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Abbrev. for High Energy Astrophysical Observatory. Any of three large (about 3000 kg) Earth satellites built by NASA for second-generation studies of X-ray sources, gamma-ray sources, and cosmic rays.

HEAO-A, or HEAO-l after launch, was put into a 400-km, 22° inclination orbit in Aug. 1977 and carried a payload of large X-ray detectors with which to conduct a more sensitive sky survey than that of Uhuru and Ariel V. HEAO-l completed two-and-a-half surveys of the sky and several extended pointings at individual sources before reentry early in 1979. The eventual outcome was a catalog of 842 X-ray sources, brighter than about 1 microjansky, together with broad-band spectra of many active galaxies, clusters, and galactic sources.

HEAO-2, the Einstein Observatory, operated from launch in Nov. 1978 until loss of control gas in Apr. 1981. The unique capabilities of its 60-cm grazing incidence telescope revolutionized X-ray astronomy in the same way that Uhuru had done a decade earlier.

HEAO-3 was launched in Sept. 1979. It carried a number of large cosmic-ray instruments to study the mass and charge distribution of primary cosmic rays, particularly the nuclei of the heavier elements, over a wide range of energies. The gamma-ray spectroscopy experiment was designed specifically to search for cosmic sources of narrow-line emission in the energy range 50 keV to 10 MeV. It consisted of a cluster of germanium solid-state detectors surrounded by an active cesium iodide scintillator shield. Apart from the study of gamma-ray line emission from solar flares, 511 keV annihilation radiation from the galactic center region, and 1.809 MeV line emission from radioactive 26Al (see gamma-ray astronomy), the spectrometer detected two strong gamma-ray line emissions from the extraordinary galactic object SS433. The various experiments on board HEAO-3 ceased operations during the years 1980-82.