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A space elevator is a proposed structure designed to transport material from a celestial body's surface into space. Many variants have been proposed, all of which involve traveling along a fixed structure instead of using rocket powered space launch. The concept most often refers to a structure that reaches from the surface of the Earth to geostationary orbit (GSO) and a counter-mass beyond.
The concept of a space elevator dates back to 1895 when Konstantin Tsiolkovsky proposed a free-standing "Tsiolkovsky" tower reaching from the surface of Earth to geostationary orbit. Most recent discussions focus on tensile structures (specifically, tethers) reaching from geostationary orbit to the ground. This structure would be held in tension between Earth and the counterweight in space like a guitar string held taut. Space elevators have also sometimes been referred to as beanstalks, space bridges, space lifts, space ladders, skyhooks, orbital towers, or orbital elevators.
Current (2009) technology is not capable of manufacturing practical engineering materials that are sufficiently strong and light to build an Earth based space elevator. This is because the total mass of conventional materials needed to construct such a structure would be far too great. However, recent conceptualizations for a space elevator are notable in their plans to use carbon nanotube-based materials as the tensile element in the tether design, since the measured strength of microscopic carbon nanotubes appears great enough to make this theoretically possible. Current technology could produce elevators for locations in the solar system with a weaker gravitational field, such as Mars.
An Orbital Ring is a concept for a space elevator that consists of a ring in low earth orbit that rotates at slightly above orbital speed, that has fixed tethers hanging down to the ground.
A launch loop or Lofstrom loop is a design for a belt based maglev orbital launch system that would be around 2,000 km (1,240 mi) long and maintained at an altitude of up to 80 km (50 mi). A launch loop would be held up at this altitude by momentum of the belt as it circulates around the structure, in effect it transfers the weight of the structure onto magnetic bearings at each end which support it.
Launch loops are intended to provide a way for non-rocket spacelaunch of vehicles weighing 5 metric tons by electromagnetically accelerating them so that they are projected into Earth orbit or even beyond. This would be achieved by the flat part of the cable which forms an acceleration track above the atmosphere.
The published cost estimates for a working launch loop are significantly lower than a space elevator and additionally has a greater launch capacity, lower payload costs and similar or greater payload masses. Unlike the space elevator no new materials need to be developed.
The system is designed to be suitable for launching humans for space tourism, space exploration and space colonization, and provides a relatively gentle 3g acceleration.