ORBITING ASTRONOMICAL OBSERVATORY

(OAO) any of a series of unmanned U.S. scientific satellites developed to observe cosmic objects from above the Earth's atmosphere. 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 kilograms), 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-3, also called Copernicus, was equipped with more powerful instruments, including a reflecting telescope with a 32-inch (81-centimetre) 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.