Any of three NASA satellites that observed the X-ray and gamma-ray sky. HEAO-1 was launched in August 1978 and functioned until January 1979 to provide almost constant monitoring of X-ray sources near the ecliptic poles (points on the celestial sphere along an imaginary line running vertically through the Earth at right angles to the ecliptic). HEAO-2, renamed the Einstein Observatory, was launched in November 1978 and ended its mission in April 1981. HEAO-3 was launched in September 1979; the satellite continued on its course until May 1981, although the mission was rendered defunct when the cryogen freezing mixture in the satellite's detectors was depleted.