View source for Interplanetary Transport Network

{{distinguish|InterPlanetary Network|Interplanetary Transport System}}
{{Short description|Low-energy trajectories in the Solar System}}
[[Image:Interplanetary Superhighway.jpg|thumb|350px|This stylized depiction of the ITN is designed to show its (often convoluted) path through the [[Solar System]]. The green ribbon represents one path from among the many that are mathematically possible along the surface of the darker green bounding tube. Locations where the ribbon changes direction abruptly represent [[trajectory]] changes at [[Lagrange points]], while constricted areas represent locations where objects linger in temporary [[orbit]] around a point before continuing on.]]

The '''Interplanetary Transport Network''' ('''ITN''')<ref name=Ross-SA-2006>{{cite journal |last=Ross |url=http://www2.esm.vt.edu/~sdross/papers/AmericanScientist2006.pdf |first=S. D. |date=2006 |doi=10.1511/2006.59.994 |title=The Interplanetary Transport Network |journal=[[American Scientist]] |volume=94 |issue=3 |pages=230–237 |access-date=2013-06-27 |archive-date=2013-10-20 |archive-url=https://web.archive.org/web/20131020185722/http://www2.esm.vt.edu/~sdross/papers/AmericanScientist2006.pdf |url-status=live }}</ref> is a collection of [[gravitation]]ally determined pathways through the [[Solar System]] that require very little [[energy]] for an object to follow.  The ITN makes particular use of [[Lagrange points]] as locations where [[trajectories]] through [[Outer space|space]] can be redirected using little or no energy. These points have the peculiar property of allowing objects to [[orbit]] around them, despite lacking an object to orbit, as these points exist where gravitational forces between two [[celestial bodies]] are equal. While it would use little energy, transport along the network would take a long time.<ref>{{Cite web |url=http://www2.esm.vt.edu/~sdross/superhighway/description.html |title=The Interplanetary Superhighway; Shane Ross; Virginia Tech. |access-date=2016-12-30 |archive-date=2019-06-15 |archive-url=https://web.archive.org/web/20190615004515/http://www2.esm.vt.edu/~sdross/superhighway/description.html |url-status=dead }}</ref>

==History==
Interplanetary transfer orbits are solutions to the gravitational [[three-body problem]], which, for the general case, does not have analytical solutions, and is addressed by [[numerical analysis]] approximations. However, a small number of exact solutions exist, most notably the five orbits referred to as "[[Lagrange point]]s", which are orbital solutions for circular orbits in the case when one body is significantly more massive.

The key to discovering the Interplanetary Transport Network was the investigation of the nature of the winding paths near the Earth-Sun and Earth-Moon Lagrange points. They were first investigated by [[Henri Poincaré]] in the 1890s. He noticed that the paths leading to and from any of those points would almost always settle, for a time, on an orbit about that point.<ref>{{cite journal |last1=Marsden |first1=J. E. |first2=S. D. |last2=Ross |date=2006 |doi=10.1090/S0273-0979-05-01085-2 |title=New methods in celestial mechanics and mission design |journal=Bull. Amer. Math. Soc. |volume=43 |pages=43–73 |doi-access=free }}</ref> There are in fact an [[Cardinality of the continuum|infinite]] number of paths taking one to the point and away from it, and all of which require nearly zero change in energy to reach. When plotted, they form a tube with the orbit about the Lagrange point at one end.

The derivation of these paths traces back to mathematicians [[Charles C. Conley]] and [[Richard P. McGehee]] in 1968.<ref name="Conley, C. C 1968">{{cite journal |last=Conley |first=C. C. |date=1968 |title=Low energy transit orbits in the restricted three-body problem |journal=SIAM Journal on Applied Mathematics |volume=16 |issue= 4|pages=732–746 |doi= 10.1137/0116060|jstor=2099124 |bibcode=1968SJAM...16..732C }}</ref>  ''[[Hiten (spacecraft)|Hiten]]'', Japan's first lunar probe, was moved into lunar orbit using similar insight into the nature of paths between the [[Earth]] and the [[Moon]].  Beginning in 1997, [[Martin Lo]], [[Shane D. Ross]], and others wrote a series of papers identifying the mathematical basis that applied the technique to the [[Genesis (spacecraft)|''Genesis'' solar wind sample return]], and to lunar and [[Jupiter|Jovian]] missions.  They referred to it as an Interplanetary Superhighway (IPS).<ref>Lo, Martin W. and Ross, Shane D. (2001) [http://www.gg.caltech.edu/~mwl/publications/papers/lunarGateway.pdf The Lunar L1 Gateway: Portal to the Stars and Beyond] {{Webarchive|url=https://web.archive.org/web/20130115075823/http://www.gg.caltech.edu/~mwl/publications/papers/lunarGateway.pdf |date=2013-01-15 }}, AIAA Space 2001 Conference, Albuquerque, New Mexico.</ref>

