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Space elevator

Diagram of a space elevator. At the bottom of the tall diagram is the Earth as viewed from high above the North Pole. About six earth-radii above the Earth an arc is drawn with the same center as the Earth. The arc depicts the level of geosynchronous orbit. About twice as high as the arc and directly above the Earth's center, a counterweight is depicted by a small square. A line depicting the space elevator's cable connects the counterweight to the equator directly below it. The system's center of mass is described as above the level of geosynchronous orbit. The center of mass is shown roughly to be about a quarter of the way up from the geosynchronous arc to the counterweight. The bottom of the cable is indicated to be anchored at the equator. A climber is depicted by a small rounded square. The climber is shown climbing the cable about one third of the way from the ground to the arc. Another note indicates that the cable rotates along with the Earth's daily rotation, and remains vertical.
A space elevator is conceived as a cable fixed to the equator and reaching into space. A counterweight at the upper end keeps the center of mass well above geostationary orbit level. This produces enough upward centrifugal force from Earth's rotation to fully counter the downward gravity, keeping the cable upright and taut. Climbers carry cargo up and down the cable.
Space elevator in motion rotating with Earth, viewed from above North Pole. A free-flying satellite (green dot) is shown in geostationary orbit slightly behind the cable.

A space elevator, also referred to as a space bridge, star ladder, and orbital lift, is a proposed type of planet-to-space transportation system,[1] often depicted in science fiction. The main component would be a cable (also called a tether) anchored to the surface and extending into space. An Earth-based space elevator cannot be constructed with a tall tower supported from below due to its immense weight—instead, it would consist of a cable with one end attached to the surface near the equator and the other end attached to a counterweight in space beyond geostationary orbit (35,786 km altitude). The competing forces of gravity, which is stronger at the lower end, and the upward centrifugal force, which is stronger at the upper end, would result in the cable being held up, under tension, and stationary over a single position on Earth. With the tether deployed, climbers (crawlers) could repeatedly climb up and down the tether by mechanical means, releasing their cargo to and from orbit.[2] The design would permit vehicles to travel directly between a planetary surface, such as the Earth's, and orbit, without the use of large rockets.

The concept of a tower reaching geosynchronous orbit was first published in 1895 by Konstantin Tsiolkovsky.[3] His proposal was for a free-standing tower reaching from the surface of Earth to the height of geostationary orbit. Like all buildings, Tsiolkovsky's structure would be under compression, supporting its weight from below. Since 1959, most ideas for space elevators have focused on purely tensile structures, with the weight of the system held up from above by centrifugal forces. In the tensile concepts, a space tether reaches from a large mass (the counterweight) beyond geostationary orbit to the ground. This structure is held in tension between Earth and the counterweight like an upside-down plumb bob. The cable thickness is adjusted based on tension; it has its maximum at a geostationary orbit and the minimum on the ground.

Available materials are not strong and light enough to make an Earth space elevator practical.[4][5][6] Some sources expect that future advances in carbon nanotubes (CNTs) could lead to a practical design.[2][7][8] Other sources believe that CNTs will never be strong enough.[9][10][11] Possible future alternatives include boron nitride nanotubes, diamond nanothreads[12][13] and macro-scale single crystal graphene.[14]

The concept is applicable to other planets and celestial bodies. For locations in the Solar System with weaker gravity than Earth's (such as the Moon or Mars), the strength-to-density requirements for tether materials are not as problematic. Currently available materials (such as Kevlar) are strong and light enough that they could be practical as the tether material for elevators there.[15]

  1. ^ "What is a Space Elevator?". The International Space Elevator Consortium. 2014. Retrieved August 22, 2020.
  2. ^ a b Edwards, Bradley Carl. The NIAC Space Elevator Program (Report). NASA Institute for Advanced Concepts. Archived from the original on May 12, 2008. Retrieved November 24, 2007.{{cite report}}: CS1 maint: bot: original URL status unknown (link)
  3. ^ Hirschfeld, Bob (January 31, 2002). "Space Elevator Gets Lift". TechTV. Archived from the original on June 8, 2005. Retrieved September 13, 2007. The concept was first described in 1895 by Russian author K. E. Tsiolkovsky in his 'Speculations about Earth and Sky and on Vesta.'
  4. ^ Fleming, Nic (February 15, 2015). "Should We give up on the dream of space elevators?". BBC. Retrieved January 4, 2021. 'This is extremely complicated. I don't think it's really realistic to have a space elevator,' said Elon Musk during a conference at MIT, adding that it would be easier to 'have a bridge from LA to Tokyo' than an elevator that could take material into space.
  5. ^ Donahue, Michelle Z. (January 21, 2016). "People Are Still Trying to Build a Space Elevator". Smithsonian Magazine. Retrieved January 4, 2020. 'We understand it's a difficult project,' YojiIshikawa says. 'Our technology is very low. If we need to be at 100 to get an elevator built – right now we are around a 1 or 2. But we cannot say this project is not possible.'
  6. ^ "Why the world still awaits its first space elevator". The Economist. January 30, 2018. Retrieved January 4, 2020. The chief obstacle is that no known material has the necessary combination of lightness and strength needed for the cable, which has to be able to support its own weight. Carbon nanotubes are often touted as a possibility, but they have only about a tenth of the necessary strength-to-weight ratio and cannot be made into filaments more than a few centimetres long, let alone thousands of kilometres. Diamond nanothreads, another exotic form of carbon, might be stronger, but their properties are still poorly understood.
  7. ^ Cite error: The named reference Smitherman was invoked but never defined (see the help page).
  8. ^ Cite error: The named reference universetoday was invoked but never defined (see the help page).
  9. ^ Aron, Jacob (June 13, 2016). "Carbon nanotubes too weak to get a space elevator off the ground". New Scientist. Retrieved January 3, 2020. Feng Ding of the Hong Kong Polytechnic University and his colleagues simulated CNTs with a single atom out of place, turning two of the hexagons into a pentagon and heptagon, and creating a kink in the tube. They found this simple change was enough to cut the ideal strength of a CNT to 40 GPa, with the effect being even more severe when they increased the number of misaligned atoms... That's bad news for people who want to build a space elevator, a cable between the Earth and an orbiting satellite that would provide easy access to space. Estimates suggest such a cable would need a tensile strength of 50 GPa, so CNTs were a promising solution, but Ding's research suggests they won't work.
  10. ^ Christensen, Billn (June 2, 2006). "Nanotubes Might Not Have the Right Stuff". Space.com. Retrieved January 3, 2020. recent calculations by Nicola Pugno of the Polytechnic of Turin, Italy, suggest that carbon nanotube cables will not work... According to their calculations, the cable would need to be twice as strong as that of any existing material including graphite, quartz, and diamond.
  11. ^ Whittaker, Clay (June 15, 2016). "Carbon Nanotubes Can't Handle a Space Elevator". Popular Science. Retrieved January 3, 2020. Alright, space elevator plans are back to square one, people. Carbon nanotubes probably aren't going to be our material solution for a space elevator, because apparently even a minuscule (read: atomic) flaw in the design drastically decreases strength.
  12. ^ Cite error: The named reference SCIAM_DN was invoked but never defined (see the help page).
  13. ^ Cite error: The named reference Xtech_DN was invoked but never defined (see the help page).
  14. ^ "Space Elevator Technology and Graphene: An Interview with Adrian Nixon". July 23, 2018.
  15. ^ Moravec, Hans (1978). Non-Synchronous Orbital Skyhooks for the Moon and Mars with Conventional Materials. Carnegie Mellon University. frc.ri.cmu.edu.

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