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: The Orbiting Astronomical Observatory 2 (OAO-2, Stargazer) / Image: NASA