As used in the space program, this refers not to the orbit of the Moon about the Earth, but to orbits by various manned or unmanned spacecraft around the Moon. The altitude at apoapsis (point farthest from the surface) for a lunar orbit is known as apolune, apocynthion or aposelene, while the periapsis (point closest to the surface) is known as perilune, pericynthion or periselene.
Low Lunar orbit (LLO)—orbits below Script error altitude—are of particular interest in exploration of the moon, but suffer from gravitational perturbation effects that make most unstable, and leave only a few orbital inclinations possible for indefinite frozen orbits, useful for long-term stays in LLO.
The Soviet Union sent the first spacecraft to the vicinity of the Moon, the robotic vehicle Luna 1, on January 4, 1959. It passed within Script error of the Moon's surface, but did not achieve lunar orbit. Luna 3, launched on October 4, 1959, was the first robotic spacecraft to complete a circumlunar free return trajectory, still not a lunar orbit, but a figure-8 trajectory which swung around the far side of the Moon and returned to the Earth. This craft provided the first pictures of the far side of the Lunar surface.
The first United States spacecraft to orbit the Moon was Lunar Orbiter 1 on August 14, 1966. The first orbit was an elliptical orbit, with an apolune of Script error and a perilune of Script error. Then the orbit was circularized at around Script error to obtain suitable imagery. Five such spacecraft were launched over a period of thirteen months, all of which successfully mapped the Moon, primarily for the purpose of finding suitable Apollo program landing sites.
The most recent was the Lunar Atmosphere and Dust Environment Explorer (LADEE), which became a ballistic impact experiment in 2014.
Human crewed spacecraftEdit
The Apollo program's Command/Service Module (CSM) remained in a lunar parking orbit while the Lunar Module (LM) landed. The combined CSM/LM would first enter an elliptical orbit, nominally Script error by Script error, which was then changed to a circular parking orbit of about Script error. Orbital periods vary according to the sum of apoapsis and periapsis, and for the CSM were about two hours. The LM began its landing sequence with a Descent Orbit Insertion (DOI) burn to lower their periapsis to about Script error, chosen to avoid hitting lunar mountains reaching heights of Script error. After the second landing mission, the procedure was changed on Apollo 14 to save more of the LM fuel for its powered descent, by using the CSM's fuel to perform the DOI burn, and later raising its periapsis back to a circular orbit after the LM had made its landing. 
Gravitational anomalies slightly distorting the orbits of some Lunar Orbiters led to the discovery of mass concentrations (dubbed mascons), beneath the lunar surface caused by large impacting bodies at some remote time in the past. These anomalies are significant enough to cause a lunar orbit to change significantly over the course of several days. The Apollo 11 first manned landing mission employed the first attempt to correct for the perturbation effect (the frozen orbits were not known at that time). The parking orbit was "circularized" at Script error by Script error, which was expected to become the nominal circular Script error when the LM made its return rendezvous with the CSM. But the effect was overestimated by a factor of two; at rendezvous the orbit was calculated to be Script error by Script error. 
Study of the mascons' effect on lunar spacecraft led to the discovery in 2001 of "frozen orbits" occurring at four orbital inclinations: 27º, 50º, 76º, and 86º, in which a spacecraft can stay in a low orbit indefinitely. The Apollo 15 subsatellite PFS-1 and the Apollo 16 subsatellite PFS-2, both small satellites released from the Apollo Service Module, contributed to this discovery. PFS-1 ended up in a long-lasting orbit, at 28 degrees inclination, and successfully completed its mission after one and a half years. PFS-2 was placed in a particularly unstable orbital inclination of 11 degrees, and lasted only 35 days in orbit before crashing into the Lunar surface.
- ↑ 1.0 1.1 1.2 1.3 Bizarre Lunar Orbits. NASA Science: Science News. NASA (2006-11-06). Retrieved on 2012-12-09.
- ↑ 2.0 2.1 2.2 Wade, Mark. Luna. Encyclopedia Astronautica. Retrieved on 2007-02-17.
- ↑ 3.0 3.1 Byers, Bruce K. (1976-12-14). APPENDIX C [367-373 RECORD OF UNMANNED LUNAR PROBES, 1958-1968: Soviet Union]. DESTINATION MOON: A History of the Lunar Orbiter Program. National Aeronautics and Space Administration. Retrieved on 2007-02-17.
- ↑ 4.0 4.1 Wade, Mark. Lunar Orbiter. Encyclopedia Astronautica. Retrieved on 2007-02-17.
- ↑ Byers, Bruce K. (1976-12-14). CHAPTER IX: MISSIONS I, II, III: APOLLO SITE SEARCH AND VERIFICATION, The First Launch. DESTINATION MOON: A History of the Lunar Orbiter Program. National Aeronautics and Space Administration. Retrieved on 2007-02-17.
- ↑ Jones, Eric M. (1976-12-14). The First Lunar Landing. Apollo 11 Lunar Surface Journal. National Aeronautics and Space Administration. Retrieved on 2014-11-09.
- ↑ Apollo 11 Mission Report (PDF) pp. 4-3 to 4-4. NASA.