EXOSAT

(eks -oh-sat) The first European Space Agency mission in X-ray astronomy, a sophisticated 400-kg satellite launched in May 1983 carrying a payload of two grazing incidence telescopes, a large (1800 cm²) array of proportional counters, and a (90 cm²) gas scintillation proportional counter. The unusual orbit of EXOSAT, initially 380 km perigee, 195 000 km apogee, gave it the unique capability of long (˜80 hr) observations, uninterrupted by Earth occultation. With apogee over N Europe, direct contact with the ground station at Villafranca (Madrid) furthermore allowed EXOSAT to be operated directly by the astronomer, much like a ground-based telescope. Although problems occurred with two detectors early in the mission, rendering one telescope inoperable, the spacecraft systems, including on-board computer and 3-axis attitude control system, worked well until exhaustion of attitude control gas in Mar. 1986. Outstanding results were obtained on a wide range of supernova remnants, X-ray binaries, X-ray burst sources, active galaxies, and clusters of galaxies. The discovery of quasi-periodic oscillations (QPOs) uncovered an important property of accreting binary systems not known before EXOSAT. See also EXOSAT Database.