KENNEDY SPACE CENTER, FLA. - In the Orbiter Processing Facility bay 1, the main engine is in place on Discovery. The main engine configuration is manufactured by Boeing Rocketdyne in Canoga Park, Calif., and includes a Pratt & Whitney high-pressure fuel turbo pump. Each space shuttle main engine is 14 feet (4.3 meters) long, weighs about 6,700 pounds (3,039 kilograms), and is 7.5 feet (2.3 meters) in diameter at the end of the nozzle. Discovery is being processed for the second return-to-flight mission STS-121. KSC-06pd0020

The Space Shuttle program was the United States government's manned launch vehicle program from 1981 to 2011, administered by NASA and officially beginning in 1972. The Space Shuttle system—composed of an orbiter launched with two reusable solid rocket boosters and a disposable external fuel tank— carried up to eight astronauts and up to 50,000 lb (23,000 kg) of payload into low Earth orbit (LEO). When its mission was complete, the orbiter would re-enter the Earth's atmosphere and lands as a glider. Although the concept had been explored since the late 1960s, the program formally commenced in 1972 and was the focus of NASA's manned operations after the final Apollo and Skylab flights in the mid-1970s. It started with the launch of the first shuttle Columbia on April 12, 1981, on STS-1. and finished with its last mission, STS-135 flown by Atlantis, in July 2011.