Artwork of the Compton Observatory satellite (CO) formerly called Gamma Ray Observatory (GRO). CO is one of NASA's four Great Observatories, the others are the Hubble Telescope, the AXAF X-ray observatory and the SIRTF infra-red observatory. CO carries four experiments: the Oriented Scintillation Spectrometer Experiment (OSSE) looking at low- energy gamma rays; the Imaging Compton Telescope (Comptel) for medium energy gamma rays; the Energetic Gamma Ray Experiment Telescope (Egret) for highest energies; and the Burst and Transient Source Experiment (BATSE), designed for the shortest- lived events. CO was launched by Shuttle Atlantis on April 9th 1991.