COS-B was the first European Space Research Organisation (ESRO) mission to study cosmic gamma ray sources.

COS-B was first put forward by the European scientific community in the mid-1960s and approved by the ESRO council in 1969.

The mission consisted of a satellite containing gamma-ray detectors, which was launched by NASA on behalf of the ESRO on 9 August 1975.

Scientific results included the 2CG Catalogue listing around 25 gamma ray sources and a map of the Milky Way.

COS-B was launched from Vandenberg Air Force Base on 9 August 1975 on a Delta 2913 rocket.