The Space Shuttle orbiter is the orbital spacecraft of the Space Shuttle program operated by NASA, the space agency of the United States. The orbiter is a reusable winged "space-plane", a mixture of rockets, spacecraft, and aircraft. This space-plane can carry crews and payloads into Earth orbit, perform on-orbit operations, then re-enter the atmosphere and land as a glider, returning her crew and any on-board payload to the Earth.
A total of six Orbiters were built for flight: Atlantis, Challenger, Columbia, Discovery, Endeavour and Enterprise. All were built by the southern California based Rockwell International company. The first Orbiter to fly, Enterprise, took her maiden flight in 1977. Built solely for unpowered atmospheric test flights and landings, her take-off was from the back of a modified Boeing-747 cargo plane, the Shuttle Carrier Aircraft, while the remaining Orbiters were built for orbital space flights, launched vertically as part of the full Space Shuttle package.
Columbia was the first Orbiter to launch into space as a Space Shuttle, in 1981. The first launches of Challenger, Discovery, and finally Atlantis, followed in 1983, 1984 and 1985 respectively. In 1986, Challenger was destroyed in an accident after launch. Endeavour was built as Challenger's replacement, and was first launched in 1992. In 2003, Columbia was destroyed during re-entry, leaving just three remaining Orbiters. Two were to be used for the last time in flights during 2010, Atlantis in May, Discovery in November. Endeavour is scheduled to make its final flight in January 2011.
In addition to their crews and payloads, the reusable Space Shuttle Orbiter carries most of the Space Shuttle System's liquid-fueled rocket propulsion system, but both the liquid hydrogen fuel and the liquid oxygen oxidizer for her three main rocket engines is fed from an external cryogenic propellant tank, and there are also two reusable large solid-fueled rocket boosters that help to lift both the Orbiter and her external propellant tanks during approximately the first two minutes of her ascent into outer space.