It was the heaviest astrophysical payload ever flown at that time at {{convert|16300|kg}}.

Costing $617&nbsp;million,<ref name=sfn2000/> the CGRO was part of [[NASA]]'s [[Great Observatories program|Great Observatories]] series, along with the [[Hubble Space Telescope]], the [[Chandra X-ray Observatory]], and the [[Spitzer Space Telescope]].<ref>{{Cite web|url=https://www.nasa.gov/audience/forstudents/postsecondary/features/F_NASA_Great_Observatories_PS.html|title=NASA – NASA's Great Observatories|first=Kathy Forsythe : MSFC|last=Barry Logan : MSFC|website=www.nasa.gov|access-date=2020-11-02|archive-date=2011-08-20|archive-url=https://web.archive.org/web/20110820072915/http://www.nasa.gov/audience/forstudents/postsecondary/features/F_NASA_Great_Observatories_PS.html|url-status=dead}}</ref> It was the second of the series to be launched into space, following the Hubble Space Telescope. The CGRO was named after [[Arthur Compton]], an American physicist and former chancellor of [[Washington University in St. Louis]] who received the Nobel prize for work involved with gamma-ray physics. CGRO was built by [[TRW Inc.|TRW]] (now [[Northrop Grumman]] Aerospace Systems) in [[Redondo Beach, California]]. CGRO was an international collaboration and additional contributions came from the [[European Space Agency]] and various universities, as well as the U.S. [[Naval Research Laboratory]].

Successors to CGRO include the ESA [[INTEGRAL]] spacecraft (launched 2002), NASA's [[Swift Gamma-Ray Burst Mission]] (launched 2004), ASI  [[AGILE (satellite)]] (launched 2007) and NASA's [[Fermi Gamma-ray Space Telescope]] (launched 2008); all remain operational as of May 2023.

==Instruments==
[[File:Compton Gamma Ray Observatory cutaway (labelled).jpg|thumb|left|Compton Gamma Ray Observatory cutaway]]
CGRO carried a complement of four instruments that covered an unprecedented six orders of the [[electromagnetic spectrum]], from 20 [[electronvolt|keV]] to 30 GeV (from 0.02 MeV to 30000 MeV). Those are presented below in order of increasing spectral energy coverage:

===BATSE===
The '''Burst and Transient Source Experiment''' ('''BATSE''') by NASA's [[Marshall Space Flight Center]] searched the sky for [[gamma-ray burst]]s (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.<ref name=nasa-cgro-nra-g>[https://heasarc.gsfc.nasa.gov/docs/cgro/nra/appendix_g.html#V. BATSE GUEST INVESTIGATOR PROGRAM]</ref> Each module consisted of both a [[Gamma spectroscopy#Scintillation detectors|NaI(Tl)]] Large Area Detector (LAD) covering the 20 keV to ~2 MeV range, 50.48&nbsp;cm in dia by 1.27&nbsp;cm thick, and a 12.7&nbsp;cm dia by 7.62&nbsp;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 [[telemetry]] 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.

===OSSE===
The '''Oriented Scintillation Spectrometer Experiment''' ('''OSSE''') by the [[Naval Research Laboratory]] 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 [[Gamma spectroscopy#Scintillation detectors|NaI(Tl)]] 12 in (303&nbsp;mm) in diameter, by 4 in (102&nbsp;mm) thick, optically coupled at the rear to a 3 in (76.2&nbsp;mm) thick [[Caesium iodide|CsI]](Na) crystal of similar diameter, viewed by seven [[photomultiplier tube]]s, operated as a [[Phoswich Detector|phoswich]]: 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 [[Electronic anticoincidence|anticoincidence]] 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 [[slew (spacecraft)|slew]] with a speed of approximately 2 degrees per second.

===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 scattering|Compton scatter]] in a forward detector module, where the interaction energy ''E<sub>1</sub>'', 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, ''E<sub>2</sub>'', would be measured. From these two energies, ''E<sub>1</sub>'' and ''E<sub>2</sub>'', the Compton scattering angle, angle θ, can be determined, along with the total energy, ''E<sub>1</sub> + E<sub>2</sub>'', of the incident photon. The positions of the interactions, in both the front and rear scintillators, was also measured. The [[Euclidean vector|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===
{| class=wikitable style="text-align:center; font-size:11px; float:right; margin:2px"
|- style="font-size: smaller;"
| colspan=8 align=center|'''Instruments'''
|-
! Instrument || Observing
|-
| BATSE || 0.02 – 8 MeV
|-
| OSSE || 0.05 – 10 MeV
|-
| COMPTEL || 0.75 – 30 MeV
|-
| EGRET || 20 – 30 000 MeV
|-
|}
{{Main|Energetic Gamma Ray Experiment Telescope}}
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 (particle physics)|calorimeter]] [[scintillation detector]] at the rear of the instrument.

==Results==
[[File:Moon egret.jpg|thumb|The Moon as seen by the Compton Gamma Ray Observatory, in gamma rays of greater than 20 MeV. These are produced by [[cosmic ray]] bombardment of its surface. The [[Sun]], which has no similar surface of high [[atomic number]] to act as target for cosmic rays, cannot be seen at all at these energies, which are too high to emerge from primary nuclear reactions, such as solar nuclear fusion.<ref>{{Cite web|url=https://heasarc.gsfc.nasa.gov/docs/cgro/epo/news/gammoon.html|title=CGRO SSC >> EGRET Detection of Gamma Rays from the Moon|website=heasarc.gsfc.nasa.gov}}</ref>]]

===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 {{chem|26|Al}} (a radioactive [[isotopes of aluminium|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 repeater|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===
{{Main|GRB 990123}}
[[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 flash|terrestrial gamma ray sources]] in 1994 that came from [[Cumulonimbus cloud|thunderclouds]]

== History ==

;Proposal: Work started in 1977.

