Editing Space elevator (section)

==Structure==
[[Image:SpaceElevatorClimbing.jpg|thumb|right|upright=1.2|One concept for the space elevator has it tethered to a mobile seagoing platform.]]
There are a variety of space elevator designs proposed for many planetary bodies. Almost every design includes a base station, a cable, climbers, and a counterweight. For an Earth Space Elevator the Earth's rotation creates upward [[centrifugal force]]<!--"upward" is a continuously changing direction which implies an accelerated reference frame where "c.f." is unquestionable (see http://xkcd.com/123/) --> on the counterweight. The counterweight is held down by the cable while the cable is held up and taut by the counterweight. The base station anchors the whole system to the surface of the Earth. Climbers climb up and down the cable with cargo.

===Base station===
Modern concepts for the base station/anchor are typically mobile stations, large oceangoing vessels or other mobile platforms. Mobile base stations would have the advantage over the earlier stationary concepts (with land-based anchors) by being able to maneuver to avoid high winds, storms, and [[space debris]]. Oceanic anchor points are also typically in [[international waters]], simplifying and reducing the cost of negotiating territory use for the base station.<ref name="Edwards" />

Stationary land-based platforms would have simpler and less costly logistical access to the base. They also would have the advantage of being able to be at high altitudes, such as on top of mountains. In an alternate concept, the base station could be a tower, forming a space elevator which comprises both a compression tower close to the surface, and a tether structure at higher altitudes.<ref name="JBIS1999" /> Combining a compression structure with a tension structure would reduce loads from the atmosphere at the Earth end of the tether, and reduce the distance into the Earth's gravity field that the cable needs to extend, and thus reduce the critical strength-to-density requirements for the cable material, all other design factors being equal.

===Cable===
[[File:Kohlenstoffnanoroehre Animation.gif|thumb|upright|[[Carbon nanotubes]] are one of the candidates for a cable material.<ref name="physorg_obayashi"/>]]
[[Image:SpaceElevatorAnchor.jpg|thumb|upright|A seagoing anchor station would also act as a deep-water [[seaport]].]]
A space elevator cable would need to carry its own weight as well as the additional weight of climbers. The required strength of the cable would vary along its length. This is because at various points it would have to carry the weight of the cable below, or provide a downward force to retain the cable and counterweight above. Maximum tension on a space elevator cable would be at geosynchronous altitude so the cable would have to be thickest there and taper as it approaches Earth. Any potential cable design may be characterized by the taper factor – the ratio between the cable's radius at geosynchronous altitude and at the Earth's surface.<ref>{{cite web |title=NAS-97-029: NASA Applications of Molecular Nanotechnology |author=Globus, Al |display-authors=etal |publisher=NASA |url=http://www.nas.nasa.gov/assets/pdf/techreports/1997/nas-97-029.pdf |access-date=27 September 2008 |archive-date=8 April 2016 |archive-url=https://web.archive.org/web/20160408064557/http://www.nas.nasa.gov/assets/pdf/techreports/1997/nas-97-029.pdf |url-status=dead }}</ref>

The cable would need to be made of a material with a high [[specific strength|tensile strength/density ratio]]. For example, the Edwards space elevator design assumes a cable material with a tensile strength of at least 100 [[gigapascal]]s.<ref name="Edwards"/> Since Edwards consistently assumed the density of his carbon nanotube cable to be 1300&nbsp;kg/m<sup>3</sup>,<ref name="EDWARDS_PHASE_I_2000_472Edwards.html"/> that implies a specific strength of 77 megapascal/(kg/m<sup>3</sup>). This value takes into consideration the entire weight of the space elevator. An untapered space elevator cable would need a material capable of sustaining a length of {{convert|4,960|km|mi|sp=us}} of its own weight ''at [[sea level]]'' to reach a [[geostationary]] altitude of {{convert|35786|km|mi|0|abbr=on}} without yielding.<ref>This 4,960 km "escape length" (calculated by [[Arthur C. Clarke]] in 1979) is much shorter than the actual distance spanned because [[Centrifugal force (fictitious)|centrifugal forces]] increase (and gravity decreases) dramatically with height: {{cite web |last=Clarke |first=A. C. |year=1979 |title=The space elevator: 'thought experiment', or key to the universe? |url=http://www.islandone.org/LEOBiblio/CLARK2.HTM |url-status=dead |archive-url=https://web.archive.org/web/20140103033306/http://www.islandone.org/LEOBiblio/CLARK2.HTM |archive-date=3 January 2014 |access-date=5 January 2010}}</ref> Therefore, a material with very high strength and lightness is needed.

