Lunar orbit
In astronomy and spaceflight, a lunar orbit (also known as a selenocentric orbit) is an orbit of an object around Earth's Moon. In general these orbits are not circular. When farthest from the Moon (at apoapsis) a spacecraft is said to be at apolune, apocynthion, or aposelene. When closest to the Moon (at periapsis) it is said to be at perilune, pericynthion, or periselene. These derive from names or epithets of the moon goddess.
For the orbit of the Moon around the Earth, see Orbit of the Moon.
Lunar orbit insertion (LOI) is an orbit insertion maneuver used to achieve lunar orbit.[1]
Low lunar orbit (LLO) is an orbit below 100 km (62 mi) altitude. These have a period of about 2 hours.[2] 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.[2]
Stable low orbits[edit]
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.[2] 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.[2]