Skyhooks are a class of cable based techniques for lifting 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 high altitude aircraft and placing them in orbit.
Another concept is orbital rings with geostationary 'spokes'.
See tether propulsion for more details on various types of skyhooks.
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 7-8km/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.
Current materials technology does not permit these kinds of structures to be practical, although advanced carbon nanotubes could in principle have the specific strength necessary to permit this.
The rotation speed of these kinds of skyhooks depends on the altitude that the centre is maintained at, and is lower at higher altitudes. When the centre of mass is placed at the Clarke orbit then the system is then geostationary and the cable rotates once per day with the Earth, and is usually referred to as a space elevator.
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 are solid or quasisolid rings that spin at faster than orbital speed and are placed in low earth orbit, and would support geostationary skyhooks that hang down to the ground that could be used to carry payloads to high altitude.
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 Steve Barnes. The destructive potential of a falling skyhook is also explored, and the potential for this to be exploited by terrorists.
Published - July 2009
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