Compton Gamma Ray Observatory

Compton Gamma Ray Observatory

Artist's conception of the CGRO spacecraft
Mission type Astronomy
Operator NASA
COSPAR ID 1991-027B
SATCAT № 21225
Website cossc.gsfc.nasa.gov
Mission duration 9 years, 2 months
Spacecraft properties
Manufacturer TRW Inc.
Launch mass 17,000 kilograms (37,000 lb)
Power 2.000 Watts
Start of mission
Launch date 5 April 1991, 14:22:45 (1991-04-05UTC14:22:45Z) UTC
Rocket Space Shuttle Atlantis
STS-37
Launch site Kennedy LC-39B
End of mission
Decay date 4 June 2000, 23:29:55 (2000-06-04UTC23:29:56) UTC
Orbital parameters
Reference system Geocentric
Regime Low Earth
Eccentricity 0.006998
Perigee 237 kilometres (147 mi)
Apogee 768 kilometres (477 mi)
Inclination 28.4610 degrees
Period 91.59 minutes
RAAN 68.6827 degrees
Main Telescopes (Four)
Type Scintillation detectors
Focal length Varied by instrument
Collecting area Varied by instrument
Wavelengths Gamma
Instruments
BATSE, OSSE, COMPTEL, EGRET

The Compton Gamma Ray Observatory (CGRO) was a space observatory detecting light from 20 keV to 30 GeV in Earth orbit from 1991 to 2000. It featured four main telescopes in one spacecraft, covering X-rays and gamma rays, including various specialized sub-instruments and detectors. Following 14 years of effort, the observatory was launched from Space Shuttle Atlantis during STS-37 on April 5, 1991, and operated until its deorbit on June 4, 2000.[1] It was deployed in low earth orbit at 450 km (280 mi) to avoid the Van Allen radiation belt. It was the heaviest astrophysical payload ever flown at that time at 17,000 kilograms (37,000 lb).

Costing $617 million,[2] the CGRO was part of NASA's "Great Observatories" series, along with the Hubble Space Telescope, the Chandra X-ray Observatory, and the Spitzer Space Telescope.[3] It was the second of the series to be launched into space, following the Hubble Space Telescope. CGRO was named after Arthur Holly Compton (Washington University in St. Louis), Nobel prize winner, for work involved with gamma ray physics. CGRO was built by 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.

Instruments

CGRO carried a complement of four instruments that covered an unprecedented six decades of the electromagnetic spectrum, from 20 keV to 30 GeV (from 0.02 MeV to 30000 MeV). In order of increasing spectral energy coverage:

BATSE

The Compton Gamma Ray Observatory during deployment from Space Shuttle Atlantis

OSSE

COMPTEL

Comparison
Instrument Observing
BATSE 0.02 - 8 MeV
OSSE 0.05 - 10 MeV
COMPTEL 0.75 - 30 MeV
EGRET 20 - 30 000 MeV

EGRET

Results

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.[4]

Basic results

GRB 990123

Main article: 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

Orbital boost

Compton Gamma Ray Observatory being deployed in 1991

By Oct 1993 the 450 km orbit had decayed to 340 km and a reboost rocket firing took it back up to 450 km.[5] A second reboost from 440 km to 515 km was done between April and June 1997.[6] After this it was hoped CGRO would operate until 2007.

De-orbit

After one of its 3 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 was preferable to letting the craft come down on its own at random.[2] Unlike the Hubble Space Telescope, it was not designed for on-orbit repair and refurbishment. 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 harmlessly into the Pacific Ocean.[7]

This de-orbit was NASA's first intentional controlled de-orbit of a satellite. [8]

See also

References

External links

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