==Paths==
As it turns out, it is very easy to transit from a path leading to the point to one leading back out. This makes sense, since the orbit is unstable, which implies one will eventually end up on one of the outbound paths after spending no energy at all. [[Edward Belbruno]] coined the term "[[Weak Stability Boundary|weak stability boundary]]"<ref name="Belbruno2000">{{cite web|author1=Edward A. Belbruno|author2=John P. Carrico|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|access-date=2017-08-29|archive-date=2008-11-20|archive-url=https://web.archive.org/web/20081120191431/http://astrogatorsguild.com/wp-content/papers/0800_wsb.pdf|url-status=live}}</ref> or "fuzzy boundary"<ref name="Frank">{{cite journal|last=Frank|first=Adam|date=September 1994|title=Gravity's Rim|journal=[[Discover (magazine)|Discover]]|url=http://discovermagazine.com/1994/sep/gravitysrim419/|access-date=29 August 2017|archive-date=22 October 2019|archive-url=https://web.archive.org/web/20191022002414/http://discovermagazine.com/1994/sep/gravitysrim419|url-status=live}}</ref> for this effect.

With careful calculation, one can pick ''which'' outbound path one wants. This turns out to be useful, as many of these paths lead to some interesting points in space, such as the Earth's Moon or between the [[Galilean moons]] of [[Jupiter]], within a few months or years.<ref>Ross, S.D., W.S. Koon, M.W. Lo and J.E. Marsden (2003) [http://www.esm.vt.edu/~sdross/papers/AAS-03-143.pdf Design of a Multi-Moon Orbiter] {{webarchive|url=https://web.archive.org/web/20070108230403/http://www.esm.vt.edu/~sdross/papers/AAS-03-143.pdf |date=2007-01-08 }}. 13th AAS/AIAA Space Flight Mechanics Meeting, [[Ponce, Puerto Rico]], Paper No. AAS 03–143.</ref>

For trips from Earth to other planets, they are not useful for crewed or uncrewed probes, as the trip would take many generations. Nevertheless, they have already been used to transfer spacecraft to the Earth–Sun {{L1}} point, a useful point for studying the Sun that was employed in a number of recent missions, including the [[Genesis (spacecraft)|''Genesis'' mission]], the first to return [[solar wind]] samples to Earth.<ref>Lo, M. W., et al. 2001. Genesis Mission Design, ''The Journal of the Astronautical Sciences'' 49:169–184.</ref> The network is also relevant to understanding Solar System dynamics;<ref>Belbruno, E., and B.G. Marsden. 1997. [https://dx.doi.org/10.1086/118359 Resonance Hopping in Comets]. [[The Astronomical Journal]] 113:1433–1444</ref><ref>{{Cite journal|doi = 10.1063/1.166509|pmid = 12779398|title = Heteroclinic connections between periodic orbits and resonance transitions in celestial mechanics|journal = Chaos: An Interdisciplinary Journal of Nonlinear Science|volume = 10|issue = 2|pages = 427–469|year = 2000|last1 = Koon|first1 = Wang Sang|last2 = Lo|first2 = Martin W.|last3 = Marsden|first3 = Jerrold E.|last4 = Ross|first4 = Shane D.|bibcode = 2000Chaos..10..427K|url = https://authors.library.caltech.edu/28344/1/LOMjpltr99-0910.pdf|access-date = 2019-09-23|archive-date = 2018-07-19|archive-url = https://web.archive.org/web/20180719043232/https://authors.library.caltech.edu/28344/1/LOMjpltr99-0910.pdf|url-status = live}}</ref> [[Comet Shoemaker–Levy 9]] followed such a trajectory on its collision path with Jupiter.<ref>Smith, D. L. 2002. [http://pr.caltech.edu/periodicals/EandS/articles/LXV4/exit.html Next Exit 0.5 Million Kilometers] {{Webarchive|url=https://web.archive.org/web/20030329081959/http://pr.caltech.edu/periodicals/EandS/articles/LXV4/exit.html |date=2003-03-29 }}. Engineering and Science LXV(4):6–15</ref><ref>Ross, S. D. 2003. [http://www.esm.vt.edu/~sdross/papers/ross-barcelona-2002.pdf Statistical theory of interior–exterior transition and collision probabilities for minor bodies in the solar system] {{webarchive|url=https://web.archive.org/web/20070108230644/http://www.esm.vt.edu/~sdross/papers/ross-barcelona-2002.pdf |date=2007-01-08 }}, Libration Point Orbits and Applications (Eds. G Gomez, M.W. Lo and J.J. Masdemont), [[World Scientific]], pp. 637–652.</ref>

