In astronomy, lunar orbit (also known as a selenocentric orbit) is the
orbit of an object around the Moon.
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, from names or epithets
of the moon goddess.
Low lunar orbit (LLO)—orbits below 100 km (62 mi)
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.
1 Robotic spacecraft
2 Human crewed spacecraft
3 Perturbation effects
4 See also
Soviet Union sent the first spacecraft to the vicinity of the
Moon, the robotic vehicle Luna 1, on January 4, 1959. It passed
within 6,000 kilometres (3,200 nmi; 3,700 mi) 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
returned to the 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 in April 1966. It studied micrometeoroid flux, and lunar
environment until May 30, 1966.
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 1,008 nautical miles (1,867 km; 1,160 mi) and
a perilune of 102.1 nautical miles (189.1 km; 117.5 mi).
Then the orbit was circularized at around 170 nautical miles
(310 km; 200 mi) 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
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 spacecraft
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 170 nautical miles
(310 km; 200 mi) by 60 nautical miles (110 km;
69 mi), which was then changed to a circular parking orbit of
about 60 nautical miles (110 km; 69 mi). 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 50,000
feet (15 km; 8.2 nmi), chosen to avoid hitting lunar
mountains reaching heights of 20,000 feet (6.1 km; 3.3 nmi).
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
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 66
nautical miles (122 km; 76 mi) by 54 nautical miles
(100 km; 62 mi), which was expected to become the nominal
circular 60 nautical miles (110 km; 69 mi) 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 63.2
nautical miles (117.0 km; 72.7 mi) by 56.8 nautical miles
(105.2 km; 65.4 mi). 
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 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.
List of orbits
Mass concentration (astronomy)
^ a b c d "Bizarre Lunar Orbits".
NASA Science: Science News. NASA.
2006-11-06. Retrieved 2012-12-09. Lunar mascons make most low lunar
orbits unstable ... As a satellite passes 50 or 60 miles overhead, the
mascons pull it forward, back, left, right, or down, the exact
direction and magnitude of the tugging depends on the satellite's
trajectory. Absent any periodic boosts from onboard rockets to correct
the orbit, most satellites released into low lunar orbits (under about
60 miles or 100 km) will eventually crash into the Moon. ... [There
are] a number of 'frozen orbits' where a spacecraft can stay in a low
lunar orbit indefinitely. They occur at four inclinations: 27°, 50°,
76°, and 86° — the last one being nearly over the lunar poles. The
orbit of the relatively long-lived
Apollo 15 subsatellite
PFS-1 had an
inclination of 28°, which turned out to be close to the inclination
of one of the frozen orbits—but poor
PFS-2 was cursed with an
inclination of only 11°.
^ a b c Wade, Mark. "Luna". Encyclopedia Astronautica. Archived from
the original on 2012-02-04. Retrieved 2007-02-17.
^ a b 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 2007-02-17.
^ a b Wade, Mark. "Lunar Orbiter". Encyclopedia Astronautica.
^ 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 2007-02-17.
^ Jones, Eric M. (1976-12-14). "The First Lunar Landing". Apollo 11
Lunar Surface Journal. National Aeronautics and Space Administration.
Apollo 11 Mission Report" (PDF). NASA. pp. 4–3 to
Elliptical / Highly elliptical
Inclined / Non-inclined
Orbit of the Moon
About other points
a Semi-major axis
b Semi-minor axis
Q, q Apsides
Ω Longitude of the ascending node
ω Argument of periapsis
ϖ Longitude of the periapsis
M Mean anomaly
ν, θ, f True anomaly
E Eccentric anomaly
L Mean longitude
l True longitude
T Orbital period
n Mean motion
v Orbital speed
Collision avoidance (spacecraft)
Low energy transfer
Transposition, docking, and extraction
Celestial coordinate system
Equatorial coordinate system
Interplanetary Transport Network
Kepler's laws of planetary motion
Orbital state vectors
Specific orbital energy
Specific relative angular momentum