# Low-energy transfer

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{{Short description|Fuel-efficient orbital maneuver}}
[[File:Animation of GRAIL-A trajectory.gif |280px|thumb |right |An example of Low-energy transfer to the Moon<br>{{legend2|magenta| GRAIL-A}}{{·}}{{legend2| Lime |[Moon](/source/Moon)}}{{·}}{{legend2| RoyalBlue|[Earth](/source/Earth)}}]]
A '''low-energy transfer''', or low-energy [trajectory](/source/trajectory), is a route in [space](/source/outer_space) that allows [spacecraft](/source/spacecraft) to change [orbit](/source/orbit)s using significantly less fuel than traditional transfers.<ref name=Capture>{{cite book|last=Belbruno|first=Edward|author-link= Edward Belbruno |title=Capture Dynamics and Chaotic Motions in Celestial Mechanics: With Applications to the Construction of Low Energy Transfers|publisher=[Princeton University Press](/source/Princeton_University_Press)|date=2004|pages=224|isbn=978-0-691-09480-9|url=http://press.princeton.edu/titles/7687.html}}</ref><ref name=FlyMe>{{cite book|last=Belbruno|first=Edward|author-link=Edward Belbruno|title=Fly Me to the Moon: An Insider's Guide to the New Science of Space Travel|publisher=[Princeton University Press](/source/Princeton_University_Press)|date=2007|pages=[https://archive.org/details/flymetomoonusing00belb/page/176 176]|isbn=978-0-691-12822-1|url=https://archive.org/details/flymetomoonusing00belb/page/176|url-access=registration}}</ref> These routes work in the [Earth](/source/Earth)–[Moon](/source/Moon) system and also in other systems, such as between the [moons of Jupiter](/source/moons_of_Jupiter). The drawback of such trajectories is that they take longer to complete than higher-energy (more-fuel) transfers, such as [Hohmann transfer orbit](/source/Hohmann_transfer_orbit)s.

Low-energy transfers are also known as [weak stability boundary](/source/weak_stability_boundary) trajectories, and include [ballistic capture](/source/ballistic_capture) trajectories.

Low-energy transfers follow special pathways in space, sometimes referred to as the [Interplanetary Transport Network](/source/Interplanetary_Transport_Network).  Following these pathways allows for long distances to be traversed for little change in velocity, or {{nowrap|[delta-v](/source/delta-v)}}.

==Example missions==
[[File:Animation of SLIM around Earth.gif|thumb|right|[SLIM](/source/Smart_Lander_for_Investigating_Moon)'s trajectory included a low energy transfer]]
Missions that have used low-energy transfers include:

* ''[Hiten](/source/Hiten_(spacecraft))'', from [JAXA](/source/Japanese_Aerospace_Exploration_Agency)
* ''[SMART-1](/source/SMART-1)'', from [ESA](/source/European_Space_Agency)
* ''[Genesis](/source/Genesis_(spacecraft))'', from [NASA](/source/NASA).<ref name="nasa-jpl-genesis-itn">[http://www.nasa.gov/mission_pages/genesis/media/jpl-release-071702.html Interplanetary Superhighway Makes Space Travel Simpler] // NASA 07.17.02: "Lo conceived the theory of the Interplanetary Superhighway. Lo and his colleagues have turned the underlying mathematics of the Interplanetary Superhighway into a tool for mission design called "LTool," ... The new LTool was used by JPL engineers to redesign the flight path for the Genesis mission"</ref>
* ''[GRAIL](/source/Gravity_Recovery_and_Interior_Laboratory)'', from [NASA](/source/NASA).<ref>{{cite web|url=http://moon.mit.edu/design.html |title=GRAIL Design at MIT Website |access-date=2012-01-22}}</ref><ref>{{cite web |url=http://www.spaceflight101.com/grail-mission-design-timeline.html |title=Spaceflight101 GRAIL Mission Design |access-date=2012-01-22 |url-status=dead |archive-url=https://web.archive.org/web/20120719225135/http://www.spaceflight101.com/grail-mission-design-timeline.html |archive-date=2012-07-19 }}</ref>
* [Danuri](/source/Danuri) from [KARI](/source/Korea_Aerospace_Research_Institute)<ref>{{Cite web|url=https://www.koreaherald.com/view.php?ud=20220606000159|title=''Danuri'' all set for Korea's first moon exploration|website=www.kari.re.kr|date=6 June 2022 |language=en|access-date=2022-07-30}}</ref>

On-going missions that use low-energy transfers include:

* [BepiColombo](/source/BepiColombo), from [ESA](/source/European_Space_Agency)/[JAXA](/source/JAXA)<ref>{{Cite web|url=http://www.esa.int/Science_Exploration/Space_Science/BepiColombo_overview2|title=BepiColombo overview|website=www.esa.int|language=en|access-date=2019-12-03}}</ref>
* [CAPSTONE](/source/CAPSTONE) from [NASA](/source/NASA)
* ''[SLIM](/source/Smart_Lander_for_Investigating_Moon)'', from [JAXA](/source/Japanese_Aerospace_Exploration_Agency)

Proposed missions using low-energy transfers include:

* [European Student Moon Orbiter](/source/European_Student_Moon_Orbiter) (ESMO)
* [Mars Direct](/source/Mars_Direct)

==History==

Low-energy transfers to the Moon were first demonstrated in 1991 by the Japanese spacecraft ''[Hiten](/source/Hiten_(spacecraft))'', which was designed to swing by the Moon but not to enter orbit. The Hagoromo subsatellite was released by Hiten on its first swing-by and may have successfully entered lunar orbit, but suffered a communications failure.

