From the December 2002 issue

A history of gamma-ray satellites

Browse a list of key satellites from the brief history of orbiting gamma-ray observatories.
By | Published: December 6, 2002
Long before they could detect gamma rays, scientists suspected this type of radiation was produced throughout the universe. To identify these emissions, Earth-bound detectors would have to be placed outside our atmosphere, which typically absorbs gamma rays. In 1961, the Explorer XI satellite carried the first gamma-ray telescope into Earth orbit, which gathered fewer than 100 cosmic gamma-ray photons.

While astronomers use Earth-based observatories to identify gamma ray bursts, they still utilize orbiting telescopes for studying gamma rays. These satellites have made significant contributions to gamma-ray astronomy in their five-decade history.

Listed below are gamma-ray satellites that have helped us develop a better understanding of these high-energy photons.

Explorer XI
Explorer XI
Explorer XI
Launch Year: 1961
Agency: NASA

Carrying a telescope built by Massachusetts Institute of Technology (MIT) scientists William L. Kraushaar and George W. Clark, Explorer XI was the first satellite dedicated to gamma-ray astronomy. Placed in an eccentric orbit around Earth, the satellite searched for the signs of cosmic-ray interaction with interstellar material. Because Explorer could not be actively directed, the spacecraft rotated end over end, providing a rough scan of the celestial sphere. The satellite detected 22 cosmic gamma-ray events before its power source deteriorated.

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ESA’s first gamma-ray satellite was COS-B. Launched during 1975, it put European astronomers at the forefront of the field.
Launch Year: 1975
Agency: European Space Agency (ESA)

Launched from California, Cos-B was ESA’s first satellite dedicated to a single experiment – in this case, detecting gamma rays. The Caravane Collaboration, a collection of research laboratories throughout Western Europe, directed the project. Over six years, Cos-B provided important gamma-ray data, most significantly the first complete map of the Milky Way in gamma rays.

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Russian Space Agency
Launch Year: 1989
Agency: Russian Space Agency

Granat was used to study both interstellar x rays and gamma rays. The instruments on board were developed by Russia and France. For the first few years of its nine years of service, the satellite made targeted observations. In 1994, Granat began operating in survey mode because its fuel supply was exhausted so the spacecraft could no longer aim. Besides its cosmic ray detections, a highlight of the mission is its deep imaging of the galactic center.

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Compton Gamma Ray Observatory
Astronauts deployed the Compton Gamma Ray Observatory from the cargo bay of Space Shuttle Atlantis on April 5, 1991. The spacecraft, with its solar panels unfurled, is shown here over western Africa just before its release.
Compton Gamma Ray Observatory (CGRO)
Launch Year: 1991
Agency: NASA

Compared to Explorer XI’s weight of roughly 82 pounds, CGRO was a behemoth at 17 tons, making it the largest astrophysical payload of its time. Gamma-ray instruments comprised almost half of the satellite’s bulk. By using these larger telescopes, CGRO could gather more gamma-ray photons than previous satellites. The instruments studied photon energies ranging from 20,000 electron volts (20 keV) to more than 30 billion electron volts (30 GeV). CGRO’s productive career lasted until 2000, when NASA directed the satellite into Earth’s atmosphere. Before its death, CGRO showed us that our universe is a violent and rapidly changing place with its detection of gamma rays, identification of gamma-ray bursts, and the discovery of a new class of quasars. A number of European countries in association with the ESA contributed to this mission’s success.

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Institute for Space and Astronautical Sciences
Launch Year: 1991
Agency: Japan’s Institute for Space and Astronautical Sciences (ISAS)

Japanese for “sunbeam,” Yohkoh studied x rays and gamma rays produced by the sun. A gamma-ray detector from NASA monitored gamma-ray emissions. The satellite functioned reliably until December 14, 2001. During the annular eclipse that day, Yohkoh fell into the shadow of the eclipsed sun, causing the spacecraft to discharge its batteries and lose its ability to point toward the sun.

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HETE-2 was launched in October 2000. It replaces the original High Energy Transient Explorer (HETE) satellite, which failed to be released from its launch vehicle in 1996.
High Energy Transient Explorer 2 (HETE-2)
Launch Year: 2000
Agency: NASA and MIT Center for Space Research

Three years after the malfunction of HETE’s launch vehicle, astronomers in France, Italy, Japan, and the United States collaborated for a second attempt. So far so good with this productive observatory, designed to detect and localize gamma-ray bursts. In October, HETE detected a gamma-ray burst that lasted for 100 seconds. This discovery led to the most detailed observations of a gamma-ray burst.

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With a suite of four instruments, Integral will provide new insights into the most violent and exotic cosmic objects, such as black holes, neutron stars, active galactic nuclei, supernovae, and mysterious gamma-ray bursts (the most energetic phenomena in the universe).
ESA / D. Ducros
International Gamma Ray Astrophysics Laboratory (Integral)
Launch Year: 2002
Agency: ESA

Integral’s telescope possesses two imagers that can scan 20,000 to 10 million electrons, making it the most sensitive gamma-ray observatory ever launched. Astronomers expect Integral will be key in not only gamma-ray detections, but also in providing clues about the explosions from which these high-energy photons originated.

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NASA's Swift Gamma-ray Explorer
The Swift gamma-ray observatory is an international endeavor led by NASA. The craft will be launched in 2003.
D. Armbrecht (Spectrum Astro) / NASA
Launch Year: Planned for 2003
Agency: NASA

Swift will search for and investigate gamma-ray bursts while focusing on the afterglow of these explosions.

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