A NASA satellite that observed solar activity at wavelengths from gamma rays to white light. SMM, also known as Solar Max, was launched in 1980 February. A failure of its pointing system after nine months led to a repair mission in 1984 April by Space Shuttle astronauts. SMM continued operations until it re-entered the Earth’s atmosphere in 1989 December.

http://solarscience.msfc.nasa.gov/SMM.shtml NASA page on SMM and its findings.