NASA’s HEAO-3 Program launched on 20 September 1979 and operated until 29 May 1981. It carried three high-energy astrophysics instruments: a high-resolution gamma-ray spectrometer surveying hard X-rays and gamma rays, plus two experiments measuring cosmic-ray composition. The mission’s largest-in-orbit germanium spectrometer and major cosmic-ray detectors supported a sky survey of gamma-ray narrow-line emission and cosmic-ray studies.