A space tether is a long cable, constructed of thin strands of high-strength fibre used to couple spacecraft to each other or to other masses, and providing a mechanical connection that enables the transfer of energy and momentum and can be used as a form of space propulsion. Konstantin Tsiolkovsky first conceived the idea after seeing the Eiffel Tower in 1889 - he then turned it into a giant tower extending out of the atmosphere, anchoring a "celestial castle" in geosynchronous orbit. Almost 100 years later, Arthur C. Clarke gave the concept of the space elevator greater public awareness and Robert L. Forward furthered the ideas in his books. In 1973, Mario Grossi suggested flying a wire from the Space Shuttle, which would act as an antenna emitting low-frequency radio waves, allowing the Shuttle to "trawl" the atmosphere and collect data. In 1975, Jerome Pearson suggested a tapered-cross- section tether, stronger than a straight strand, with a counterweight extending halfway to the Moon.

There are two general categories of tethers: Momentum-Exchange Tethers, which allow momentum and energy to be transferred between objects in space; and Electrodynamic Tethers, which interact with the Earth's magnetosphere to generate power or propulsion. Space tethers can be used for a diverse range of applications, including studying plasma physics and electrical generation in the upper atmosphere, space elevators, orbiting or deorbiting of space vehicles and payloads, planetary exploration, and mining of asteroids. In the century since their conception, space tethers have not been fully utilized. As the convergence of materials and technology continues, however, there will be numerous opportunities to use tethers in space.

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