Series of US astronomical satellites. After initial failures, OAO 2 was launched on 7 December 1968 carrying a PAYLOAD of telescopes and ultra-violet detection equipment. Although designed for one year of operation it continued to function until 13 February 1973. Its achievements include the detection of a huge hydrogen cloud round comet Tago-Sato-Kosaka, and the first ultra-violet observation made from above the atmosphere of a SUPERNOVA.
The last of the series, OAO 3 (actually the fourth OAO, but the second successful one) was launched on 21 August 1972, and named Copernicus in honour of the Polish astronomer, the 500th anniversary of whose birth was in 1973. It carried ultra-violet and X-ray detectors, and its achievements include the detection of molecular hydrogen and deuterium in interstellar clouds, and the increasing period of the binary X-ray source, Cygnus X-3. The pointing accuracy of the instrumentation turned out to be 0.03 second of arc, three times better than specification.