For comparison, metals like titanium, steel or aluminium alloys have [[specific strength|breaking lengths]] of only 20–30&nbsp;km (0.2–0.3 MPa/(kg/m<sup>3</sup>)). Modern [[Man-made fibers|fiber]] materials such as [[kevlar]], [[fibreglass|fiberglass]] and [[Carbon fiber|carbon/graphite fiber]] have breaking lengths of 100–400&nbsp;km (1.0–4.0 MPa/(kg/m<sup>3</sup>)). Nanoengineered materials such as [[carbon nanotubes]] and, more recently discovered, [[graphene]] ribbons (perfect two-dimensional sheets of carbon) are expected to have breaking lengths of 5000–6000&nbsp;km (50–60 MPa/(kg/m<sup>3</sup>)), and also are able to conduct electrical power.{{Citation needed|date=April 2014}}

For a space elevator on Earth, with its comparatively high gravity, the cable material would need to be stronger and lighter than currently available materials.<ref name="Huff.3353697" /> For this reason, there has been a focus on the development of new materials that meet the demanding specific strength requirement. For high specific strength, carbon has advantages because it is only the sixth element in the [[periodic table]]. Carbon has comparatively few of the [[nucleons|protons and neutrons]] which contribute most of the dead weight of any material. Most of the interatomic [[Chemical bond|bonding forces]] of any element are contributed by only the [[Valence electron|outer few]] electrons. For carbon, the strength and stability of those bonds is high compared to the mass of the atom. The challenge in using carbon nanotubes remains to extend to macroscopic sizes the production of such material that are still perfect on the microscopic scale (as microscopic [[Crystallographic defects|defects]] are most responsible for material weakness).<ref name="Huff.3353697">{{cite news |first=Jillian |last=Scharr |title=Space Elevators On Hold At Least Until Stronger Materials Are Available, Experts Say |newspaper=Huffington Post |date=29 May 2013 |url=https://www.huffingtonpost.com/2013/05/29/space-elevators-stronger-materials_n_3353697.html}}</ref><ref>{{cite journal |last=Feltman |first=R. |title=Why Don't We Have Space Elevators? |journal=Popular Mechanics |date=7 March 2013 |url=http://www.popularmechanics.com/science/space/nasa/why-dont-we-have-space-elevators-15185070}}</ref><ref>{{cite news |last=Templeton |first=Graham |url=http://www.extremetech.com/extreme/176625-60000-miles-up-geostationary-space-elevator-could-be-built-by-2035-says-new-study |title=60,000 miles up: Space elevator could be built by 2035, says new study |work=Extreme Tech |date=6 March 2014 |access-date=14 April 2014}}</ref> As of 2014, carbon nanotube technology allowed growing tubes up to a few tenths of meters.<ref>{{cite journal| first1=X.| last1=Wang| title=Fabrication of Ultralong and Electrically Uniform Single-Walled Carbon Nanotubes on Clean Substrates| volume=9| pages=3137–3141| year=2009| doi=10.1021/nl901260b| journal=Nano Letters| last2=Li| first2=Q.| last3=Xie| first3=J.| last4=Jin| first4=Z.| last5=Wang| first5=J.| last6=Li| first6=Y.| last7=Jiang| first7=K.| last8=Fan| first8=S.| issue=9| pmid=19650638| bibcode=2009NanoL...9.3137W| url=http://www.chem.pku.edu.cn/page/liy/labhomepage/publications/2009/2009NL.pdf| url-status=dead| archive-url=https://web.archive.org/web/20170808164154/http://www.chem.pku.edu.cn/page/liy/labhomepage/publications/2009/2009NL.pdf| archive-date=8 August 2017| citeseerx=10.1.1.454.2744}}</ref>

In 2014, [[carbon nanothread|diamond nanothreads]] were first synthesized.<ref name="SCIAM_DN">{{cite magazine |url=http://www.scientificamerican.com/article/liquid-benzene-squeezed-to-form-diamond-nanothreads/ |title=Liquid Benzene Squeezed to Form Diamond Nanothreads |first=Julia |last=Calderone |date=26 September 2014 |magazine=[[Scientific American]] |access-date=22 July 2018}}</ref> Since they have strength properties similar to carbon nanotubes, diamond nanothreads were quickly seen as candidate cable material as well.<ref name="Xtech_DN">{{cite news |url=http://www.extremetech.com/extreme/190691-new-diamond-nanothreads-could-be-the-key-material-for-building-a-space-elevator |title=New diamond nanothreads could be the key material for building a space elevator |first=Sebastian |last=Anthony |date=23 September 2014 |publisher=Zeff Davis, LLC |newspaper=Extremetech |access-date=22 July 2018}}</ref>

===Climbers===
[[Image:SpaceElevatorInClouds.jpg|thumb|upright|A conceptual drawing of a space elevator climber ascending through the clouds.]]

