https://en.wikipedia.org/wiki/Interplanetary_Transport_Network said:
Interplanetary Transport 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. ... While they use little energy, the transport can take a very long time.
History
The key to discovering the Interplanetary Transport Network was the investigation of the exact nature of the winding paths near the Earth-Sun and Earth-Moon Lagrange points. They were first investigated by Jules-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.[3] 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. ... 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)[5]
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 between the Galilean moons of Jupiter.[6] As a result, for the cost of reaching the Earth–Sun L2 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. But the trip from earth to Mars or other distant location would likely take thousands of years.
The transfers are so low-energy that they make travel to almost any point in the Solar System possible.[citation needed] On the downside, these transfers are very slow. For trips from Earth to other planets, they are not useful for manned or unmanned 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 mission, the first to return solar wind samples to Earth. The network is also relevant to understanding Solar System dynamics;[8][9] Comet Shoemaker–Levy 9 followed such a trajectory on its collision path with Jupiter.[10][11] In a more recent example, the 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.
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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 unstable orbits around the Lagrange points – points in space where the gravity between various bodies balances with the centrifugal force of an object there. ... appear to be the lowest-energy transfers possible, even lower than the common Hohmann transfer orbit that has dominated orbital navigation in the past.
