Space elevator 29192 226012804 2008-07-16T13:05:23Z Michaellaine 7485543 /* General */ [[Image:Space elevator structural diagram.png|thumb|A space elevator would consist of a cable [4] anchored to the [[Earth]]'s surface [6], reaching into [[outer space|space]]. By attaching a counterweight [3] at the end (or by further extending the cable for the same purpose), inertia ensures that the cable remains stretched taut, countering the gravitational pull on the lower sections, thus allowing the elevator to remain in [[geostationary]] orbit [1]. Once beyond the gravitational midpoint [2], carriage [5] would be accelerated further by the planet's rotation. (Diagram not to scale.)]] A '''space elevator''' is a proposed [[megastructure]] designed to transport [[material]] from a [[Astronomical object|celestial body]]'s [[surface]] into [[outer space|space]] as a way of [[non-rocket spacelaunch]]. The term most often refers to a structure that reaches from the surface of the Earth to [[geostationary orbit]] (GSO) and a counter-mass beyond. The concept of a structure reaching to geostationary orbit was first conceived 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 = 2007-09-13 |last = Hirschfeld |first = Bob |date = 2002-01-31 |work = TechTV |publisher = G4 Media, Inc. |archiveurl = http://web.archive.org/web/20050608080057/http://www.g4tv.com/techtvvault/features/35657/Space_Elevator_Gets_Lift.html |archivedate = 2005-06-08 |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> who proposed a compression structure, or "Tsiolkovsky tower." Most recent discussions focus on [[Tension (physics)|tensile]] structures ([[Tether satellite|tethers]]) reaching from geostationary orbit to the ground. Space [[elevators]] have also sometimes been referred to as '''beanstalks''', '''space bridges''', '''space lifts''', '''space ladders''', '''skyhooks''', '''orbital towers''', or '''orbital elevators'''. <!-- please do not rearrange, the order is determined by Google popularity- see 'naming issues' in talk ---> The most common proposal is a [[tether]], usually in the form of a [[cable]] or [[ribbon]], spanning from the surface near the equator to a point beyond [[geostationary orbit]]. As the planet rotates, the inertia at the end of the tether counteracts gravity, and also keeps the cable taut. Vehicles can then climb the tether and reach orbit without the use of rocket propulsion. Such a structure could hypothetically permit delivery of [[cargo]] and people to orbit at a fraction of the cost of launching payloads by rocket. Current technology is not capable of manufacturing materials that are sufficiently strong and light enough to build an Earth based space elevator as the total mass of conventional materials needed to construct such a structure would be far too great. Recent proposals for a space elevator are notable in their plans to use [[carbon nanotube]]-based materials as the tensile element in the tether design, since the theoretical strength of carbon nanotubes appears great enough to make this practical. Current technology may be able to support elevators in other locations in the solar system however, and other designs for space elevators exist that use current materials. ==Geostationary orbital tethers== This concept, also called an '''orbital space elevator''', '''geostationary orbital [[tether propulsion|tether]]''', or a '''beanstalk''', is a subset of the [[skyhook (structure)|skyhook]] concept, and is what people normally think of when the phrase 'Space elevator' is used (although there are [[#Alternatives to geosynchronous tether concepts|variants]]). Construction would be a vast project: a tether would have to be built of a [[material]] that could endure tremendous [[stress (physics)|stress]] while also being light-weight, cost-effective, and manufacturable in great quantities. Today's materials [[technology]] does not meet these requirements, although [[carbon nanotube]] technology shows great promise. A considerable number of other novel engineering problems would also have to be solved to make a space elevator practical. Not all problems regarding feasibility have yet been addressed. Nevertheless, the [[LiftPort Group]] believes that the necessary technology might be developed as early as 2008<ref edwards-we>{{cite web |url=http://liftport.com/research2.php |title=Space Elevator Concept |publisher=LiftPort Group |accessdate=2007-07-28}} 'COUNTDOWN TO LIFT: October 27, 2031'</ref> and that by developing the technology, the first space elevator could be operational by 2014.