View source for Lunar orbit

{{short description|Orbit of an object around the Moon}}
{{For|the orbit of the Moon around the Earth|Orbit of the Moon}}
[[File:Earth rising behind the Moon.jpg|thumb|[[Orion capsule]] of [[Artemis 1]] above the Moon in December 2022.]]

In [[astronomy]] and [[spaceflight]], a '''lunar orbit''' (also known as a '''selenocentric orbit''') is an [[orbit]] by 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]].

'''Lunar orbit insertion''' ('''LOI''') is an [[orbit insertion]] maneuver used to achieve lunar orbit.<ref>{{cite book| year=2008 | chapter=Entering lunar orbit: the LOI manoeuvre | last=Woods | first=W.D. | title=How Apollo Flew to the Moon | publisher=Springer Praxis Books | series=Space Exploration | pages=189–210 | doi=10.1007/978-0-387-74066-9_8 | isbn=978-0-387-71675-6 }}</ref>

'''Low lunar orbit''' ('''LLO''') is an orbit below {{convert|100|km|mi|abbr=on}} altitude. These have a period of about 2 hours.<ref name="nasa20061106" /> They are of particular interest in the [[exploration of the Moon]], but suffer from gravitational [[perturbation (astronomy)|perturbation]]s that make most unstable, and leave only a few orbital [[trajectory|trajectories]] possible for indefinite ''[[frozen orbit]]s''. These would be useful for long-term stays in LLO.<ref name=nasa20061106/>

==Perturbation effects and low orbits==
{{further|Lunar mascons}}
Most lunar low orbits below {{convert|100|km|mi|abbr=on}} are unstable.<ref name=nasa20061106/>
[[File:Earth, Moon and Lunar Module, AS11-44-6643 c.jpg|thumb|[[Lunar Module Eagle|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 concentration (astronomy)|mass concentration]]s (dubbed '''mascons''') beneath the lunar surface caused by large impacting bodies at some remote time in the past.<ref name=nasa20061106/><ref>{{Cite journal|last1=Konopliv|first1=A. S.|last2=Asmar|first2=S. W.|last3=Carranza|first3=E.|last4=Sjogren|first4=W. L.|last5=Yuan|first5=D. N.|date=2001-03-01|title=Recent Gravity Models as a Result of the Lunar Prospector Mission|journal=Icarus|volume=150|issue=1|pages=1–18|doi=10.1006/icar.2000.6573|bibcode=2001Icar..150....1K|issn=0019-1035}}</ref>  
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.<ref name="nasa20061106" />
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 {{convert|66|by|54|nmi|km mi}}, which was expected to become the nominal circular {{convert|60|nmi|km mi}} when the LM made its return [[Space rendezvous|rendezvous]] with the CSM. But the effect was overestimated by a factor of two; at rendezvous, the orbit was calculated to be {{convert|63.2|by|56.8|nmi|km mi}}.
<ref name="Apollo 11 Mission Report">{{Cite web|url=https://history.nasa.gov/alsj/a11/A11_MissionReport.pdf|title=Apollo 11 Mission Report|publisher=[[NASA]]|pages = 4–3 to 4–4}}</ref>

==Stable low orbits==
Study of the mascons' effect on lunar spacecraft led to the discovery in 2001 of [[frozen orbit]]s occurring at four [[orbital inclination]]s: 27°, 50°, 76°, and 86°, in which a spacecraft can stay in a low orbit indefinitely.<ref name=nasa20061106>
{{cite web |title=Bizarre Lunar Orbits|url=http://science.nasa.gov/headlines/y2006/06nov_loworbit.htm |url-status=dead |archive-url=https://web.archive.org/web/20061110094207/http://science.nasa.gov/headlines/y2006/06nov_loworbit.htm |archive-date=2006-11-10 |work=Science@NASA |last=Bell |first=Trudy E. |date=2006-11-06 |publisher=[[NASA]] |access-date=2012-12-09 |quote=''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°.''}}</ref> The [[Apollo 15]] subsatellite [[PFS-1]] and the [[Apollo 16]] subsatellite [[PFS-2]], both small satellites released from the Apollo [[Apollo Command/Service Module#Service Module (SM)|Service Module]], contributed to this discovery. PFS-1 ended up in a long-lasting orbit, at 28° [[Orbital inclination|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.<ref name=nasa20061106/>

==Lunar high orbits==
For lunar orbits with altitudes in the {{convert|500|to|20,000|km|mi|abbr=on}} range, the gravity of Earth leads to orbit [[Perturbation (astronomy)|perturbations]]. At altitudes higher than that perturbed two-body [[astrodynamics]] models are insufficient and [[three-body problem|three-body]] models are required.<ref name=ely-2005>{{cite journal |first=Todd |last=Ely |title=Stable Constellations of Frozen Elliptical Inclined Lunar Orbits|date=July 2005 |journal=The Journal of the Astronautical Sciences |volume=53 |issue=3 |pages=301–316 |doi=10.1007/BF03546355 |bibcode=2005JAnSc..53..301E |url=https://www.researchgate.net/publication/264815005}}</ref>

