NASA's Space Shuttle is an operational, reusable and human-rated spacecraft. Designed to take off like a rocket and maneuver in space like a spaceship, it can return to Earth and land on a runway like an airplane. The idea of a winged spacecraft that could make airplane-like landings goes back to the early 1960s. The North American X-15 program, the McDonnell ASSET re-entry vehicle and the Boeing X-20 Dyna-Soar project, explored the concept of a reusable spacecraft. But, it wasn't until after Project Apollo ended that NASA began development of a spacecraft that would serve as an Earth-to-orbit space truck. This program evolved into the Space Transportation System, or STS, known today as the Space Shuttle. In 1972, NASA selected North American Rockwell as the prime contractor to build the shuttle. North American not only had experience in building Apollo spacecraft, but it also built the X-15 rocket plane, the first winged aircraft to fly to space and land on a runway. Boeing and McDonnell Douglas were key program partners. The STS consists of a delta-winged orbiter, a huge detachable fuel tank and two detachable solid rocket boosters. The shuttle orbiter and the solid rocket boosters are reusable. The external fuel tank is expended during each launch. The shuttle orbiter is the only part that actually goes into space. The first orbiter, the Enterprise, was used for flight tests in the atmosphere. It was followed by Columbia, which made the first space flight in April 1981. The orbiters Challenger, Discovery, and Atlantis followed. A fifth operational shuttle, Endeavor, was added in 1991. Once in orbit, the shuttle travels at 17,500 miles per hour, and can reach altitudes up to 600 miles above the Earth.