<ref>{{cite web |url=http://www.space.com/businesstechnology/technology/space_elevator_020327-1.html |title=The Space Elevator Comes Closer to Reality |first=Leonard |last=David |year=2002 |accessdtate=2007-07-28 }} '(Bradley Edwards said) In 12 years, we could be launching tons of payload every three days'</ref><ref>{{cite web |url=http://www.isr.us/research_es_se.asp |title=The Space Elevator |publisher=Institute for Scientific Research, Inc. |accessdate=2006-03-05}}</ref> ==History== ===Early concepts=== [[Image:Tsiolkovsky.jpg|thumb|[[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]] to consider a tower that reached all the way into space, built from the ground up to an altitude of [[1 E7 m|35,790 kilometers]] above sea level ([[geostationary orbit]]). He noted that a "celestial castle" at the top of such a spindle-shaped cable would have the "castle" orbiting [[Earth]] in a geo stationary orbit (i.e. the castle would remain over the same spot on Earth's surface). Tsiolkovsky's tower would be able to launch objects into orbit without a rocket. Since the elevator would attain orbital velocity as it rode up the cable, an object released at the tower's top would also have the orbital velocity necessary to remain in geostationary orbit. Unlike more recent concepts for space elevators, Tsiolkovsky's (conceptual) tower was a compression structure, rather than a tension (or "tether") structure. ===Twentieth century=== Building a compression structure from the ground up proved an unrealistic task; there was no material in existence with enough compressive strength to support its own weight under such conditions.<ref name="JBIS1999"/> 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 center of gravity of the cable motionless relative to Earth. Artsutanov's idea was introduced to the Russian-speaking public in an interview published in the Sunday supplement of ''[[Komsomolskaya Pravda]]'' (usually translated as "Young Person's Pravda" in English) in 1960,<ref name="artsutanov">{{cite web |url=http://www.liftport.com/files/Artsutanov_Pravda_SE.pdf |title=To the Cosmos by Electric Train |year=1960 |publisher=Young Person's Pravda |last=Artsutanov |first=Yu |format=PDF |accessdate=2006-03-05}}</ref> but was not available in English until much later. He also proposed tapering the cable thickness so that the tension in the cable was constant&mdash;this gives a thin cable at ground level, thickening up towards [[geostationary orbit|GEO]]. Making a cable over 35,000 [[kilometer]]s long is a difficult task. 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 |title=Satellite Elongation into a True 'Sky-Hook' |year=1966 |publisher=Science, |volume = 11 |author=Isaacs, J. D., A. C. Vine, H. Bradner and G. E. Bachus |}}</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, and found that the strength required would be twice that of any 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"/> a tapered cross section that 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 144,000 kilometers (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 force (fictitious)|centrifugal force]]s 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]]al or lunar ore. In 1977, [[Hans Moravec]] published an article called "A Non-Synchronous Orbital Skyhook", in which he proposed an alternative space elevator concept, using a rotating cable,<ref>Hans P. Moravec, "A Non-Synchronous Orbital Skyhook," ''Journal of the Astronautical Sciences'', Vol. 25, October-December 1977</ref> in which the rotation speed exactly matches the orbital speed in such a way that the instantaneous velocity at the point where the cable was at the closest point to the Earth was zero. This concept is an early version of a [[tether propulsion|space tether]] transportation system. 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. ===21st century=== 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 an orbital skyhook 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 turning the concept into a reality.