Although the Moon's [[Hill sphere]] extends to a radius of {{cvt|60000|km|mi}},<ref>{{cite web | author = Follows, Mike | date = 4 October 2017 | title = Ever Decreasing Circles | work = [[NewScientist.com]] | url = https://www.newscientist.com/lastword/mg23631461-200-ever-decreasing-circles/ | access-date = 23 July 2023 | quote = The moon's Hill sphere has a radius of 60,000 kilometres, about one-sixth of the distance between it and Earth.}} For mean distance and mass data for the bodies (for verification of the foregoing citation), see {{cite web | author = Williams, David R. | date = 20 December 2021 | title = Moon Fact Sheet | work = [[NASA.gov]] | publisher = NASA Goddard Space Flight Center | location = Greenbelt, MD | url = https://nssdc.gsfc.nasa.gov/planetary/factsheet/moonfact.html | access-date = 23 July 2023}}</ref> the gravity of Earth intervenes enough to make lunar orbits unstable at a distance of {{cvt|690|km|mi}}.<ref name="Phys.org 2006 k599">{{cite web | title=A New Paradigm for Lunar Orbits | website=Phys.org | date=2006-12-01 | url=https://phys.org/news/2006-12-paradigm-lunar-orbits.html | access-date=2023-11-05}}</ref>

The [[Lagrange point]]s of the Earth-Moon system can provide stable orbits in the lunar vicinity, such as [[halo orbit]]s and [[distant retrograde orbit]]s.

Some halo orbits remain over particular regions of the lunar surface. These can be used by lunar [[relay satellite]]s 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 {{cvt|65000|km|mi}} from the Moon.<ref>{{cite web |title=NSSDCA - Queqiao |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=QUEQIAO |website=nasa.gov |access-date=May 10, 2025}}</ref>

[[File:LL2 Halo Example Synodic.gif|thumb|An example of a halo orbit at the second lunar lagrange point.]]

Since 2022 ([[CAPSTONE]]) [[near-rectilinear halo orbit]]s, using as well a Lagrange point, have been used and are planned to be employed by the [[Lunar Gateway]].
[[File:Near Rectilinear Halo Orbit (NRHO).png|thumb|Near-rectilinear halo orbit ([[Near-rectilinear halo orbit|NRHO]]) in cislunar space, as illustrated by A.I. Solutions, Inc. using the [[FreeFlyer]] software.]]
[[File:Lunar-L1-and-L2-northern-and-southern-NRHOs.png|thumb|Overview of NRHOs around the Moon]]

==Orbital transfer==
{{Main|Trans-lunar injection|orbit insertion}}
There are three main ways to get to lunar orbit from Earth: direct transfer, [[low thrust relative orbital transfer|low thrust transfer]] and [[low-energy transfer]]. These take 3–4 days, {{Fix|text=word missing}} months or 2.5–4 months respectively.<ref name="The Aerospace Corporation 2023 j509">{{cite web | author=The Aerospace Corporation | title=It's International Moon Day! Let's talk about Cislunar Space. | website=Medium | date=2023-07-20 | url=https://medium.com/the-aerospace-corporation/its-international-moon-day-let-s-talk-about-cislunar-space-9d108f1a1b0b | access-date=2023-11-07}}</ref>

[[File:Animation of Lunar Reconnaissance Orbiter trajectory around Earth.gif|thumb|right|Animation of [[Lunar Reconnaissance Orbiter|LRO]] trajectory around Earth. Using a direct transfer, it arrived on moon in four and a half days<br/>{{legend2|magenta|[[Lunar Reconnaissance Orbiter]]}}{{·}}{{legend2|RoyalBlue|Earth}}{{·}}{{legend2|DarkGoldenrod|Moon}}]]

[[File:Animation of Chandrayaan-3 around Earth.gif|thumb|right|[[Chandrayaan-3]]'s trajectory included multiple orbit raising maneuvers to get to the Moon]]

[[File:Animation of SLIM around Earth.gif|thumb|right|[[Smart Lander for Investigating Moon|SLIM]]'s trajectory included [[low energy transfer]]]]

==History of missions to lunar orbit==
{{Further|List of extraterrestrial orbiters#Moon}}

