A European gamma-ray satellite, launched in 1975 August and operated until 1982. It mapped the gamma-ray sky in the energy range 70–5000 MeV (2.5 × 10−7 to 2 × 10−5 nm) with a spark-chamber detector. It also detected several individual sources, including the Vela and Crab Pulsars and the quasar 3C 273.
http://sci.esa.int/cos-b/31447-summary/ ESA mission website.