Lunar Orbiter

Any of a series of five US photographic reconnaissance satellites put into orbit around the Moon 1966–67 to map landing sites for the Apollo programme. A total of 1,950 high-resolution images covering more than 99% of the lunar surface were transmitted back to Earth, some showing features as small as a metre in size. Precise tracking of the orbital motions of the satellites allowed scientists to map the gravitational field of the Moon and discover mascons (mass concentrations – regions of higher-density rock) beneath the lunar surface.