===First orbiters===
[[File:First View of Earth from Moon.jpg|thumb|First image of Earth from around another [[astronomical object]] (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).
<ref>{{cite web |last=Stein |first=Ben P. |title=45 Years Ago: How the 1st Photo of Earth From the Moon Happened |website=Space.com |date=August 23, 2011 |url=https://www.space.com/12707-earth-photo-moon-nasa-lunar-orbiter-1-anniversary.html |access-date=October 7, 2020}}</ref><ref name=Smith>{{Cite web|url=https://www.smithsonianmag.com/smart-news/fifty-years-ago-this-photo-captured-first-view-of-earth-from-the-moon-180960222/|title=Fifty Years Ago, This Photo Captured the First View of Earth From the Moon|date=August 23, 2016|archive-url=https://web.archive.org/web/20160825094218/https://www.smithsonianmag.com/smart-news/fifty-years-ago-this-photo-captured-first-view-of-earth-from-the-moon-180960222/|archive-date=August 25, 2016|url-status=live}}</ref>]]
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.<ref name="mark">{{cite web
 |last        = Wade
 |first       = Mark
 |title       = Luna
 |publisher   = [[Encyclopedia Astronautica]]
 |url         = http://www.astronautix.com/project/luna.htm
 |access-date  = 2007-02-17
 |url-status     = dead
 |archive-url  = https://web.archive.org/web/20120111203409/http://www.astronautix.com/project/luna.htm
 |archive-date = 2012-01-11
}}</ref> It passed within {{convert|6,000|km|nmi mi}} of the Moon's surface, but did not achieve lunar orbit.<ref name="mark"/> [[Luna 3]], launched on October 4, 1959, was the first robotic spacecraft to complete a [[Circumlunar trajectory|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.<ref name="mark"/>

[[Luna 10]] became the first spacecraft to actually orbit the [[Moon]] and any extraterrestrial body in April 1966.<ref name="beyers">{{cite web
 | last = Byers
 | first = Bruce K.
 | title = APPENDIX C [367-373] RECORD OF UNMANNED LUNAR PROBES, 1958-1968: Soviet Union
 | work = DESTINATION MOON: A History of the Lunar Orbiter Program
 | publisher = National Aeronautics and Space Administration
 | date = 1976-12-14
 | url = http://www.hq.nasa.gov/office/pao/History/TM-3487/app-c.htm#soviet
 | access-date = 2007-02-17
 | archive-date = 2021-01-26
 | archive-url = https://web.archive.org/web/20210126065856/https://www.hq.nasa.gov/office/pao/History/TM-3487/app-c.htm#soviet
 | url-status = dead
  }}</ref> It studied [[micrometeoroid]] flux, and lunar environment until May 30, 1966.<ref name="beyers"/> 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.<ref name="orbiter">{{cite web
 | last = Wade
 | first = Mark
 | title = Lunar Orbiter
 | publisher = Encyclopedia Astronautica
 | url = http://www.astronautix.com/craft/lunbiter.htm
 | archive-url = https://web.archive.org/web/20020821173650/http://astronautix.com/craft/lunbiter.htm
 | url-status = dead
 | archive-date = August 21, 2002
 | access-date =  2007-02-17}}</ref> The first orbit was an [[elliptical orbit]], with an [[apolune]] of {{convert|1,008|nmi|km mi}} and a [[perilune]] of {{convert|102.1|nmi|km mi}}.<ref>{{cite web
 | last = Byers
 | first = Bruce K.
 | title = CHAPTER IX: MISSIONS I, II, III: APOLLO SITE SEARCH AND VERIFICATION, The First Launch
 | work = DESTINATION MOON: A History of the Lunar Orbiter Program
 | publisher = National Aeronautics and Space Administration
 | date = 1976-12-14
 | url = http://www.hq.nasa.gov/office/pao/History/TM-3487/ch9-2.htm
 | access-date = 2007-02-17
 | archive-date = 2020-09-27
 | archive-url = https://web.archive.org/web/20200927145230/https://www.hq.nasa.gov/office/pao/History/TM-3487/ch9-2.htm
 | url-status = dead
  }}</ref> Then the orbit was circularized at around {{convert|170|nmi|km 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 finding suitable [[Apollo program]] landing sites.<ref name="orbiter"/>

===Crewed and later orbiters===
The [[Apollo program]]'s [[Apollo Command/Service Module|Command/Service Module]] (CSM) remained in a lunar parking orbit while the [[Apollo Lunar Module|Lunar Module]] (LM) landed. 
The combined CSM/LM would first enter an elliptical orbit, nominally {{convert|170|by|60|nmi|km mi}}, which was then changed to a circular parking orbit of about {{convert|60|nmi|km 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 {{convert|50,000|ft|km nmi}}, chosen to avoid hitting [[List of mountains on the Moon|lunar mountains]] reaching heights of {{convert|20,000|ft|km 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.<ref name="a11fj">{{cite web
 | last = Jones
 | first = Eric M.
 | title = The First Lunar Landing
 | work = Apollo 11 Lunar Surface Journal
 | publisher = National Aeronautics and Space Administration
 | date = 1976-12-14
 | url = https://www.hq.nasa.gov/alsj/a11/a11.landing.html
 | access-date = 2014-11-09}}</ref>

==See also==
*[[Cislunar space]]
*[[List of orbits]]
*[[Orbital mechanics]]
*[[Distant retrograde orbit]]
*[[Near-rectilinear halo orbit]]

==References==
{{reflist}}

{{orbits}}

[[Category:Orbit of the Moon]]