n.
Vehicle designed to operate, with or without a crew, in a controlled flight pattern above Earth's lower atmosphere.
Since streamlining is not needed in the high vacuum of this environment, a spacecraft's shape is designed according to its mission (see space exploration ). Most spacecraft are not self-propelled; they are accelerated to the necessary high velocity by staged rocket s, which are jettisoned when their fuel is used up. A major exception, the space shuttle orbiter, uses three onboard liquid-fuel main engines supplied by a disposable external tank and a pair of solid-fuel boosters to reach space. The spacecraft goes into an orbit around Earth or, if given enough velocity, it continues toward another destination in space. The craft may have its own small rocket engines for orienting and maneuvering. For internal power, Earth-orbiting spacecraft use solar cells and storage batteries, fuel cells, or a combination, whereas craft designed for deep-space missions usually carry thermoelectric generators heated by a radioactive element. The enormous complexity of design, particularly of manned spacecraft with their millions of components, requires a high degree of miniaturization and reliability. See also launch vehicle .