Skyhook (structure)

A skyhook is a proposed momentum exchange tether that aims to reduce the cost of placing payloads into space. A heavy orbiting station is connected to a cable which extends down towards the upper atmosphere. Payloads, which are much lighter than the station, are hooked to the end of the cable as it passes, and are then flung into orbit by rotation of the cable around the centre of mass. The station can then be reboosted to its original altitude by electromagnetic propulsion, rocket propulsion, or by deorbiting another object equal in mass to the payload.

A skyhook differs from a geostationary orbit space elevator in that a skyhook would be much shorter and would not come in contact with the surface of the Earth. A skyhook would require a suborbital launch vehicle to reach its lower end, while a space elevator would not.

Different synchronous non-rotating orbiting skyhook concepts and versions have been proposed, starting with Isaacs in 1966, Artsutanov in 1967, Pearson and Colombo in 1975, Kalaghan in 1978, and Braginski in 1985. The versions with best potential involve a much shorter tether in low Earth orbit which rotates in its orbital plane and whose ends brush the upper Earth atmosphere, with the rotational motion cancelling the orbital motion at ground level. These "rotating" skyhook versions were proposed by Moravec in 1976, and Sarmont in 1994.

When the Italian scientist Giuseppe Colombo proposed in the early 1970s the idea of using a tidally stabilized tether for downward-looking Earth observation satellites, NASA officially began to assess in 1979 the possible scientific applications for long tethers in space and whether the development of a tethered system was justified. This resulted in a Shuttle-based tether system: the TSS-1R mission, launched 22 February 1996 on STS-75 that focused in characterizing basic space tether behavior and space plasma physics. The Italian satellite was deployed to a distance of 19.7 km (12.2 mi) from the Space Shuttle.

An engineer speculated in 1994 that the skyhook could be cost competitive with what is realistically thought to be achievable using a space elevator.

In 2000 and 2001, Boeing Phantom Works, with a grant from NASA Institute for Advanced Concepts, performed a detailed study of the engineering and commercial feasibility of various skyhook designs. They studied in detail a specific variant of this concept, called "Hypersonic Space Tether Orbital Launch System" or HASTOL. This design called for a hypersonic ramjet or scramjet aircraft to intercept a rotating hook while flying at Mach 10.

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