The EXOSAT mission was approved in 1973, when the intention was to use lunar occultation to obtain precise positional information for the relatively small number of X-ray sources then known.

In fact, the European Space Agency's X-ray observatory EXOSAT was operational between May 1983 and April 1986. Its goal was to make 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.