<ref>Science @ NASA, [http://science.nasa.gov/headlines/y2000/ast07sep_1.htm Audacious & Outrageous: Space Elevators], September 2000</ref> The publication he edited compiling information from the workshop, "Space Elevators: An Advanced Earth-Space Infrastructure for the New Millennium",<ref>{{cite web | title = Space Elevators: An Advanced Earth-Space Infrastructure for the New Millennium | url = http://www.affordablespaceflight.com/spaceelevator.html}}</ref> provides an introduction to the state of the technology at the time, and summarizes the findings. Another American scientist, [[Bradley C. Edwards]], suggested creating a 100,000 km long paper-thin ribbon using nanotube fibers, suggesting that this structure would stand a greater chance of surviving impacts by meteoroids. Supported by the [[NASA Institute for Advanced Concepts]], the work of Edwards 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>Bradley Edwards, Eureka Scientific, [http://www.niac.usra.edu/studies/472Edwards.html NIAC Phase I study]</ref><ref>Bradley Edwards, Eureka Scientific, [http://www.niac.usra.edu/studies/521Edwards.html NIAC Phase II study]</ref> The largest holdup to Edwards' proposed design is the technological limits of the tether material. His calculations call for a fiber composed of epoxy-bonded [[carbon nanotube]]s with a minimal tensile strength of 130 [[Pascal (unit)|GPa]] (including a [[safety factor]] of 2); however, tests in 2000 of individual single-walled carbon nanotubes (SWCNTs), which should be notably stronger than an epoxy-bonded rope, indicated the strongest measured as 52 GPa.<ref name="Yu 2000 PRL">{{cite journal | first = Min-Feng | last = Yu | coauthors = Files, Bradley S.; Arepalli, Sivaram; Ruoff, Rodney S. | title = Tensile Loading of Ropes of Single Wall Carbon Nanotubes and their Mechanical Properties | journal = Phys. Rev. Lett. | volume = 84 | pages = 5552–5555 | date = 2000 | url = http://prola.aps.org/abstract/PRL/v84/i24/p5552_1 | doi = 10.1103/PhysRevLett.84.5552}}</ref> Multi-walled carbon nanotubes have been measured with tensile strengths up to 63 GPa.<ref> {{cite journal | author = Min-Feng Yu, Oleg Lourie, Mark J. Dyer, Katerina Moloni, Thomas F. Kelly, Rodney S. Ruoff | year = 2000 | title = Strength and Breaking Mechanism of Multiwalled Carbon Nanotubes Under Tensile Load | journal = Science | volume = no. 287 | issue = 5453 | pages = pp. 637–640 | url = http://sciencemag.org/cgi/content/abstract/287/5453/637 | pmid = 10649994 | doi = 10.1126/science.287.5453.637 }} </ref> In order to speed development of space elevators, proponents are planning several 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 |accessdate=2006-03-05}}</ref><ref>{{cite web |url=http://www.elevator2010.org/ |title=The Space Elevator - Elevator:2010 |accessdate=2006-03-05}}</ref> Among them are [[Elevator:2010]] which will organize annual competitions for climbers, ribbons and power-beaming systems, the Robolympics Space Elevator Ribbon Climbing competition,<ref>{{cite web |url=http://robolympics.net/rules/climbing.shtml |title=Space Elevator Ribbon Climbing Robot Competition Rules |accessdate=2006-03-05}}</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=2006-03-05}}</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=2006-03-05}}</ref> In [[2005]], "the [[LiftPort Group]] of space elevator companies has 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 100,000 km (62,000 mile) space elevator, this move will allow it to make money in the short term and conduct research and development into new production methods.The space elevator is proposed to launch in 2010."<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=2006-03-05}}</ref> 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 5 cm wide and 1 mm (approx. 6 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=[[2006-02-15]] |work=NewScientist.com |publisher=[[New Scientist]] |first=Kimm |last=Groshong |accessdate=2006-03-05}}</ref> On [[August 24]], [[2006]] the Japanese National Museum of Emerging Science and Technology in Tokyo has started to show the animation movie 'Space Elevator', based on ATA Space Elevator Project, also directed and edited by project leader, [[Dr. Serkan Anilir]]. This movie shows a possible image about the cities of future, placing the space elevator tower as a new infrastructure into the city planning, and aims to contribute children education. Currently, the movie is shown in all science museums in Japan.