[Edward Belbruno](/source/Edward_Belbruno) and James Miller of the [Jet Propulsion Laboratory](/source/Jet_Propulsion_Laboratory) had heard of the failure, and helped to salvage the mission by developing a ballistic capture trajectory that would enable the main Hiten probe to itself enter lunar orbit.  The trajectory they developed for ''[Hiten](/source/Hiten_(spacecraft))'' used Weak Stability Boundary Theory and required only a small perturbation to the elliptical swing-by orbit, sufficiently small to be achievable by the spacecraft's thrusters.<ref name=Capture/> This course would result in the probe being captured into temporary lunar orbit using zero {{nowrap|delta-v}}, but required five months instead of the usual three days for a Hohmann transfer.<ref name="GR">{{cite journal|last=Frank|first=Adam|date=September 1994|title=Gravity's Rim|journal=[Discover](/source/Discover_(magazine))|url=http://discovermagazine.com/1994/sep/gravitysrim419/}}</ref>

==Delta-v savings==

From low Earth orbit to lunar orbit, the {{nowrap|[delta-v](/source/delta-v)}} savings approach 25% on the burn applied after leaving low Earth orbit, compared to the retrograde burn applied near the Moon in the traditional {{nowrap|[trans-lunar injection](/source/trans-lunar_injection)}}, and allow for a doubling of payload.<ref name="Calculation">{{cite web|author1=Edward A. Belbruno  |author2=John P. Carrico |name-list-style=amp |title=Calculation of Weak Stability Boundary Ballistic Lunar Transfer Trajectories|url=http://astrogatorsguild.com/wp-content/papers/0800_wsb.pdf|publisher=AIAA/AAS Astrodynamics Specialist Conference|date=2000}}</ref>

Robert Farquhar has described a 9-day route from low earth orbit to lunar capture that takes 3.5&nbsp;km/s.<ref>{{cite web |url=http://www.lpi.usra.edu/lunar/documents/NASA%20TN%20D-6365.pdf |title=THE UTILIZATION OF HALO ORBITS IN ADVANCED LUNAR OPERATIONS |last=Farquhar |first=Robert |date= 1971|website=www.lpi.usra.edu |access-date=2020-08-02}}</ref> Belbruno's routes from low Earth orbit require a 3.1&nbsp;km/s burn for trans lunar injection, a delta-''v'' saving of not more than 0.4&nbsp;km/s. However, the latter require no large delta-''v ''change after leaving low Earth orbit, which may have operational benefits if using an upper stage with limited restart or in-orbit endurance capability, which would require the spacecraft to have a separate main propulsion system for capture.<ref>{{Cite book|title=Low-Energy Lunar Trajectory Design|last1=Parker|first1=Jeffrey|pages=24|last2=Anderson|first2=Rodney|date=25 June 2014 |isbn=9781118855317 |url=https://books.google.com/books?id=epTqAwAAQBAJ}}</ref>

For rendezvous with the Martian moons, the savings are 12% for Phobos and 20% for Deimos. Rendezvous is targeted because the stable pseudo-orbits around the Martian moons do not spend much time within 10&nbsp;km of the surface.<ref>{{cite web|author1=A. L. Genova |author2=S. V. Weston |author3=L. J. Simurda  |name-list-style=amp |title=Human & robotic mission applications of low-energy transfers to Phobos & Deimos |url=http://multimedia.seti.org/PhD2011/abstracts/PhD2-11-010.pdf |date=2011 |url-status=dead |archive-url=https://web.archive.org/web/20120425154155/http://multimedia.seti.org/PhD2011/abstracts/PhD2-11-010.pdf |archive-date=April 25, 2012 }}</ref>

==See also==
{{Portal|Spaceflight}}
* [Bi-elliptic transfer](/source/Bi-elliptic_transfer)
* [Delta-v budget](/source/Delta-v_budget)
* [Gravity assist](/source/Gravity_assist)
* [Interplanetary Transport Network](/source/Interplanetary_Transport_Network)
* [Orbital mechanics](/source/Orbital_mechanics)

==References==
{{Reflist}}

==External links==
* [https://web.archive.org/web/20150908032524/http://www.siam.org/news/news.php?id=242 Celestial Mechanics Theory Meets the Nitty-Gritty of Trajectory Design]
* [https://web.archive.org/web/20140308132852/http://www.astrodynamics.eu/Astrodynamics.eu/Talks_files/Topputo-Talk-2005-2.pdf Earth-to-Moon Low Energy Transfers Targeting L1 Hyperbolic Transit Orbit] June 2005
* [https://web.archive.org/web/20070311001950/http://www.annalsnyas.org/cgi/content/abstract/1065/1/1 Low Energy Trajectories and Chaos: Applications to Astrodynamics and Dynamical Astronomy]
* [https://www.sciencenews.org/article/navigating-celestial-currents Navigating Celestial Currents]

{{Orbits}}

{{DEFAULTSORT:Low Energy Transfers}}
Category:Astrodynamics

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Adapted from the Wikipedia article [Low-energy transfer](https://en.wikipedia.org/wiki/Low-energy_transfer) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Low-energy_transfer?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.