The study of gamma rays from space. Much of the radiation detected comes from collisions between hydrogen gas and cosmic rays in our galaxy. Some sources have been identified, including the Crab nebula and the Vela pulsar (the most powerful gamma-ray source detected).
Gamma rays are difficult to detect and are generally studied by use of balloon-borne detectors and artificial satellites. The first gamma-ray satellites were SAS II (1972) and COS B (1975), although gamma-ray detectors were carried on the Apollo 15 and 16 missions. SAS II failed after only a few months, but COS B continued working until 1982, carrying out a complete survey of the galactic disc.
The US Gamma Ray Observatory was launched by US space shuttle Atlantis in April 1991 to study the gamma-ray sky for five years. The observatory cost $617 million and at 17 tons was the heaviest payload ever carried by space shuttle.