<ref>[https://www.miraikan.jst.go.jp/e/event/2007/0924_plan_01.html Miraikan Event<!-- Bot generated title -->]</ref> The x-Tech Projects company has also been founded to pursue the prospect of a commercial Space Elevator. In [[2007]], [[Elevator:2010]] held the 2007 Space Elevator games which featured US$500,000 awards for each of the two competitions, (US$1,000,000 total) as well as an additional US$4,000,000 to be awarded over the next five years for space elevator related technologies.<ref>http://www.elevator2010.org/competition.html</ref> No teams won the competition, but a team from [[MIT]] entered the first 2-gram, 100% carbon nanotube entry into the competition.<ref>[http://www.spaceward.org/games07Wrapup.html The Spaceward Foundation<!-- Bot generated title -->]</ref> ==Physics and structure== [[Image:SpaceElevatorClimbing.jpg||thumb|right|One concept for the space elevator has it tethered to a mobile seagoing platform.]] The centrifugal force of earth's rotation is the main principle behind the elevator. As the earth rotates the centrifugal force tends to align the nanotube in a stretched manner.There are a variety of tether designs. Almost every design includes a base station, a cable, climbers, and a counterweight. ===Base station=== The base station designs typically fall into two categories&mdash;mobile and stationary. Mobile stations are typically large oceangoing vessels,<ref name="niac">{{cite web |url=http://www.spaceelevator.com/docs/521Edwards.pdf |title=The Space Elevator NIAC Phase II Final Report |publisher=NASA |accessdate=2007-06-12}}</ref> though airborne stations have been proposed as well.{{Fact|date=June 2007}} Stationary platforms would generally be located in high-altitude locations, such as on top of mountains, or even potentially on high towers.<ref name="JBIS1999"/> Mobile platforms have the advantage of being able to maneuver to avoid high winds, storms, and [[space debris]]. While stationary platforms don't have these advantages, they typically would have access to cheaper and more reliable power sources, and require a shorter cable. While the decrease in cable length may seem minimal (typically no more than a few kilometers), that can significantly reduce the minimal width of the cable at the center, and reduce the minimal length of cable reaching beyond geostationary orbit significantly. ===Cable=== The cable must be made of a material with a large [[tensile strength]]/density ratio. A space elevator can be made relatively economically feasible if a cable with a density similar to [[graphite]] and a tensile strength of ~65&ndash;120 [[gigapascal|GPa]] can be mass-produced at a reasonable price. [[Image:Kohlenstoffnanoroehre Animation.gif|thumb|right|[[Carbon nanotube]]s would be a highly useful material for creating a space elevator]] By comparison, most steel has a tensile strength of under 2 GPa, and the strongest steel resists no more than 5.5 GPa, but steel is dense. The much lighter material [[Kevlar]] has a tensile strength of 2.6&ndash;4.1 GPa, while [[quartz]] fiber{{Fact|date=July 2007}} and carbon nanotubes<ref name="demczyk">{{cite web |url=http://www.glue.umd.edu/~cumings/PDF%20Publications/16.MSE%20A334demczyk.pdf |title=Direct mechanical measurement of the tensile strength and elastic modulus of multiwalled carbon nanotubes |first=B.G. |last=Demczyk |accessdate=2007-07-15 |year=2002}}"2–5 GPa for fibers [2,3] and up to 20 GPa for ‘whiskers’", "Depending on the choice of this surface, σT can range from E/7 to E/5 (0.14–0.177 TPa)"</ref> can reach upwards of 20 GPa; the tensile strength of [[diamond]] filaments would theoretically be minimally higher. [[Carbon nanotube]]s' theoretical tensile strength has been estimated between 140 and 177 GPa (depending on plane shape),<ref name="demczyk"/> and its observed tensile strength has been variously measured from 63 to 150 GPa, close to the requirements for space elevator structures.