Basic Information
Lagrange Points (also known as L Points, Lagrangian Points, or Libration Points) are specific points in space where an object can orbit safely in relation to two other objects. For example, a space station could be built orbiting the earth at an Earth-Moon Lagrange Point. Such a space station would not have to worry about eventually crashing into the moon. At least, not without other mitigating circumstances. In the absence of other influences, it would maintain it's position relative to both the earth and the moon.
Any system of two objects, where one orbits another, should have 5 viable Lagrange points where a third smaller object could orbit without danger.
- In line with the two larger objects, closer to the medium than larger object, but between them. This is known as L1.
- In line with the two larger objects, closer to the medium than larger object, but on the far side where its view of the larger object were eclipsed by the medium object. This is known as L2.
- In line with the two larger objects, closer to the larger one than the medium one, and on the far side of the larger object such that all view of the medium object were entire eclipsed. This point, nearly as far away from the large object as the medium object is from the large object, is known as L3.
- At the same distance from both the large and medium object that they are from each other, in the orbital path of the medium object. This two points (one ahead of the medium objects motion, and one behind it) are known as L4 and L5 (respectively).
The existence of these points has be proven practically as well as mathematically.
See also: Movement of the Earth
Sources
Wikipedia has an exhaustive article on Lagrange points, from which the above information was drawn:
http://en.wikipedia.org/wiki/Lagrange_points
The Cyberpunk 2020 sourcebook Near Orbit explains Lagrange points well, and describes possible space stations at many of them.
Game and Story Use
- Lagrange points are the logical placement for space stations in a hard-science setting.
- Earth-Moon L1 is a good way station for travelers to and from the moon, and such a station may prove to be an important first step to colonizing the moon.
- Earth-Moon L2 is a good place for make an observatory for viewing other celestial bodies without the moon ever eclipsing your view.
- Sun-Earth L1 is a good spot to observe the sun from, and Sun-Earth L2 would be another, more distant, place for a celestial observatory.
- Earth-Moon L3 might be a good station for servicing vessels departing for or arriving from another world, as it would be clear of the gravitational influence of the moon.
- Sun-Earth L3 is the location of a Counter-Earth or parallel world in many pulp sci-fi tales.
- L4 and L5 are just locations for stable orbits with earth-like (or moon-like) conditions in regards to radiation and sunlight exposure, except of course they'd be lacking the shielding provided by the Earth's atmosphere. It may be possible they could serve various navigational beacon or flight plan functions, as well.
- As the L4 and L5 points are stable, they are good places for finding ancient alien artifacts left behind by precursor species.
- The terms trojan planet or trojan asteroid are used to refer to smaller natural bodies occuring at the L4 and L5 points of a star-planet system.