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Orbiting Astronomical Observatory (OAO)

The OAO satellites were a series of four American space observatories launched by NASA between 1966 and 1972, which provided the first high-quality observations of many objects in ultraviolet light. Although two OAO missions were failures, the success of the other two increased awareness within the astronomical community of the benefits of space-based observations, and led to the instigation of the Hubble Space Telescope.

OAO-3 (Copernicus) was launched on 21 August 1972, and proved to be the most successful of the OAO missions. It was a collaborative effort between NASA and the UK's Science Research Council (later known as the Science and Engineering Research Council), and carried an X-ray detector built by University College London's Mullard Space Science Laboratory in addition to an 80 cm UV telescope built by Princeton University. After its launch, it was named Copernicus to mark the 500th anniversary of the birth of Nicolaus Copernicus.

Copernicus operated until February 1981, and returned high resolution spectra of hundreds of stars along with extensive X-ray observations. Among the significant discoveries made by Copernicus were the discovery of several long-period pulsars, with rotation times of many minutes instead of the more typical second or less.