INTRODUCTION
Space Shuttle, spacecraft designed for transporting humans and cargo to and from orbit around Earth. The United States National Aeronautics and Space Administration (NASA) developed the shuttle in the 1970s to serve as a reusable rocket and spacecraft. This objective differed significantly from that of previous space programs in which the launch and space vehicles could be used only once. After ten years of preparation, the first space shuttle, Columbia, was launched on April 12, 1981. Today NASA has three space shuttles: Discovery, acquired in 1983; Atlantis, which arrived in 1985; and Endeavour, which joined the fleet in 1991. The Union of Soviet Socialist Republics (USSR) started a shuttle program in 1988 with the Buran space shuttle, but the program was halted in 1993. The space shuttle was initially used to deploy satellites in orbit; to carry scientific experiments such as Spacelab, a modular arrangement of experiments installed in the shuttle's cargo bay; and to carry out military missions. As the program has matured, the space shuttle also has been used to service and repair orbiting satellites, to retrieve and return to Earth previously deployed spacecraft, and to help build and maintain the International Space Station (see Space Station). The space shuttle carries a wide range of equipment, known as the payload, into space, ranging from communication, military, and astronomical satellites; space experiments for studying the apparent weightlessness (called “microgravity”) experienced aboard a shuttle flight; and human experimental facilities. Often, NASA collaborates with other countries by allowing them to use shuttle cargo space for special projects. The space shuttle is designed to leave Earth as a vertically launched rocket weighing up to 2.0 million kg (4.5 million lb) with 3 million kg (7 million lb) of thrust from its multiple propulsion systems. The orbiter segment returns from space—withstanding the...