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A space elevator is a hypothetical structure designed to transport material from a planet's surface into space. Many different types of space elevator structures have been proposed. They all share the goal of replacing rocket propulsion with the traversal of a fixed structure via a mechanism not unlike an elevator, hence its name, in order to move material into or beyond orbit. Space elevators have also sometimes been referred to as space bridges, beanstalks, space ladders or space lifts. The most common proposal is a tether (usually 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. 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.