Lunar orbit

in lunar orbit during Apollo 11, July 1969 Gravitational anomalies slightly distorting the orbits of some early lunar orbiters led to the discovery of mass concentrations (dubbed “mascons”) beneath the lunar surface. These mascons were caused by large bodies which impacted the Moon at some remote time in the past. These anomalies are large enough to 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. ==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 (CAPSTONE) near-rectilinear halo orbits, using as well a Lagrange point, have been used and are planned to be employed by the Lunar Gateway. ) 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. trajectory around Earth. Using a direct transfer, it arrived on moon in four and a half days 's trajectory included multiple orbit raising maneuvers to get to the Moon 's trajectory included low energy transfer ==History of missions to lunar orbit==

First orbiters (the Moon), and first picture of both Earth and the Moon from space, by Lunar Orbiter 1 (not to be confused with the later Earthrise image). The Soviet Union sent the first spacecraft to the vicinity of the Moon (or any extraterrestrial object), the robotic vehicle Luna 1, on January 4, 1959. It passed within of the Moon's surface, but did not achieve lunar orbit. It studied micrometeoroid flux, and lunar environment until May 30, 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. ==Sun-synchronous lunar orbits==

A Sun-synchronous lunar orbit is a proposed type of lunar orbit in which the orbital plane remains at nearly the same orientation relative to the Sun over time. To do this, the orbital plane would need to shift by about 1° per day, similar to a sun-synchronous orbit around Earth. Unlike Earth's sun-synchronous orbits, lunar Sun-synchronous orbits do not occur naturally, because the Lunar gravity field is highly uneven, such an orbit would generally require active control to maintain. Earth and the Sun’s gravity would also affect it through third-body perturbations. sun-synchronous data centers ==See also==