English: The Third Orbiting Solar Observatory, OSO-3, was launched on 8 March 1967, into a nearly circular orbit of mean altitude 550 km, inclined at 33 degrees with respect to the equatorial plane. The satellite had two principle components, a continuously spinning wheel in which the hard X-ray experiment is mounted with a radial view, and a sail component which was served to acquire the sun during the orbit day. The attitude control system maintained the scan plane orientation to within a few degrees of the sun. The spin-period of the satellite of roughly 1.7 s allowed about 1 cm2/s exposure per revolution to sources on the scan plane. Celestial sources near the ecliptic plane such as Sco X-1 transited the instrument scan plane twice a year. Only real-time data were received from the satellite after the last tape recorder failure on 27 June 1968. The last data transmission occurred 10 November 1969. OSO-3 descended into the atmosphere on 4 April 1982.