High Energy Astrophysical Observatory

Name of three NASA orbiting observatories launched between 1977 and 1979, two of which were for X-ray astronomy and the third for gamma-ray astronomy. HEAO-1, launched in September 1977, conducted an all-sky survey for X-ray sources which resulted in the most complete catalogue up to that time. HEAO-2 became known as the Einstein Observatory. HEAO-3 was launched in September 1979 and carried a gamma-ray spectrometer and two cosmic ray instruments.