The High Energy Astronomy Observatory Program was a NASA program of the late 1970s and early 1980s that included a series of three large low-Earth-orbiting spacecraft for X-ray and Gamma-Ray astronomy and Cosmic-Ray investigations. After launch, they were denoted HEAO 1, HEAO 2 (also known as The Einstein Observatory), and HEAO 3, respectively. The large (~3000 kg) satellites were 3-axis stabilized to arc-minute accuracy, with fixed solar panels. All three observatories were launched from Cape Canaveral, Florida on Atlas-Centaur SLV-3D launch vehicles into near-circular orbits with initial altitudes slightly above 500 km.