Old page wikitext, before the edit (old_wikitext ) | '{{Use mdy dates|date=June 2013}}
[[File:Space elevator structural diagram--corrected for scale+CM+etc.svg|thumb|upright=1.2|alt=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.]]
[[File:Space elevator in motion viewed from above north pole.ogv|thumbtime=28|thumb|upright=1.2|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''' is a proposed type of planet-to-space transportation system.<ref>{{cite web|url=http://www.isec.org/index.php/what-is-a-space-elevator |title=What is a Space Elevator? |publisher=The International Space Elevator Consortium |date=April 11, 2012}}</ref> The main component would be a cable (also called a [[space tether|tether]]) anchored to the surface and extending into space. The design would permit vehicles to travel along the cable from a planetary surface, such as the Earth's, directly into space or orbit, [[non-rocket spacelaunch|without the use of large rockets]]. An Earth-based space elevator would consist of a cable with one end attached to the surface near the equator and the other end in space beyond [[geostationary orbit]] (35,786 km altitude). The competing forces of gravity, which is stronger at the lower end, and the outward/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 could repeatedly climb the tether to space by mechanical means, releasing their cargo to orbit. Climbers could also descend the tether to return cargo to the surface from orbit.<ref name=Edwards>Edwards, Bradley Carl. [http://www.niac.usra.edu/studies/521Edwards.html "The NIAC Space Elevator Program"]. NASA Institute for Advanced Concepts</ref>
The concept of a tower reaching geosynchronous orbit was first published in 1895 by [[Konstantin Tsiolkovsky]].<ref>{{cite web|url = http://www.g4tv.com/techtvvault/features/35657/Space_Elevator_Gets_Lift.html|title = Space Elevator Gets Lift|accessdate = September 13, 2007|last = Hirschfeld|first = Bob|date = January 31, 2002 |publisher = [[TechTV]] |archiveurl = https://web.archive.org/web/20050608080057/http://www.g4tv.com/techtvvault/features/35657/Space_Elevator_Gets_Lift.html|archivedate = June 8, 2005|quote = The concept was first described in 1895 by Russian author K. E. Tsiolkovsky in his 'Speculations about Earth and Sky and on Vesta.'}}</ref> 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 (physical)|compression]], supporting its weight from below. Since 1959, most ideas for space elevators have focused on purely [[Tension (physics)|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]].
To construct a space elevator on Earth, the cable material would need to be both stronger and lighter (have greater [[specific strength]]) than any known material. Development of new materials that meet the demanding specific strength requirement must happen before designs can progress beyond discussion stage. [[Carbon nanotube]]s (CNTs) have been identified as possibly being able to meet the specific strength requirements for an Earth space elevator.<ref name=Edwards/><ref name=BBCfuture>{{cite web |url=http://www.bbc.com/future/story/20150211-space-elevators-a-lift-too-far |title=Should We give up on the dream of space elevators? |first=Nic |last=Fleming |date=February 15, 2015 |accessdate=July 22, 2018 |publisher=[[BBC]]}}</ref> Other materials considered have been [[boron nitride nanotube]]s, and [[carbon nanothread|diamond nanothreads]], which were first constructed in 2014<ref name=SCIAM_DN /><ref name=Xtech_DN />. In 2018 single-crystal [[Graphene]] was also proposed as a potential material<ref>{{Cite web|url=https://www.azom.com/article.aspx?ArticleID=16371|title=Space Elevator Technology and Graphene: An Interview with Adrian Nixon|last=|first=|date=|website=|archive-url=|archive-date=|dead-url=|access-date=}}</ref>.
The concept is applicable to other planets and [[Astronomical object|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.<ref>[[Hans Moravec|Moravec, Hans]] (1978). [http://www.frc.ri.cmu.edu/~hpm/project.archive/1976.skyhook/papers/scasci.txt ''Non-Synchronous Orbital Skyhooks for the Moon and Mars with Conventional Materials'']. Carnegie Mellon University. frc.ri.cmu.edu</ref>
==History==
===Early concepts===
[[Image:Tsiolkovsky.jpg|thumb|left|upright|[[Konstantin Tsiolkovsky]]]]
The key concept of the space elevator appeared in 1895 when [[Russia]]n scientist [[Konstantin Tsiolkovsky]] was inspired by the [[Eiffel Tower]] in [[Paris]]. He considered a similar tower that reached all the way into space and was built from the ground up to the altitude of 35,786 kilometers, the height of [[geostationary orbit]].<ref name="NASASci">{{cite web|url=https://science.nasa.gov/headlines/y2000/ast07sep_1.htm |title=The Audacious Space Elevator |last= |first= |date= |website= |publisher=NASA Science News |dead-url=yes |accessdate=September 27, 2008 |archiveurl=https://web.archive.org/web/20080919070924/https://science.nasa.gov/headlines/y2000/ast07sep_1.htm |archivedate=September 19, 2008 |df= }}</ref> He noted that the top of such a tower would be circling [[Earth]] as in a geostationary orbit. Objects would attain horizontal velocity as they rode up the tower, and an object released at the tower's top would have enough horizontal velocity to remain there in geostationary orbit. Tsiolkovsky's conceptual tower was a compression structure, while modern concepts call for a [[tensile structure]] (or "tether").
===20th century===
Building a compression structure from the ground up proved an unrealistic task as there was no material in existence with enough compressive strength to support its own weight under such conditions.<ref name="JBIS1999">{{cite journal
|author1=Landis, Geoffrey A. |author2=Cafarelli, Craig
|lastauthoramp=yes | year = 1999
| title = The Tsiolkovski Tower Reexamined
| journal = Journal of the British Interplanetary Society
| volume = 52
| pages = 175–180
| others = Presented as paper IAF-95-V.4.07, 46th International Astronautics Federation Congress, Oslo Norway, October 2–6, 1995
|bibcode = 1999JBIS...52..175L }}
</ref> In 1959 another Russian scientist, [[Yuri N. Artsutanov]], suggested a more feasible proposal. Artsutanov suggested using a geostationary [[satellite]] as the base from which to deploy the structure downward. By using a [[counterweight]], a cable would be lowered from geostationary orbit to the surface of Earth, while the counterweight was extended from the satellite away from Earth, keeping the cable constantly over the same spot on the surface of the Earth. Artsutanov's idea was introduced to the Russian-speaking public in an interview published in the Sunday supplement of ''[[Komsomolskaya Pravda]]'' in 1960,<ref name="artsutanov">{{cite web
|url=http://liftport.com/files/Artsutanov_Pravda_SE.pdf
|archiveurl=https://web.archive.org/web/20060506100948/http://liftport.com/files/Artsutanov_Pravda_SE.pdf
|archivedate=May 6, 2006 |title=To the Cosmos by Electric Train
|work=liftport.com
|year=1960
|publisher=Young Person's Pravda
|last=Artsutanov
|first=Yu
|accessdate=March 5, 2006}}</ref> but was not available in English until much later. He also proposed tapering the cable thickness so that the stress in the cable was constant. This gave a thinner cable at ground level that became thickest at the level of geostationary orbit.
Both the tower and cable ideas were proposed in the quasi-humorous [[Daedalus (Ariadne)|''Ariadne'' column]] in ''[[New Scientist]]'', December 24, 1964.
In 1966, Isaacs, Vine, Bradner and Bachus, four [[United States|American]] engineers, reinvented the concept, naming it a "Sky-Hook", and published their analysis in the journal [[Science (journal)|''Science'']].<ref>{{cite journal
|title=Satellite Elongation into a True 'Sky-Hook'
|year=1966
|journal= Science
|volume = 151
| doi = 10.1126/science.151.3711.682
|author=Isaacs, J. D. |author2= A. C. Vine, H. Bradner and G. E. Bachus|bibcode = 1966Sci...151..682I
|issue=3711
|pages=682–3
|last3=Bradner
|last4=Bachus
|pmid=17813792
}}</ref> They decided to determine what type of material would be required to build a space elevator, assuming it would be a straight cable with no variations in its cross section area, and found that the [[specific strength|strength]] required would be twice that of any then-existing material including [[graphite]], [[quartz]], and [[diamond]].
In 1975 an American scientist, [[Jerome Pearson]], reinvented the concept yet again, publishing his analysis in the journal ''[[Acta Astronautica]]''. He designed<ref name="pearson">
{{cite journal
| author = Pearson, J.
| year = 1975
| title = The orbital tower: a spacecraft launcher using the Earth's rotational energy
| url = http://www.star-tech-inc.com/papers/tower/tower.pdf
| journal = Acta Astronautica
| volume = 2
| pages = 785–799
| doi = 10.1016/0094-5765(75)90021-1
| format = PDF <!--Retrieved from CrossRef by DOI bot-->
| issue = 9–10
| bibcode = 1975AcAau...2..785P
| citeseerx = 10.1.1.530.3120
}}
</ref> a cross-section-area altitude profile that tapered and would be better suited to building the elevator. The completed cable would be thickest at the geostationary orbit, where the tension was greatest, and would be narrowest at the tips to reduce the amount of weight per unit area of cross section that any point on the cable would have to bear. He suggested using a counterweight that would be slowly extended out to {{convert|144,000|km|mi|abbr=off|sp=us}}, almost half the distance to the [[Moon]] as the lower section of the elevator was built. Without a large counterweight, the upper portion of the cable would have to be longer than the lower due to the way [[gravity|gravitational]] and centrifugal forces change with distance from Earth. His analysis included disturbances such as the gravitation of the Moon, wind and moving payloads up and down the cable. The weight of the material needed to build the elevator would have required thousands of [[Space Shuttle]] trips, although part of the material could be transported up the elevator when a minimum strength strand reached the ground or be manufactured in space from [[Asteroid mining|asteroidal]] or [[In-situ resource utilization|lunar ore]].
After the development of [[carbon nanotubes]] in the 1990s, engineer David Smitherman of [[NASA]]/Marshall's Advanced Projects Office realized that the high strength of these materials might make the concept of a space elevator feasible, and put together a workshop at the [[Marshall Space Flight Center]], inviting many scientists and engineers to discuss concepts and compile plans for an elevator to turn the concept into a reality.
In 2000, another American scientist, [[Bradley C. Edwards]], suggested creating a {{convert|100,000|km|mi|abbr=on}} long paper-thin ribbon using a carbon nanotube composite material.<ref name=EDWARDS_PHASE_I_2000_472Edwards.html>Bradley C. Edwards, "[http://www.niac.usra.edu/studies/472Edwards.html The Space Elevator]"</ref> He chose the wide-thin ribbon-like cross-section shape rather than earlier circular cross-section concepts because that shape would stand a greater chance of surviving impacts by meteoroids. The ribbon cross-section shape also provided large surface area for climbers to climb with simple rollers. Supported by the [[NASA Institute for Advanced Concepts]], Edwards' work was expanded to cover the deployment scenario, climber design, power delivery system, [[Space debris|orbital debris]] avoidance, anchor system, surviving [[atomic oxygen]], avoiding lightning and hurricanes by locating the anchor in the western equatorial Pacific, construction costs, construction schedule, and environmental hazards.<ref name=Edwards/><ref>[http://www.nss.org/resources/library/spaceelevator/2000-SpaceElevator-NASA-CP210429.pdf "Space Elevators: An Advanced Earth-Space Infrastructure for the New Millennium"], NASA/CP-2000-210429, Marshall Space Flight Center, Huntsville, Alabama, 2000</ref><ref>Science @ NASA, [https://science.nasa.gov/headlines/y2000/ast07sep_1.htm "Audacious & Outrageous: Space Elevators"] {{webarchive|url=https://web.archive.org/web/20080919070924/https://science.nasa.gov/headlines/y2000/ast07sep_1.htm |date=September 19, 2008 }}, September 2000</ref><ref>{{cite web | title = Space Elevators: An Advanced Earth-Space Infrastructure for the New Millennium | url = http://www.affordablespaceflight.com/spaceelevator.html| archiveurl = https://web.archive.org/web/20070221162221/http://www.affordablespaceflight.com/spaceelevator.html| archivedate = February 21, 2007|work=affordablespaceflight.com}}</ref>
===21st century===
To speed space elevator development, proponents have organized several [[Space Elevator Competitions|competitions]], similar to the [[Ansari X Prize]], for relevant technologies.<ref>{{cite web
|url=http://msnbc.msn.com/id/5792719/
|title=Space elevator contest proposed
|first=Alan
|last=Boyle
|publisher=MSNBC
|date=August 27, 2004}}</ref><ref>{{cite web
|url=http://www.elevator2010.org/
|title=The Space Elevator – Elevator:2010
|accessdate=March 5, 2006}}</ref> Among them are [[Elevator:2010]], which organized annual competitions for climbers, ribbons and power-beaming systems from 2005 to 2009, the Robogames Space Elevator Ribbon Climbing competition,<ref>{{cite web
|url=http://robogames.net/rules/climbing.php
|title=Space Elevator Ribbon Climbing Robot Competition Rules
|accessdate=March 5, 2006 |archiveurl = https://web.archive.org/web/20050206100051/http://robolympics.net/rules/climbing.shtml|archivedate=February 6, 2005 }}</ref> as well as NASA's [[Centennial Challenges]] program, which, in March 2005, announced a partnership with the Spaceward Foundation (the operator of Elevator:2010), raising the total value of prizes to US$400,000.<ref>{{cite web
|url=http://www.nasa.gov/home/hqnews/2005/mar/HQ_m05083_Centennial_prizes.html
|title=NASA Announces First Centennial Challenges' Prizes
|year=2005
|accessdate=March 5, 2006}}</ref><ref>{{cite web
|url=http://www.space.com/news/050323_centennial_challenge.html
|title=NASA Details Cash Prizes for Space Privatization
|first=Robert Roy
|last=Britt
|publisher=Space.com
|accessdate=March 5, 2006}}</ref>
The first European Space Elevator Challenge (EuSEC) to establish a climber structure took place in August 2011.<ref>{{cite web
|title=What's the European Space Elevator Challenge?
