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A skyhook is a proposed space transportation concept that its promoters say will make Earth to orbit, and interplanetary spaceflight affordable, thereby opening the way for the commercial development of lunar mining, asteroid mining, space-based solar power stations, space colonies, and colonies on the Moon, Mars, and in the asteroids. Skyhooks are often confused with an Earth surface to geostationary orbit space elevator, but they are different. A skyhook is a much shorter version of the space elevator that does not reach down to the surface of the Earth, is much lighter in mass, and can be affordably built with existing materials and technology. It works by starting from a relatively low altitude orbit and hanging a cable down to just above the Earth’s atmosphere. Since the lower end of the cable is moving at less than orbital velocity for its altitude, a launch vehicle flying to the bottom of the skyhook can carry a larger payload than it could otherwise carry to orbit. When the skyhook is long enough, single stage to skyhook flight with a reusable sub-orbital launch vehicle becomes possible.

The most important difference between a skyhook and a space elevator is that a skyhook can be built with presently available materials, while a space elevator cannot.

The following is a direct quote from “Hypersonic Airplane Space Tether Orbital Launch System” (HASTOL), a NASA funded study on rotating and non-rotating skyhooks.

“The fundamental conclusion of the Phase I HASTOL study effort is that the concept is technically feasible. We have evaluated a number of alternate system configurations that will allow hypersonic air-breathing vehicle technologies to be combined with orbiting, spinning space tether technologies to provide a method of moving payloads from the surface of the Earth into Earth orbit. For more than one HASTOL architecture concept, we have developed a design solution using existing, or near-term technologies. We expect that a number of the other HASTOL architecture concepts will prove similarly technically feasible when subjected to detailed design studies. The systems are completely reusable and have the potential of drastically reducing the cost of Earth-to-orbit space access. “

The following is a quote from Keith Henson, co-founder of the L-5 Society regarding space elevators.

“No current material exists with sufficiently high tensile strength and sufficiently low density out of which we could construct the cable . There’s nothing in sight that’s strong enough to do it – not even carbon nanotubes.”

Skyhooks come in two types: rotating and non-rotating. While no skyhook capable of capturing an arriving spacecraft has been built so far, there have been a number of flight experiments exploring various aspects of the skyhook/space tether concept.

Non-rotating skyhooks

A non-rotating skyhook is a vertically oriented, gravity gradient stabilized, tether whose lower endpoint does not reach the surface of the planet it is orbiting. As a result it appears to be hanging from the sky, hence the name skyhook. The idea of using a tidal stabilized tether for downward looking Earth observation satellites was first proposed by the Italian scientist Giuseppe Colombo.

The idea of using a non-rotating skyhook as part of a space transportation system where sub-orbital launch vehicles would fly to the bottom end of the tether, and spacecraft bound for higher orbit, or returning from higher orbit, would use the upper end of the tether, was first proposed by E. Sarmont in 1990, and expanded on in a second paper published in 1994. Other scientists and engineers, as well as NASA, Lockheed Martin, and former astronaut Bruce McCandless II have also investigated, validated, and added to the concept.

In addition, NASA representatives who have reviewed this concept have described it as, “The first idea we have seen that offers a believable path to $100 per pound to orbit.”

The non-rotating skyhook is not a space elevator. A non-rotating skyhook does not reach down to the surface of the Earth. The lower end of the non-rotating skyhook is above the upper edge of the atmosphere and requires a high-speed aircraft/sub-orbital launch vehicle to get there. Since the lower end of the non-rotating skyhook is moving at less than orbital velocity for its altitude, a launch vehicle flying to the bottom of the non-rotating skyhook can carry a larger payload than it could carry to orbit on its own. When the cable is long enough, single-stage to skyhook flight with a reusable sub-orbital launch vehicle becomes possible. In addition, unlike a space elevator that remains over the same spot on the Earth, a non-rotating skyhook circles the planet every few hours. This allows the non-rotating skyhook to serve as a terminal for sub-orbital launch vehicles arriving from just about anywhere on Earth. This type of skyhook can start out as short as 200 km and grow to over 4,000 km in length using a bootstrap method that takes advantage of the reduction in launch costs that come with each increase in tether length.

Note: The speed for orbit at the lower endpoint altitude is approximately 7.8 km/s. The tip speed of the lower end of a 200 km long non-rotating skyhook is approximately 7.5 km/s. The tip speed of the lower end of a 4,000 km long non-rotating skyhook is approximately 5.3 km/s.

