Orbiting Astronomical Observatory

Orbiting Astronomical Observatory (OAO), any of a series of four unmanned U.S. scientific satellites developed to observe cosmic objects from above the Earth’s atmosphere. OAO-1 was launched on April 8, 1966, but its power supply failed shortly after liftoff. OAO-2, launched Dec. 7, 1968, carried two large telescopes and a complement of spectrometers and other auxiliary devices. It weighed more than 4,200 pounds (1,900 kg), the heaviest satellite orbited up to that time. OAO-2 was...