|url=http://eusec.warr.de/?eusec|publisher=European Space Elevator Challenge
|accessdate=April 21, 2011}}</ref>
In 2005, "the [[LiftPort Group]] of space elevator companies announced that it will be building a carbon nanotube manufacturing plant in [[Millville, New Jersey]], to supply various glass, plastic and metal companies with these strong materials. Although LiftPort hopes to eventually use carbon nanotubes in the construction of a {{convert|100,000|km|mi|abbr=on}} space elevator, this move will allow it to make money in the short term and conduct research and development into new production methods."<ref>{{cite web
|url=http://www.universetoday.com/am/publish/liftport_manufacture_nanotubes.html?2742005
|title=Space Elevator Group to Manufacture Nanotubes
|year=2005
|publisher=Universe Today
|accessdate=March 5, 2006}}</ref> Their announced goal was a space elevator launch in 2010. On February 13, 2006 the LiftPort Group announced that, earlier the same month, they had tested a mile of "space-elevator tether" made of carbon-fiber composite strings and fiberglass tape measuring {{convert|5|cm|in|abbr=on}} wide and 1 mm (approx. 13 sheets of paper) thick, lifted with balloons.<ref>{{cite news
|url=http://www.newscientistspace.com/article/dn8725.html
|title=Space-elevator tether climbs a mile high
|date=February 15, 2006
|work=NewScientist.com
|publisher=New Scientist
|first=Kimm
|last=Groshong
|accessdate=March 5, 2006}}</ref>
In 2007, [[Elevator:2010]] held the 2007 Space Elevator games, which featured US$500,000 awards for each of the two competitions, ($1,000,000 total) as well as an additional $4,000,000 to be awarded over the next five years for space elevator related technologies.<ref>[https://web.archive.org/web/20100118153108/http://www.spaceward.org/elevator2010 Elevator:2010 – The Space Elevator Challenge]. spaceward.org</ref> No teams won the competition, but a team from [[MIT]] entered the first 2-gram (0.07 oz), 100-percent carbon nanotube entry into the competition.<ref>[https://web.archive.org/web/20071101081423/http://www.spaceward.org/games07Wrapup.html Spaceward Games 2007]. The Spaceward Foundation</ref> Japan held an international conference in November 2008 to draw up a timetable for building the elevator.<ref name=JapanUKTimes>{{cite news | title = Japan hopes to turn sci-fi into reality with elevator to the stars | url=http://www.thetimes.co.uk/tto/news/world/article1967078.ece | work=The Times | location=London | first=Leo | last=Lewis | date=September 22, 2008 | accessdate=May 23, 2010}} Lewis, Leo; News International Group; accessed September 22, 2008.</ref>
In 2008 the book ''Leaving the Planet by Space Elevator'' by Dr. Brad Edwards and Philip Ragan was published in Japanese and entered the Japanese best-seller list.<ref name=Leaving>{{cite web | title = Leaving the Planet by Space Elevator | url = http://www.leavingtheplanet.com/}} Edwards, Bradley C. and Westling, Eric A. and Ragan, Philip; Leasown Pty Ltd.; accessed September 26, 2008.</ref> <ref>{{Cite book|title=Space Elevator: Leaving the Planet by Space Elevator|date=2008|isbn=9784270003350|location=東京|language=Japanese|last1 = エドワーズ|first1 = ブラッドリー・C|last2=フィリップ・レーガン}}</ref> This led to Shuichi Ono, chairman of the Japan Space Elevator Association, unveiling a space-elevator plan, putting forth what observers considered an extremely low cost estimate of a trillion yen (£5 billion / $8 billion) to build one.<ref name=JapanUKTimes/>
In 2012, the [[Obayashi Corporation]] announced that in 38 years it could build a space elevator using carbon nanotube technology.<ref>{{cite news| url=http://www.physorg.com/news/2012-02-japan-builder-eyes-space-elevator.html | work=PhysOrg.com | title=Going up: Japan builder eyes space elevator | date=February 22, 2012}}</ref> At 200 kilometers per hour, the design's 30-passenger climber would be able to reach the GEO level after a 7.5 day trip.<ref>{{cite news| url=http://www.ibtimes.com/articles/302223/20120221/space-elevator-60000-miles-reality-obayashi-nanotube.htm | title=Space Elevator That Soars 60,000 Miles into Space May Become Reality by 2050 | date=February 21, 2012}}</ref> No cost estimates, finance plans, or other specifics were made. This, along with timing and other factors, hinted that the announcement was made largely to provide publicity for the opening of one of the company's other projects in Tokyo.<ref>{{cite web|last=Boucher |first=Marc |url=http://www.spaceelevator.com/2012/02/obayashi-and-the-space-elevator---a-story-of-hype.html#more |title=Obayashi and the Space Elevator – A Story of Hype|work= The Space Elevator Reference |date=February 23, 2012 |accessdate=August 14, 2012 |deadurl=yes |archiveurl=https://web.archive.org/web/20120621201313/http://www.spaceelevator.com/2012/02/obayashi-and-the-space-elevator---a-story-of-hype.html |archivedate=June 21, 2012 }}</ref>
In 2013, the International Academy of Astronautics published a technological feasibility assessment which concluded that the critical capability improvement needed was the tether material, which was projected to achieve the necessary strength-to-weight ratio within 20 years. The four-year long study looked into many facets of space elevator development including missions, development schedules, financial investments, revenue flow, and benefits. It was reported that it would be possible to operationally survive smaller impacts and avoid larger impacts, with meteors and space debris, and that the estimated cost of lifting a kilogram of payload to GEO and beyond would be $500.<ref>{{Cite book|title = Space Elevators: An Assessment of the Technological Feasibility and the Way Forward|last1 = Swan|last2 =Raitt|last3 =Swan|last4 = Penny|last5 =Knapman|first1 = Peter A.|first2 = David I.|first3 = Cathy W.|first4 = Robert E.|first5 = John M.|publisher = International Academy of Astronautics|year = 2013|isbn = 9782917761311|location = Virginia, US|pages = 10–11, 207–208}}</ref><ref name=":0" />
In 2014, Google X's Rapid Evaluation R&D team began the design of a Space Elevator, eventually finding that no one had yet manufactured a perfectly formed carbon nanotube strand longer than a meter. They thus decided to put the project in "deep freeze" and also keep tabs on any advances in the carbon nanotube field.<ref>{{cite web|last=Gayomali|first=Chris|title=Google X Confirms The Rumors: It Really Did Try To Design A Space Elevator|url=http://www.fastcompany.com/3029138/world-changing-ideas/google-x-confirms-the-rumors-it-really-did-try-to-design-a-space-elevat?partner=rss|work=Fast Company|accessdate=17 April 2014|date=15 April 2014}}</ref>
In 2018, researchers at Japan's [[Shizuoka University]] launched STARS-Me, two [[CubeSat]]s connected by a tether, which a mini-elevator will travel on.<ref>{{Cite web | url=https://www.nbcnews.com/mach/science/colossal-elevator-space-could-be-going-sooner-you-ever-imagined-ncna915421 | title=A colossal elevator to space could be going up sooner than you ever imagined}}</ref><ref>{{cite web
|url=https://www.curbed.com/2018/9/12/17851500/space-elevator-japan-news
|title=Japan is trying to build an elevator to space
|publisher=Curbed.com
|first=Meghan
|last=Barber
|date=September 12, 2018
|accessdate=September 18, 2018
}}</ref> The experiment was launched as a test bed for a larger structure.<ref>https://gizmodo.com/japan-testing-miniature-space-elevator-near-the-interna-1828800558</ref>
==In fiction==
{{main|Space elevators in fiction}}
In 1979, space elevators were introduced to a broader audience with the simultaneous publication of [[Arthur C. Clarke]]'s novel, ''[[The Fountains of Paradise]]'', in which engineers construct a space elevator on top of a mountain peak in the fictional island country of "Taprobane" (loosely based on [[Sri Lanka]], albeit moved south to the Equator), and [[Charles Sheffield]]'s first novel, ''[[The Web Between the Worlds]]'', also featuring the building of a space elevator. Three years later, in [[Robert A. Heinlein]]'s 1982 novel ''[[Friday (novel)|Friday]]'' the principal character makes use of the "Nairobi Beanstalk" in the course of her travels. In [[Kim Stanley Robinson]]'s 1993 novel ''[[Red Mars]]'', colonists build a space elevator on Mars that allows both for more colonists to arrive and also for natural resources mined there to be able to leave for Earth. In [[David Gerrold]]'s 2000 novel, ''[[Jumping Off The Planet]]'', a family excursion up the Ecuador "beanstalk" is actually a child-custody kidnapping. Gerrold's book also examines some of the industrial applications of a mature elevator technology. The concept of a space elevator, called the [[Old Man's War#Beanstalk|Beanstalk]], is also depicted in John Scalzi's 2005 novel, ''[[Old Man's War]].'' In a biological version, [[Joan Slonczewski]]'s 2011 novel ''The Highest Frontier'' depicts a college student ascending a space elevator constructed of self-healing cables of anthrax bacilli. The engineered bacteria can regrow the cables when severed by space debris.
==Physics==
===Apparent gravitational field===
An Earth space elevator cable rotates along with the rotation of the Earth. Therefore the cable, and objects attached to it, would experience upward centrifugal force in the direction opposing the downward gravitational force. The higher up the cable the object is located, the less the gravitational pull of the Earth, and the stronger the upward centrifugal force due to the rotation, so that more centrifugal force opposes less gravity. The centrifugal force and the gravity are balanced at geosynchronous equatorial orbit (GEO). Above GEO, the centrifugal force is stronger than gravity, causing objects attached to the cable there to pull ''upward'' on it.
The net force for objects attached to the cable is called the ''apparent gravitational field''. The apparent gravitational field for attached objects is the (downward) gravity minus the (upward) centrifugal force. The apparent gravity experienced by an object on the cable is zero at GEO, downward below GEO, and upward above GEO.
The apparent gravitational field can be represented this way:{{rp | Ref<ref name="aravind"/> Table 1}}
{{block indent|The downward force of actual [[Newton's law of universal gravitation|gravity]] ''decreases'' with height: [[Newton's law of universal gravitation|<math>g_r = -GM/r^2</math>]]}}
{{block indent|The upward [[centrifugal force]] due to the planet's rotation ''increases'' with height: [[Centrifugal force|<math>a = \omega^2 r</math>]]}}
{{block indent|Together, the apparent gravitational field is the sum of the two:
{{block indent|<math>g = -\frac{GM}{r^2} + \omega^2 r</math>}}}}
where
{{block indent|''g'' is the acceleration of ''apparent'' gravity, pointing down (negative) or up (positive) along the vertical cable (m s<sup>−2</sup>),}}
{{block indent|''g<sub>r</sub>'' is the gravitational acceleration due to Earth's pull, pointing down (negative)(m s<sup>−2</sup>),}}
{{block indent|''a'' is the centrifugal acceleration, pointing up (positive) along the vertical cable (m s<sup>−2</sup>),}}
{{block indent|''G'' is the [[gravitational constant]] (m<sup>3</sup> s<sup>−2</sup> kg<sup>−1</sup>)}}
{{block indent|''M'' is the mass of the Earth (kg)}}
{{block indent|''r'' is the distance from that point to Earth's center (m),}}
{{block indent|''ω'' is Earth's rotation speed (radian/s).}}
At some point up the cable, the two terms (downward gravity and upward centrifugal force) are equal and opposite. Objects fixed to the cable at that point put no weight on the cable. This altitude (r<sub>1</sub>) depends on the mass of the planet and its rotation rate. Setting actual gravity equal to centrifugal acceleration gives:{{rp | Ref<ref name="aravind"/> page 126}}
{{block indent|<math>r_1 = \left(\frac{GM}{\omega^2}\right)^\frac{1}{3}</math>}}
On Earth, this distance is {{convert|35786|km|mi|0|abbr=on}} above the surface, the altitude of geostationary orbit.{{rp | Ref<ref name="aravind"/> Table 1}}
On the cable ''below'' geostationary orbit, downward gravity would be greater than the upward centrifugal force, so the apparent gravity would pull objects attached to the cable downward. Any object released from the cable below that level would initially accelerate downward along the cable. Then gradually it would deflect eastward from the cable. On the cable ''above'' the level of stationary orbit, upward centrifugal force would be greater than downward gravity, so the apparent gravity would pull objects attached to the cable ''upward''. Any object released from the cable ''above'' the geosynchronous level would initially accelerate ''upward'' along the cable. Then gradually it would deflect westward from the cable.
===Cable section===
Historically, the main technical problem has been considered the ability of the cable to hold up, with tension, the weight of itself below any given point. The greatest tension on a space elevator cable is at the point of geostationary orbit, {{convert|35786|km|mi|0|abbr=on}} above the Earth's equator. This means that the cable material, combined with its design, must be strong enough to hold up its own weight from the surface up to {{convert|35786|km|mi|0|abbr=on}}. A cable which is thicker in cross section area at that height than at the surface could better hold up its own weight over a longer length. How the cross section area tapers from the maximum at {{convert|35786|km|mi|0|abbr=on}} to the minimum at the surface is therefore an important design factor for a space elevator cable.
To maximize the usable excess strength for a given amount of cable material, the cable's cross section area would need to be designed for the most part in such a way that the [[Stress (mechanics)|stress]] (i.e., the tension per unit of cross sectional area) is constant along the length of the cable.<ref name="aravind"/><ref>Artuković, Ranko (2000). [http://www.zadar.net/space-elevator/ "The Space Elevator".] zadar.net</ref> The constant-stress criterion is a starting point in the design of the cable cross section area as it changes with altitude. Other factors considered in more detailed designs include thickening at altitudes where more space junk is present, consideration of the point stresses imposed by climbers, and the use of varied materials.<ref name=PhaseII/> To account for these and other factors, modern detailed designs seek to achieve the largest ''[[Factor of safety#Margin of safety|safety margin]]'' possible, with as little variation over altitude and time as possible.<ref name=PhaseII/> In simple starting-point designs, that equates to constant-stress.
In the constant-stress case, the cross-section-area can be described by the differential equation as:
{{block indent|<math>\frac{dA}{A} = \frac{\rho g R^2}{T} \left(\frac{1}{r^2} - \frac{r}{R_g^3} \right)dr</math>{{rp | Ref<ref name="aravind"/> equation 6}}}}
where
{{block indent|''g'' is the acceleration along the radius (m·s<sup>−2</sup>),}}
{{block indent|''A'' is the cross-section area of the cable at any given point r, (m<sup>2</sup>),}}
{{block indent|''ρ'' is the density of the material used for the cable (kg·m<sup>−3</sup>),}}
{{block indent|''R'' is the earth's equatorial radius,}}
{{block indent|<math>R_g</math> is the radius of geosynchronous orbit,}}
{{block indent|1=''T'' is the stress the cross-section area can bear without [[Yield (engineering)|yielding]] (N·m<sup>−2</sup>=kg·m<sup>−1</sup>·s<sup>−2</sup>), its elastic limit.}}
For a constant-stress cable with no safety margin, the cross-section-area profile as a function of distance from Earth's center can be solved with
{{block indent|<math>A( r ) = A_s exp\left[ \frac{\rho g R^2}{T}\left( \frac{1}{R}+\frac{R^2}{2R_g^3}-\frac{1}{r}-\frac{r^2}{2R_g^3} \right) \right]</math>{{rp | Ref<ref name="aravind"/> equation 7}}}}
Safety margin can be accounted for by dividing T by the desired safety factor. <ref name="aravind"/>
===Cable materials===
Using the above taper formula to solve for the specific case of earth equatorial surface (<math>R=6378</math> km) and Earth geosynchronous orbit (<math>R_g = 42164</math> km), specific materials can be examined:<ref group=note>Specific substitutions used to produce the factor {{val|4.85|e=7}}:{{block indent|<math>A = A_s exp \left[ \frac{\rho \times 9.81 \times (6.378\times 10^6)^2 } {T} \left(
\frac{1}{6.378\times 10^6} + \frac{(6.378\times 10^6)^2}{(4.2164\times 10^7)^3} - \frac{1}{4.2164\times 10^7} - \frac{(4.2164\times 10^7)^2}{2 (4.2164\times 10^7)^3}\right)\right]</math>}}</ref>
{{block indent|<math>A = A_s exp [\frac{\rho}{T}\times 4.85\times 10^7]</math> }}
A table of values for taper for various materials are:
{| class="wikitable" style="text-align:left"
|+Taper ratios by materials{{rp | Ref<ref name="aravind"/> Table 2}}
|-
!Material!!Tensile strength<br>(MPa)!!Density<br>(kg/m^3)!![[Specific strength]]<br>(MPa)/(kg/m^3)!! Taper ratio
|-
|Steel || 5,000 || 7,900 || 0.63 || {{val|1.6|e=33}}
|-
|Kevlar || 3,600 || 1,440 || 2.5 || {{val|2.5|e=8}}
|-
|Carbon nanotubes || 130,000 || 1,300 || 100|| 1.6
|}
The taper factor results in large increases in cross-section-area unless the specific strength of the material used approaches 48 (MPa)/(kg/m^3). Low specific strength materials require very large taper ratios which equates to large (or astronomical) total mass of the cable with associated large or impossible costs.
