Interplanetary Superhighway

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Artist's concept of the Interplanetary Superhighway

The Interplanetary Superhighway has come to denote a set of transfer orbits between various planets and moons in the solar system. These transfers have particularly low delta-v requirements, and appear to be the lowest energy transfers, even lower than the common Hohmann transfer orbit that has dominated orbital dynamics in the past.

The Interplanetary Superhighway is based around a series of orbital paths predicted by chaos theory, leading to and from the unstable orbits around the Lagrange points — points in space where the gravity between various bodies balances out. There are a number of these around the Earth, created by the balance of forces between the Earth, Moon and Sun. For instance, the L1 point lies at the point between the Earth and Moon where the gravity of the two balances.

Although the forces balance at these points, they are not stable equilibrium points. If a spacecraft placed at the L1 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 L1 point. However the entire system is in motion, so the spacecraft will not actually hit the Moon, but travel in a winding path off into space. There is, however, a semi-stable orbit around each of these points. The orbits for two of the points, L4 and L5, are stable, but the orbits for L1 through L3 are stable only on the order of months.

The key to the Interplanetary Superhighway was investigating the exact nature of these winding paths near the points. They were first investigated by Jules-Henri Poincaré in the 1890s, and he noticed that the paths leading to and from any of these points would almost always settle, for a time, on the orbit around it. There are in fact an infinite number of paths taking you to the point and back away from it, and all of them require no energy to reach. When plotted, they form a tube with the orbit around the point at one end.

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 you'll eventually end up on one of the outbound paths after spending no energy at all. However, with careful calculation you can pick which outbound path you want. This turned out to be quite exciting, because many of these paths lead right by some interesting points in space, like Mars. That means that for the cost of getting to the Earth-Sun L2 point (Lagrange points exist for all bodies in orbit of each other, Earth-Moon, Earth-Sun, Mars-Sun etc.) which is rather low, one can travel to a huge number of very interesting points, almost for free.

The transfers are so low-energy 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 out of the Earth-Sun L1 point, a useful point for studying the Sun that was used in a number of recent missions, including the Genesis mission. The Solar and Heliospheric Observatory is here. The Interplanetary Superhighway is also relevant to understanding solar system dynamics; Comet Shoemaker-Levy 9 followed such a trajectory to collide with Jupiter.

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