KENNEDY SPACE CENTER, FLA. - In NASA Kennedy Space Center's Orbiter Processing Facility bay 3, United Space Alliance shuttle technicians remove the hard cover from a window on Space Shuttle Discovery to enable STS-121 crew members to inspect the window from the cockpit. Launch of Space Shuttle Discovery on mission STS-121, the second return-to-flight mission, is scheduled no earlier than May. KSC-06pd0316

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.