==Structure==
[[Image:SpaceElevatorClimbing.jpg|thumb|right|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 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 an advantage of being able to be at high altitude, 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 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]]
[[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 carefully 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 name=NASA97029>{{cite web|url=http://www.nas.nasa.gov/assets/pdf/techreports/1997/nas-97-029.pdf
|title=NAS-97-029: NASA Applications of Molecular Nanotechnology
|author=Globus, Al
|display-authors=etal
|publisher=NASA
|accessdate=September 27, 2008}}</ref>
The cable would need to be made of a material with a large [[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 kg/m^3,<ref name=EDWARDS_PHASE_I_2000_472Edwards.html /> that implies a specific strength of 77 megapascal/(kg/m^3). 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 force]]s increase (and gravity decreases) dramatically with height: {{cite web|url= http://www.islandone.org/LEOBiblio/CLARK2.HTM|title = The space elevator: 'thought experiment', or key to the universe?''|last = Clarke|first = A.C.|year = 1979}}</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 km (0.2 - 0.3 MPa/(kg/m^3)). Modern [[Man-made fibers|fibre]] materials such as [[kevlar]], [[fibreglass]] and [[Carbon fiber|carbon/graphite fibre]] have breaking lengths of 100–400 km (1.0 - 4.0 MPa/(kg/m^3)). 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 km (50 - 60 MPa/(kg/m^3)), 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 6th 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=http://www.huffingtonpost.com/2013/05/29/space-elevators-stronger-materials_n_3353697.html }}</ref>
<ref name="pop.15185070">{{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 name="extreme.176625">{{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 |accessdate=2014-04-19 }}</ref> As of 2014, carbon nanotube technology allowed growing tubes up to a few tenths of meters.<ref>{{cite journal| first=X.| last=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| deadurl=yes| archiveurl=https://web.archive.org/web/20170808164154/http://www.chem.pku.edu.cn/page/liy/labhomepage/publications/2009/2009NL.pdf| archivedate=August 8, 2017| df=mdy-all| citeseerx=10.1.1.454.2744}}</ref>
In 2014, [[carbon nanothread|diamond nanothreads]] were first synthesized.<ref name=SCIAM_DN>{{cite web |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=September 26, 2014 |publisher=[[Scientific American]] |accessdate=July 22, 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 web |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=September 23, 2014 |publisher=Zeff Davis, LLC |website=Extreme Tech |accessdate=July 22, 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" >Lang, David D. [http://spaceelevatorwiki.com/wiki/images/2/2b/Paper_Lang_Climber_Transit.pdf Space Elevator Dynamic Response to In-Transit Climbers].</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 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 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 name="Gassend_fall">
{{cite web
|first=Blaise
|last=Gassend
|title=Falling Climbers
|url=http://gassend.net/spaceelevator/falling-climbers/index.html
|accessdate=December 16, 2013
}}</ref><ref name=Skyway_to_LEO>
{{cite web
|publisher=Endless Skyway|title=Space elevator to low orbit?
|url=http://www.endlessskyway.com/2010/05/space-elevator-to-low-orbit.html
|accessdate=December 16, 2013
}}</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 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 like an inverted 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>[http://gassend.net/spaceelevator/center-of-mass/index.html "Why the Space Elevator's Center of Mass is not at GEO" by Blaise Gassend]. Gassend.net. Retrieved on September 30, 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.{{citation needed|date=April 2014}} 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|author1=Bill Fawcett, Michael Laine |author2=Tom Nugent jr. |lastauthoramp=yes |title=LIFTPORT|date=2006|publisher=Meisha Merlin Publishing, Inc.|location=Canada|isbn=978-1-59222-109-7|page=103}}</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 get that 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 km it is possible to use solar energy to power the climber<ref>{{cite web
|url = http://isec.org/pdfs/isec_reports/2013_ISEC_Design_Considerations_for_Space_Elevator_Tether_Climbers_Final_Report.pdf
|title = Design Consideration for Space Elevator Tether Climbers
|last1 = Swan, P. A.
|last2 = Swan, C. W.
|last3 = Penny, R. E.
|last4 = Knapman, J. M.
|last5 = Glaskowsky, P. N.
|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
|deadurl = yes
|archiveurl = https://web.archive.org/web/20170116175959/http://isec.org/pdfs/isec_reports/2013_ISEC_Design_Considerations_for_Space_Elevator_Tether_Climbers_Final_Report.pdf
|archivedate = January 16, 2017
|df = mdy-all
}}</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/>
*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, USA: 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.
==Launching into deep space==
An object attached to a space elevator at a radius of approximately 53,100 km would be at [[escape velocity]] when released. Transfer orbits to the L1 and L2 [[Lagrangian point]]s could be attained by release at 50,630 and 51,240 km, respectively, and transfer to lunar orbit from 50,960 km.<ref>{{cite web|url=http://www.spaceelevator.com/docs/iac-2004/iac-04-iaa.3.8.3.04.engel.pdf |title=IAC-04-IAA.3.8.3.04 Lunar transportation scenarios utilising the space elevator |author=Engel, Kilian A. |publisher=www.spaceelevator.com |deadurl=yes |archiveurl=https://web.archive.org/web/20120424230830/http://www.spaceelevator.com/docs/iac-2004/iac-04-iaa.3.8.3.04.engel.pdf |archivedate=April 24, 2012 }}</ref>
At the end of Pearson's {{convert|144,000|km|mi|abbr=on}} cable, the tangential velocity is 10.93 kilometers per second (6.79 mi/s). That is more than enough to [[escape velocity|escape]] Earth's gravitational field and send probes at least as far out as [[Jupiter]]. Once at Jupiter, a [[gravitational assist]] maneuver could permit solar escape velocity to be reached.<ref name="aravind">{{cite journal|title=The physics of the space elevator|author=Aravind, P. K.|url=http://users.wpi.edu/~paravind/Publications/PKASpace%20Elevators.pdf|year=2007|journal=American Journal of Physics|volume=45|issue=2|doi=10.1119/1.2404957|page=125|bibcode = 2007AmJPh..75..125A}}</ref>
==Extraterrestrial elevators==
A space elevator could also be constructed on other planets, asteroids and moons.
A [[Mars|Martian]] tether could be much shorter than one on Earth. Mars' surface gravity is 38 percent of Earth's, while it rotates around its axis in about the same time as Earth. Because of this, Martian [[areostationary orbit|stationary orbit]] is much closer to the surface, and hence the elevator could be much shorter. Current materials are already sufficiently strong to construct such an elevator.<ref>Forward, Robert L. and Moravec, Hans P. (March 22, 1980) [http://www.frc.ri.cmu.edu/~hpm/project.archive/1976.skyhook/1982.articles/elevate.800322 Space Elevators]. Carnegie Mellon University. "Interestingly enough, they are already more than strong enough for constructing skyhooks on the moon and Mars."</ref> Building a Martian elevator would be complicated by the Martian moon [[Phobos (moon)|Phobos]], which is in a low orbit and intersects the Equator regularly (twice every orbital period of 11 h 6 min).
On the near side of the Moon, the strength-to-density required of the tether of a [[lunar space elevator]] exists in currently available materials. A lunar space elevator would be about {{convert|50,000|km|mi|sp=us}} long. Since the Moon does not rotate fast enough, there is no effective lunar-stationary orbit, but the [[Lagrangian point]]s could be used. The near side would extend through the Earth-Moon [[Inner lagrangian point|L1]] point from an anchor point near the center of the visible part of Earth's Moon.<ref name="Pearson 2005"/>
On the far side of the Moon, a lunar space elevator would need to be very long—more than twice the length of an Earth elevator—but due to the low gravity of the Moon, could also be made of existing engineering materials.<ref name="Pearson 2005">{{cite web| url=http://www.niac.usra.edu/files/studies/final_report/1032Pearson.pdf| year= 2005| title=Lunar Space Elevators for Cislunar Space Development Phase I Final Technical Report| last1=pearson |first1=Jerome| first2=Eugene |last2=Levin |first3=John |last3=Oldson |first4=Harry |last4=Wykes}}</ref>
Rapidly spinning asteroids or moons could use cables to eject materials to convenient points, such as Earth orbits;<ref>Ben Shelef, the Spaceward Foundation [http://www.spaceward.org/documents/papers/ASE.pdf Asteroid Slingshot Express - Tether-based Sample Return]</ref> or conversely, to eject materials to send a portion of the mass of the asteroid or moon to Earth orbit or a [[Lagrangian point]]. [[Freeman Dyson]], a physicist and mathematician, has suggested{{Citation needed|date=September 2008}} using such smaller systems as power generators at points distant from the Sun where solar power is uneconomical.
A space elevator using presently available engineering materials could be constructed between mutually tidally locked worlds, such as [[Pluto]] and [[Charon (moon)|Charon]] or the components of binary asteroid [[90 Antiope]], with no terminus disconnect, according to Francis Graham of Kent State University.<ref>{{cite book|author=Graham FG |title=45th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit|doi=10.2514/6.2009-4906|chapter=Preliminary Design of a Cable Spacecraft Connecting Mutually Tidally Locked Planetary Bodies|year=2009|isbn=978-1-60086-972-3}}</ref> However, spooled variable lengths of cable must be used due to ellipticity of the orbits.
==Construction==
{{Main article|Space elevator construction}}
The construction of a space elevator would need reduction of some technical risk. Some advances in engineering, manufacturing and physical technology are required.<ref name=Edwards/> Once a first space elevator is built, the second one and all others would have the use of the previous ones to assist in construction, making their costs considerably lower. Such follow-on space elevators would also benefit from the great reduction in technical risk achieved by the construction of the first space elevator.<ref name=Edwards/>
Prior to the work of Edwards in 2000<ref name=EDWARDS_PHASE_I_2000_472Edwards.html /> most concepts for constructing a space elevator had the cable manufactured in space. That was thought to be necessary for such a large and long object and for such a large counterweight. Manufacturing the cable in space would be done in principle by using an [[asteroid]] or [[Near-Earth object]] for source material.<ref name=SMITHERMAN>D.V. Smitherman (Ed.), [http://www.nss.org/resources/library/spaceelevator/2000-SpaceElevator-NASA-CP210429.pdf Space Elevators: An Advanced Earth-Space Infrastructure for the New Millennium], NASA/CP-2000-210429, Marshall Space Flight Center, Huntsville, Alabama, 2000</ref><ref>Hein, A.M., [https://www.academia.edu/2111184/A.M._Hein_Producing_a_Space_Elevator_Tether_using_a_NEO_A_Preliminary_Assessment_ Producing a Space Elevator Tether Using a NEO: A Preliminary Assessment], International Astronautical Congress 2012, IAC-2012, Naples, Italy, 2012</ref> These earlier concepts for construction require a large preexisting space-faring infrastructure to maneuver an asteroid into its needed orbit around Earth. They also required the development of technologies for manufacture in space of large quantities of exacting materials.<ref name=ISEC_SE_way_forward_2013>Space Elevators: An Assessment of the Technological Feasibility and the Way Forward, Page 326, http://www.virginiaedition.com/media/spaceelevators.pdf</ref>
Since 2001, most work has focused on simpler methods of construction requiring much smaller space infrastructures. They conceive the launch of a long cable on a large spool, followed by deployment of it in space.<ref name=Edwards/><ref name=EDWARDS_PHASE_I_2000_472Edwards.html /><ref name=ISEC_SE_way_forward_2013 /> The spool would be initially parked in a geostationary orbit above the planned anchor point. A long cable would be dropped "downward" (toward Earth) and would be balanced by a mass being dropped "upward" (away from Earth) for the whole system to remain on the geosynchronous orbit. Earlier designs imagined the balancing mass to be another cable (with counterweight) extending upward, with the main spool remaining at the original geosynchronous orbit level. Most current designs elevate the spool itself as the main cable is paid out, a simpler process. When the lower end of the cable is long enough to reach the surface of the Earth (at the equator), it would be anchored. Once anchored, the center of mass would be elevated more (by adding mass at the upper end or by paying out more cable). This would add more tension to the whole cable, which could then be used as an elevator cable.
One plan for construction uses conventional rockets to place a "minimum size" initial seed cable of only 19,800 kg.<ref name=Edwards/> This first very small ribbon would be adequate to support the first 619 kg climber. The first 207 climbers would carry up and attach more cable to the original, increasing its cross section area and widening the initial ribbon to about 160 mm wide at its widest point. The result would be a 750-ton cable with a lift capacity of 20 tons per climber.
===Safety issues and construction challenges===
{{Main article|Space elevator safety}}
For early systems, transit times from the surface to the level of geosynchronous orbit would be about five days. On these early systems, the time spent moving through the [[Van Allen radiation belts]] would be enough that passengers would need to be protected from radiation by shielding, which would add mass to the climber and decrease payload.<ref name=firstfloor>{{cite web|url=https://www.newscientist.com/article/dn10520 |title=Space elevators: 'First floor, deadly radiation!' |accessdate=January 2, 2010 |date=November 13, 2006|work=New Scientist |publisher=Reed Business Information Ltd.}}</ref>
A space elevator would present a navigational hazard, both to aircraft and spacecraft. Aircraft could be diverted by [[air-traffic control]] restrictions. All objects in stable orbits that have [[perigee]] below the maximum altitude of the cable that are not synchronous with the cable would impact the cable eventually, unless avoiding action is taken. One potential solution proposed by Edwards is to use a movable anchor (a sea anchor) to allow the tether to "dodge" any space debris large enough to track.<ref name=Edwards/>
Impacts by space objects such as meteoroids, micrometeorites and orbiting man-made debris pose another design constraint on the cable. A cable would need to be designed to maneuver out of the way of debris, or absorb impacts of small debris without breaking.
===Economics===
{{Main article|Space elevator economics}}
With a space elevator, materials might be sent into orbit at a fraction of the current cost. As of 2000, conventional rocket designs cost about US$25,000 per [[kilogram]] (US$11,000 per [[Pound (mass)|pound]]) for transfer to geostationary orbit.<ref>{{cite web|url=http://www.domain-b.com/companies/companies_f/futron_corporation/20021018_countdown.html |title=Delayed countdown |accessdate=June 3, 2009 |date=October 18, 2002|work=Fultron Corporation |publisher=The Information Company Pvt Ltd}}</ref> Current space elevator proposals envision payload prices starting as low as $220 per kilogram ($100 per [[Pound (mass)|pound]]),<ref>{{cite web|url=http://www.spaceward.org/elevator-faq |title=The Space Elevator FAQ |accessdate=June 3, 2009 |author=The Spaceward Foundation |location=Mountain View, CA |deadurl=yes |archiveurl=https://web.archive.org/web/20090227115101/http://www.spaceward.org/elevator-faq |archivedate=February 27, 2009 }}</ref> similar to the $5–$300/kg estimates of the [[Launch loop]], but higher than the $310/ton to 500 km orbit quoted<ref>{{cite web |url=http://www.jerrypournelle.com/archives2/archives2view/view306.html#Friday |title=Friday's VIEW post from the 2004 Space Access Conference |date=April 23, 2003| accessdate=January 1, 2010 |first=Jerry|last=Pournelle}}</ref> to Dr. [[Jerry Pournelle]] for an orbital airship system.
