A series of US astronomical satellites launched between 1966 and 1972. OAO-1, launched on 8 April 1966, developed faults the day after and was a write-off. OAO-2 was launched on 7 December 1968. It carried instruments for photometry and sky survey work in the ultraviolet, and continued to be operational until February 1973. The next launch, in 1970, was a total failure. The final satellite in the series, OAO-3, was renamed Copernicus after launch on 21 August 1972. Its main purpose was ultraviolet astronomy, for which it carried a 0.8-metre 31-inch telescope complete with a spectrograph. It also carried a British X-ray astronomy instrument with which important observations were made.