Compton Gamma Ray Observatory

I

Introduction

Compton Gamma Ray Observatory (Compton GRO), United States astronomical satellite that detected and studied gamma rays, a type of electromagnetic radiation with wavelengths shorter than those of visible light and X rays. Earth’s atmosphere blocks most gamma rays from telescopes on the ground, so scientists use satellites such as GRO for gamma-ray astronomy. Gamma rays originate in some of the universe's most violent events. Astronomers who study these events find gamma rays useful because gamma rays can pass through the dust between stars (see Interstellar Matter). This dust blocks other forms of radiation created by energetic events. So, by using a gamma-ray telescope, astronomers can study energetic processes that would otherwise be hidden, such as activity in the heart of the Milky Way Galaxy. GRO's four scientific instruments detected the entire range of gamma-ray emissions with greater precision than previously possible. The National Aeronautics and Space Administration (NASA) launched Compton GRO in 1991; the satellite’s mission ended in 2000.

II

Compton GRO Spacecraft

Compton GRO weighed almost 16 metric tons—as much as a fully loaded semi-truck trailer—when the space shuttle Atlantis deployed it on April 7, 1991. The spacecraft was the heaviest object that a shuttle had ever launched. GRO’s scientific instruments made up more than 5 metric tons of the satellite’s total mass. The rectangular satellite measured 4.5 m (15 ft) high and 9 m (30 ft) long—as large as a school bus. Two solar panels (see Solar Energy), which generated power for GRO’s scientific instruments and communication systems, extended from the satellite’s sides like wings. The solar panels made the satellite’s width 21 m (69 ft).

A

Spacecraft Systems

GRO returned data to scientists on Earth through its high-gain antenna, which extended from the top front of the spacecraft. The antenna sent a signal from the observatory to one of the communications satellites that are part of NASA’s Tracking and Data Relay Satellite System (TDRSS). The TDRSS satellite then relayed the data to a station on the ground. When GRO’s mission began, its scientific instruments fed data to two tape recorders aboard the observatory. When the tape recorders had collected about three hours worth of data, the recorders played back the data at high speed, sending the signals to a TDRSS satellite. GRO’s tape recorders began to fail in 1992, so the observatory’s scientific instruments were forced to send data directly to the TDRSS system. To decrease the time that the observatory was out of touch with Earth during each orbit, NASA launched two additional TDRSS satellites, in 1993 and 1995.

The GRO had its own propulsion system. This set of small thruster rockets allowed the observatory to boost itself into a higher orbit if required. The satellite carried four fuel tanks for the thrusters. Three gyroscopes aboard GRO enabled engineers on Earth to guide and navigate the satellite.

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B

Scientific Instruments

The instruments that the Compton Gamma-Ray Observatory carried measured a wide range of wavelengths within the gamma-ray spectrum. Gamma rays are electromagnetic waves with wavelengths smaller than 1 picometer (pm; one picometer equals 1 trillionth of a meter or 4 billionths of an inch). The Imaging Compton Telescope (COMPTEL) instrument measured the energy and direction of incoming mid-wavelength gamma rays, from about 0.00041 to about 0.0016 pm (about 1.6 x 10^-12 to about 6.24 x 10^-12 in). Scientists in the United States, the former West Germany, and The Netherlands designed and developed COMPTEL. The drum-shaped instrument had two arrays of detectors, one above the other. The double array enabled the car-sized telescope to determine both the energy and the direction of incoming gamma rays. The detector, mounted at the center of the top of the satellite, could see an area of sky about 57° wide by about 57° tall at any one time.

The Burst and Transient Source Experiment (BATSE) monitored the sky for short, intense bursts of gamma rays. Designed by scientists in the United States, the BATSE consisted of eight identical modules mounted on the spacecraft's corners. The area of sky that each BATSE detector covered overlapped with that of other BATSE detectors. As a result, gamma rays from the same source could hit more than one detector, which helped pinpoint the location of the source. Each BATSE detector contained two scintillation counters (see Particle Detectors: Scintillation Counter), particle detectors that produce a flash of light when a gamma ray hits the detector. Other devices in the instrument detected and analyzed the light that the scintillation counters produced. The detectors were sensitive to wavelengths from the short-wavelength X-ray range through all of the gamma-ray range.

The Oriented Scintillation Spectrometer Experiment (OSSE) determined how much gamma-ray radiation came from a source and how much came from background radiation in the sky around the source. OSSE consisted of four identical instruments in a car-sized triangular box mounted at one end of the satellite’s topside. Each instrument had an independent pointing system, but the detectors usually acted in pairs. In each pair of detectors, one focused on the gamma-ray source while the other took readings on the level of gamma-ray radiation present in the sky around the source—known as background radiation. Having these two readings allowed astronomers to subtract the background radiation level from the total radiation amount, thereby obtaining precise measurements of the radiation from the source. The OSSE often used data from the BATSE detectors to decide where to point its instruments. Like the BATSE detectors, the OSSE detectors used scintillation counters. The wavelength sensitivity of the OSSE detectors was much like that of the BATSE instruments, but the OSSE detectors could provide more information about a source than the BATSE detectors could.

The drum-shaped Energetic Gamma Ray Experiment Telescope (EGRET) was on one end of GRO’s topside. EGRET was a large spark chamber, a particle detector in which a gamma ray ionizes gas to produce an electrical signal. EGRET focused on short-wavelength gamma rays, from about 0.0001 to about 0.04 pm (about 4 x 10^-15 to about 2 x 10^-12 in). EGRET collected enough data to compose a map of the high-energy gamma-ray universe. The telescope could pinpoint a source’s location five times more accurately than any previous gamma-ray telescope.