The Orbiting Astronomical Observatory 2, launched on December 7, 1968. An Atlas-Centaur rocket launched it into a nearly circular 750-kilometre altitude Earth orbit. Data was collected in ultraviolet on many sources including comets, planets, and galaxies. It had two major instrument sets facing in opposite directions, the Smithsonian Astrophysical Observatory and the Wisconsin Experiment Package.
One discovery was large halos of hydrogen gas around comets, and it also observed Nova Serpentis, which was a nova discovered in 1970.
Featured Image: An illustration of OAO 2, nicknamed Stargazer, in orbit around Earth / Image: via nasa.gov