COS-B was the first European Space Research Organisation 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 August 9, 1975. The mission was completed on April 25, 1982, after the satellite had been operational for more than 6.5 years, four years longer than planned and had increased the amount of data on gamma rays by a factor of 25. Scientific results included the 2CG Catalogue listing around 25 gamma ray sources and a map of the Milky Way. The satellite also observed the X-ray binary Cygnus X-3.