EXOSAT ( European X - Ray Observatory Satellite ) was the first X-ray observatory of the European Space Agency ESA. The main contractor was the German company Messerschmitt -Bolkow -Blohm (MBB ).

EXOSAT should be started first scientific satellite with the then- new European rocket Ariane 1. He then started, however, on 26 May 1983 with a Delta 3914 rocket from Vandenberg Space station in the United States in a highly eccentric orbit between 347 km and 191,709 km altitude and 72.5 ° inclination equator. The circulation time was 90.6 hours. EXOSAT was to failure of the position control on 9 April 1986 and became operational on May 6, again in 1986 into the Earth's atmosphere.

EXOSAT had three identically oriented instruments for the energy ranges 0.05-2.5 keV, 2-20 keV and 1-50 keV. Due to the special orbit uninterrupted observations of variable objects were long, possible. Coverings of X-ray sources by the Moon were used to locate these in more detail and to determine their structure better than the telescope resolution appropriately.

EXOSAT discovered quasi-periodic brightness variations in X-ray binaries and made ​​a total of 1,780 observations of X-ray sources, such as active galactic nuclei, the coronae of stars, white dwarfs, variable stars, clusters of galaxies and supernova remnants.