any of a series of five unmanned U.S. spacecraft placed in orbit around the Moon from 1966 to 1967. The orbiters obtained 1,950 wide-angle and high-resolution photographs of much of the Moon's surface, including the polar regions and the far side, some from as close as 28.5 miles (45.6 km) above the surface. These pictures enabled the selection of five primary landing sites for the manned Apollo missions and also made possible the construction of lunar maps with as much as 100 times the detail available from Earth-based telescopic observations. See also Moon exploration.