Philip Ragan, co-author of the book ''Leaving the Planet by Space Elevator'', states that "The first country to deploy a space elevator will have a 95 percent cost advantage and could potentially control all space activities."<ref>{{cite news |url=http://www.news.com.au/news/race-to-build-worlds-first-space-elevator/story-fna7dq6e-1111118059040 |title=Race on to build world's first space elevator |date=November 17, 2008|work=news.com.au|first=Andrew|last=Ramadge|author2=Schneider, Kate }}</ref>
==International Space Elevator Consortium (ISEC)==
The International Space Elevator Consortium (ISEC) is a US Non-Profit [[501(c)(3) organization|501(c)(3)]] Corporation<ref>{{Cite web|url=https://apps.irs.gov/app/eos/displayAll.do?dispatchMethod=displayAllInfo&Id=4984679&ein=800302896&country=US&deductibility=all&dispatchMethod=searchAll&isDescending=false&city=&ein1=80-0302896&postDateFrom=&exemptTypeCode=al&submitName=Search&sortColumn=orgName&totalResults=1&names=&resultsPerPage=25&indexOfFirstRow=0&postDateTo=&state=IL|title=ISEC IRS filing|last=|first=|date=|website=apps.irs.gov|archive-url=|archive-date=|dead-url=|access-date=2019-02-09}}</ref> formed to promote the development, construction, and operation of a space elevator as "a revolutionary and efficient way to space for all humanity"<ref name=isec>{{cite web | url=http://www.isec.org/index.php/what-is-isec | work=ISEC | title=About us | accessdate=2 June 2012 | deadurl=yes | archiveurl=https://web.archive.org/web/20120707201835/http://www.isec.org/index.php/what-is-isec | archivedate=July 7, 2012 | df=mdy-all }}</ref>. It was formed after the Space Elevator Conference in [[Redmond, Washington]] in July 2008 and became an affiliate organization with the [[National Space Society]]<ref>{{Cite web|title = NSS Affiliates|url = http://www.nss.org/about/affiliates.html|website = www.nss.org|accessdate = 2015-08-30}}</ref> in August 2013<ref name=isec />. ISEC hosts an annual Space Elevator conference at the [[Seattle Museum of Flight]] <ref>{{Cite web|url=https://www.space.com/27225-space-elevator-technology.html|title=Space Elevator Advocates Take Lofty Look at Innovative Concepts|last=Tech|first=Leonard David 2014-09-22T11:59:53Z|website=Space.com|language=en|access-date=2019-02-13}}</ref><ref>{{Cite web|url=https://space.nss.org/the-international-space-elevator-consortium-isec-2017-space-elevator-conference/|title=The International Space Elevator Consortium (ISEC) 2017 Space Elevator Conference{{!}}National Space Society|last=Society|first=National Space|language=en-US|access-date=2019-02-13}}</ref><ref>{{Cite web|url=http://spaceref.com/space-elevator/annual-space-elevator-conference-set-for-august-25-27.html|title=Annual Space Elevator Conference Set for August 25-27 - SpaceRef|website=spaceref.com|access-date=2019-02-13}}</ref>.
ISEC coordinates with the two other major societies focusing on space elevators: the Japanese Space Elevator Association<ref>{{Cite web|title = Japan Space Elevator Association|url = http://www.jsea.jp/links/|website = 一般|JSEA 一般社団法人 宇宙エレベーター協会|accessdate = 2015-08-30}}</ref> and EuroSpaceward.<ref>{{Cite web|url = http://www.eurospaceward.org/|title = Eurospaceward|date = 2015-08-30|accessdate = 2015-08-30|website = Eurospaceward|publisher = }}</ref> ISEC supports symposia and presentations at the International Academy of Astronautics<ref>{{Cite web|url = http://iaaweb.org/content/view/624/823/|title = Homepage of the Study Group 3.24, Road to Space Elevator Era|date = 2014-10-02|accessdate = 2015-08-30|website = The International Academy of Astronautics (IAA)|publisher = The International Academy of Astronautics (IAA)|last = Akira|first = Tsuchida}}</ref> and the International Astronautical Federation Congress<ref>{{Cite web|url = http://www.iafastro.org/events/iac/iac-2014/meetings/|title = IAC 2014 Meeting Schedule|date = |accessdate = 2015-08-30|website = International Astronautical Federation|publisher = }}</ref> each year. The organization published two issues of a peer-reviewed journal on space elevators called "CLIMB"<ref name="isec" /><ref>{{cite web | url=http://www.spaceelevator.com/2012/01/first-issue-of-the-space-elevator-journal-released.html | title=First Issue of the Space Elevator Journal Released | work=The Space Elevator Reference | date=20 January 2012 | first=Marc | last=Boucher | accessdate=2 June 2012 | deadurl=yes | archiveurl=https://web.archive.org/web/20120513064307/http://www.spaceelevator.com/2012/01/first-issue-of-the-space-elevator-journal-released.html | archivedate=May 13, 2012 | df=mdy-all }}</ref><ref>{{Cite web | url=http://www.isec.org/index.php/store/climb-the-space-elevator-journal | title=CLIMB - the Space Elevator Journal}}</ref> and a magazine "Via Ad Astra"<ref>{{Cite book|url=https://www.worldcat.org/oclc/1020867745|title=VIA AD ASTRA - VOL 1|last=ISEC.|date=2015|publisher=LULU COM|isbn=132964123X|location=[Place of publication not identified],|oclc=1020867745}}</ref>.
ISEC also conducts one-year studies focusing on individual topics. The process involves experts for one year of discussions on the topic of choice and culminates in a draft report that is presented and reviewed at the ISEC Space Elevator conference workshop to allow input from space elevator enthusiasts and other experts. Study Reports are usually published early the following year, to date these are as follows : <ref>https://isec.org/isec-reports/</ref>
* 2010 - Space Elevator Survivability, Space Debris Mitigation <ref name=":0">Swan, P., Penny, R., Swan, C. "Space Elevator Survivability, Space Debris Mitigation", Lulu.com Publishers, 2011</ref>
* 2012 - Space Elevator Concept of Operations <ref>Swan, P., Penny, R., Swan, C. "Space Elevator Concept of Operations" Lulu.com Publishers, 2013</ref>
* 2013 - Design Consideration for Space Elevator Tether Climbers <ref>Swan, P., Penny, R., Swan, C. "Design Considerations for Space Elevator Tether Climbers" Lulu.com Publishers, 2014</ref>,
* 2014 - Space Elevator Architectures and Roadmaps <ref>Fitzgerald, M., Swan, P., Penny, R., Swan, C. "Space Elevator Architectures and Roadmaps", Lulu.com Publishers, 2015</ref>
* 2015 - Design Characteristics of a Space Elevator Earth Port <ref>Hall, V., Glaskowsky, P., Schaeffer, S. "Design Characteristics of a Space Elevator Earth Port", Lulu.com Publishers, 2016</ref>
* 2016 - Design Considerations for the Space Elevator Apex Anchor and GEO Node <ref>Fitzgerald, M. ''et al.'' "Design Considerations for the Space Elevator Apex Anchor and GEO Node", Lulu.com Publishers, 2017</ref>
* 2017 - Design Considerations for a Software Space Elevator Simulator <ref>Wright, D., Avery, S., Knapman, J., Lades, M., Roubekas, P., Swan, P. " Design Considerations for a Software Space Elevator Simulator," www.lulu.com, 2018</ref>
* 2018 - Design Considerations for the Multi-Stage Space Elevator <ref>Knapman, J., Glaskowsky, P., Gleeson, D., Hall, V., Wright, D., Fitzgerald, M., Swan, P. "Design Considerations for the Multi-stage Space Elevator," www.lulu.com, 2019, {{ISBN|978-0-359-33232-8}}</ref>
==Related concepts==
The conventional current concept of a "Space Elevator" has evolved from a static compressive structure reaching to the level of GEO, to the modern baseline idea of a static tensile structure anchored to the ground and extending to well above the level of GEO. In the current usage by practitioners (and in this article), a "Space Elevator" means the Tsiolkovsky-Artsutanov-Pearson type as considered by the International Space Elevator Consortium. This conventional type is a static structure fixed to the ground and extending into space high enough that cargo can climb the structure up from the ground to a level where simple release will put the cargo into an [[orbit]].<ref>"CLIMB: The Journal of the International Space Elevator Consortium", Volume 1, Number 1, December 2011, This journal is cited as an example of what is generally considered to be under the term "Space Elevator" by the international community. [http://www.isec.org/index.php?option=com_content&view=article&id=28&Itemid=31]</ref>
Some concepts related to this modern baseline are not usually termed a "Space Elevator", but are similar in some way and are sometimes termed "Space Elevator" by their proponents. For example, [[Hans Moravec]] published an article in 1977 called "A Non-Synchronous Orbital [[Skyhook (structure)|Skyhook]]" describing a concept using a rotating cable.<ref>{{cite journal|author=Moravec, Hans P. |title=A Non-Synchronous Orbital Skyhook|journal=Journal of the Astronautical Sciences|volume=25|date= October–December 1977|bibcode=1977JAnSc..25..307M|pages=307–322}}</ref> The rotation speed would exactly match the orbital speed in such a way that the tip velocity at the lowest point was zero compared to the object to be "elevated". It would dynamically grapple and then "elevate" high flying objects to orbit or low orbiting objects to higher orbit.
The original concept envisioned by Tsiolkovsky was a compression structure, a concept similar to an [[Radio masts and towers|aerial mast]]. While such structures might reach [[Karman line|space]] (100 km, 62 mi), they are unlikely to reach geostationary orbit. The concept of a Tsiolkovsky tower combined with a classic space elevator cable (reaching above the level of GEO) has been suggested.<ref name="JBIS1999"/> Other ideas use very tall compressive towers to reduce the demands on launch vehicles.<ref name=TorontoProposal /> The vehicle is "elevated" up the tower, which may extend as high as [[Karman line|above the atmosphere]], and is launched from the top. Such a tall tower to access near-space altitudes of {{convert|20|km|mi|abbr=on}} has been proposed by various researchers.<ref name=TorontoProposal>{{cite journal|doi=10.1016/j.actaastro.2009.02.018|url=http://pi.library.yorku.ca/dspace/bitstream/handle/10315/2587/AA_3369_Quine_Space_Elevator_Final_2009.pdf|bibcode=2009AcAau..65..365Q|title=A free-standing space elevator structure: A practical alternative to the space tether|year=2009|last1=Quine|first1=B.M.|last2=Seth|first2=R.K.|last3=Zhu|first3=Z.H.|journal=Acta Astronautica|volume=65|issue=3–4|page=365|citeseerx=10.1.1.550.4359}}</ref><ref name="landis1996">Landis, Geoffrey, "Compression Structures for Earth Launch," 7th Advanced Space Propulsion Workshop, Jet Propulsion Laboratory, April 9–11, 1996; also [http://arc.aiaa.org/doi/abs/10.2514/6.1998-3737 paper AIAA-98-3737], 24th AIAA/ASME/SAE/ASEE Joint Propulsion Conference, 1998.</ref><ref>Hjelmstad, Keith, [http://hieroglyph.asu.edu/wp-content/uploads/2014/08/Hjelmstad-on-Stephenson-Structural-Design-of-the-Tall-Tower.pdf "Structural Design of the Tall Tower"], ''Hieroglyph'', 11/30/2013. (retrieved 1 Sept 2015)</ref>
Other concepts for [[non-rocket spacelaunch]] related to a space elevator (or parts of a space elevator) include an [[orbital ring]], a pneumatic space tower,<ref>[http://www.zdnet.com/blog/emergingtech/scientists-envision-inflatable-alternative-to-tethered-space-elevator/1600 Scientists envision inflatable alternative to tethered space elevator], ''[[ZDNet]]'', June 17, 2009. Retrieved February 2013.</ref> a [[space fountain]], a [[launch loop]], a [[Skyhook (structure)|skyhook]], a [[space tether]], and a buoyant "SpaceShaft".<ref>[http://ksj.mit.edu/tracker/2009/07/space-shaft-or-story-would-have-been-bit/ Space Shaft: Or, the story that would have been a bit finer, if only one had known...], "Knight Science Journalism Tracker (MIT)", July 1, 2009</ref>
==Notes==
{{reflist|group=note}}
==References==
{{Reflist|25em}}
==Further reading==
{{Refbegin}}
* Edwards BC, Ragan P. "Leaving The Planet By Space Elevator" Seattle, USA: Lulu; 2006. {{ISBN|978-1-4303-0006-9}}
* Edwards BC, Westling EA. ''The Space Elevator: A Revolutionary Earth-to-Space Transportation System.'' San Francisco, USA: Spageo Inc.; 2002. {{ISBN|0-9726045-0-2}}.
*[http://www.nss.org/resources/library/spaceelevator/2000-SpaceElevator-NASA-CP210429.pdf A conference publication based on findings from the Advanced Space Infrastructure Workshop on Geostationary Orbiting Tether "Space Elevator" Concepts] (PDF), held in 1999 at the NASA Marshall Space Flight Center, Huntsville, Alabama. Compiled by D.V. Smitherman, Jr., published August 2000.
*"The Political Economy of Very Large Space Projects" [http://www.jetpress.org/volume4/space.htm HTML] [http://www.jetpress.org/volume4/space.pdf PDF], John Hickman, Ph.D. ''[[Journal of Evolution and Technology]]'' Vol. 4 – November 1999.
*[http://spectrum.ieee.org/aerospace/space-flight/a-hoist-to-the-heavens A Hoist to the Heavens] By Bradley Carl Edwards
*Ziemelis K. (2001) "Going up". In [[New Scientist]] '''2289''': 24–27. [http://www.spaceref.com/news/viewnews.html?id=337 Republished in SpaceRef]. Title page: "The great space elevator: the dream machine that will turn us all into astronauts."
*[https://web.archive.org/web/20101104104658/http://www.space.com/businesstechnology/technology/space_elevator_020327-1.html The Space Elevator Comes Closer to Reality]. An overview by Leonard David of space.com, published March 27, 2002.
* Krishnaswamy, Sridhar. Stress Analysis — [https://web.archive.org/web/20060519133820/http://www.cqe.northwestern.edu/sk/C62/OrbitalTower_ME362.pdf The Orbital Tower] (PDF)
* [[LiftPort]]'s Roadmap for Elevator To Space [https://web.archive.org/web/20070710032602/http://www.liftport.com/papers/SE_Roadmap_v1beta.pdf SE Roadmap] (PDF)
* [https://web.archive.org/web/20080403061917/http://space.newscientist.com/article/dn13552-space-elevators-face-wobble-problem.html Space Elevators Face Wobble Problem]: New Scientist
* Alexander Bolonkin, “Non Rocket Space Launch and Flight”. Elsevier, 2005. 488 pgs. {{ISBN|978-0-08044-731-5}}. https://archive.org/details/Non-rocketSpaceLaunchAndFlight,
{{Refend}}
==External links==
{{Portal|Spaceflight|Science}}
{{Commons category|Space elevators}}
{{Spoken Wikipedia|Space_elevator.ogg|2006-05-29}}
* [http://spaceelevatorwiki.com/ Space Elevator Engineering-Development wiki]
* [https://web.archive.org/web/20080919070924/https://science.nasa.gov/headlines/y2000/ast07sep_1.htm Audacious & Outrageous: Space Elevators]
* [http://www.bildung-kultur.org/167/ Ing-Math.Net (Germany)] – Ing-Math.Net (German Max-Born Space Elevator Team 2006) (German)
* [https://web.archive.org/web/20111206185615/http://www.warr.de/spaceelevator Project of the Scientific Workgroup for Rocketry and Spaceflight](WARR) (German)
* [http://economist.com/science/tq/displayStory.cfm?story_id=7001786 The Economist: Waiting For The Space Elevator] (June 8, 2006 – subscription required)
* [https://web.archive.org/web/20060806021241/http://www.radio.cbc.ca/programs/quirks/archives/01-02/nov0301.htm CBC Radio Quirks and Quarks November 3, 2001] ''Riding the Space Elevator''
* [http://www.timesonline.co.uk/tol/driving/features/article5529668.ece Times of London Online: Going up ... and the next floor is outer space]
* [http://www.islandone.org/LEOBiblio/CLARK1.HTM ''The Space Elevator: 'Thought Experiment', or Key to the Universe?'']. By Sir Arthur C. Clarke. Address to the XXXth International Astronautical Congress, Munich, September 20, 1979.
* [http://www.zadar.net/space-elevator/ The Space Elevator – Physical Principles] The math and the numbers for actual materials.
{{Space elevator}}
{{Non-rocket spacelaunch}}
{{Emerging technologies}}
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[[Category:Space elevator| ]]
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[[Category:Space colonization]]
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[[File:Space elevator structural diagram--corrected for scale+CM+etc.svg|thumb|upright=1.2|alt=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 shit
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.]]