;Funding and Development:  CGRO was designed for in-orbit refuelling/servicing.<ref name="C-1">{{cite news |last1=Cowing |first1=Keith |title=NASA Preparing Plans for Destructive Reentry to End Compton Gamma Ray Observatory's Mission |url=https://spaceref.com/uncategorized/nasa-preparing-plans-for-destructive-reentry-to-end-compton-gamma-ray-observatorys-mission/ |work=SpaceRef |date=January 14, 2000 |archive-url=https://archive.today/20231228162531/https://spaceref.com/uncategorized/nasa-preparing-plans-for-destructive-reentry-to-end-compton-gamma-ray-observatorys-mission/ |archive-date=December 28, 2023 |url-status=live }}</ref>

;Construction and test:

;Launch and Commissioning: Launched 7 April 1991. Fuel line problems were found soon after launch which discouraged frequent orbital reboosts.

;Communications: <!-- up to TDRS satellites in GEO, or down to TDRS or DSN ground stations ? -->

;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.<ref name="nasa-199403">{{cite web |title=March 1994 – Gamma Ray Observatory Remote Terminal System (GRTS) Declared Operational |url=https://www.nasa.gov/image-article/march-1994-gamma-ray-observatory-remote-terminal-system-grts-declared-operational/ |publisher=[[NASA]] |archive-url=https://web.archive.org/web/20231228162852/https://www.nasa.gov/image-article/march-1994-gamma-ray-observatory-remote-terminal-system-grts-declared-operational/ |archive-date=December 28, 2023 |date=March 1994 |url-status=live}}</ref>

===Orbital re-boost===
[[File:1991 s37 GRO copy.jpg|thumb|Compton Gamma Ray Observatory being deployed from Space Shuttle ''Atlantis'' in 1991 in Earth orbit]]
It was deployed to an altitude of 450&nbsp;km on April 7, 1991, when it was first launched.<ref name="heasarc.gsfc.nasa.gov">{{Cite web |url=https://heasarc.gsfc.nasa.gov/docs/cgro/epo/news/reboost.html |title=CGRO SSC >> Successful Reboost of Compton Gamma Ray Observatory |website=heasarc.gsfc.nasa.gov |archive-url=https://web.archive.org/web/20231127091941/https://heasarc.gsfc.nasa.gov/docs/cgro/epo/news/reboost.html |archive-date=November 27, 2023 |url-status=live |date=August 1, 2005 |publisher=[[NASA]] }}</ref> Over time the orbit decayed and needed re-boosting to prevent atmospheric entry sooner than desired.<ref name="heasarc.gsfc.nasa.gov"/> It was reboosted twice using onboard propellant: in October 1993 from 340&nbsp;km to 450&nbsp;km altitude, and in June 1997 from 440&nbsp;km to 515&nbsp;km altitude, to potentially extend operation to 2007.<ref name="heasarc.gsfc.nasa.gov"/>

==De-orbit==
After one of its three [[gyroscope]]s 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.<ref name=sfn2000>{{Cite web|url=https://spaceflightnow.com/cgrodeorbit/index.html|title=Spaceflight Now &#124; CGRO Deorbit &#124; NASA space telescope heads for fiery crash into Pacific|website=spaceflightnow.com}}</ref> 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.<ref>{{Cite news|url=https://www.nytimes.com/2000/06/05/us/satellite-marked-for-extinction-plunges-into-the-sea-on-target.html|title=Satellite Marked for Extinction Plunges Into the Sea, on Target (Published 2000)|newspaper=The New York Times|agency=Associated Press|date=June 5, 2000}}</ref>

This de-orbit was NASA's first intentional controlled de-orbit of a satellite.
<ref>{{Cite journal
  | title = Entry Debris Field estimation methods and application to Compton Gamma Ray Observatory
  | journal = 2001 Flight Mechanics Symposium
 | date = June 2001
 | publisher = Mission Operations Directorate Nasa Johnson Space Center
  | url = https://ntrs.nasa.gov/citations/20010084992
  | last1 = Mrozinski
 | first1 = Richard B.
 }}</ref>

==See also==
{{Portal|Spaceflight}}
*[[Gamma-ray astronomy]]
*[[Great Observatories program|NASA – Great Observatories program]]
*[[List of heaviest spacecraft]]

==References==
{{Reflist}}

==External links==
{{Commons category}}
*[https://web.archive.org/web/19961209190738/http://cossc.gsfc.nasa.gov/ NASA Compton Gamma Ray Observatory site]
*[http://heasarc.gsfc.nasa.gov/docs/cgro/images/epo/gallery/cgro/ NASA CGRO images]
*[http://heasarc.gsfc.nasa.gov/docs/cgro/batse/ Mapping of BATSE GRB detections]
*[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]

{{GSFC}}
{{Space observatories}}
{{TRW}}
{{Orbital launches in 1991}}
{{Authority control}}

[[Category:Spacecraft launched in 1991]]
[[Category:Satellites formerly orbiting Earth]]
[[Category:TRW Inc.]]
[[Category:Spacecraft which reentered in 2000]]
[[Category:Gamma-ray telescopes]]
[[Category:Goddard Space Flight Center]]
[[Category:Space telescopes]]
[[Category:Spacecraft launched by the Space Shuttle]]
[[Category:Great Observatories program]]
[[Category:Destroyed spacecraft]]