At its longest, the non-rotating skyhook is approximately 1/25th the length of the 100,000 km long space elevator. As a result, it is much lighter in mass, and can be affordably built with existing commercially available carbon fiber materials. Analysis has also shown that this savings in cost to build for the non-rotating skyhook more than makes up for the additional cost of the sub-orbital launch vehicle that it requires. As a result, a mature non-rotating skyhook with reusable single stage sub-orbital launch vehicle is considered to be cost competitive with what is thought to be realistically achievable using a space elevator, assuming a space elevator can ever be built.

Another advantage of the non-rotating skyhook is that once it is long enough, the upper end of the cable as shown in figure 2, will be moving at just short of escape velocity for its altitude. This means that a spacecraft such as the Orion spacecraft, could be placed on either a free-return orbit to the Moon, or on course for a Near Earth Asteroid, without the need for an expensive expendable upper stage for boosting it to escape velocity. Elimination of the expendable upper stage and all the propellant it will require will also dramatically reduce the number of flights to the lower end of the non-rotating skyhook, which will further reduce the cost of such a mission.

This ability to capture sub-orbital launch vehicles coming up from the Earth at the lower end of the cable, and to launch spacecraft to higher orbits from the upper end of the cable, requires energy. Energy that comes from either a solar powered ion propulsion system or an electrodynamic propulsion system on the skyhook. The advantage of this over current launch systems is the greatly improved operating efficiency and reduced cost of either of these propulsion systems compared to conventional expendable rockets. While these high efficiency, low thrust, propulsion systems cannot be used for a planetary surface to orbit launch system due to their low thrust, they are perfect for use on an orbiting skyhook due to their ability to gradually store up energy by raising the orbital altitude of the skyhook between arriving and departing flights.

Additional information on the Earth orbiting non-rotating skyhook concept can be found in the following non-internet available articles.

A non-rotating skyhook system for Mars

In 1984, Paul Penzo of JPL, proposed a planetary surface to escape velocity tether transportation system for Mars that consists of two non-rotating skyhooks; one attached to the Martian moon Phobos, and the other attached to the Martian moon Deimos. With this system, a spacecraft arriving at Mars, either direct from Earth, or from an Earth-Mars Cycler spacecraft as it swings by Mars, docks at the upper end of the non-rotating skyhook attached to the outer moon Deimos. The people and cargo on that spacecraft then transfer to an elevator on the skyhook that will take them down to the lower end of the Deimos skyhook. There they board a small orbital transfer vehicle that will take them to the upper end of the non-rotating skyhook that is attached to the inner moon Phobos. Again they transfer to an elevator that will take them to the lower end of the Phobos skyhook where they will transfer to the reusable single stage Mars Lander that will carry them to the Martian surface. Passengers and cargo from the Martian surface that are bound for either the asteroids, or for Earth, would ride the system in reverse.

One of the advantages of this concept is that neither skyhook will require a propulsion system for orbital re-boost or for orbit control, as they both will use the Martian moon they are attached to as a momentum bank to make up any discrepancies in the upward versus downward mass flow of people and cargo. This elimination of the propulsion systems for the two skyhooks also makes for a significant reduction in the cost to build, and the cost to operate. Like the non-rotating skyhook for Earth, this two-stage non-rotating skyhook system for Mars can be affordably built with existing materials and technology.

Rotating skyhooks

A rotating tether 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 in the direction of Earth's rotation, 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 exchange tether. If it is used to lift many payloads into orbit its own orbit will degrade. If it catches fast moving 'rocks' on the high end of the skyhook, their kinetic energy would help lift the skyhook into higher orbits.

However, a Boeing study in 2000 assessed that "Trying to lower the tether tip speed to 4.0 km/s (13 000 ft/s or Mach 13) would require a skyhook tether mass greater than 200 times the payload mass." The Boeing team stated that "If the mass of the tether alone started to exceed 200 times the mass of the payload, then that was an indication the particular scenario being considered was not engineeringly feasible using presently available materials". As of 2013, a tether material with the required strength has not been developed yet.

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.
• 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".
• 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.

See also

• Mass driver
• Orbital rings
• Railgun
• Space elevator
• Space fountain
• Space tether
• Space tether missions