===Further explanation===
The ITN is based around a series of orbital paths predicted by [[chaos theory]] and the [[restricted three-body problem]] leading to and from the orbits around the Lagrange points – points in space where the [[gravity]] between various bodies balances with the centrifugal force of an object there. For any two bodies in which one body orbits around the other, such as a star/planet or planet/moon system, there are five such points, denoted {{L1}} through {{L5}}. For instance, the Earth–Moon {{L1}} point lies on a line between the two, where gravitational forces between them exactly balance with the centrifugal force of an object placed in orbit there. These five points have particularly low [[delta-v]] requirements, and appear to be the lowest-energy transfers possible, even lower than the common [[Hohmann transfer orbit]] that has dominated orbital navigation since the start of space travel.

Although the forces balance at these points, the first three points (the ones on the line between a certain large mass, e.g. a [[star]], and a smaller, orbiting mass, e.g. a [[planet]]) are not stable [[Mechanical equilibrium|equilibrium]] points. If a [[spacecraft]] placed at the Earth–Moon {{L1}} point is given even a slight nudge away from the equilibrium point, the spacecraft's trajectory will diverge away from the {{L1}} point. The entire system is in motion, so the spacecraft will not actually hit the Moon, but will travel in a winding path, off into space. There is, however, a semi-stable orbit around each of these points, called a [[halo orbit]]. The orbits for two of the points, {{L4}} and {{L5}}, are stable, but the halo orbits for {{L1}} through {{L3}} are stable only on the order of [[month]]s.

In addition to orbits around Lagrange points, the rich dynamics that arise from the gravitational pull of more than one mass yield interesting trajectories, also known as [[low energy transfers]].<ref name="Conley, C. C 1968"/> For example, the gravity environment of the Sun–Earth–Moon system allows spacecraft to travel great distances on very little fuel,{{citation needed|reason=From LEO, 3.2 km/s suffices for escape. To reach Sun earth L1, 3.1 km/s needs to be invested. the fuel savings isn't that great|date=November 2015}} albeit on an often circuitous route.

==Missions==
Launched in 1978, the [[ISEE-3]] spacecraft was sent on a mission to orbit around one of the Lagrange points.<ref>{{cite journal |last1=Farquhar |first1=R. W. |first2=D. P. |last2=Muhonen |first3=C. |last3=Newman |first4=H. |last4=Heuberger |date=1980 |title=Trajectories and Orbital Maneuvers for the First Libration-Point Satellite |journal=Journal of Guidance and Control |volume=3 |issue= 6|pages=549–554 |doi= 10.2514/3.56034|bibcode = 1980JGCD....3..549F }}</ref> The spacecraft was able to maneuver around the Earth's neighborhood using little fuel by taking advantage of the unique [[gravity]] environment. After the primary mission was completed, ISEE-3 went on to accomplish other goals, including a flight through the [[geomagnetic]] tail and a comet flyby. The mission was subsequently renamed the [[International Cometary Explorer]] (ICE).

The first low energy transfer using what would later be called the ITN was the rescue of [[Japan]]'s ''[[Hiten (spacecraft)|Hiten]]'' lunar mission in 1991.<ref>{{cite book |last=Belbruno |first=E. |date=2004 |url=http://www.pupress.princeton.edu/titles/7687.html |title=Capture Dynamics and Chaotic Motions in Celestial Mechanics: With the Construction of Low Energy Transfers |publisher=[[Princeton University Press]] |isbn=9780691094809 |access-date=2006-09-25 |archive-url=https://web.archive.org/web/20141202074354/http://www.pupress.princeton.edu/titles/7687.html |archive-date=2014-12-02 |url-status=dead }}</ref> 

Another example of the use of the ITN was [[NASA]]'s 2001–2003 [[Genesis (spacecraft)|Genesis mission]], which orbited the Sun–Earth {{L1}} point for over two years collecting material, before being redirected to the {{L2}} Lagrange point, and finally redirected from there back to Earth.<ref name=Ross-SA-2006/> 

The 2003–2006 ''[[SMART-1]]'' of the [[European Space Agency]] used another low energy transfer from the ITN.{{cn|date=July 2020}} 

