Interplanetary Transport Network
 Not to be confused with InterPlanetary Network
The Interplanetary Transport Network (ITN)^{[1]} is a collection of gravitationally 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 space are 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. While they use little energy, the transport can take a very long time.
Contents
 History 1
 Further explanation 2
 See also 3
 Sources and notes 4
 External links 5
History
The key to discovering the Interplanetary Transport Network was the investigation of the exact nature of the winding paths near the EarthSun and EarthMoon Lagrange points. They were first investigated by JulesHenri 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.^{[2]} There are in fact an infinite number of paths taking one to the point and away from it, and all of which require no 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.^{[3]} 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 solar wind sample return, and to lunar and Jovian missions. They referred to it as an Interplanetary Superhighway (IPS)^{[4]}
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. However, with careful calculation, one can pick which outbound path one wants. This turned out to be useful, as many of these paths lead to some interesting points in space, such as the Earth's Moon or the Galilean moons of Jupiter.^{[5]} As a result, for the cost of reaching the Earth–Sun L_{2} point, which is rather low energy value, one can travel to a huge number of very interesting points for a little or no additional fuel cost.
The transfers are so lowenergy that they make travel to almost any point in the Solar System possible. On the downside, these transfers are very slow, and only useful for automated probes. Nevertheless, they have already been used to transfer spacecraft to the Earth–Sun L_{1} point, a useful point for studying the Sun that was employed in a number of recent missions, including the Genesis mission, the first to return solar wind samples to Earth.^{[6]} The Solar and Heliospheric Observatory began operations at L1 in 1996. The network is also relevant to understanding Solar System dynamics;^{[7]}^{[8]} Comet Shoemaker–Levy 9 followed such a trajectory on its collision path with Jupiter.^{[9]}^{[10]} In a more recent example, the Chinese spacecraft Chang'e 2 used the ITN to travel from lunar orbit to the EarthSun L2 point, then on to fly by the asteroid 4179 Toutatis.
Further explanation
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.^{[3]} For example, the gravity environment of the Sun–Earth–Moon system allows spacecraft to travel great distances on very little fuel, albeit on an often circuitous route. Launched in 1978, the ISEE3 spacecraft was sent on a mission to orbit around one of the Lagrange points.^{[11]} 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, ISEE3 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 lunar mission in 1991.^{[12]} Another example of the use of the ITN was NASA's 2001–2003 Genesis mission, which orbited the Sun–Earth L_{1} point for over two years collecting material, before being redirected to the L_{2} Lagrange point, and finally redirected from there back to Earth. The 2003–2006 SMART1 of the European Space Agency used another low energy transfer from the ITN.
The ITN is based around a series of orbital paths predicted by chaos theory and the restricted threebody problem leading to and from the unstable 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 three such points, denoted L_{1} through L_{3}. For instance, the Earth–Moon L_{1} 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. For two bodies whose ratio of masses exceeds 24.96, there are two additional stable points denoted as L_{4} and L_{5}. These five points have particularly low deltav requirements, and appear to be the lowestenergy transfers possible, even lower than the common Hohmann transfer orbit that has dominated orbital navigation in the past.
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 equilibrium points. If a spacecraft placed at the Earth–Moon L_{1} point is given even a slight nudge towards the Moon, for instance, the Moon's gravity will now be greater and the spacecraft will be pulled away from the L_{1} 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 semistable orbit around each of these points, called a halo orbit. The orbits for two of the points, L_{4} and L_{5}, are stable, but the halo orbits for L_{1} through L_{3} are stable only on the order of months.
See also
 Gravitational slingshot
 Gravitational keyhole
 Lowenergy transfer
 Orbital mechanics
 Interplanetary spaceflight
 Hill sphere
 Horseshoe orbit
 Halo orbit
Sources and notes
 ^ Ross, S. D. (2006). "The Interplanetary Transport Network" (PDF).
 ^ Marsden, J. E.; Ross, S. D. (2006). "New methods in celestial mechanics and mission design". Bull. Amer. Math. Soc. 43: 43–73.
 ^ ^{a} ^{b} Conley, C. C. (1968). "Low energy transit orbits in the restricted threebody problem". SIAM Journal on Applied Mathematics 16: 732–746.
 ^ Lo, Martin W. and Ross, Shane D. 2001. The Lunar L1 Gateway: Portal to the Stars and Beyond, AIAA Space 2001 Conference, Albequerque, New Mexico.
 ^ Ross, S.D., W.S. Koon, M.W. Lo and J.E. Marsden. 2003. Design of a MultiMoon Orbiter. 13th AAS/AIAA Space Flight Mechanics Meeting, Ponce, Puerto Rico. Paper No. AAS 03–143.
 ^ Lo, M. W., et al. 2001. Genesis Mission Design, The Journal of the Astronautical Sciences 49:169–184.
 ^ Belbruno, E., and B.G. Marsden. 1997. Resonance Hopping in Comets. The Astronomical Journal 113:1433–1444
 ^ W.S. Koon, M.W. Lo, J.E. Marsden, and S.D. Ross. 2000. Heteroclinic connections between periodic orbits and resonance transitions in celestial mechanics. Chaos 10:427–469
 ^ Smith, D. L. 2002. Next Exit 0.5 Million Kilometers. Engineering and Science LXV(4):6–15
 ^ Ross, S. D. 2003. Statistical theory of interior–exterior transition and collision probabilities for minor bodies in the solar system, Libration Point Orbits and Applications (Eds. G Gomez, M.W. Lo and J.J. Masdemont), World Scientific, pp. 637–652.
 ^ Farquhar, R. W.; Muhonen, D. P.; Newman, C.; Heuberger, H. (1980). "Trajectories and Orbital Maneuvers for the First LibrationPoint Satellite". Journal of Guidance and Control 3: 549–554.
 ^ Belbruno, E. (2004). Capture Dynamics and Chaotic Motions in Celestial Mechanics: With the Construction of Low Energy Transfers. Princeton University Press.
External links
 "The Interplanetary Transport Network" // American Scientist, May–June 2006 (Subscription)
 "Ride the celestial subway" New Scientist, 27 March 2006
 "Tube Route" Science, 18 November 2005
 "Navigating Celestial Currents" Science News, 18 April 2005
 "Next Exit 0.5 Million Kilometers" Engineering and Science, 2002
 "Mathematics Unites The Heavens And The Atom" Space Daily, 28 September 2005
 "Asteroids Lost in Space" Physical Review Focus, 14 June 2002
 Interplanetary Transport Network lecture (YouTube) by Shane D. Ross
 "Cylindrical manifolds and tube dynamics in the restricted threebody problem"  PhD dissertation by Shane D. Ross
 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
 The Dynamical Mechanism of Ballistic Lunar Capture Transfers in the FourBody Problem from the Perspective of Invariant Manifolds and Hill's Regions by Edward Belbruno
 Dynamical Systems, the ThreeBody Problem, and Space Mission Design, by Wang Sang Koon, Martin W. Lo, Jerrold E. Marsden, Shane D. Ross (book available as PDF). ISBN 9780615240954
 20071008 audio interview with Belbruno on lowenergy transfer