<ref name="demczyk"/><ref>{{cite web |url=http://depts.washington.edu/polylab/cn.html |title=Carbon Nanotube POF |first=Jordan |last=Mills |accessdate=2007-07-15 |year=2002}}</ref> Even the strongest fiber made of nanotubes is likely to have notably less strength than its components. Improving tensile strength depends on further research on purity and different types of nanotubes. Designs call for single-walled carbon nanotubes. While multi-walled nanotubes are easier to produce and have similar tensile strengths, there is a concern that the interior tubes would not be sufficiently coupled to the outer tubes to help hold the tension. However, if the nanotubes are long enough, even weak [[Van der Waals forces]] will be sufficient to keep them from slipping, and the full strength of individual nanotubes (single or multiwalled) could be realized macroscopically by spinning them into a yarn. It has also been proposed to chemically interlink the nanotubes in some way, but it is likely that this would greatly compromise their strength. One such proposal is to take advantage of the high pressure interlinking properties of carbon nanotubes of a single variety.<ref> {{cite journal | author = T. Yildirim, O. Gülseren, Ç. Kılıç, S. Ciraci | year = 2000 | title = Pressure-induced interlinking of carbon nanotubes | journal = Phys. Rev. B | volume = 62 | pages = 12648–12651 | url = http://link.aps.org/abstract/PRB/v62/p12648 | doi = 10.1103/PhysRevB.62.12648 <!--Retrieved from CrossRef by DOI bot--> }} </ref> While this would cause the tubes to lose some tensile strength by the trading of [[sp² bond]] (graphite, nanotubes) for [[sp³ bond|sp³]] (diamond), it will enable them to be held together in a single fiber by more than the usual, weak [[Van der Waals force]] (VdW), and allow manufacturing of a fiber of any length. [[Image:SpaceElevatorAnchor.jpg||thumb|right|A seagoing anchor station would incidentally act as a deep-water [[seaport]].]] The technology to spin regular VdW-bonded yarn from carbon nanotubes is just in its infancy: the first success in spinning a long yarn, as opposed to pieces of only a few centimeters, was reported in March 2004; but the strength/weight ratio was not as good as Kevlar due to the inconsistent quality and short length of the tubes being held together by VdW. As of [[as of 2006|2006]], carbon nanotubes cost $25/gram, and even a space elevator that did not reach GEO would have a mass of 20,000 kg. However, this price is declining, and large-scale production could result in strong [[economies of scale]].<ref>{{cite web|url=http://www.domitech.es/recensteam/recensteam/documentos/RecensTeam%20paper.pdf|title=UPC Team Recens’ Answer to NASA's Beam Power Space Elevator Challenge|publisher=[[Polytechnic University of Catalonia]]|date=[[March 26]], [[2007]]|accessdate=2008-02-11}}</ref> Carbon nanotube fiber is an area of energetic worldwide research because the applications go much further than space elevators. Other suggested application areas include suspension bridges, new composite materials, lighter aircraft and rockets, armor technologies, and computer processor interconnects. This is good news for space elevator proponents because it is likely to push down the price of the cable material further. ===Cable taper=== Due to its enormous length a space elevator cable must be carefully designed to carry its own weight as well as the smaller weight of climbers. The required strength of the cable will vary along its length, since at various points it has to carry the weight of the cable below, or provide a [[centripetal force]] to retain the cable and counterweight above. In an ideal cable, the actual strength of the cable at any given point would equal to the required strength at that point (plus a safety margin). This implies a tapered design. Using a model that takes into account the Earth's gravitational and "centrifugal" forces (and neglecting the smaller solar and lunar effects), it is possible to show<ref name="pearson"> {{cite journal | author = J. Pearson | year = 1975 | title = [http://www.star-tech-inc.com/papers/tower/tower.pdf The orbital tower: a spacecraft launcher using the Earth's rotational energy] | journal = Acta Astronautica | volume = 2 | pages = 785–799 | doi = 10.1016/0094-5765(75)90021-1 <!