A space elevator cannot be an elevator in the typical sense (with moving cables) due to the need for the cable to be significantly wider at the center than at the tips. While various designs employing moving cables have been proposed, most cable designs call for the "elevator" to climb up a stationary cable.

Climbers cover a wide range of designs. On elevator designs whose cables are planar ribbons, most propose to use pairs of rollers to hold the cable with friction.

Climbers would need to be paced at optimal timings so as to minimize cable stress and oscillations and to maximize throughput. Lighter climbers could be sent up more often, with several going up at the same time. This would increase throughput somewhat, but would lower the mass of each individual payload.<ref name="LangGTOSS">{{cite web |url=http://spaceelevatorwiki.com/wiki/images/2/2b/Paper_Lang_Climber_Transit.pdf |last=Lang |first=David D. |title=Space Elevator Dynamic Response to In-Transit Climbers |access-date=9 February 2016 |archive-date=28 May 2016 |archive-url=https://web.archive.org/web/20160528232403/http://spaceelevatorwiki.com/wiki/images/2/2b/Paper_Lang_Climber_Transit.pdf |url-status=dead }}</ref>

[[File:Space elevator balance of forces--circular Earth--more accurate force vectors.svg|thumb|upright=1.2|As the car climbs, the cable takes on a slight lean due to the Coriolis force. The top of the cable travels faster than the bottom. The climber is accelerated horizontally as it ascends by the Coriolis force which is imparted by angles of the cable. The lean-angle shown is exaggerated.]]
The horizontal speed, i.e. due to orbital rotation, of each part of the cable increases with altitude, proportional to distance from the center of the Earth, reaching low [[orbital speed]] at a point approximately 66 percent of the height between the surface and geostationary orbit, or a height of about 23,400&nbsp;km. A payload released at this point would go into a highly eccentric elliptical orbit, staying just barely clear from atmospheric reentry, with the [[periapsis]] at the same altitude as low earth orbit (LEO) and the [[apoapsis]] at the release height. With increasing release height the orbit would become less eccentric as both periapsis and apoapsis increase, becoming circular at geostationary level.<ref>{{cite web |first=Blaise |last=Gassend |title=Falling Climbers |url=http://gassend.net/spaceelevator/falling-climbers/index.html |access-date=16 December 2013}}</ref><ref>{{cite web |title=Space elevator to low orbit? |url=http://www.endlessskyway.com/2010/05/space-elevator-to-low-orbit.html |date=19 May 2010 |website=Endless Skyway |access-date=16 December 2013 |archive-url=https://web.archive.org/web/20131216184533/http://www.endlessskyway.com/2010/05/space-elevator-to-low-orbit.html |archive-date=16 December 2013 |url-status=dead}}</ref>

When the payload has reached GEO, the horizontal speed is exactly the speed of a circular orbit at that level, so that if released, it would remain adjacent to that point on the cable. The payload can also continue climbing further up the cable beyond GEO, allowing it to obtain higher speed at jettison. If released from 100,000&nbsp;km, the payload would have enough speed to reach the asteroid belt.<ref name="PhaseII" />

As a payload is lifted up a space elevator, it would gain not only altitude, but horizontal speed (angular momentum) as well. The angular momentum is taken from the Earth's rotation. As the climber ascends, it is initially moving slower than each successive part of cable it is moving on to. This is the [[Coriolis force]]: the climber "drags" (westward) on the cable, as it climbs, and slightly decreases the Earth's rotation speed. The opposite process would occur for descending payloads: the cable is tilted eastward, thus slightly increasing Earth's rotation speed.

The overall effect of the <!--n.b. the elevator is in a non inertial reference frame, so centrifugal is correct--->centrifugal force acting on the cable would cause it to constantly try to return to the energetically favorable vertical orientation, so after an object has been lifted on the cable, the counterweight would swing back toward the vertical, a bit like a pendulum.<ref name="LangGTOSS" /> Space elevators and their loads would be designed so that the center of mass is always well-enough above the level of geostationary orbit<ref>{{cite web |url=http://gassend.net/spaceelevator/center-of-mass/index.html |title=Why the Space Elevator's Center of Mass is not at GEO |first=Blaise |last=Gassend |access-date=30 September 2011}}</ref> to hold up the whole system. Lift and descent operations would need to be carefully planned so as to keep the pendulum-like motion of the counterweight around the tether point under control.<ref>{{cite journal|doi=10.1016/j.actaastro.2008.10.003|title=The effect of climber transit on the space elevator dynamics|year=2009|last1=Cohen|first1=Stephen S.|last2=Misra|first2=Arun K.|journal=Acta Astronautica|volume=64|issue=5–6|pages=538–553|bibcode=2009AcAau..64..538C}}</ref>

