Space elevator

Real World Theory
A space elevator is a theoretical structure designed to transport material from a planet's surface into space. A space elevator would consist of a cable attached to the surface on the equator and reaching outwards into space. By positioning it so that the total centrifugal force exceeds the total gravity, either by extending the cable or attaching a counterweight, the elevator would stay in place geosynchronously. Once moved far enough, climbers would be accelerated further by the planet's rotation.

The most common proposal is a tether, usually in the form of a cable or ribbon, that spans from the surface to a point beyond geosynchronous orbit. As the planet rotates, the inertia at the end of the tether counteracts gravity and keeps the tether taut. Vehicles can then climb the tether and escape the planet's gravity without the use of rockets. While the engineering of a structure is beyond current knowledge, as it requires an extremely light by extremely strong material (current estimates require a material ~2 g/cm³ in density and a tensile strength of ~70 Gpa), such a structure could eventually permit delivery of great quantities of cargo and people to orbit, and at costs only a fraction of those associated with current means.

In Halo
In Halo, there have been multiple space elevators constructed by the UNSC, both on Earth and her colonies. There are at least two on Earth, the New Mombasa Orbital Elevator in New Mombasa and the Centennial Orbital Elevator in Havana. Before it's loss in 2525, Harvest had seven elevators linked to the Tiara, the orbital port of Harvest. While there are certainly more space elevators constructed by UNSC, as of the events of Halo 3, all known elevators have been destroyed.

Space elevators consist of both elevator strands and a rigid support structure (which must be made of some unknown super strong material), the purpose of which could be conjectured to facilitate repairs on the strands, or even protect from micrometeroids, space debris, etc. The strands themselves are constructed of carbon nano-fiber (presumably a more advanced version of carbon nano-tubes), and vehicles using the structure are derive power from strands of superconducting material.

Links

 * Centennial Orbital Elevator
 * New Mombasa Orbital Elevator
 * Tiara
 * socialtext.net article