--Retrieved from CrossRef by DOI bot--> }} </ref> that the optimal cross-sectional area of the cable as a function of height is given by: [[Image:SpaceElevatorCableTaper3DPlot2.jpg|thumb|right|Cable [[Taper Plot]]]] :<math> A(r) = A_{0} \ \exp \left[ \frac{\rho}{s} \left[ \begin{matrix}\frac{1}{2}\end{matrix} \omega^{2} \left(r_{0}^{2} - r^2\right) + g_{0}r_{0} \left(1 - \frac{r_{0}}{r}\right) \right] \right] </math> where <math>A(r)</math> is the cross-sectional area as a function of distance <math> r </math> from the Earth's ''center''. The constants in the equation are: * <math>A_{0}</math> is the cross-sectional area of the cable on the earth's surface. * <math>\rho</math> is the density of the material the cable is made out of. * <math>s</math> is the tensile strength of the material. * <math>\omega</math> is the angular velocity of the Earth about its axis, 7.292&nbsp;&times;&nbsp;10<sup>−5</sup>&nbsp;[[radian per second|rad·s<sup>−1</sup>]]. * <math>r_{0}</math> is the distance between the Earth's center and the base of the cable. It is approximately the Earth's [[equator]]ial radius, 6378&nbsp;km. * <math>g_{0}</math> is the acceleration due to [[gravity]] at the cable's base, 9.780&nbsp;m·s<sup>−2</sup>. This equation gives a shape where the cable thickness initially increases rapidly in an exponential fashion, but slows at an altitude a few times the Earth's radius, and then gradually becomes parallel when it finally reaches maximum thickness at [[geostationary orbit]]. The cable thickness then decreases again out from geostationary orbit. The relative thickness at all points is determined by the strength density ratio. This is shown in the figure to the right. Thus the taper of the cable from base to GEO (''r'' = 42,164&nbsp;km), :<math> \frac{A(r_{\mathrm{GEO}})}{A_0} = \exp \left[ \frac{\rho}{s} \times 4.832 \times 10^{7} \, \mathrm{ {m^2}\!\!\cdot\!{s^{-2}} } \right] </math> Using the density and tensile strength of steel, and assuming a diameter of 1&nbsp;cm at ground level, yields a diameter of several hundred kilometers at geostationary orbit height, showing that steel, and indeed all materials used in present day mechanical engineering, are unsuitable for building a space elevator. The equation shows us that there are four ways of achieving a more reasonable thickness at geostationary orbit: * Using a lower density material. Not much scope for improvement as the range of densities of most solids that come into question is rather narrow, somewhere between 1000&nbsp;kg·m<sup>−3</sup> and 5000&nbsp;kg·m<sup>−3</sup>. * Using a higher strength material. This is the area where most of the research is focused. Carbon nanotubes are tens of times stronger than the strongest types of steel, hugely reducing the cable's cross-sectional area at geostationary orbit. * Increasing the height of a tip of the base station, where the base of cable is attached. If the cable is properly tapered, however (see next point) this will not make much difference unless a tower of the order of 1000 km is built. * Making the cable as thin as possible at its base. It still has to be thick enough to carry a payload however, so the minimum thickness at base level also depends on tensile strength. A cable made of carbon nanotubes (a type of [[fullerene]]), would typically be just a millimeter wide at the base{{Fact|date=February 2008}}. ===Climbers=== [[Image:SpaceElevatorInClouds.jpg|thumb|right|Most space elevator designs call for a '''climber''' to move autonomously along a stationary cable.]] 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 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. Usually, elevators are designed for climbers to move only upwards, because that is where most of the payload goes. For returning payloads, atmospheric reentry on a heat shield is a very competitive option, which also avoids the problem of docking to the elevator in space. Climbers must be paced at optimal timings so as to minimize cable stress and oscillations and to maximize throughput. Lighter climbers can be sent up more often, with several going up at the same time. This increases throughput somewhat, but lowers the mass of each individual payload. ===Powering climbers=== Both power and energy are significant issues for climbers- the climbers need to gain a large amount of potential energy as quickly as possible to clear the cable for the next payload. Nuclear energy and solar power have been proposed, but generating enough energy to reach the top of the elevator in any reasonable time without weighing too much is not feasible.