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he Compton Gamma Ray Observatory (CGRO) is the second of the great observatory series of four spacecraft NASA plans to launch. Launched in 1991, the CGRO is a complex spacecraft fitted with four different gamma-ray detectors, each of which concentrates on different but overlapping energy ranges. The instruments are the largest of their kind that have ever flown in space; each instrument weighs about 6 tons, and three of them are about the size of a subcompact car. Size is important because gamma rays can only be detected when they interact with matter. The bigger the masses of the detectors, the greater the number of gamma rays they can detect.

Outer space is filled with electromagnetic radiation that tells the story of the birth and death of stars and galaxies. A small portion of that radiation is visible to our eyes. The rest can be detected only with special instruments. In a chart of the electromagnetic spectrum, gamma rays fall at the far right end after visible light, ultraviolet light, and x rays. Gamma rays have very short wavelengths and are extremely energetic, but most of them do not penetrate Earth’s atmosphere. The only way for astronomers to view these waves is to send instruments into space.

The process for gamma-ray detection is similar to the way fluorescent paints convert ultraviolet light to visible light. When gamma rays interact with crystals, liquids, and other materials, they produce flashes of light that are recorded by electronic sensors. Astronomers can determine how energetic a particular ray is from the intensity of the flash—the brighter the flash of light from the interaction, the higher the energy of the ray.

The CGRO helps astronomers learn about the most powerful celestial bodies and events in the universe. It observes momentous gamma-ray bursts, such as those near the large Magellanic Cloud, which radiate more gamma rays in 0.2 second than our Sun does in 1,000 years. The CGRO gathers data to test theories on supernovae and the structure and dynamics of galaxies. The data collected on pulsars will allow scientists to explain how pulsars can produce more energy over their lifetime than the explosion it took to create them. The CGRO also monitors quasars, the luminous bodies with unusually high-energy outputs commonly found in the center of galaxies. In addition, the observatory views very high-temperature emissions data from black holes, which will reveal information on the origin of the universe and matter distribution.

The four different kinds of gamma ray detectors on the CGRO are the Burst and Transient Source Experiment (BATSE), the Oriented Scintillation Spectrometer Experiment (OSSE), the Imaging Compton Telescope (COMPTEL), and the Energetic Gamma Ray Experiment Telescope (EGRET). The following are brief descriptions of these detectors:


 * BATSE consists of eight detectors, placed on the corners of the spacecraft, which monitor as much of the sky as possible for gamma ray bursts, because gamma-ray bursts are brief, random events. These bursts are in the lower energy range of gamma rays. However, because BATSE is the instrument with the widest view range when it detects higher range gamma rays, it signals the other instruments.
 * OSSE uses four very precise crystal detectors primarily for plotting radioactive emissions from supernovae, pulsars, and novae. This experiment provides such information as temperature, particle velocities, and magnetic field strength.
 * COMPTEL studies gamma rays with a higher energy range than OSSE. COMPTEL is a liquid detector that acts like a camera. Gamma rays enter through an initial detector, which is similar to a lens, and then pass through a second detector, which acts like film. In this way, COMPTEL reconstructs wide-field-view images of the sky. COMPTEL observes point sources, such as neutron stars, galaxies, and other diffuse emissions.
 * EGRET detects the highest energy gamma rays, which are associated with the most energetic processes that occur in nature. EGRET was designed to collect data on quasars, black holes, stellar and galactic explosions, matter and antimatter annihilation, and high-energy portions of gamma-ray bursts and solar flares. The highly sensitive instruments of EGRET can observe fainter sources than previously possible and with greater accuracy.