The last of NASA's three High Energy Astronomy Observatories, HEAO 3 was launched on 20 September 1979 on an Atlas-Centaur launch vehicle, into a nearly circular, 43.6-degree inclination low-Earth orbit with an initial perigee of 486.4 km. The normal operating mode was a continuous celestial scan, spinning approximately once every 20 min about the spacecraft's z-axis, which was nominally pointed at the Sun. The total mass of the observatory at launch was 2,660.0 kilograms (5,864.3 lb).