Lunar orbit
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In astronomy, lunar orbit refers just to the orbit of the Moon around the Earth. See The Moon's orbit.
As used in the space program, this refers not to the orbit of Earth's Moon, but to orbits around that Moon by various manned or unmanned spacecraft. The first spacecraft to reach lunar orbit was the Soviet unmanned vehicle Lunik 1 in 1959. The vehicle flew once around the Moon, returning low quality photographs.
The first spacecraft the enter a true lunar orbit, in the sense that it was a satellite continuously going around the Moon, was the USA's Lunar Orbiter 1 in 1965. This was the first of five similar spacecraft launched over a period of thirteen months, all of which successfully mapped the Moon. This was in preparation for the Apollo Project, searching for a suitable landing site.
The Apollo Project's Command and Service Module remained in lunar orbit while the Lunar Module landed. Selection of an orbit was constrained on the low side by lunar mountains reaching heights of 20,000 feet (nearly seven kilometers), and on the high side by the need to obtain suitable imagery. Lunar Orbiters were mostly around 200 miles altitude while the Apollo Project used roughly 65 miles. Orbital periods were roughly two hours.
Gravitational anomalies slightly distorting the orbits of the Lunar Orbiters led to the discovery of mascons, concentrations of mass beneath the lunar surface caused by large impacting bodies at some remote time in the past. These anomalies were too small to be of significance for the short time the Apollo Project's spacecraft were in lunar orbit.
