COS -B (Cosmic Ray Satellite B ) was a scientific satellite of the European Space Agency ( ESA) to study cosmic gamma radiation.

COS -B was taken on August 9, 1975 a NASA Delta rocket from the launch site Vandenberg in a highly eccentric orbit with 37 hours turnaround time, and until 25 April 1982. Main instrument was a large, direction -resolving detector ( spark chamber ) for gamma rays between 30 MeV and 5 GeV, an additional detector for X-ray radiation was present. COS -B was a project of ESA with contributions in the research institutes for instrumentation.

From the observations of COS -B, the first map of the gamma radiation of the Milky Way, the 2CG catalog of cosmic gamma-ray sources 25, and studies emerged of individual objects, such as pulsars and the first extragalactic gamma source 3C ​​273