[[File:Space elevator in motion viewed from above north pole.ogv|thumbtime=28|thumb|upright=1.2|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''' is a proposed type of planet-to-space transportation system.<ref>{{cite web|url=http://www.isec.org/index.php/what-is-a-space-elevator |title=What is a Space Elevator? |publisher=The International Space Elevator Consortium |date=April 11, 2012}}</ref> The main component would be a cable (also called a [[space tether|tether]]) anchored to the surface and extending into space. The design would permit vehicles to travel along the cable from a planetary surface, such as the Earth's, directly into space or orbit, [[non-rocket spacelaunch|without the use of large rockets]]. An Earth-based space elevator would consist of a cable with one end attached to the surface near the equator and the other end in space beyond [[geostationary orbit]] (35,786 km altitude). The competing forces of gravity, which is stronger at the lower end, and the outward/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 could repeatedly climb the tether to space by mechanical means, releasing their cargo to orbit. Climbers could also descend the tether to return cargo to the surface from orbit.<ref name=Edwards>Edwards, Bradley Carl. [http://www.niac.usra.edu/studies/521Edwards.html "The NIAC Space Elevator Program"]. NASA Institute for Advanced Concepts</ref>
The concept of a tower reaching geosynchronous orbit was first published in 1895 by [[Konstantin Tsiolkovsky]].<ref>{{cite web|url = http://www.g4tv.com/techtvvault/features/35657/Space_Elevator_Gets_Lift.html|title = Space Elevator Gets Lift|accessdate = September 13, 2007|last = Hirschfeld|first = Bob|date = January 31, 2002 |publisher = [[TechTV]] |archiveurl = https://web.archive.org/web/20050608080057/http://www.g4tv.com/techtvvault/features/35657/Space_Elevator_Gets_Lift.html|archivedate = June 8, 2005|quote = The concept was first described in 1895 by Russian author K. E. Tsiolkovsky in his 'Speculations about Earth and Sky and on Vesta.'}}</ref> 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 (physical)|compression]], supporting its weight from below. Since 1959, most ideas for space elevators have focused on purely [[Tension (physics)|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]].
To construct a space elevator on Earth, the cable material would need to be both stronger and lighter (have greater [[specific strength]]) than any known material. Development of new materials that meet the demanding specific strength requirement must happen before designs can progress beyond discussion stage. [[Carbon nanotube]]s (CNTs) have been identified as possibly being able to meet the specific strength requirements for an Earth space elevator.<ref name=Edwards/><ref name=BBCfuture>{{cite web |url=http://www.bbc.com/future/story/20150211-space-elevators-a-lift-too-far |title=Should We give up on the dream of space elevators? |first=Nic |last=Fleming |date=February 15, 2015 |accessdate=July 22, 2018 |publisher=[[BBC]]}}</ref> Other materials considered have been [[boron nitride nanotube]]s, and [[carbon nanothread|diamond nanothreads]], which were first constructed in 2014<ref name=SCIAM_DN /><ref name=Xtech_DN />. In 2018 single-crystal [[Graphene]] was also proposed as a potential material<ref>{{Cite web|url=https://www.azom.com/article.aspx?ArticleID=16371|title=Space Elevator Technology and Graphene: An Interview with Adrian Nixon|last=|first=|date=|website=|archive-url=|archive-date=|dead-url=|access-date=}}</ref>.
The concept is applicable to other planets and [[Astronomical object|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.<ref>[[Hans Moravec|Moravec, Hans]] (1978). [http://www.frc.ri.cmu.edu/~hpm/project.archive/1976.skyhook/papers/scasci.txt ''Non-Synchronous Orbital Skyhooks for the Moon and Mars with Conventional Materials'']. Carnegie Mellon University. frc.ri.cmu.edu</ref>
==History==
===Early concepts===
[[Image:Tsiolkovsky.jpg|thumb|left|upright|[[Konstantin Tsiolkovsky]]]]
The key concept of the space elevator appeared in 1895 when [[Russia]]n scientist [[Konstantin Tsiolkovsky]] was inspired by the [[Eiffel Tower]] in [[Paris]]. He considered a similar tower that reached all the way into space and was built from the ground up to the altitude of 35,786 kilometers, the height of [[geostationary orbit]].<ref name="NASASci">{{cite web|url=https://science.nasa.gov/headlines/y2000/ast07sep_1.htm |title=The Audacious Space Elevator |last= |first= |date= |website= |publisher=NASA Science News |dead-url=yes |accessdate=September 27, 2008 |archiveurl=https://web.archive.org/web/20080919070924/https://science.nasa.gov/headlines/y2000/ast07sep_1.htm |archivedate=September 19, 2008 |df= }}</ref> He noted that the top of such a tower would be circling [[Earth]] as in a geostationary orbit. Objects would attain horizontal velocity as they rode up the tower, and an object released at the tower's top would have enough horizontal velocity to remain there in geostationary orbit. Tsiolkovsky's conceptual tower was a compression structure, while modern concepts call for a [[tensile structure]] (or "tether").
===20th century===
Building a compression structure from the ground up proved an unrealistic task as there was no material in existence with enough compressive strength to support its own weight under such conditions.<ref name="JBIS1999">{{cite journal
|author1=Landis, Geoffrey A. |author2=Cafarelli, Craig
|lastauthoramp=yes | year = 1999
| title = The Tsiolkovski Tower Reexamined
| journal = Journal of the British Interplanetary Society
| volume = 52
| pages = 175–180
| others = Presented as paper IAF-95-V.4.07, 46th International Astronautics Federation Congress, Oslo Norway, October 2–6, 1995
|bibcode = 1999JBIS...52..175L }}
</ref> In 1959 another Russian scientist, [[Yuri N. Artsutanov]], suggested a more feasible proposal. Artsutanov suggested using a geostationary [[satellite]] as the base from which to deploy the structure downward. By using a [[counterweight]], a cable would be lowered from geostationary orbit to the surface of Earth, while the counterweight was extended from the satellite away from Earth, keeping the cable constantly over the same spot on the surface of the Earth. Artsutanov's idea was introduced to the Russian-speaking public in an interview published in the Sunday supplement of ''[[Komsomolskaya Pravda]]'' in 1960,<ref name="artsutanov">{{cite web
|url=http://liftport.com/files/Artsutanov_Pravda_SE.pdf
|archiveurl=https://web.archive.org/web/20060506100948/http://liftport.com/files/Artsutanov_Pravda_SE.pdf
|archivedate=May 6, 2006 |title=To the Cosmos by Electric Train
|work=liftport.com
|year=1960
|publisher=Young Person's Pravda
|last=Artsutanov
|first=Yu
|accessdate=March 5, 2006}}</ref> but was not available in English until much later. He also proposed tapering the cable thickness so that the stress in the cable was constant. This gave a thinner cable at ground level that became thickest at the level of geostationary orbit.
Both the tower and cable ideas were proposed in the quasi-humorous [[Daedalus (Ariadne)|''Ariadne'' column]] in ''[[New Scientist]]'', December 24, 1964.
In 1966, Isaacs, Vine, Bradner and Bachus, four [[United States|American]] engineers, reinvented the concept, naming it a "Sky-Hook", and published their analysis in the journal [[Science (journal)|''Science'']].<ref>{{cite journal
|title=Satellite Elongation into a True 'Sky-Hook'
|year=1966
|journal= Science
|volume = 151
| doi = 10.1126/science.151.3711.682
|author=Isaacs, J. D. |author2= A. C. Vine, H. Bradner and G. E. Bachus|bibcode = 1966Sci...151..682I
|issue=3711
|pages=682–3
|last3=Bradner
|last4=Bachus
|pmid=17813792
}}</ref> They decided to determine what type of material would be required to build a space elevator, assuming it would be a straight cable with no variations in its cross section area, and found that the [[specific strength|strength]] required would be twice that of any then-existing material including [[graphite]], [[quartz]], and [[diamond]].
In 1975 an American scientist, [[Jerome Pearson]], reinvented the concept yet again, publishing his analysis in the journal ''[[Acta Astronautica]]''. He designed<ref name="pearson">
{{cite journal
| author = Pearson, J.
| year = 1975
| title = The orbital tower: a spacecraft launcher using the Earth's rotational energy
| url = http://www.star-tech-inc.com/papers/tower/tower.pdf
| journal = Acta Astronautica
| volume = 2
| pages = 785–799
| doi = 10.1016/0094-5765(75)90021-1
| format = PDF <!--Retrieved from CrossRef by DOI bot-->
| issue = 9–10
| bibcode = 1975AcAau...2..785P
| citeseerx = 10.1.1.530.3120
}}
</ref> a cross-section-area altitude profile that tapered and would be better suited to building the elevator. The completed cable would be thickest at the geostationary orbit, where the tension was greatest, and would be narrowest at the tips to reduce the amount of weight per unit area of cross section that any point on the cable would have to bear. He suggested using a counterweight that would be slowly extended out to {{convert|144,000|km|mi|abbr=off|sp=us}}, almost half the distance to the [[Moon]] as the lower section of the elevator was built. Without a large counterweight, the upper portion of the cable would have to be longer than the lower due to the way [[gravity|gravitational]] and centrifugal forces change with distance from Earth. His analysis included disturbances such as the gravitation of the Moon, wind and moving payloads up and down the cable. The weight of the material needed to build the elevator would have required thousands of [[Space Shuttle]] trips, although part of the material could be transported up the elevator when a minimum strength strand reached the ground or be manufactured in space from [[Asteroid mining|asteroidal]] or [[In-situ resource utilization|lunar ore]].
After the development of [[carbon nanotubes]] in the 1990s, engineer David Smitherman of [[NASA]]/Marshall's Advanced Projects Office realized that the high strength of these materials might make the concept of a space elevator feasible, and put together a workshop at the [[Marshall Space Flight Center]], inviting many scientists and engineers to discuss concepts and compile plans for an elevator to turn the concept into a reality.
In 2000, another American scientist, [[Bradley C. Edwards]], suggested creating a {{convert|100,000|km|mi|abbr=on}} long paper-thin ribbon using a carbon nanotube composite material.<ref name=EDWARDS_PHASE_I_2000_472Edwards.html>Bradley C. Edwards, "[http://www.niac.usra.edu/studies/472Edwards.html The Space Elevator]"</ref> He chose the wide-thin ribbon-like cross-section shape rather than earlier circular cross-section concepts because that shape would stand a greater chance of surviving impacts by meteoroids. The ribbon cross-section shape also provided large surface area for climbers to climb with simple rollers. Supported by the [[NASA Institute for Advanced Concepts]], Edwards' work was expanded to cover the deployment scenario, climber design, power delivery system, [[Space debris|orbital debris]] avoidance, anchor system, surviving [[atomic oxygen]], avoiding lightning and hurricanes by locating the anchor in the western equatorial Pacific, construction costs, construction schedule, and environmental hazards.<ref name=Edwards/><ref>[http://www.nss.org/resources/library/spaceelevator/2000-SpaceElevator-NASA-CP210429.pdf "Space Elevators: An Advanced Earth-Space Infrastructure for the New Millennium"], NASA/CP-2000-210429, Marshall Space Flight Center, Huntsville, Alabama, 2000</ref><ref>Science @ NASA, [https://science.nasa.gov/headlines/y2000/ast07sep_1.htm "Audacious & Outrageous: Space Elevators"] {{webarchive|url=https://web.archive.org/web/20080919070924/https://science.nasa.gov/headlines/y2000/ast07sep_1.htm |date=September 19, 2008 }}, September 2000</ref><ref>{{cite web | title = Space Elevators: An Advanced Earth-Space Infrastructure for the New Millennium | url = http://www.affordablespaceflight.com/spaceelevator.html| archiveurl = https://web.archive.org/web/20070221162221/http://www.affordablespaceflight.com/spaceelevator.html| archivedate = February 21, 2007|work=affordablespaceflight.com}}</ref>
===21st century===
To speed space elevator development, proponents have organized several [[Space Elevator Competitions|competitions]], similar to the [[Ansari X Prize]], for relevant technologies.<ref>{{cite web
|url=http://msnbc.msn.com/id/5792719/
|title=Space elevator contest proposed
|first=Alan
|last=Boyle
|publisher=MSNBC
|date=August 27, 2004}}</ref><ref>{{cite web
|url=http://www.elevator2010.org/
|title=The Space Elevator – Elevator:2010
|accessdate=March 5, 2006}}</ref> Among them are [[Elevator:2010]], which organized annual competitions for climbers, ribbons and power-beaming systems from 2005 to 2009, the Robogames Space Elevator Ribbon Climbing competition,<ref>{{cite web
|url=http://robogames.net/rules/climbing.php
|title=Space Elevator Ribbon Climbing Robot Competition Rules
|accessdate=March 5, 2006 |archiveurl = https://web.archive.org/web/20050206100051/http://robolympics.net/rules/climbing.shtml|archivedate=February 6, 2005 }}</ref> as well as NASA's [[Centennial Challenges]] program, which, in March 2005, announced a partnership with the Spaceward Foundation (the operator of Elevator:2010), raising the total value of prizes to US$400,000.<ref>{{cite web
|url=http://www.nasa.gov/home/hqnews/2005/mar/HQ_m05083_Centennial_prizes.html
|title=NASA Announces First Centennial Challenges' Prizes
|year=2005
|accessdate=March 5, 2006}}</ref><ref>{{cite web
|url=http://www.space.com/news/050323_centennial_challenge.html
|title=NASA Details Cash Prizes for Space Privatization
|first=Robert Roy
|last=Britt
|publisher=Space.com
|accessdate=March 5, 2006}}</ref>
The first European Space Elevator Challenge (EuSEC) to establish a climber structure took place in August 2011.<ref>{{cite web
|title=What's the European Space Elevator Challenge?