References

1. ^ Bogar, Thomas J.; Bangham, Michal E.; Forward, Robert L.; Lewis, Mark J. (7 January 2000). "Hypersonic Airplane Space Tether Orbital Launch System" (PDF). Research Grant No. 07600-018l Phase I Final Report. NASA Institute for Advanced Concepts. Retrieved 2014-03-20. {{cite conference}}: |format= requires |url= (help)
2. ^ Dvorsky, G. (13 February 2013). "Why we'll probably never build a space elevator". io9.com.
3. ^ Feltman, R. (7 March 2013). "Why Don't We Have Space Elevators?". Popular Mechanics.
4. ^ Scharr, Jillian (29 May 2013). "Space Elevators On Hold At Least Until Stronger Materials Are Available, Experts Say". Huffington Post.
5. ^ Templeton, Graham (6 March 2014). "60,000 miles up: Space elevator could be built by 2035, says new study". Extreme Tech. Retrieved 2014-04-19.
6. ^ Chen, Yi; Huang, Rui; Ren, Xianlin; He, Liping; He, Ye (2013). "History of the Tether Concept and Tether Missions: A Review". ISRN Astronomy and Astrophysics. 2013. doi:10.1155/2013/502973. Retrieved 2014-03-07.{{cite journal}}: CS1 maint: unflagged free DOI (link) Cite error: The named reference "isrn.502973" was defined multiple times with different content (see the help page).
7. ^ Cosmo, M.; Lorenzini, E. (December 1997). Tethers in Space Handbook (PDF) (Third ed.). Smithsonian Astrophysical Observatory.
8. Sarmont, E. (26 May 1990). An Orbiting Skyhook: Affordable Access to Space. International Space Development Conference. Anaheim California.
9. ^ Sarmont, E. (October 1994). "How an Earth Orbiting Tether Makes Possible an Affordable Earth-Moon Space Transportation System". SAE 942120.
10. ^ Wilson, N. (August 1998). "Space Elevators, Space Hotels and Space Tourism". SpaceFuture.com.
11. ^ Smitherman, D. V. "Space Elevators, An Advanced Earth-Space Infrastructure for the New Millennium". NASA/CP-2000-210429. Archived from the original on 2007-02-21. {{cite web}}: Unknown parameter |deadurl= ignored (|url-status= suggested) (help)
12. Mottinger, T., Marshall, L., “The Bridge to Space – A space access architecture”, AIAA 2000-5138
13. Mottinger, T., Marshall, L., “The Bridge to Space Launch System”, CP552, Space Technology and Applications International Forum, 2001
14. Marshall, L., Ladner, D., McCandless, B., "The Bridge to Space: Elevator Sizing & Performance Analysis", CP608, Space Technology and Applications International Forum, 2002
15. Stasko, S., Flandro, G., “The Feasibility of an Earth Orbiting Tether Propulsion System”, AIAA 2004-3901
16. Sarmont, E., ”Affordable to the Individual Spaceflight”, accessed Feb. 6, 2014
17. Sovey, J.S., Rawlin, V.K., and Patterson, M.J., "Ion Propulsion Development Projects in U.S.: Space Electric Rocket Test 1 to Deep Space 1", Journal of Propulsion and Power, Vol. 17, No. 3, May-June 2001, pp.517-526
18. Cartmell, M. P.; McKenzie, D. J. (2008). "A review of space tether research". Progress in Aerospace Sciences. 44 (1): 1–21.
19. Colombo, G.; Gaposchkin, E. M.; Grossi, M. D.; Weiffenbach, G. C. (1975). "The sky-hook: a shuttle-borne tool for low-orbital-altitude research". Meccanica. 10 (1): 3–20.
20. Johnson, L.; Gilchrist, B.; Estes, R. D.; Lorenzini, E. (1999). "Overview of future NASA tether applications". Advances in Space Research. 24 (8): 1055–1063.
21. Levin, E. M. (2007), Dynamic Analysis of Space Tether Missions, Washington, DC: American Astronautical Society
22. Penzo, P., "Tethers for Mars Space Operations", The Case for Mars II, Vol. 62, Science and Technology Series, July 1984, pp 445-465
23. Penzo, P., Carroll, J., "Mars Moons Tether Transport System", Tethers in Space Handbook, 3rd Edition, Dec. 1997, pp 70-71
24. Isaacs, J. D.; Vine, A. C.; Bradner, H.; Bachus, G. E. (1966). "Satellite elongation into a true "sky-hook"". Science. 151 (3711): 682–683.
25. "Extract: The Last Theorem by Arthur C Clarke and Frederik Pohl". The Daily Telegraph.

External links

• Moravec, Hans (1976). "Skyhook proposal".
• Moravec, Hans (1981). "Skyhook proposal".

v t e Non-rocket spacelaunch
Spaceflight
Static structures	Compressive Space tower Pneumatic freestanding tower Tensile Orbiting skyhooks Skyhook Momentum exchange tether Space elevators Space elevator
Dynamic structures	Space fountain Orbital ring Launch loop Endo-atmospheric tether
Projectile launchers	Electrical Coilgun Mass driver Railgun StarTram Chemical Space gun Blast wave accelerator Ram accelerator Mechanical Slingatron
Reaction drives	Air launch Spaceplanes Laser propulsion Beam-powered propulsion
Buoyant lifting	Balloon Buoyant space port High-altitude platform
See also Rocket sled launch Megascale engineering Category

• Megastructures
• Space elevator
• Spacecraft propulsion
• Vertical transport devices