||This article includes a list of references, but its sources remain unclear because it has insufficient inline citations. (July 2010)|
Proposals for skyhooks include designs that employ tethers spinning at hypersonic speed for catching high speed payloads or high altitude aircraft and placing them in orbit.
Types of skyhooks
The non-rotating Skyhook was first proposed for use as part of an Earth-to-orbit / orbit-to-escape-velocity space transportation system by E. Sarmont at the 1990 International Space Development Conference. In this concept a suborbital launch vehicle would fly to the bottom end of a skyhook, while spacecraft bound for higher orbit, or returning from higher orbit, would use the upper end.
'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.
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.
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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.
- In the Star Wars expanded universe, skyhooks are common above Coruscant. They are frequently private retreats owned by corporations or wealthy individuals.
- In the LucasArts video game Star Wars: The Force Unleashed a skyhook is being constructed on the planet Kashyyyk.
- 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.
- 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,[clarification needed]
- Skyhook construction 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.
- Chen, Yi; Rui Huang, Xianlin Ren, Liping He, and Ye He (2013). "History of the Tether Concept and Tether Missions: A Review". ISRN Astronomy and Astrophysics 2013. 10.1155/2013/502973. Retrieved 2014-03-07.
- H. Moravec, "A non-synchronous orbital skyhook". Journal of the Astronautical Sciences, vol. 25, no. 4, pp. 307–322, 1977.
- M. P. Cartmell and D. J. McKenzie, "A review of space tether research," Progress in Aerospace Sciences, vol. 44, no. 1, pp. 1–21, 2008.
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- .M. L. Cosmo and E. C. Lorenzini, Tethers in Space Handbook, NASA Marshall Space Flight Center, Huntsville, Ala, USA, 3rd edition, 1997.
- .L. Johnson, B. Gilchrist, R. D. Estes, and E. Lorenzini, "Overview of future NASA tether applications," Advances in Space Research, vol. 24, no. 8, pp. 1055–1063, 1999.
- E. M. Levin, "Dynamic Analysis of Space Tether Missions", American Astronautical Society, Washington, DC, USA, 2007.
- Hypersonic Airplane Space Tether Orbital Launch (HASTOL) System: Interim Study Results
- Sarmont, E., "An Orbiting Skyhook: Affordable Access to Space", International Space Development Conference, Anaheim California, May 26, 1990 
- .J. D. Isaacs, A. C. Vine, H. Bradner, and G. E. Bachus, “Satellite elongation into a true "sky-hook",” Science, vol. 151, no. 3711, pp. 682–683, 1966.
- Extract: The Last Theorem by Arthur C Clarke and Frederik Pohl