|url=http://eusec.warr.de/?eusec|publisher=European Space Elevator Challenge
|accessdate=April 21, 2011}}</ref>
In 2005, "the [[LiftPort Group]] of space elevator companies announced that it will be building a carbon nanotube manufacturing plant in [[Millville, New Jersey]], to supply various glass, plastic and metal companies with these strong materials. Although LiftPort hopes to eventually use carbon nanotubes in the construction of a {{convert|100,000|km|mi|abbr=on}} space elevator, this move will allow it to make money in the short term and conduct research and development into new production methods."<ref>{{cite web
|url=http://www.universetoday.com/am/publish/liftport_manufacture_nanotubes.html?2742005
|title=Space Elevator Group to Manufacture Nanotubes
|year=2005
|publisher=Universe Today
|accessdate=March 5, 2006}}</ref> Their announced goal was a space elevator launch in 2010. On February 13, 2006 the LiftPort Group announced that, earlier the same month, they had tested a mile of "space-elevator tether" made of carbon-fiber composite strings and fiberglass tape measuring {{convert|5|cm|in|abbr=on}} wide and 1 mm (approx. 13 sheets of paper) thick, lifted with balloons.<ref>{{cite news
|url=http://www.newscientistspace.com/article/dn8725.html
|title=Space-elevator tether climbs a mile high
|date=February 15, 2006
|work=NewScientist.com
|publisher=New Scientist
|first=Kimm
|last=Groshong
|accessdate=March 5, 2006}}</ref>
In 2007, [[Elevator:2010]] held the 2007 Space Elevator games, which featured US$500,000 awards for each of the two competitions, ($1,000,000 total) as well as an additional $4,000,000 to be awarded over the next five years for space elevator related technologies.<ref>[https://web.archive.org/web/20100118153108/http://www.spaceward.org/elevator2010 Elevator:2010 – The Space Elevator Challenge]. spaceward.org</ref> No teams won the competition, but a team from [[MIT]] entered the first 2-gram (0.07 oz), 100-percent carbon nanotube entry into the competition.<ref>[https://web.archive.org/web/20071101081423/http://www.spaceward.org/games07Wrapup.html Spaceward Games 2007]. The Spaceward Foundation</ref> Japan held an international conference in November 2008 to draw up a timetable for building the elevator.<ref name=JapanUKTimes>{{cite news | title = Japan hopes to turn sci-fi into reality with elevator to the stars | url=http://www.thetimes.co.uk/tto/news/world/article1967078.ece | work=The Times | location=London | first=Leo | last=Lewis | date=September 22, 2008 | accessdate=May 23, 2010}} Lewis, Leo; News International Group; accessed September 22, 2008.</ref>
In 2008 the book ''Leaving the Planet by Space Elevator'' by Dr. Brad Edwards and Philip Ragan was published in Japanese and entered the Japanese best-seller list.<ref name=Leaving>{{cite web | title = Leaving the Planet by Space Elevator | url = http://www.leavingtheplanet.com/}} Edwards, Bradley C. and Westling, Eric A. and Ragan, Philip; Leasown Pty Ltd.; accessed September 26, 2008.</ref> <ref>{{Cite book|title=Space Elevator: Leaving the Planet by Space Elevator|date=2008|isbn=9784270003350|location=東京|language=Japanese|last1 = エドワーズ|first1 = ブラッドリー・C|last2=フィリップ・レーガン}}</ref> This led to Shuichi Ono, chairman of the Japan Space Elevator Association, unveiling a space-elevator plan, putting forth what observers considered an extremely low cost estimate of a trillion yen (£5 billion / $8 billion) to build one.<ref name=JapanUKTimes/>
In 2012, the [[Obayashi Corporation]] announced that in 38 years it could build a space elevator using carbon nanotube technology.<ref>{{cite news| url=http://www.physorg.com/news/2012-02-japan-builder-eyes-space-elevator.html | work=PhysOrg.com | title=Going up: Japan builder eyes space elevator | date=February 22, 2012}}</ref> At 200 kilometers per hour, the design's 30-passenger climber would be able to reach the GEO level after a 7.5 day trip.<ref>{{cite news| url=http://www.ibtimes.com/articles/302223/20120221/space-elevator-60000-miles-reality-obayashi-nanotube.htm | title=Space Elevator That Soars 60,000 Miles into Space May Become Reality by 2050 | date=February 21, 2012}}</ref> No cost estimates, finance plans, or other specifics were made. This, along with timing and other factors, hinted that the announcement was made largely to provide publicity for the opening of one of the company's other projects in Tokyo.<ref>{{cite web|last=Boucher |first=Marc |url=http://www.spaceelevator.com/2012/02/obayashi-and-the-space-elevator---a-story-of-hype.html#more |title=Obayashi and the Space Elevator – A Story of Hype|work= The Space Elevator Reference |date=February 23, 2012 |accessdate=August 14, 2012 |deadurl=yes |archiveurl=https://web.archive.org/web/20120621201313/http://www.spaceelevator.com/2012/02/obayashi-and-the-space-elevator---a-story-of-hype.html |archivedate=June 21, 2012 }}</ref>
In 2013, the International Academy of Astronautics published a technological feasibility assessment which concluded that the critical capability improvement needed was the tether material, which was projected to achieve the necessary strength-to-weight ratio within 20 years. The four-year long study looked into many facets of space elevator development including missions, development schedules, financial investments, revenue flow, and benefits. It was reported that it would be possible to operationally survive smaller impacts and avoid larger impacts, with meteors and space debris, and that the estimated cost of lifting a kilogram of payload to GEO and beyond would be $500.<ref>{{Cite book|title = Space Elevators: An Assessment of the Technological Feasibility and the Way Forward|last1 = Swan|last2 =Raitt|last3 =Swan|last4 = Penny|last5 =Knapman|first1 = Peter A.|first2 = David I.|first3 = Cathy W.|first4 = Robert E.|first5 = John M.|publisher = International Academy of Astronautics|year = 2013|isbn = 9782917761311|location = Virginia, US|pages = 10–11, 207–208}}</ref><ref name=":0" />
In 2014, Google X's Rapid Evaluation R&D team began the design of a Space Elevator, eventually finding that no one had yet manufactured a perfectly formed carbon nanotube strand longer than a meter. They thus decided to put the project in "deep freeze" and also keep tabs on any advances in the carbon nanotube field.<ref>{{cite web|last=Gayomali|first=Chris|title=Google X Confirms The Rumors: It Really Did Try To Design A Space Elevator|url=http://www.fastcompany.com/3029138/world-changing-ideas/google-x-confirms-the-rumors-it-really-did-try-to-design-a-space-elevat?partner=rss|work=Fast Company|accessdate=17 April 2014|date=15 April 2014}}</ref>
In 2018, researchers at Japan's [[Shizuoka University]] launched STARS-Me, two [[CubeSat]]s connected by a tether, which a mini-elevator will travel on.<ref>{{Cite web | url=https://www.nbcnews.com/mach/science/colossal-elevator-space-could-be-going-sooner-you-ever-imagined-ncna915421 | title=A colossal elevator to space could be going up sooner than you ever imagined}}</ref><ref>{{cite web
|url=https://www.curbed.com/2018/9/12/17851500/space-elevator-japan-news
|title=Japan is trying to build an elevator to space
|publisher=Curbed.com
|first=Meghan
|last=Barber
|date=September 12, 2018
|accessdate=September 18, 2018
}}</ref> The experiment was launched as a test bed for a larger structure.<ref>https://gizmodo.com/japan-testing-miniature-space-elevator-near-the-interna-1828800558</ref>
==In fiction==
{{main|Space elevators in fiction}}
In 1979, space elevators were introduced to a broader audience with the simultaneous publication of [[Arthur C. Clarke]]'s novel, ''[[The Fountains of Paradise]]'', in which engineers construct a space elevator on top of a mountain peak in the fictional island country of "Taprobane" (loosely based on [[Sri Lanka]], albeit moved south to the Equator), and [[Charles Sheffield]]'s first novel, ''[[The Web Between the Worlds]]'', also featuring the building of a space elevator. Three years later, in [[Robert A. Heinlein]]'s 1982 novel ''[[Friday (novel)|Friday]]'' the principal character makes use of the "Nairobi Beanstalk" in the course of her travels. In [[Kim Stanley Robinson]]'s 1993 novel ''[[Red Mars]]'', colonists build a space elevator on Mars that allows both for more colonists to arrive and also for natural resources mined there to be able to leave for Earth. In [[David Gerrold]]'s 2000 novel, ''[[Jumping Off The Planet]]'', a family excursion up the Ecuador "beanstalk" is actually a child-custody kidnapping. Gerrold's book also examines some of the industrial applications of a mature elevator technology. The concept of a space elevator, called the [[Old Man's War#Beanstalk|Beanstalk]], is also depicted in John Scalzi's 2005 novel, ''[[Old Man's War]].'' In a biological version, [[Joan Slonczewski]]'s 2011 novel ''The Highest Frontier'' depicts a college student ascending a space elevator constructed of self-healing cables of anthrax bacilli. The engineered bacteria can regrow the cables when severed by space debris.
==Physics==
===Apparent gravitational field===
An Earth space elevator cable rotates along with the rotation of the Earth. Therefore the cable, and objects attached to it, would experience upward centrifugal force in the direction opposing the downward gravitational force. The higher up the cable the object is located, the less the gravitational pull of the Earth, and the stronger the upward centrifugal force due to the rotation, so that more centrifugal force opposes less gravity. The centrifugal force and the gravity are balanced at geosynchronous equatorial orbit (GEO). Above GEO, the centrifugal force is stronger than gravity, causing objects attached to the cable there to pull ''upward'' on it.
The net force for objects attached to the cable is called the ''apparent gravitational field''. The apparent gravitational field for attached objects is the (downward) gravity minus the (upward) centrifugal force. The apparent gravity experienced by an object on the cable is zero at GEO, downward below GEO, and upward above GEO.
The apparent gravitational field can be represented this way:{{rp | Ref<ref name="aravind"/> Table 1}}
{{block indent|The downward force of actual [[Newton's law of universal gravitation|gravity]] ''decreases'' with height: [[Newton's law of universal gravitation|<math>g_r = -GM/r^2</math>]]}}
{{block indent|The upward [[centrifugal force]] due to the planet's rotation ''increases'' with height: [[Centrifugal force|<math>a = \omega^2 r</math>]]}}
{{block indent|Together, the apparent gravitational field is the sum of the two:
{{block indent|<math>g = -\frac{GM}{r^2} + \omega^2 r</math>}}}}
where
{{block indent|''g'' is the acceleration of ''apparent'' gravity, pointing down (negative) or up (positive) along the vertical cable (m s<sup>−2</sup>),}}
{{block indent|''g<sub>r</sub>'' is the gravitational acceleration due to Earth's pull, pointing down (negative)(m s<sup>−2</sup>),}}
{{block indent|''a'' is the centrifugal acceleration, pointing up (positive) along the vertical cable (m s<sup>−2</sup>),}}
{{block indent|''G'' is the [[gravitational constant]] (m<sup>3</sup> s<sup>−2</sup> kg<sup>−1</sup>)}}
{{block indent|''M'' is the mass of the Earth (kg)}}
{{block indent|''r'' is the distance from that point to Earth's center (m),}}
{{block indent|''ω'' is Earth's rotation speed (radian/s).}}
At some point up the cable, the two terms (downward gravity and upward centrifugal force) are equal and opposite. Objects fixed to the cable at that point put no weight on the cable. This altitude (r<sub>1</sub>) depends on the mass of the planet and its rotation rate. Setting actual gravity equal to centrifugal acceleration gives:{{rp | Ref<ref name="aravind"/> page 126}}
{{block indent|<math>r_1 = \left(\frac{GM}{\omega^2}\right)^\frac{1}{3}</math>}}
On Earth, this distance is {{convert|35786|km|mi|0|abbr=on}} above the surface, the altitude of geostationary orbit.{{rp | Ref<ref name="aravind"/> Table 1}}
On the cable ''below'' geostationary orbit, downward gravity would be greater than the upward centrifugal force, so the apparent gravity would pull objects attached to the cable downward. Any object released from the cable below that level would initially accelerate downward along the cable. Then gradually it would deflect eastward from the cable. On the cable ''above'' the level of stationary orbit, upward centrifugal force would be greater than downward gravity, so the apparent gravity would pull objects attached to the cable ''upward''. Any object released from the cable ''above'' the geosynchronous level would initially accelerate ''upward'' along the cable. Then gradually it would deflect westward from the cable.
===Cable section===
Historically, the main technical problem has been considered the ability of the cable to hold up, with tension, the weight of itself below any given point. The greatest tension on a space elevator cable is at the point of geostationary orbit, {{convert|35786|km|mi|0|abbr=on}} above the Earth's equator. This means that the cable material, combined with its design, must be strong enough to hold up its own weight from the surface up to {{convert|35786|km|mi|0|abbr=on}}. A cable which is thicker in cross section area at that height than at the surface could better hold up its own weight over a longer length. How the cross section area tapers from the maximum at {{convert|35786|km|mi|0|abbr=on}} to the minimum at the surface is therefore an important design factor for a space elevator cable.
To maximize the usable excess strength for a given amount of cable material, the cable's cross section area would need to be designed for the most part in such a way that the [[Stress (mechanics)|stress]] (i.e., the tension per unit of cross sectional area) is constant along the length of the cable.<ref name="aravind"/><ref>Artuković, Ranko (2000). [http://www.zadar.net/space-elevator/ "The Space Elevator".] zadar.net</ref> The constant-stress criterion is a starting point in the design of the cable cross section area as it changes with altitude. Other factors considered in more detailed designs include thickening at altitudes where more space junk is present, consideration of the point stresses imposed by climbers, and the use of varied materials.<ref name=PhaseII/> To account for these and other factors, modern detailed designs seek to achieve the largest ''[[Factor of safety#Margin of safety|safety margin]]'' possible, with as little variation over altitude and time as possible.<ref name=PhaseII/> In simple starting-point designs, that equates to constant-stress.
In the constant-stress case, the cross-section-area can be described by the differential equation as:
{{block indent|<math>\frac{dA}{A} = \frac{\rho g R^2}{T} \left(\frac{1}{r^2} - \frac{r}{R_g^3} \right)dr</math>{{rp | Ref<ref name="aravind"/> equation 6}}}}
where
{{block indent|''g'' is the acceleration along the radius (m·s<sup>−2</sup>),}}
{{block indent|''A'' is the cross-section area of the cable at any given point r, (m<sup>2</sup>),}}
{{block indent|''ρ'' is the density of the material used for the cable (kg·m<sup>−3</sup>),}}
{{block indent|''R'' is the earth's equatorial radius,}}
{{block indent|<math>R_g</math> is the radius of geosynchronous orbit,}}
{{block indent|1=''T'' is the stress the cross-section area can bear without [[Yield (engineering)|yielding]] (N·m<sup>−2</sup>=kg·m<sup>−1</sup>·s<sup>−2</sup>), its elastic limit.}}
For a constant-stress cable with no safety margin, the cross-section-area profile as a function of distance from Earth's center can be solved with
{{block indent|<math>A( r ) = A_s exp\left[ \frac{\rho g R^2}{T}\left( \frac{1}{R}+\frac{R^2}{2R_g^3}-\frac{1}{r}-\frac{r^2}{2R_g^3} \right) \right]</math>{{rp | Ref<ref name="aravind"/> equation 7}}}}
Safety margin can be accounted for by dividing T by the desired safety factor. <ref name="aravind"/>
===Cable materials===
Using the above taper formula to solve for the specific case of earth equatorial surface (<math>R=6378</math> km) and Earth geosynchronous orbit (<math>R_g = 42164</math> km), specific materials can be examined:<ref group=note>Specific substitutions used to produce the factor {{val|4.85|e=7}}:{{block indent|<math>A = A_s exp \left[ \frac{\rho \times 9.81 \times (6.378\times 10^6)^2 } {T} \left(
\frac{1}{6.378\times 10^6} + \frac{(6.378\times 10^6)^2}{(4.2164\times 10^7)^3} - \frac{1}{4.2164\times 10^7} - \frac{(4.2164\times 10^7)^2}{2 (4.2164\times 10^7)^3}\right)\right]</math>}}</ref>
{{block indent|<math>A = A_s exp [\frac{\rho}{T}\times 4.85\times 10^7]</math> }}
A table of values for taper for various materials are:
{| class="wikitable" style="text-align:left"
|+Taper ratios by materials{{rp | Ref<ref name="aravind"/> Table 2}}
|-
!Material!!Tensile strength<br>(MPa)!!Density<br>(kg/m^3)!![[Specific strength]]<br>(MPa)/(kg/m^3)!! Taper ratio
|-
|Steel || 5,000 || 7,900 || 0.63 || {{val|1.6|e=33}}
|-
|Kevlar || 3,600 || 1,440 || 2.5 || {{val|2.5|e=8}}
|-
|Carbon nanotubes || 130,000 || 1,300 || 100|| 1.6
|}
The taper factor results in large increases in cross-section-area unless the specific strength of the material used approaches 48 (MPa)/(kg/m^3). Low specific strength materials require very large taper ratios which equates to large (or astronomical) total mass of the cable with associated large or impossible costs.
