Lunar Orbiter was the project that mapped the Moon in preparation for the Apollo landings. A total of five Lunar Orbiters were flown to the Moon. They were built for NASA's Langley Research Center by Boeing and launched to the Moon on Atlas-Agena rockets. The first three orbited around the Moon's equator and provided detailed photographic coverage of the primary Apollo landing sites, including stereo images. Because of the success of these earlier missions, the final two Lunar Orbiters were placed into a polar orbit so that virtually the entire surface of the Moon was mapped. In addition to images of the Apollo landing sites, Lunar Orbiter provided many breathtaking photographs, including features on the farside.
In addition to the near global photographic coverage of the Moon, Lunar Orbiter provided additional information that aided Apollo. Sensors on board indicated that radiation levels near the Moon would pose no danger to the astronauts. Analysis of the spacecraft orbits found evidence of gravity perturbations, which suggested that the Moon was not gravatationally uniform. Instead it appeared as if buried concentrations of mass were under the mare deposits. By discovering and defining these "mascons," Lunar Orbiter made it possible for the Apollo missions to conduct highly accurate landings and precision rendezvous.
After depleting their film supplies, all five Lunar Orbiters were proposely crashed onto the Moon to prevent their radio transmitters from interferring from future spacecraft.
Transferred from the Boeing Company through NASA - Langley Research Center