Lunar orbit
In astronomy and spaceflight, a lunar orbit is an orbit by an object around Earth's Moon. In general these orbits are not circular. When farthest from the Moon a spacecraft is said to be at apolune, apocynthion, or aposelene. When closest to the Moon it is said to be at perilune, pericynthion, or periselene. These derive from names or epithets of the moon goddess.
Lunar orbit insertion is an orbit insertion maneuver used to achieve lunar orbit.
Low lunar orbit is an orbit below altitude. These have a period of about 2 hours. They are of particular interest in the exploration of the Moon, but suffer from gravitational perturbations that make most unstable, and leave only a few orbital trajectories possible for indefinite frozen orbits. These would be useful for long-term stays in LLO.
Perturbation effects and low orbits
Most lunar low orbits below are unstable.File:Earth, Moon and Lunar Module, AS11-44-6643 c.jpg|thumb|Lunar Module Eagle in lunar orbit during Apollo 11, July 1969
Gravitational anomalies slightly distorting the orbits of some Lunar Orbiters led to the discovery of mass concentrations beneath the lunar surface caused by large impacting bodies at some remote time in the past.
These anomalies are large enough to cause a lunar orbit to change significantly over the course of several days. They can cause a plumb bob to hang about a third of a degree off vertical, pointing toward the mascon, and increase the force of gravity by one-half percent.
The Apollo 11 first manned landing mission employed the first attempt to correct for the perturbation effect. The parking orbit was "circularized" at, which was expected to become the nominal circular 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.
Stable low orbits
Study of the mascons' effect on lunar spacecraft led to the discovery in 2001 of frozen orbits occurring at four orbital inclinations: 27°, 50°, 76°, and 86°, 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° inclination, and successfully completed its mission after one and a half years. PFS-2 was placed in a particularly unstable orbital inclination of 11°, and lasted only 35 days in orbit before crashing into the lunar surface.Lunar high orbits
For lunar orbits with altitudes in the range, the gravity of Earth leads to orbit perturbations. At altitudes higher than that perturbed two-body astrodynamics models are insufficient and three-body models are required.Although the Moon's Hill sphere extends to a radius of, the gravity of Earth intervenes enough to make lunar orbits unstable at a distance of.
The Lagrange points of the Earth-Moon system can provide stable orbits in the lunar vicinity, such as halo orbits and distant retrograde orbits.
Some halo orbits remain over particular regions of the lunar surface. These can be used by lunar relay satellites to communicate with surface stations on the far side of the Moon. The first to do this was the 2019 Queqiao relay satellite. It was placed around Earth-Moon L2 at roughly from the Moon.
Since 2022 near-rectilinear halo orbits, using as well a Lagrange point, have been used and are planned to be employed by the Lunar Gateway.
File:Near Rectilinear Halo Orbit.png|thumb|Near-rectilinear halo orbit in cislunar space, as illustrated by A.I. Solutions, Inc. using the FreeFlyer software.
Orbital transfer
There are three main ways to get to lunar orbit from Earth: direct transfer, low thrust transfer and low-energy transfer. These take 3–4 days, months or 2.5–4 months respectively.File:Animation of SLIM around Earth.gif|thumb|right|SLIM's trajectory included low energy transfer
History of missions to lunar orbit
First orbiters
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 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 Earth. This craft provided the first pictures of the far side of the Lunar surface.Luna 10 became the first spacecraft to actually orbit the Moon and any extraterrestrial body in April 1966. It studied micrometeoroid flux, and lunar environment until May 30, 1966. A follow-on mission, Luna 11, was launched on August 24, 1966, and studied lunar gravitational anomalies, radiation and solar wind measurements.
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 and a perilune of. Then the orbit was circularized at around 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.
Crewed and later orbiters
The Apollo program's Command/Service Module remained in a lunar parking orbit while the Lunar Module landed.The combined CSM/LM would first enter an elliptical orbit, nominally, which was then changed to a circular parking orbit of about. 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 burn to lower their periapsis to about, chosen to avoid hitting lunar mountains reaching heights of. 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.