Climber speed would be limited by the Coriolis force, available power, and by the need to ensure the climber's accelerating force does not break the cable. Climbers would also need to maintain a minimum average speed in order to move material up and down economically and expeditiously.<ref>{{Cite web|last=Courtland|first=Rachel|title=Space elevator trips could be agonisingly slow|url=https://www.newscientist.com/article/dn16223-space-elevator-trips-could-be-agonisingly-slow/|access-date=2021-05-28|website=New Scientist|language=en-US}}</ref> At the speed of a very fast car or train of {{convert|300|km/h|mph|abbr=on}} it will take about 5 days to climb to geosynchronous orbit.<ref>{{cite book |last1=Fawcett |first1=Bill |title=LIFTPORT |last2=Laine |first2=Michael |last3=Nugent Jr. |first3=Tom |date=2006 |publisher=Meisha Merlin Publishing, Inc. |isbn=978-1-59222-109-7 |location=Canada |page=103 |language=en |name-list-style=amp}}</ref>

===Powering climbers===
Both power and energy are significant issues for climbers – the climbers would need to gain a large amount of potential energy as quickly as possible to clear the cable for the next payload.

Various methods have been proposed to provide energy to the climber:
* Transfer the energy to the climber through [[wireless energy transfer]] while it is climbing.
* Transfer the energy to the climber through some material structure while it is climbing.
* Store the energy in the climber before it starts – requires an extremely high [[specific energy]] such as nuclear energy.
* Solar power – After the first 40&nbsp;km it is possible to use solar energy to power the climber<ref>{{cite web |last1=Swan |first1=P. A. |last2=Swan |first2=C. W. |last3=Penny |first3=R. E. |last4=Knapman |first4=J. M. |last5=Glaskowsky |first5=P. N. |title=Design Consideration for Space Elevator Tether Climbers |url=http://isec.org/pdfs/isec_reports/2013_ISEC_Design_Considerations_for_Space_Elevator_Tether_Climbers_Final_Report.pdf |url-status=dead |archive-url=https://web.archive.org/web/20170116175959/http://isec.org/pdfs/isec_reports/2013_ISEC_Design_Considerations_for_Space_Elevator_Tether_Climbers_Final_Report.pdf |archive-date=16 January 2017 |publisher=[[International Space Elevator Consortium|ISEC]] |quote=During the last ten years, the assumption was that the only power available would come from the surface of the Earth, as it was inexpensive and technologically feasible. However, during the last ten years of discussions, conference papers, IAA Cosmic Studies, and interest around the globe, many discussions have led some individuals to the following conclusions: • Solar Array technology is improving rapidly and will enable sufficient energy for climbing • Tremendous advances are occurring in lightweight deployable structures.}}</ref>

Wireless energy transfer such as [[laser power beaming]] is currently considered the most likely method, using megawatt-powered free electron or solid state lasers in combination with adaptive mirrors approximately {{convert|10|m|ft|abbr=on}} wide and a photovoltaic array on the climber tuned to the laser frequency for efficiency.<ref name="Edwards" /> For climber designs powered by power beaming, this efficiency is an important design goal. Unused energy would need to be re-radiated away with heat-dissipation systems, which add to weight.

Yoshio Aoki, a professor of precision machinery engineering at [[Nihon University]] and director of the Japan Space Elevator Association, suggested including a second cable and using the conductivity of carbon nanotubes to provide power.<ref name="JapanUKTimes" />

===Counterweight===
[[File:Nasa space elev.jpg|thumb|Space elevator with space station]]
Several solutions have been proposed to act as a counterweight:
* a heavy, captured [[asteroid]]<ref name="NASASci" /><ref>{{cite web|url=https://www.popsci.com/building-hanging-from-an-asteroid/ |title=This building hanging from an asteroid is absurd – but let's take it seriously for a second |work=Popular Science |first=Sara |last=Chodosh |date=29 March 2017 |language=en|access-date=4 September 2019}}</ref>
* a [[space dock]], [[space station]] or [[spaceport]] positioned past geostationary orbit
* a further upward extension of the cable itself so that the net upward pull would be the same as an equivalent counterweight
* parked spent climbers that had been used to thicken the cable during construction, other junk, and material lifted up the cable for the purpose of increasing the counterweight.<ref name="PhaseII">Edwards BC, Westling EA. (2002) ''The Space Elevator: A Revolutionary Earth-to-Space Transportation System.'' San Francisco, California: Spageo Inc. {{ISBN|0-9726045-0-2}}.</ref>
Extending the cable has the advantage of some simplicity of the task and the fact that a payload that went to the end of the counterweight-cable would acquire considerable velocity relative to the Earth, allowing it to be launched into interplanetary space. Its disadvantage is the need to produce greater amounts of cable material as opposed to using just anything available that has mass.