The European Space Agency's X-ray observatory EXOSAT was operational between May 1983 and April 1986 and during this time made 1780 observations in the X-ray band of many classes of objects, including active galactic nuclei, white dwarfs, stars, supernova remnants, clusters of galaxies, cataclysmic variables and X-ray binaries.

A unique feature of EXOSAT was its highly eccentric orbit with an apogee of 190 000 km, a perigee of 350 km and an orbital period of 90 hours. Although this orbit subjected EXOSAT to somewhat higher backgrounds that were dependent on solar activity, it provided periods of up to several days of uninterrupted viewing of a source. Low earth orbit missions must deal with Earth occultations for <30 minutes every ~95 minutes, except for sources near the orbital poles.