A space elevator is a proposed non-rocket space launch structure (a structure designed to transport material from a celestial body's surface into space). Non-rocket space launch (NRS) is the idea of reaching outer space specifically from the Earth's surface predominantly without the use of conventional chemical rockets, which currently is the only method in use. Many elevator variants have been suggested, all of which involve travelling along a fixed structure instead of using rocket-powered space launch, most often a cable that reaches from the surface of the Earth on or near the equator to geostationary orbit (GSO) and a counterweight outside of the geostationary orbit. A geostationary orbit (or Geostationary Earth Orbit - GEO) is a geosynchronous orbit (orbit around the Earth with an orbital period that matches the Earth's sidereal rotation period) directly above the Earth's equator (0 degree latitude), with a period equal to the Earth's rotational period and an orbital eccentricity (orbital eccentricity of an astronomical body is the amount by which its orbit deviates from a perfect circle of approximately zero).
While some variants of the space elevator concept are technologically feasible, current technology is not capable of producing practical engineering materials that are sufficiently strong and light to build an Earth-based space elevator of the geostationary orbital tether type. Recent concepts for a space elevator are notable for their plans to use carbon nanotube or boron nitride nanotube based materials.
In 1895, the primary concept of the space elevator appeared, when Russian scientist Konstantin Tsiolkovsky was enthused by the Eiffel Tower in Paris to consider a tower that reached all the way into space, built from the ground up to an altitude of 35,790 kilometers (22,238 mi) above sea level (geostationary orbit). Unlike more recent concepts for space elevators, Tsiolkovsky's (conceptual) tower was a compression structure, rather than a tension (or "tether") structure. In 1959 another Russian scientist, Yuri N. Artsutanov, suggested a more feasible proposal. Artsutanov suggested using a geostationary satellite as the base from which to deploy the structure downward. By using a counterweight, a cable would be lowered from geostationary orbit to the surface of Earth, while the counterweight was extended from the satellite away from Earth, keeping the center of gravity of the cable constant relative to Earth.
Making a cable over 35,000 kilometers (22,000 miles) long is a difficult task. In 1966, Isaacs, Vine, Bradner and Bachus, four American engineers, reinvented the concept, naming it a "Sky-Hook," and published their analysis in the journal Science. After the development of carbon nanotubes in the 1990s, engineer David Smitherman of NASA/Marshall's Advanced Projects published a book which provides an introduction to the state of the technology at the time, and summarizes the findings. In 2008 the book "Leaving the Planet by Space Elevator", by Dr. Brad Edwards and Philip Ragan, was published in Japanese and entered the Japanese best seller list. This has led to a Japanese announcement of intent to build a space Elevator at a projected price tag of 5 billion dollars.
This notion, also known an orbital space elevator, geostationary orbital tether, or a beanstalk, is a division of the skyhook concept. Skyhooks are a theoretical class of cable based techniques intended to lift payloads to high altitudes and speeds. Construction would be a large project: the minimum length of an Earth-based space elevator is well over 38,000 km (24,000 mi) long. The tether would have to be built of a material that could bear tremendous stress while also being light-weight, cost-effective, and manufacturable in great quantities. Materials available at present do not meet these requirements, although carbon nanotube technology shows great guarantee.
The centrifugal force of earth's rotation is the main principle behind the elevator. As the earth rotates, the centrifugal force tends to align the nanotube in a stretched manner. There are a variety of tether designs. Almost every design includes a base station, a cable, climbers, and a counterweight.
The construction of a space elevator would be a vast project requiring advances in engineering, manufacturing, and physical technology.
A space elevator could also be constructed on other planets, asteroids and moons.
Mars' surface gravity is 38% of Earth's, while it rotates around its axis in about the same time as Earth. Current materials are already sufficiently strong to construct such an elevator. A lunar space elevator can possibly be built with currently available technology about 50,000 kilometers (31,000 miles) long extending through the Earth-Moon L1 (the L1 point lies on the line defined by the two large masses M1 and M2, and between them) point from an anchor point near the center of the visible part of Earth's moon. However, the lack of an atmosphere allows for other, perhaps better, alternatives to rockets. A lunar space elevator is a proposed cable running from the surface of the Moon into space.