SPACECRAFT

vehicle designed to operate, with or without a crew, in a controlled flight pattern, in Earth orbit or in outer space. Early conceptions of space flight, such as those of Jules Verne and other writers of fiction, usually pictured the craft as streamlined. Actual vehicles, however, have been designed with a variety of shapes depending on the mission because streamlining has no particular advantage in the vacuum of space. The first spacecraft, the Soviet Sputnik 1, was launched Oct. 4, 1957, into an elliptical Earth orbit with an apogee (farthest point from the Earth) of 940 km (584 miles) and a perigee (nearest point) of 230 km (143 miles). Sputnik 1 weighed 83.6 kg (184 pounds). It was soon followed by many more unmanned Soviet and U.S. spacecraft and, within four years (April 12, 1961), by the first manned spacecraft, Vostok 1, which carried the Soviet cosmonaut Yury A. Gagarin into orbit. Within the next few years numerous other manned and unmanned spacecraft were launched, each designed to perform a variety of scientific or engineering tasks. Spacecraft are basically nonpowered; that is, they depend on the initial velocity provided by a launching rocket and usually a second-stage rocket, both of which automatically separate and fall to the Earth when their fuel is depleted, while the spacecraft either enters an orbit around the Earth or, if given momentum, continues toward a destination in space. The spacecraft itself may be equipped with small liquid-fuel rocket engines for purposes of maneuvering; in the case of the U.S. Apollo moon-exploration program, the lunar landing vehicle was powered with rockets to permit it to lift off the Moon and return to the orbiting Apollo craft, which in turn had sufficient rocket power to pull itself out of lunar orbit for the return journey to Earth. The enormous complexity of design, particularly of manned spacecraft, which involve literally millions of components, requires a high degree of miniaturization and achievement of reliability factors of more than 99.9999 percent. An automobile whose parts had equivalent reliability should operate for 100 years before its first malfunction. Spacecraft require a local source of electrical power to operate the equipment they carry. The large majority of unmanned craft utilize panels of solar cells, often in conjunction with storage batteries. On manned spacecraft, including space stations such as Skylab, fuel cells are generally employed, sometimes in combination with solar cells. Small nuclear reactors were used to provide electrical power for the U.S. Pioneer and Voyager deep-space probes. On such craft a plutonium-fueled reactor produces enough energy to operate all telemetry, photographic, and scientific equipment for hundreds of years.