==Structure==
[[Image:SpaceElevatorClimbing.jpg|thumb|right|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 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 an advantage of being able to be at high altitude, 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 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]]
[[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 carefully 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 name=NASA97029>{{cite web|url=http://www.nas.nasa.gov/assets/pdf/techreports/1997/nas-97-029.pdf
|title=NAS-97-029: NASA Applications of Molecular Nanotechnology
|author=Globus, Al
|display-authors=etal
|publisher=NASA
|accessdate=September 27, 2008}}</ref>
The cable would need to be made of a material with a large [[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 kg/m^3,<ref name=EDWARDS_PHASE_I_2000_472Edwards.html /> that implies a specific strength of 77 megapascal/(kg/m^3). 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 force]]s increase (and gravity decreases) dramatically with height: {{cite web|url= http://www.islandone.org/LEOBiblio/CLARK2.HTM|title = The space elevator: 'thought experiment', or key to the universe?''|last = Clarke|first = A.C.|year = 1979}}</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 km (0.2 - 0.3 MPa/(kg/m^3)). Modern [[Man-made fibers|fibre]] materials such as [[kevlar]], [[fibreglass]] and [[Carbon fiber|carbon/graphite fibre]] have breaking lengths of 100–400 km (1.0 - 4.0 MPa/(kg/m^3)). 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 km (50 - 60 MPa/(kg/m^3)), 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 6th 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=http://www.huffingtonpost.com/2013/05/29/space-elevators-stronger-materials_n_3353697.html }}</ref>
<ref name="pop.15185070">{{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 name="extreme.176625">{{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 |accessdate=2014-04-19 }}</ref> As of 2014, carbon nanotube technology allowed growing tubes up to a few tenths of meters.<ref>{{cite journal| first=X.| last=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| deadurl=yes| archiveurl=https://web.archive.org/web/20170808164154/http://www.chem.pku.edu.cn/page/liy/labhomepage/publications/2009/2009NL.pdf| archivedate=August 8, 2017| df=mdy-all| citeseerx=10.1.1.454.2744}}</ref>
In 2014, [[carbon nanothread|diamond nanothreads]] were first synthesized.<ref name=SCIAM_DN>{{cite web |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=September 26, 2014 |publisher=[[Scientific American]] |accessdate=July 22, 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 web |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=September 23, 2014 |publisher=Zeff Davis, LLC |website=Extreme Tech |accessdate=July 22, 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" >Lang, David D. [http://spaceelevatorwiki.com/wiki/images/2/2b/Paper_Lang_Climber_Transit.pdf Space Elevator Dynamic Response to In-Transit Climbers].</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 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 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 name="Gassend_fall">
{{cite web
|first=Blaise
|last=Gassend
|title=Falling Climbers
|url=http://gassend.net/spaceelevator/falling-climbers/index.html
|accessdate=December 16, 2013
}}</ref><ref name=Skyway_to_LEO>
{{cite web
|publisher=Endless Skyway|title=Space elevator to low orbit?
|url=http://www.endlessskyway.com/2010/05/space-elevator-to-low-orbit.html
|accessdate=December 16, 2013
}}</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 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 like an inverted 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>[http://gassend.net/spaceelevator/center-of-mass/index.html "Why the Space Elevator's Center of Mass is not at GEO" by Blaise Gassend]. Gassend.net. Retrieved on September 30, 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.{{citation needed|date=April 2014}} 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|author1=Bill Fawcett, Michael Laine |author2=Tom Nugent jr. |lastauthoramp=yes |title=LIFTPORT|date=2006|publisher=Meisha Merlin Publishing, Inc.|location=Canada|isbn=978-1-59222-109-7|page=103}}</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 get that 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 km it is possible to use solar energy to power the climber<ref>{{cite web
|url = http://isec.org/pdfs/isec_reports/2013_ISEC_Design_Considerations_for_Space_Elevator_Tether_Climbers_Final_Report.pdf
|title = Design Consideration for Space Elevator Tether Climbers
|last1 = Swan, P. A.
|last2 = Swan, C. W.
|last3 = Penny, R. E.
|last4 = Knapman, J. M.
|last5 = Glaskowsky, P. N.
|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
|deadurl = yes
|archiveurl = https://web.archive.org/web/20170116175959/http://isec.org/pdfs/isec_reports/2013_ISEC_Design_Considerations_for_Space_Elevator_Tether_Climbers_Final_Report.pdf
|archivedate = January 16, 2017
|df = mdy-all
}}</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/>
*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, USA: 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.
==Launching into deep space==
An object attached to a space elevator at a radius of approximately 53,100 km would be at [[escape velocity]] when released. Transfer orbits to the L1 and L2 [[Lagrangian point]]s could be attained by release at 50,630 and 51,240 km, respectively, and transfer to lunar orbit from 50,960 km.<ref>{{cite web|url=http://www.spaceelevator.com/docs/iac-2004/iac-04-iaa.3.8.3.04.engel.pdf |title=IAC-04-IAA.3.8.3.04 Lunar transportation scenarios utilising the space elevator |author=Engel, Kilian A. |publisher=www.spaceelevator.com |deadurl=yes |archiveurl=https://web.archive.org/web/20120424230830/http://www.spaceelevator.com/docs/iac-2004/iac-04-iaa.3.8.3.04.engel.pdf |archivedate=April 24, 2012 }}</ref>
At the end of Pearson's {{convert|144,000|km|mi|abbr=on}} cable, the tangential velocity is 10.93 kilometers per second (6.79 mi/s). That is more than enough to [[escape velocity|escape]] Earth's gravitational field and send probes at least as far out as [[Jupiter]]. Once at Jupiter, a [[gravitational assist]] maneuver could permit solar escape velocity to be reached.<ref name="aravind">{{cite journal|title=The physics of the space elevator|author=Aravind, P. K.|url=http://users.wpi.edu/~paravind/Publications/PKASpace%20Elevators.pdf|year=2007|journal=American Journal of Physics|volume=45|issue=2|doi=10.1119/1.2404957|page=125|bibcode = 2007AmJPh..75..125A}}</ref>
==Extraterrestrial elevators==
A space elevator could also be constructed on other planets, asteroids and moons.
A [[Mars|Martian]] tether could be much shorter than one on Earth. Mars' surface gravity is 38 percent of Earth's, while it rotates around its axis in about the same time as Earth. Because of this, Martian [[areostationary orbit|stationary orbit]] is much closer to the surface, and hence the elevator could be much shorter. Current materials are already sufficiently strong to construct such an elevator.<ref>Forward, Robert L. and Moravec, Hans P. (March 22, 1980) [http://www.frc.ri.cmu.edu/~hpm/project.archive/1976.skyhook/1982.articles/elevate.800322 Space Elevators]. Carnegie Mellon University. "Interestingly enough, they are already more than strong enough for constructing skyhooks on the moon and Mars."</ref> Building a Martian elevator would be complicated by the Martian moon [[Phobos (moon)|Phobos]], which is in a low orbit and intersects the Equator regularly (twice every orbital period of 11 h 6 min).
On the near side of the Moon, the strength-to-density required of the tether of a [[lunar space elevator]] exists in currently available materials. A lunar space elevator would be about {{convert|50,000|km|mi|sp=us}} long. Since the Moon does not rotate fast enough, there is no effective lunar-stationary orbit, but the [[Lagrangian point]]s could be used. The near side would extend through the Earth-Moon [[Inner lagrangian point|L1]] point from an anchor point near the center of the visible part of Earth's Moon.<ref name="Pearson 2005"/>
On the far side of the Moon, a lunar space elevator would need to be very long—more than twice the length of an Earth elevator—but due to the low gravity of the Moon, could also be made of existing engineering materials.<ref name="Pearson 2005">{{cite web| url=http://www.niac.usra.edu/files/studies/final_report/1032Pearson.pdf| year= 2005| title=Lunar Space Elevators for Cislunar Space Development Phase I Final Technical Report| last1=pearson |first1=Jerome| first2=Eugene |last2=Levin |first3=John |last3=Oldson |first4=Harry |last4=Wykes}}</ref>
Rapidly spinning asteroids or moons could use cables to eject materials to convenient points, such as Earth orbits;<ref>Ben Shelef, the Spaceward Foundation [http://www.spaceward.org/documents/papers/ASE.pdf Asteroid Slingshot Express - Tether-based Sample Return]</ref> or conversely, to eject materials to send a portion of the mass of the asteroid or moon to Earth orbit or a [[Lagrangian point]]. [[Freeman Dyson]], a physicist and mathematician, has suggested{{Citation needed|date=September 2008}} using such smaller systems as power generators at points distant from the Sun where solar power is uneconomical.
A space elevator using presently available engineering materials could be constructed between mutually tidally locked worlds, such as [[Pluto]] and [[Charon (moon)|Charon]] or the components of binary asteroid [[90 Antiope]], with no terminus disconnect, according to Francis Graham of Kent State University.<ref>{{cite book|author=Graham FG |title=45th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit|doi=10.2514/6.2009-4906|chapter=Preliminary Design of a Cable Spacecraft Connecting Mutually Tidally Locked Planetary Bodies|year=2009|isbn=978-1-60086-972-3}}</ref> However, spooled variable lengths of cable must be used due to ellipticity of the orbits.
==Construction==
{{Main article|Space elevator construction}}
The construction of a space elevator would need reduction of some technical risk. Some advances in engineering, manufacturing and physical technology are required.<ref name=Edwards/> Once a first space elevator is built, the second one and all others would have the use of the previous ones to assist in construction, making their costs considerably lower. Such follow-on space elevators would also benefit from the great reduction in technical risk achieved by the construction of the first space elevator.<ref name=Edwards/>
Prior to the work of Edwards in 2000<ref name=EDWARDS_PHASE_I_2000_472Edwards.html /> most concepts for constructing a space elevator had the cable manufactured in space. That was thought to be necessary for such a large and long object and for such a large counterweight. Manufacturing the cable in space would be done in principle by using an [[asteroid]] or [[Near-Earth object]] for source material.<ref name=SMITHERMAN>D.V. Smitherman (Ed.), [http://www.nss.org/resources/library/spaceelevator/2000-SpaceElevator-NASA-CP210429.pdf Space Elevators: An Advanced Earth-Space Infrastructure for the New Millennium], NASA/CP-2000-210429, Marshall Space Flight Center, Huntsville, Alabama, 2000</ref><ref>Hein, A.M., [https://www.academia.edu/2111184/A.M._Hein_Producing_a_Space_Elevator_Tether_using_a_NEO_A_Preliminary_Assessment_ Producing a Space Elevator Tether Using a NEO: A Preliminary Assessment], International Astronautical Congress 2012, IAC-2012, Naples, Italy, 2012</ref> These earlier concepts for construction require a large preexisting space-faring infrastructure to maneuver an asteroid into its needed orbit around Earth. They also required the development of technologies for manufacture in space of large quantities of exacting materials.<ref name=ISEC_SE_way_forward_2013>Space Elevators: An Assessment of the Technological Feasibility and the Way Forward, Page 326, http://www.virginiaedition.com/media/spaceelevators.pdf</ref>
Since 2001, most work has focused on simpler methods of construction requiring much smaller space infrastructures. They conceive the launch of a long cable on a large spool, followed by deployment of it in space.<ref name=Edwards/><ref name=EDWARDS_PHASE_I_2000_472Edwards.html /><ref name=ISEC_SE_way_forward_2013 /> The spool would be initially parked in a geostationary orbit above the planned anchor point. A long cable would be dropped "downward" (toward Earth) and would be balanced by a mass being dropped "upward" (away from Earth) for the whole system to remain on the geosynchronous orbit. Earlier designs imagined the balancing mass to be another cable (with counterweight) extending upward, with the main spool remaining at the original geosynchronous orbit level. Most current designs elevate the spool itself as the main cable is paid out, a simpler process. When the lower end of the cable is long enough to reach the surface of the Earth (at the equator), it would be anchored. Once anchored, the center of mass would be elevated more (by adding mass at the upper end or by paying out more cable). This would add more tension to the whole cable, which could then be used as an elevator cable.
One plan for construction uses conventional rockets to place a "minimum size" initial seed cable of only 19,800 kg.<ref name=Edwards/> This first very small ribbon would be adequate to support the first 619 kg climber. The first 207 climbers would carry up and attach more cable to the original, increasing its cross section area and widening the initial ribbon to about 160 mm wide at its widest point. The result would be a 750-ton cable with a lift capacity of 20 tons per climber.
===Safety issues and construction challenges===
{{Main article|Space elevator safety}}
For early systems, transit times from the surface to the level of geosynchronous orbit would be about five days. On these early systems, the time spent moving through the [[Van Allen radiation belts]] would be enough that passengers would need to be protected from radiation by shielding, which would add mass to the climber and decrease payload.<ref name=firstfloor>{{cite web|url=https://www.newscientist.com/article/dn10520 |title=Space elevators: 'First floor, deadly radiation!' |accessdate=January 2, 2010 |date=November 13, 2006|work=New Scientist |publisher=Reed Business Information Ltd.}}</ref>
A space elevator would present a navigational hazard, both to aircraft and spacecraft. Aircraft could be diverted by [[air-traffic control]] restrictions. All objects in stable orbits that have [[perigee]] below the maximum altitude of the cable that are not synchronous with the cable would impact the cable eventually, unless avoiding action is taken. One potential solution proposed by Edwards is to use a movable anchor (a sea anchor) to allow the tether to "dodge" any space debris large enough to track.<ref name=Edwards/>
Impacts by space objects such as meteoroids, micrometeorites and orbiting man-made debris pose another design constraint on the cable. A cable would need to be designed to maneuver out of the way of debris, or absorb impacts of small debris without breaking.
===Economics===
{{Main article|Space elevator economics}}
With a space elevator, materials might be sent into orbit at a fraction of the current cost. As of 2000, conventional rocket designs cost about US$25,000 per [[kilogram]] (US$11,000 per [[Pound (mass)|pound]]) for transfer to geostationary orbit.<ref>{{cite web|url=http://www.domain-b.com/companies/companies_f/futron_corporation/20021018_countdown.html |title=Delayed countdown |accessdate=June 3, 2009 |date=October 18, 2002|work=Fultron Corporation |publisher=The Information Company Pvt Ltd}}</ref> Current space elevator proposals envision payload prices starting as low as $220 per kilogram ($100 per [[Pound (mass)|pound]]),<ref>{{cite web|url=http://www.spaceward.org/elevator-faq |title=The Space Elevator FAQ |accessdate=June 3, 2009 |author=The Spaceward Foundation |location=Mountain View, CA |deadurl=yes |archiveurl=https://web.archive.org/web/20090227115101/http://www.spaceward.org/elevator-faq |archivedate=February 27, 2009 }}</ref> similar to the $5–$300/kg estimates of the [[Launch loop]], but higher than the $310/ton to 500 km orbit quoted<ref>{{cite web |url=http://www.jerrypournelle.com/archives2/archives2view/view306.html#Friday |title=Friday's VIEW post from the 2004 Space Access Conference |date=April 23, 2003| accessdate=January 1, 2010 |first=Jerry|last=Pournelle}}</ref> to Dr. [[Jerry Pournelle]] for an orbital airship system.
