NASA / SCIENCE PHOTO LIBRARY NASA / SCIENCE PHOTO LIBRARY
Thrusters of Shuttle Discovery's Reaction Control Subsystem (RCS) firing during orbit. This photograph was taken during Mission STS-39 of 28 April to 6 May 1991. The RCS is used to provide altitude control and fine tuning of the orbit; the system comprises a total of 44 thrusters across the Shuttle. In Discovery's cargo bay are two payloads carried for the US Department of Defense: in the foreground is the top of the STP-1 payload; in the background is the AFP-675 experiment package. STS-39 was the first unclassified military-sponsored flight of the Space Shuttle.
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