The Compton Gamma Ray Observatory
An Orbital Telescope Searching for Remote Very High Energy Sources
Aug 30, 2009
The Compton Gamma Ray Observatory (GRO) launched in 1991, and de-orbited in 2000 was the second of NASA's Great Orbital Observatories. It was a sophisticated satellite observatory dedicated to observing the high-energy Universe. It is the second in NASA's program of orbiting telescopes following the Hubble Space Telescope. Unlike Hubble's instruments, which operate at visible and ultraviolet wavelengths, Compton operates to observe gamma rays.
Compton Scattering
Gamma-ray telescopes use a process called Compton scattering, where a gamma-ray strikes an electron and loses energy, similar to a cue ball striking an eight ball. Unlike optical light and X-rays, gamma rays cannot be captured and be reflected in mirrors. These high-energy photons would pass right through such a device.
Compton Observatory Instruments
The GRO carried a collection of four instruments used to investigate gamma-rays. These instruments are the Burst And Transient Source Experiment (BATSE) instrument, the Oriented Scintillation Spectrometer Experiment (OSSE), the Imaging Compton Telescope (COMPTEL), and the Energetic Gamma Ray Experiment Telescope (EGRET). Compton was a very heavy telescope weighing in at 17 tons.
The Burst and Transient Source Experiment (BATSE) instrument was used for detecting and locating strong transient sources. There were eight detectors, located at each corner of the satellite.
The Oriented Scintillation Spectrometer Experiment (OSSE) consisted of four NaI scintillation detectors. These allowed observations of a gamma-ray source to be alternated with observations of nearby background regions.
The Imaging Compton Telescope (COMPTEL) utilized the Compton effect. This is where gamma rays are studied from active galaxies, radioactive supernova remnants, and diffuse gamma rays from giant molecular clouds.
The Energetic Gamma Ray Experiment Telescope (EGRET) provided the highest energy gamma-ray window for the Compton Observatory. It was 10 to 20 times larger and more sensitive than previous detectors s making detailed observations of high-energy processes associated with diffuse gamma-ray emission, gamma-ray bursts, cosmic rays, pulsars, and active galaxies known as blazars.
Gamma-rays are the most energetic form of light and are produced by the hottest regions of the universe. They can be produced by violent events like supernova explosions or the destruction of atoms; also less dramatic events, such as the decay of radioactive material in space can be a source.
They produce a lot of energy. Gamma-ray bursts can release more energy in 10 seconds than the Sun will emit in over 10 billion-years. However, all of the bursts that have been observed have come from outside the Milky Way Galaxy. Scientists believe that a gamma-ray burst in the Milky Way is very rare; they will occur once every few million years.
Gamma-ray astronomy is a recent development. It did not develop until it was possible to get detectors above the atmosphere, at first using balloons and later spacecraft. In fact, the first gamma-ray telescope was lifted into orbit on the Explorer XI satellite in 1961. It picked up fewer than 100 cosmic gamma-ray photons. The Compton Observatory was de-orbited because one of its gyroscopes failed. It was going to come down before NASA could plan a repair mission.
Sources
Compton Scattering
NASA http://imagine.gsfc.nasa.gov/docs/science/how_l2/compton_scatter.html
Compton Observatory Instruments
NASA, http://heasarc.gsfc.nasa.gov/docs/cgro/cgro/
What Are Gamma-rays
NASA, http://science.hq.nasa.gov/kids/imagers/ems/gamma.html
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