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Skyhooks are a theoretical class of tether propulsion intended to lift payloads to high altitudes and speeds. The name skyhook is a reference to an imaginary hook that hangs from the sky.
Plausible near-term proposals for skyhooks include designs that employ tethers spinning at hypersonic speeds for catching payloads from very high speed, high altitude aircraft and placing them in orbit.
Another concept is orbital rings with geostationary 'spokes'.
Types of skyhooks 
The technique of hooking cargo on the ground or in the air with a capturing aircraft has been successfully used by the U.S. Air Force in the Fulton surface-to-air recovery system.
'Conventional' orbital skyhooks 
This is a type of cable that would be in orbit around the Earth, with a tip speed equal to its orbital speed (around 7–8 km/s). The tip rotates down, and as it does so, it moves backwards, slows, enters the atmosphere at low speed and picks up a payload from the ground (or the atmosphere). It then carries it up into space.
Note that the skyhook acts as a momentum bank. If it is used to lift many payloads into orbit its own orbit will degrade. However, if it catches fast moving 'junk rocks' on the high end of the skyhook, their kinetic energy will help lift the sky hook into higher orbits. Thus if a sky hook has been created, it can effectively convert waste asteroid or lunar material thrown to it, into high grade rocket fuel (that is by replacing the fuel spent to maintain its own orbit), by virtue of their kinetic energy. Another way around the energy loss would be to use electrodynamic tether technology to reboost the skyhook once it has lost some momentum (see tether propulsion).
The rotation rate of these kinds of skyhooks depends on the altitude that the center is maintained at, and is lower at higher altitudes. When the center of mass is placed in geostationary orbit then the cable rotates once per day with the Earth, and is usually referred to as a space elevator. Current materials technology does not permit these kinds of 'space elevator' structures to be practical, although advanced carbon nanotubes could in principle have the specific strength necessary to permit this.
Hypersonic orbital skyhooks 
A variation of this technique that appears to be currently possible is to have the tip speed of the cable lower than the orbital speed and use hypersonic aircraft to catch the tip while it is in the upper atmosphere.
This is probably possible with current technology as it doesn't need such high strength materials for the cable.
Orbital rings 
Orbital rings are solid or quasisolid rings that would encircle the Earth and would spin at faster than orbital speed in low earth orbit. The ring would support geostationary skyhooks that hang down to the ground that could be used to carry payloads to high altitude.
In fiction 
A form of hard-structure subsonic skyhook was constructed during the events of Jack McDevitt's novel Deepsix.
In the anime Bubblegum Crisis: Tokyo 2040, the three main protagonists arrive at the series' climactic battle with Galatea in Earth orbit by commandeering a skyhook transit system.
Turn-A Gundam, anime series, depicts an ancient hypersonic skyhook which has been maintained operationally by nanomachines over thousands of years. An ancient mass driver is also used for transporting space-vessels from earth's surface to the skyhook.
The planet of Tara K. Harper's Grey Ones series features a number of skyhook stations. The tethers are apparently no longer functioning, but large terminal structures still exist. They are used as a plot device, as ferrous weapons are impossible to use near the structures due to their intense magnetic fields.
A skyhook figures prominently in Arthur C. Clarke's posthumous novel The Last Theorem, which he co-wrote with Frederik Pohl. The novel describes the skyhook as a means of interplanetary travel rather than simply a means to reach orbit. It is used as a means of transport by athletes and delegates to the "first-ever lunar Olympics".
The construction of skyhooks, including a space elevator and several other orbital devices for launching craft into orbit and interplanetary travel, as well as decelerating and capturing craft on arrival, is a central theme in the science fiction novel The Barsoom Project, the second book in the Dream Park series, by Larry Niven and Steven Barnes. The destructive potential of a falling skyhook is also explored, and the potential for this to be exploited by terrorists.
In the Charlie and the Chocolate Factory sequel by Roald Dahl, Charlie and the Great Glass Elevator, Willy Wonka mentions in passing that the titular elevator flies by skyhooks. He is not, however, referring to such a device as described in this article or indeed to any specific device; he is merely giving a deliberately absurd answer to a question to avoid giving a real one. Insofar as any basis for the elevator's propulsion can be discerned - the book is, after all, a work of children's fantasy - it appears to fly by some combination of rockets and an unstated magical principle.
See also 
- A paper by Hans Moravec: http://www.frc.ri.cmu.edu/~hpm/project.archive/1976.skyhook/papers/scable.pox
- A draft of an article by Hans Moravec that appeared in Omni magazine