OSO 3

The Orbiting Solar Observatory 3 (OSO 3), launched on March 8, 1967, was placed into a nearly circular orbit at an altitude of 550 km with a 33° inclination to the equator. The satellite's tape recorder failed on June 28, 1968, limiting data collection to real-time acquisition during station passes thereafter. Its final data transmission occurred on November 10, 1969, and it reentered Earth's atmosphere, disintegrating on April 4, 1982.

OSO 3 featured two main components: the Sail and the Wheel. The Sail, solar-facing, housed solar panels and experiments for solar physics, while the Wheel rotated every two seconds to maintain stability and carried sky-scanning instruments.

Instrumentation included a UCSD hard X-ray experiment with a NaI(Tl) crystal, operating from 7.7 to 210 keV under Laurence E. Peterson's leadership. The Wheel also had an MIT cosmic gamma-ray instrument led by William L. Kraushaar, detecting gamma rays above 50 MeV.

Key scientific achievements included extensive hard X-ray observations of solar flares, the cosmic diffuse X-ray background, and Scorpius X-1, marking the first extrasolar X-ray source detected by a satellite observatory. The MIT instrument identified high-energy cosmic gamma rays from both galactic and extragalactic sources.