The
Lunar Orbiter program was a series of five U.S. spacecraft used for a broad-scale lunar mapping effort, in contrast to
the preceding Ranger spacecraft, which zeroed in on specific targets on their way to crash landings. The five Orbiter missions, all of them successful, were launched at approximately three-month intervals from Aug. 10, 1966, through Aug. 1, 1967.The general area of interest for Apollo (see Apollo program)
landing sites was a rectangle extending 5¡ north and south of the lunar equator and 45¡ east and west of the lunar prime (essentially earth-centered) meridian. Lunar Orbiter 1 was launched to photograph the southern part of the rectangle, while Lunar Orbiter 2 was concerned with the northern half. A dozen promising Apollo landing sites were selected from these batches of images, and the third probe in the series was used for additional photos of the chosen sites. Lunar Orbiter 4 was placed in a higher-altitude orbit around the Moon, giving wider coverage while still showing the surface from a near-vertical aspect that cannot be seen from Earth except near the center of the lunar disk. Lunar Orbiter 5 was also used to complete Apollo site studies, but the success of the four previous Lunar Orbiters enabled much of the photography of the fifth mission to be devoted to targets of primarily scientific interest. Altogether, 95 percent of the Moon's surface was photographed by the Lunar Orbiters.Some of the photos were also used to help pinpoint the locations of some of the unmanned Surveyor landing craft, and Lunar Orbiters 1 and 5 took pictures of the Earth. The Orbiters also discovered mascons (mass concentrations) on the Moon.