Editing Space elevator (section)

===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>