A series of three NASA X-ray and gamma-ray astronomy satellites. HEAO-1, launched in 1977 August, surveyed the sky in the X-ray energy range 0.2–60 keV (0.02–6.2 nm) for discrete sources and studied the X-ray background. The satellite also studied low-energy gamma rays up to about 10 MeV. It ceased operation in 1979 January. HEAO-2, also known as the Einstein Observatory, was launched in 1978 November and observed in the 0.1–4 keV (0.3–12.4 nm) range. It ceased operation in 1981 April. HEAO-3, launched in 1979 September, was devoted to gamma-ray astronomy. It ceased operation in 1981 May.

http://heasarc.gsfc.nasa.gov/docs/heao1/heao1.html Information page on HEAO 1 at Goddard Space Flight Center.

http://heasarc.gsfc.nasa.gov/docs/einstein/heao2.html Information page on HEAO 2 at Goddard Space Flight Center.

http://heasarc.gsfc.nasa.gov/docs/heao3/heao3.html Information page on HEAO 3 at Goddard Space Flight Center.