(OAO), series of four orbiting observatories (see observatory, orbiting) launched between 1966 and 1972 by the National Aeronautics and Space Administration (NASA) to provide astronomical data in the ultraviolet and X-ray wavelengths filtered out by the earth's atmosphere. Only two of the four launches were successful, however. OAO-2, launched in 1978, which carried two large telescopes in addition to a cluster of four ultraviolet telescopes, four photoelectric telescopes, and a number of spectrometers and other devices, weighed 4,200 lb (1,900 kg) and was the heaviest satellite orbited to that time. Working primarily with ultraviolet radiation, it was able to photograph very young stars. OAO-3, which was also called Copernicus, was launched in 1972. Carrying what was at that time the largest telescope ever orbited and three smaller X-ray telescopes, OAO-3 was primarily used in the study of ultraviolet radiation from interstellar gas and dust and from stars near the edges of the Milky Way. See also ultraviolet astronomy.

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