This is also called the Interplanetary Transport Network.
The Interplanetary Superhighway is a collection of interconnected trajectories and orbits that allow objects, whether natural or manmade, to reach almost anywhere in the solar system with little or no energy. This network depends heavily on the properties of Lagrange Points associated with moons and planets.
Lagrange points are locations in space in a two-body system where the forces of gravity and orbital motion balance each other. French mathematician Louis Lagrange originally worked out the orbital mechanics of the positions in 1772, thus the points bear his name.
An object placed at a Lagrange point will be in a state of gravitational equilibrium, and will orbit with the same period as the bodies in the system. In other words, in the Earth-Moon system, an object at a lagrange point will keep pace with the Moon in its orbit about Earth.
In any two body system where one body orbits the other, there are five lagrange points. The first lagrange point, usually abbreviated L1, is located directly between the primary and the satellite. In the Earth-Moon system, the L1 point is roughly 200,000 miles (323,110 kilometers) away, or roughly 84% of the way to the moon.
The L2 point lies in direct line with the L1 point, but at a distance of some 37,000 miles (60,000 kilometers) behind the moon. The L3 point also lies along the same imaginary line of L1 and L2, but on the opposite side of Earth from the moon, in the Moon’s orbit.
Objects at the L1, L2, and L3 lagrange points would have what are called metastable orbits. The forces of gravity and orbital motion are precisely balanced at these points, but even a slight nudge will send any object at them drifting off. Think of a ball balanced precisely on top of a hill; though the ball is stable and in equilibrium, even the slightest push will send it rolling off its perch.
The L4 lagrange point lies 60 degrees trailing the Moon in its orbit, and the L5 lagrange point lies 60 degrees spinward of the Moon in its orbit, about 238,000 miles from both the moon and Earth, forming an equilateral triangle with those bodies. These are also called the Trojan Points, after the asteroids Agamemnon, Achilles, and Hector that orbit in the L4 and L5 points of the Jupiter-Sun system.
Unlike the first three lagrange points, L4 and L5 can offer true orbital stability. Whereas objects at lagrange points 1 through 3 can be held akin to a ball on a hill, objects at L4 and L5 can be thought of metaphorically as a ball in a large shallow pit on top of that hill. A gentle push at an object at these point will not send it drifting away, but will instead put it into "orbit" around the lagrange point.
Because objects can orbit a lagrange point, that also means that lagrange points can be used to curve an object's trajectory. Scientists can precisely calculate the forces involved, and therefore can tell how exactly unstable each orbit is. Knowing this, they can calculate the natural exit point and trajectory for any given orbital insertion into a lagrange point.
By using this technique in series, as well as curving trajectories using traditional orbits around solid masses like planets and moons, objects can be sent on long odysseys through the solar system, able to reach almost anywhere with little to no input of energy once in the network. One of the most advantageous insertion points into the network for a spacecraft is the Earth-Sun L2 point. If a spacecraft can reach there, almost the entire solar system is opened up to it.
However, this process can be quite slow, especially for distant destinations--many years or decades or longer. What a space agency may save in fuel they end up paying in much longer total mission times. Also, some fuel aboard a spacecraft would still be needed, even if using this network. While all the different trajectories and orbits of the network can be synched up spatially, there is still often a difference in relative velocity that must be compensated for.
These trajectories and orbits are usually formally referred to as invariant manifolds, or "tubes," to help visualize--mathematically and otherwise--how the network and its various interconnections work. More detailed explanations of this are detailed in the articles linked to at the bottom of the page.
Real spacecraft that have used this network so far include NASA's Genesis mission and the ESA's SMART-1.
Taking advantage of the Interplanetary Superhighway mean a number of adjustments for future space missions. The spacecraft may need less overall fuel, saving a great deal of potential cost. but may also have to carry heavier shielding and bigger and more robust power sources in order to serve it over much longer transit times.
Much farther in the future, the superhighway could also be used for transport of low priority bulk material between one outpost or base or colony and another. For example, a mission mining the moons of Saturn may launch a steady stream of relatively small mined material through the network so that eventually it would arrive at a processing station in, say, orbit around Mars. Even though the journey would take years for any one piece of debris, the steady stream of the material arriving over that time would make the operation worthwhile financially.
The Superhighway could also be of advantage in a long-term plan to build a base of colony on Mars, sending needed cargo and materials in bulk well ahead of astronauts using this network to cut total mission costs.
LAGRANGE POINTS
http://www.esa.int/esaSC/SEMM17XJD1E_index_0.html
http://orbitalvector.com/Space%20Structures/Lagrange%20Structures/LAGRANGE%20POINT%20STRUCTURES.htm
INTERPLANETARY TRANSPORT NETWORK
http://www2.esm.vt.edu/~sdross/superhighway/description.html
http://www.jpl.nasa.gov/releases/2002/release_2002_147.html
http://en.wikipedia.org/wiki/Interplanetary_Transport_Network
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