The three High Energy Astronomical Observatories (HEAO)
launched by the National Aeronautics and Space Administration in
the late 1970s studied extraterrestrial X rays (see X-ray astronomy), cosmic rays, and gamma rays. The satellites were designed to help scientists to understand how extremely high energies are generated in space, how basic elements are formed, and how the universe evolved. They also increased scientific knowledge of the physical processes occurring in galactic nuclei, quasars, pulsars, neutron stars, supernovas and their remnants, and black holes.HEAO 1 was
launched on Aug. 12, 1977, into a nearly circular orbit of 440-km (270-mi) altitude. Its instruments mapped the
celestial sphere for X-ray sources; measured emissions and absorptions of diffuse cosmic X rays and correlated the results with radio and visible-light emissions; determined the precise celestial position of selected cosmic-ray sources and investigated their size and structure; and determined the spectra, time variations, intensity, and other properties of X rays and gamma rays. Mapping the entire celestial sphere, HEAO 1 increased the number of known celestial X-ray sources from 350 to nearly 1,500. It left orbit on Mar. 15, 1979.HEAO 2, also called the Einstein Observatory, was launched on Nov. 13, 1978, into a nearly circular orbit of 537-km (334-mi) altitude. It was a nontumbling, stable-orbiting base from which instruments could be pointed with great precision and steadiness. The X-ray telescope's resolution was 1,000 times greater than that of any X-ray instrument previously orbited. HEAO 2 determined that scattered discrete sources may account for much of the extragalactic X-ray background radiation and discovered a higher intensity of X-ray emission from the coronas of certain main-sequence stars than had been predicted. The satellite left orbit on Mar. 25, 1982.HEAO 3 was launched on Sept. 20, 1979, into a nearly circular orbit of 480-km (300-mi) altitude. Its three instruments were a gamma-ray spectrometer, a cosmic-ray isotope detector, and a cosmic-ray heavy-nuclei detector. Like HEAO 1, it was designed to scan the entire celestial sphere. It left orbit on Dec. 7, 1981.