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 able to photograph young stars that emit mostly ultraviolet light. Astronomers had detected very few such stars with ground-based telescopes because ultraviolet radiation is absorbed by the Earth's atmosphere. OAO-2 remained in operation until January 1973. OAO-B failed to reach orbit after its launch on Nov. 30, 1970. Copernicus (OAO-3) was equipped with more powerful instruments, including a reflecting telescope with a 32-inch (81-cm) mirror. Launched Aug. 21, 1972, this satellite was primarily used to study ultraviolet emissions from interstellar gas and stars in the far reaches of the Milky Way. Copernicus also carried four X-ray detectors that discovered several pulsars. Copernicus continued observing until February 1981.