Compton Gamma Ray Observatory was an artificial satellite used to study gamma rays from space. Gamma rays occur in a wide variety of heavenly bodies. Examples of sources include the sun and other active stars; and quasars, the extremely bright cores of distant, energetic galaxies.
Gamma rays are the most energetic form of electromagnetic radiation, which consists of waves of electric and magnetic energy. Other forms include rays of visible light and radio waves. Astronomers use orbiting telescopes to study gamma rays from space because Earth’s atmosphere absorbs most such rays.
The National Aeronautics and Space Administration (NASA) launched the Compton Gamma Ray Observatory from the space shuttle Atlantis in 1991. The satellite was named for American physicist Arthur H. Compton, who shared the 1927 Nobel Prize for physics. Compton received the award for his work with X rays, the second most energetic form of electromagnetic radiation.
In June 2000, NASA deliberately destroyed the Compton Gamma Ray Observatory by directing it back to Earth. As the satellite descended, much of it burned in the atmosphere. The remaining parts splashed down in the Pacific Ocean. Officials at NASA had decided to end the observatory’s mission after one of its three gyroscopes failed. Engineers had determined that two gyroscopes were necessary to control the satellite safely. Officials at NASA became concerned that if another gyroscope failed, the satellite could go out of control and crash into a populated area.
Sources of gamma rays.
Gamma rays result mainly from nuclear reactions; collisions of high-energy subatomic particles (bits of matter smaller than an atom); and radioactive decay, a breaking apart of atomic nuclei. The sun can send gamma rays into space in a solar flare, a sudden release of high-energy particles and radiation. When a star explodes as a supernova, nuclei combine to form new chemical elements. Some types of these elements are unstable and therefore decay, producing gamma rays. Another source of gamma rays is rapidly spinning, collapsed stars called pulsars. A pulsar is a type of neutron star, a small, extremely dense object.
The region of space around a black hole also produces gamma rays. A black hole is an invisible collapsed object that exerts such powerful gravitational force that not even light can escape its surface. Gamma rays also arise from collisions in the gas in the spiral arms of the Milky Way, the galaxy that is the home of our solar system.
In addition, the Compton Gamma Ray Observatory discovered many sources of gamma rays that are still unidentified. Mysterious bursts of gamma rays that occur randomly throughout the sky appear to be the most distant and the most energetic explosions ever observed.
Gamma-ray telescopes.
The Compton Gamma Ray Observatory had four telescopes, each designed for particular scientific goals. Unlike telescopes built to detect visible light, gamma-ray telescopes do not use lenses or mirrors. Gamma rays are so energetic that they pass right through most materials; a lens or mirror cannot focus them.
A gamma-ray telescope uses a special detector. First, a gamma ray interacts with atoms inside the detector. The interaction can produce either a flash of visible light or an electric spark, depending on the type of detector. Sensors record the optical or electric signal resulting from the interaction. By analyzing the data gathered by the sensors, astronomers can calculate the energy of the original ray and the direction to its source in space.
Two of the observatory’s telescopes, the Energetic Gamma Ray Experiment (EGRET) and the Imaging Compton Telescope (COMPTEL), carried out the first mapping of the entire sky in gamma rays. The Oriented Scintillation Spectrometer Experiment (OSSE) measured the energies of gamma rays coming from individual sources. The Burst and Transient Source Experiment (BATSE) searched the sky continuously for sudden events, such as solar flares and gamma-ray bursts.
Other observatories.
The Compton Gamma Ray Observatory was the second of NASA’s four “Great Observatories” to go into orbit. The others are the Hubble Space Telescope, which was launched in 1990; the Chandra X-ray Observatory, launched in 1999; and the Spitzer Space Telescope Facility, launched in 2003.
See also Black hole; Gamma rays; Milky Way; Quasar; Supernova .