In a more recent example, the [[China space program|Chinese]] spacecraft [[Chang'e 2]] used the ITN to travel from lunar orbit to the Earth-Sun {{L2}} point, then on to fly by the asteroid [[4179 Toutatis]].{{cn|date=July 2020}}

==Asteroids==
The asteroid [[39P/Oterma]]'s path from outside Jupiter's orbit, to inside, and back to outside is said to follow these low energy paths.<ref name=Ross-SA-2006/>

==See also==
{{colbegin}}
* [[Grand Tour program]], a [[NASA]] initiative to use a 175 years planet alignment that would eventually become the [[Voyager program]]
* {{annotated link|Gravitational keyhole}}
* {{annotated link|Gravity assist}}
* {{annotated link|Halo orbit}}
* {{annotated link|Hill sphere}}
* {{annotated link|Horseshoe orbit}}
* {{annotated link|Interplanetary spaceflight}}
* {{annotated link|Invariant manifold}}
* {{annotated link|Lunar cycler}}
* {{annotated link|Mars cycler}}
* {{annotated link|Orbital mechanics}}
{{colend}}

==References==
{{Reflist|2}}

==Further reading==
*[http://www2.esm.vt.edu/~sdross/books/ ''Dynamical Systems, the Three-Body Problem, and Space Mission Design''] {{Webarchive|url=https://web.archive.org/web/20180911225402/http://www2.esm.vt.edu/~sdross/books/ |date=2018-09-11 }}, by Wang Sang Koon, [[Martin Lo|Martin W. Lo]], [[Jerrold E. Marsden]], [[Shane D. Ross]] (book available as [http://www2.esm.vt.edu/~sdross/books/KoLoMaRo_DMissionBook_2011-04-25.pdf PDF] {{Webarchive|url=https://web.archive.org/web/20180911225453/http://www2.esm.vt.edu/~sdross/books/KoLoMaRo_DMissionBook_2011-04-25.pdf |date=2018-09-11 }}). {{ISBN|978-0-615-24095-4}}
* [http://resolver.caltech.edu/CaltechETD:etd-05182004-154045 "Cylindrical manifolds and tube dynamics in the restricted three-body problem" - PhD dissertation] by [[Shane D. Ross]]
* [http://www2.esm.vt.edu/~sdross/papers/AmericanScientist2006.pdf "The Interplanetary Transport Network"] {{Webarchive|url=https://web.archive.org/web/20131020185722/http://www2.esm.vt.edu/~sdross/papers/AmericanScientist2006.pdf |date=2013-10-20 }} by [[Shane D. Ross]], ''American Scientist'', May–June 2006 [https://dx.doi.org/10.1511/2006.59.994 (Subscription)]
* [https://www.newscientist.com/article/mg18925441.300.html "Ride the celestial subway" ''New Scientist'', 27 March 2006]
* [http://www.sciencemag.org/content/vol310/issue5751/r-samples.dtl#310/5751/1114a "Tube Route" ''Science'', 18 November 2005]
* [http://sciencenews.org/articles/20050416/bob9.asp "Navigating Celestial Currents" ''Science News'', 18 April 2005] {{Webarchive|url=https://web.archive.org/web/20050421000557/http://www.sciencenews.org/articles/20050416/bob9.asp |date=21 April 2005 }}

==External links==
* [http://pr.caltech.edu/periodicals/EandS/articles/LXV4/exit.html "Next Exit 0.5 Million Kilometers" Engineering and Science, 2002]
* [http://www.spacedaily.com/news/cosmology-05z.html "Mathematics Unites The Heavens And The Atom," ''Space Daily'', 28 September 2005]
* [http://focus.aps.org/story/v9/st31 "Asteroids Lost in Space" ''Physical Review Focus'', 14 June 2002]
* [https://www.youtube.com/watch?v=c00f1DiHygI Interplanetary Transport Network lecture (YouTube)] by [[Shane D. Ross]], 2004
*[https://web.archive.org/web/20141202074354/http://www.pupress.princeton.edu/titles/7687.html ''Capture Dynamics and Chaotic Motions in Celestial Mechanics: With the Construction of Low Energy Transfers''] - A mathematical analysis of aspects of the ITN, [[Edward Belbruno]] (2004)
* [https://archive.org/details/groks285 2007-10-08 audio interview with Belbruno on low-energy transfer]

{{Orbits}}

[[Category:Astrodynamics]]
[[Category:Dynamical systems]]
[[Category:Orbits]]