Compton Gamma Ray Observatory

Second of NASA’s remarkable Observatories, The Compton Gamma Ray Observatory was launched from the Space Shuttle Atlantis on 1991 April 5. The other three observatories in this special scientific spacecraft family are the Chandra X-Ray Observatory (CXO), the Hubble Space Telescope (HST), and the Spitzer Space Telescope (SST). The observatory was named in honor of Arthur Holly Compton (1892–1962), who was awarded the Nobel Prize for physics for work on gamma-ray detection techniques.

The CGRO was deployed successfully into low-Earth orbit (LEO) by the crew of space shuttle Atlantis on April 7, 1991, during shuttle mission STS-37. The observatory was then boosted to a higher circular orbit where it could accomplish its scientific mission. This large, 16,300-kg spacecraft carried a variety of sensitive instruments designed to detect gamma rays over an extensive range of energies from about 30 kilo electron volts (keV) to 30 billion electron volts (GeV). The CGRO was an extremely powerful tool for investigating some of the most puzzling astrophysical mysteries in the universe, including energetic gamma-ray bursts pulsars, quasars, and active galaxies.

Its four instruments provided unprecedented coverage of the electromagnetic spectrum, from 30 keV to 30 GeV. The Burst and Transient Source Experiment (BATSE) was designed to detect short outbursts. The Oriented Scintillation Spectrometer Experiment (OSSE) studied the spectrum of gamma-ray sources. The Imaging Compton Telescope (COMPTEL) mapped the gamma ray sky at medium energies, and the Energetic Gamma Ray Experiment Telescope (EGRET) made an all-sky map of high-energy sources.

The four CGRO instruments were much larger and more sensitive than any gamma-ray telescopes previously flown in space. The large size was necessary because the number of Gamma-ray interactions that can be recorded are directly related to the mass of the detector. Since the quantity of gamma-ray photons from celestial sources is very small compared to the number of optical photons, astrophysicists must use large instruments to detect a significant number of gamma rays in a reasonable amount of time.

The observatory re-entered the Earth’s atmosphere on 2000 June 4. During its lifetime, the telescope detected more than 400 gamma-ray sources, ten times more than were previously known. The origins of many of these sources remain unknown. Prior to Compton, 300 gamma-ray bursts had been detected; observations from the satellite recorded a further 2500. One of BATSE’s most important contributions was an all-sky map of gamma-ray burst positions, which confirmed that they originate well beyond our Galaxy. One of the major discoveries made by EGRET was a new class of quasars known as Blazars. The observatory also discovered a number of gamma-ray Pulsars, while observations of the galactic centre by OSSE revealed gamma radiation from the annihilation of positrons and electrons in the interstellar medium.

Questions to Ponder

• How do gamma ray observatories work?
• Is the CRGO operating now?
• Why was the CGRO recalled from orbit?