Space fountain

A space fountain is a proposed form of structure extending into space that, like a space elevator, can extend to geostationary orbit, but does not rely on tensile strength for support. In contrast to the space elevator design, a space fountain is a tremendously tall tower extending up from the ground. Since such a tall tower could not support its own weight using traditional materials, fast-moving pellets are projected upward from the bottom of the tower and redirected back down once they reach the top, so that the force of redirection holds the top of the tower aloft. Payloads ascend or descend by coupling with this stream of pellets or by climbing up the side of the tower. The space fountain has some advantages over a space elevator in that it does not require materials with extreme strength, can be located at any point on a planet's surface instead of just the equator, and can be raised to heights lower than the level of geostationary orbit. Its major disadvantages come about from the fact that it is an extremely-high-energy active structure. It requires constant power input to make up energy losses and remain erect. The high energy content of the kinetic component of the structure also continually threatens to cause the collapse of the tower if the containment systems fail.

The concept originated in a conversation on a computer net in the 1980s when scientists Marvin Minsky of MIT, John McCarthy, and Hans Moravec of Stanford, speculated about variations on the skyhook concept with Roderick Hyde and Lowell Wood, scientists at Lawrence Livermore National Laboratory. As a means of supporting the upper end of a traditional space elevator at an altitude much less than geostationary, they proposed a ring of space stations hovering 2,000 kilometers above Earth, motionless relative to the surface. These stations would not be in orbit; they would support themselves by deflecting a ring of fast-moving pellets circling Earth. The pellets would be moving at far greater speed than the orbital velocity for that altitude, so if the stations stopped deflecting them the pellets would move outward and the stations would fall inward.

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