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Launch date: 26-May-1983 15:18 UT
Mission end: 09-April-1986
Launch vehicle: Thor-Delta rocket
Launch mass: 500 kg
Mission phase: Archive
Orbit: 90 hour highly-eccentric Earth orbit
Initial apogee: 191 000 km
Perigee: 350 km
Achievements: Discovery of the quasi period oscillations in LMXRB and X-ray pulsars Measuring iron lines in galactic and extra galactic sources.
Obtaining low-energy high-resolution spectra

The European Space Agency's X-ray Observatory, EXOSAT, was operational from May 1983 to April 1986. During that time, EXOSAT made 1780 observations of a wide variety of objects, including active galactic nuclei, stellar coronae, cataclysmic variables, white dwarfs, X-ray binaries, clusters of galaxies, and supernova remnants.

Mission Objectives

The original objective of the EXOSAT mission was to use lunar occultation to obtain precise positional information for the relatively small number of X-ray sources then known. The mission made a number of significant scientific advances, including:

  • Discovery of the quasi period oscillations in LMXRB and X-ray pulsars
  • Comprehensive study of AGN variability
  • Observing LMXRB and CV over many orbital periods
  • Measuring iron lines in galactic and extra galactic sources
  • Obtaining low-energy high-resolution spectra

Mission Name

EXOSAT = European X-ray Observing Satellite


EXOSAT was the European Space Agency's first three-axis stabilised spacecraft, with all three instrument packages co-aligned.


The three instrument packages gave coverage between 0.05 and 50 keV. The LE (Low-energy imaging telescopes) had the highest sensitivity.

Name   Type Range Area delta E /E
LE 2 Telescopes + CMA or PSD 0.05-2 keV 10 cm² 1 <0.25 keV
ME Medium Energy Proporsional Counter 1-20/5-50 keV 800-1600 cm² 21% (6 keV)
GSPC Gas Scintillation Proportional Counter 2-16/2-32 keV 100 cm² 10% (6 keV)


EXOSAT was placed in a 90 hour highly-eccentric Earth orbit, quite different from that of any previous X-ray astronomy satellite. The initial apogee was 191 000 km and the perigee 350 km. The science instruments were operated above 50 000 km, outside the earth's radiation belts. This allowed scientific operations for up to 76 hr per 90 hr orbit, without interruption.

Operations Centre

Ground station: Villafranca. Visible 76 hours out of 91 hour orbit.
Scientific operations: European Space Operations Centre, Darmstadt.

Last Update: 1 September 2019
11-Dec-2023 02:30 UT

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