Philip Ragan, co-author of the book ''Leaving the Planet by Space Elevator'', states that "The first country to deploy a space elevator will have a 95 percent cost advantage and could potentially control all space activities."<ref>{{cite news |url=http://www.news.com.au/news/race-to-build-worlds-first-space-elevator/story-fna7dq6e-1111118059040 |title=Race on to build world's first space elevator |date=November 17, 2008|work=news.com.au|first=Andrew|last=Ramadge|author2=Schneider, Kate }}</ref>
==International Space Elevator Consortium (ISEC)==
The International Space Elevator Consortium (ISEC) is a US Non-Profit [[501(c)(3) organization|501(c)(3)]] Corporation<ref>{{Cite web|url=https://apps.irs.gov/app/eos/displayAll.do?dispatchMethod=displayAllInfo&Id=4984679&ein=800302896&country=US&deductibility=all&dispatchMethod=searchAll&isDescending=false&city=&ein1=80-0302896&postDateFrom=&exemptTypeCode=al&submitName=Search&sortColumn=orgName&totalResults=1&names=&resultsPerPage=25&indexOfFirstRow=0&postDateTo=&state=IL|title=ISEC IRS filing|last=|first=|date=|website=apps.irs.gov|archive-url=|archive-date=|dead-url=|access-date=2019-02-09}}</ref> formed to promote the development, construction, and operation of a space elevator as "a revolutionary and efficient way to space for all humanity"<ref name=isec>{{cite web | url=http://www.isec.org/index.php/what-is-isec | work=ISEC | title=About us | accessdate=2 June 2012 | deadurl=yes | archiveurl=https://web.archive.org/web/20120707201835/http://www.isec.org/index.php/what-is-isec | archivedate=July 7, 2012 | df=mdy-all }}</ref>. It was formed after the Space Elevator Conference in [[Redmond, Washington]] in July 2008 and became an affiliate organization with the [[National Space Society]]<ref>{{Cite web|title = NSS Affiliates|url = http://www.nss.org/about/affiliates.html|website = www.nss.org|accessdate = 2015-08-30}}</ref> in August 2013<ref name=isec />. ISEC hosts an annual Space Elevator conference at the [[Seattle Museum of Flight]] <ref>{{Cite web|url=https://www.space.com/27225-space-elevator-technology.html|title=Space Elevator Advocates Take Lofty Look at Innovative Concepts|last=Tech|first=Leonard David 2014-09-22T11:59:53Z|website=Space.com|language=en|access-date=2019-02-13}}</ref><ref>{{Cite web|url=https://space.nss.org/the-international-space-elevator-consortium-isec-2017-space-elevator-conference/|title=The International Space Elevator Consortium (ISEC) 2017 Space Elevator Conference{{!}}National Space Society|last=Society|first=National Space|language=en-US|access-date=2019-02-13}}</ref><ref>{{Cite web|url=http://spaceref.com/space-elevator/annual-space-elevator-conference-set-for-august-25-27.html|title=Annual Space Elevator Conference Set for August 25-27 - SpaceRef|website=spaceref.com|access-date=2019-02-13}}</ref>.
ISEC coordinates with the two other major societies focusing on space elevators: the Japanese Space Elevator Association<ref>{{Cite web|title = Japan Space Elevator Association|url = http://www.jsea.jp/links/|website = 一般|JSEA 一般社団法人 宇宙エレベーター協会|accessdate = 2015-08-30}}</ref> and EuroSpaceward.<ref>{{Cite web|url = http://www.eurospaceward.org/|title = Eurospaceward|date = 2015-08-30|accessdate = 2015-08-30|website = Eurospaceward|publisher = }}</ref> ISEC supports symposia and presentations at the International Academy of Astronautics<ref>{{Cite web|url = http://iaaweb.org/content/view/624/823/|title = Homepage of the Study Group 3.24, Road to Space Elevator Era|date = 2014-10-02|accessdate = 2015-08-30|website = The International Academy of Astronautics (IAA)|publisher = The International Academy of Astronautics (IAA)|last = Akira|first = Tsuchida}}</ref> and the International Astronautical Federation Congress<ref>{{Cite web|url = http://www.iafastro.org/events/iac/iac-2014/meetings/|title = IAC 2014 Meeting Schedule|date = |accessdate = 2015-08-30|website = International Astronautical Federation|publisher = }}</ref> each year. The organization published two issues of a peer-reviewed journal on space elevators called "CLIMB"<ref name="isec" /><ref>{{cite web | url=http://www.spaceelevator.com/2012/01/first-issue-of-the-space-elevator-journal-released.html | title=First Issue of the Space Elevator Journal Released | work=The Space Elevator Reference | date=20 January 2012 | first=Marc | last=Boucher | accessdate=2 June 2012 | deadurl=yes | archiveurl=https://web.archive.org/web/20120513064307/http://www.spaceelevator.com/2012/01/first-issue-of-the-space-elevator-journal-released.html | archivedate=May 13, 2012 | df=mdy-all }}</ref><ref>{{Cite web | url=http://www.isec.org/index.php/store/climb-the-space-elevator-journal | title=CLIMB - the Space Elevator Journal}}</ref> and a magazine "Via Ad Astra"<ref>{{Cite book|url=https://www.worldcat.org/oclc/1020867745|title=VIA AD ASTRA - VOL 1|last=ISEC.|date=2015|publisher=LULU COM|isbn=132964123X|location=[Place of publication not identified],|oclc=1020867745}}</ref>.
ISEC also conducts one-year studies focusing on individual topics. The process involves experts for one year of discussions on the topic of choice and culminates in a draft report that is presented and reviewed at the ISEC Space Elevator conference workshop to allow input from space elevator enthusiasts and other experts. Study Reports are usually published early the following year, to date these are as follows : <ref>https://isec.org/isec-reports/</ref>
* 2010 - Space Elevator Survivability, Space Debris Mitigation <ref name=":0">Swan, P., Penny, R., Swan, C. "Space Elevator Survivability, Space Debris Mitigation", Lulu.com Publishers, 2011</ref>
* 2012 - Space Elevator Concept of Operations <ref>Swan, P., Penny, R., Swan, C. "Space Elevator Concept of Operations" Lulu.com Publishers, 2013</ref>
* 2013 - Design Consideration for Space Elevator Tether Climbers <ref>Swan, P., Penny, R., Swan, C. "Design Considerations for Space Elevator Tether Climbers" Lulu.com Publishers, 2014</ref>,
* 2014 - Space Elevator Architectures and Roadmaps <ref>Fitzgerald, M., Swan, P., Penny, R., Swan, C. "Space Elevator Architectures and Roadmaps", Lulu.com Publishers, 2015</ref>
* 2015 - Design Characteristics of a Space Elevator Earth Port <ref>Hall, V., Glaskowsky, P., Schaeffer, S. "Design Characteristics of a Space Elevator Earth Port", Lulu.com Publishers, 2016</ref>
* 2016 - Design Considerations for the Space Elevator Apex Anchor and GEO Node <ref>Fitzgerald, M. ''et al.'' "Design Considerations for the Space Elevator Apex Anchor and GEO Node", Lulu.com Publishers, 2017</ref>
* 2017 - Design Considerations for a Software Space Elevator Simulator <ref>Wright, D., Avery, S., Knapman, J., Lades, M., Roubekas, P., Swan, P. " Design Considerations for a Software Space Elevator Simulator," www.lulu.com, 2018</ref>
* 2018 - Design Considerations for the Multi-Stage Space Elevator <ref>Knapman, J., Glaskowsky, P., Gleeson, D., Hall, V., Wright, D., Fitzgerald, M., Swan, P. "Design Considerations for the Multi-stage Space Elevator," www.lulu.com, 2019, {{ISBN|978-0-359-33232-8}}</ref>
==Related concepts==
The conventional current concept of a "Space Elevator" has evolved from a static compressive structure reaching to the level of GEO, to the modern baseline idea of a static tensile structure anchored to the ground and extending to well above the level of GEO. In the current usage by practitioners (and in this article), a "Space Elevator" means the Tsiolkovsky-Artsutanov-Pearson type as considered by the International Space Elevator Consortium. This conventional type is a static structure fixed to the ground and extending into space high enough that cargo can climb the structure up from the ground to a level where simple release will put the cargo into an [[orbit]].<ref>"CLIMB: The Journal of the International Space Elevator Consortium", Volume 1, Number 1, December 2011, This journal is cited as an example of what is generally considered to be under the term "Space Elevator" by the international community. [http://www.isec.org/index.php?option=com_content&view=article&id=28&Itemid=31]</ref>
Some concepts related to this modern baseline are not usually termed a "Space Elevator", but are similar in some way and are sometimes termed "Space Elevator" by their proponents. For example, [[Hans Moravec]] published an article in 1977 called "A Non-Synchronous Orbital [[Skyhook (structure)|Skyhook]]" describing a concept using a rotating cable.<ref>{{cite journal|author=Moravec, Hans P. |title=A Non-Synchronous Orbital Skyhook|journal=Journal of the Astronautical Sciences|volume=25|date= October–December 1977|bibcode=1977JAnSc..25..307M|pages=307–322}}</ref> The rotation speed would exactly match the orbital speed in such a way that the tip velocity at the lowest point was zero compared to the object to be "elevated". It would dynamically grapple and then "elevate" high flying objects to orbit or low orbiting objects to higher orbit.
The original concept envisioned by Tsiolkovsky was a compression structure, a concept similar to an [[Radio masts and towers|aerial mast]]. While such structures might reach [[Karman line|space]] (100 km, 62 mi), they are unlikely to reach geostationary orbit. The concept of a Tsiolkovsky tower combined with a classic space elevator cable (reaching above the level of GEO) has been suggested.<ref name="JBIS1999"/> Other ideas use very tall compressive towers to reduce the demands on launch vehicles.<ref name=TorontoProposal /> The vehicle is "elevated" up the tower, which may extend as high as [[Karman line|above the atmosphere]], and is launched from the top. Such a tall tower to access near-space altitudes of {{convert|20|km|mi|abbr=on}} has been proposed by various researchers.<ref name=TorontoProposal>{{cite journal|doi=10.1016/j.actaastro.2009.02.018|url=http://pi.library.yorku.ca/dspace/bitstream/handle/10315/2587/AA_3369_Quine_Space_Elevator_Final_2009.pdf|bibcode=2009AcAau..65..365Q|title=A free-standing space elevator structure: A practical alternative to the space tether|year=2009|last1=Quine|first1=B.M.|last2=Seth|first2=R.K.|last3=Zhu|first3=Z.H.|journal=Acta Astronautica|volume=65|issue=3–4|page=365|citeseerx=10.1.1.550.4359}}</ref><ref name="landis1996">Landis, Geoffrey, "Compression Structures for Earth Launch," 7th Advanced Space Propulsion Workshop, Jet Propulsion Laboratory, April 9–11, 1996; also [http://arc.aiaa.org/doi/abs/10.2514/6.1998-3737 paper AIAA-98-3737], 24th AIAA/ASME/SAE/ASEE Joint Propulsion Conference, 1998.</ref><ref>Hjelmstad, Keith, [http://hieroglyph.asu.edu/wp-content/uploads/2014/08/Hjelmstad-on-Stephenson-Structural-Design-of-the-Tall-Tower.pdf "Structural Design of the Tall Tower"], ''Hieroglyph'', 11/30/2013. (retrieved 1 Sept 2015)</ref>
Other concepts for [[non-rocket spacelaunch]] related to a space elevator (or parts of a space elevator) include an [[orbital ring]], a pneumatic space tower,<ref>[http://www.zdnet.com/blog/emergingtech/scientists-envision-inflatable-alternative-to-tethered-space-elevator/1600 Scientists envision inflatable alternative to tethered space elevator], ''[[ZDNet]]'', June 17, 2009. Retrieved February 2013.</ref> a [[space fountain]], a [[launch loop]], a [[Skyhook (structure)|skyhook]], a [[space tether]], and a buoyant "SpaceShaft".<ref>[http://ksj.mit.edu/tracker/2009/07/space-shaft-or-story-would-have-been-bit/ Space Shaft: Or, the story that would have been a bit finer, if only one had known...], "Knight Science Journalism Tracker (MIT)", July 1, 2009</ref>
==Notes==
{{reflist|group=note}}
==References==
{{Reflist|25em}}
==Further reading==
{{Refbegin}}
* Edwards BC, Ragan P. "Leaving The Planet By Space Elevator" Seattle, USA: Lulu; 2006. {{ISBN|978-1-4303-0006-9}}
* Edwards BC, Westling EA. ''The Space Elevator: A Revolutionary Earth-to-Space Transportation System.'' San Francisco, USA: Spageo Inc.; 2002. {{ISBN|0-9726045-0-2}}.
*[http://www.nss.org/resources/library/spaceelevator/2000-SpaceElevator-NASA-CP210429.pdf A conference publication based on findings from the Advanced Space Infrastructure Workshop on Geostationary Orbiting Tether "Space Elevator" Concepts] (PDF), held in 1999 at the NASA Marshall Space Flight Center, Huntsville, Alabama. Compiled by D.V. Smitherman, Jr., published August 2000.
*"The Political Economy of Very Large Space Projects" [http://www.jetpress.org/volume4/space.htm HTML] [http://www.jetpress.org/volume4/space.pdf PDF], John Hickman, Ph.D. ''[[Journal of Evolution and Technology]]'' Vol. 4 – November 1999.
*[http://spectrum.ieee.org/aerospace/space-flight/a-hoist-to-the-heavens A Hoist to the Heavens] By Bradley Carl Edwards
*Ziemelis K. (2001) "Going up". In [[New Scientist]] '''2289''': 24–27. [http://www.spaceref.com/news/viewnews.html?id=337 Republished in SpaceRef]. Title page: "The great space elevator: the dream machine that will turn us all into astronauts."
*[https://web.archive.org/web/20101104104658/http://www.space.com/businesstechnology/technology/space_elevator_020327-1.html The Space Elevator Comes Closer to Reality]. An overview by Leonard David of space.com, published March 27, 2002.
* Krishnaswamy, Sridhar. Stress Analysis — [https://web.archive.org/web/20060519133820/http://www.cqe.northwestern.edu/sk/C62/OrbitalTower_ME362.pdf The Orbital Tower] (PDF)
* [[LiftPort]]'s Roadmap for Elevator To Space [https://web.archive.org/web/20070710032602/http://www.liftport.com/papers/SE_Roadmap_v1beta.pdf SE Roadmap] (PDF)
* [https://web.archive.org/web/20080403061917/http://space.newscientist.com/article/dn13552-space-elevators-face-wobble-problem.html Space Elevators Face Wobble Problem]: New Scientist
* Alexander Bolonkin, “Non Rocket Space Launch and Flight”. Elsevier, 2005. 488 pgs. {{ISBN|978-0-08044-731-5}}. https://archive.org/details/Non-rocketSpaceLaunchAndFlight,
{{Refend}}
==External links==
{{Portal|Spaceflight|Science}}
{{Commons category|Space elevators}}
{{Spoken Wikipedia|Space_elevator.ogg|2006-05-29}}
* [http://spaceelevatorwiki.com/ Space Elevator Engineering-Development wiki]
* [https://web.archive.org/web/20080919070924/https://science.nasa.gov/headlines/y2000/ast07sep_1.htm Audacious & Outrageous: Space Elevators]
* [http://www.bildung-kultur.org/167/ Ing-Math.Net (Germany)] – Ing-Math.Net (German Max-Born Space Elevator Team 2006) (German)
* [https://web.archive.org/web/20111206185615/http://www.warr.de/spaceelevator Project of the Scientific Workgroup for Rocketry and Spaceflight](WARR) (German)
* [http://economist.com/science/tq/displayStory.cfm?story_id=7001786 The Economist: Waiting For The Space Elevator] (June 8, 2006 – subscription required)
* [https://web.archive.org/web/20060806021241/http://www.radio.cbc.ca/programs/quirks/archives/01-02/nov0301.htm CBC Radio Quirks and Quarks November 3, 2001] ''Riding the Space Elevator''
* [http://www.timesonline.co.uk/tol/driving/features/article5529668.ece Times of London Online: Going up ... and the next floor is outer space]
* [http://www.islandone.org/LEOBiblio/CLARK1.HTM ''The Space Elevator: 'Thought Experiment', or Key to the Universe?'']. By Sir Arthur C. Clarke. Address to the XXXth International Astronautical Congress, Munich, September 20, 1979.
* [http://www.zadar.net/space-elevator/ The Space Elevator – Physical Principles] The math and the numbers for actual materials.
{{Space elevator}}
{{Non-rocket spacelaunch}}
{{Emerging technologies}}
{{DEFAULTSORT:Space Elevator}}
[[Category:Space elevator| ]]
[[Category:Exploratory engineering]]
[[Category:Megastructures]]
[[Category:Space colonization]]
[[Category:Spacecraft propulsion]]
[[Category:Spaceflight technologies]]
[[Category:Vertical transport devices]]
[[Category:Space access]]
[[Category:Hypothetical technology]]
[[Category:Emerging technologies]]
[[Category:Articles containing video clips]]' |
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