<ref>[http://www.isr.us/Downloads/niac_pdf/chapter4.html NIAC Space Elevator Report chapter4]</ref> The current method of favor is laser power beaming, using megawatt powered free electron or solid state lasers in combination with adaptive mirrors approximately 10 m wide and a photovoltaic array on the climber tuned to the laser frequency for efficiency.<ref name="niac"/> A major obstacle for any climber design is the dissipation of the substantial amount of waste heat generated due to the less than perfect efficiency of any of the power methods. ===Counterweight=== There have been several methods proposed for dealing with the counterweight need: a heavy object, such as a captured asteroid or a [[space station]], positioned past geostationary orbit, or extending the cable itself well past geostationary orbit. The latter idea has gained more support in recent years due to the relative 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. Additionally, [[#Brad Edwards' proposal|Brad Edwards]] has proposed that initially elevators would be up-only, and that the elevator cars that are used to thicken up the cable could simply be parked at the top of the cable and act as a counterweight. ===Angular momentum, speed and cable lean=== [[Image:Space elevator balance of forces.png|thumb|250px|As the car climbs, the elevator takes on a 1 degree lean, due to the top of the elevator traveling faster than the bottom around the Earth (Coriolis effect). This diagram is not to scale.]] The horizontal speed of each part of the cable increases with altitude, proportional to distance from the center of the Earth, reaching [[orbital velocity]] at geostationary orbit. Therefore as a payload is lifted up a space elevator, it needs to gain not only altitude but [[angular momentum]] (horizontal speed) as well. This angular momentum is taken from the Earth's own rotation. As the climber ascends it is initially moving slightly more slowly than the cable that it moves onto ([[Coriolis effect]]) and thus the climber "drags" on the cable, carrying the cable with it very slightly to the west (and necessarily pulling the counterweight slightly to the west, shown as an offset of the counterweight in the diagram to right, slightly changing the motion of the counterweight). At a 200 km/h climb speed this generates a 1 degree lean on the lower portion of the cable. The horizontal component of the tension in the non-vertical cable applies a sideways pull on the payload, accelerating it eastward (see diagram) and this is the source of the speed that the climber needs. Conversely, the cable pulls westward on Earth's surface, insignificantly slowing the Earth, from [[Newton's laws of motion#Newton's third law: law of reciprocal actions|Newton's 3rd law]]. Meanwhile, 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 causes it to constantly try to return to the energetically favourable vertical orientation, so after an object has been lifted on the cable the counterweight will swing back towards the vertical like an inverted pendulum. Provided that the Space Elevator is designed so that the center of weight always stays above geostationary orbit<ref>[http://gassend.com/spaceelevator/center-of-mass/index.html "Why the Space Elevator's Center of Mass is not at GEO" by Blaise Gassend]</ref> for the maximum climb speed of the climbers, the elevator cannot fall over. Lift and descent operations must be carefully planned so as to keep the pendulum-like motion of the counterweight around the tether point under control. By the time the payload has reached GEO the angular momentum (horizontal speed) is enough that the payload is in orbit. The opposite process would occur for payloads descending the elevator, tilting the cable eastwards and insignificantly increasing Earth's rotation speed. ===Launching into outer space=== The velocities that might be attained at the end of Pearson's 144,000&nbsp;km cable can be determined. The tangential velocity is 10.93 kilometers per second which is more than enough to [[escape velocity|escape]] Earth's gravitational field and send probes as far out as [[Saturn (planet)|Saturn]]. If an object were allowed to slide freely along the upper part of the tower, a velocity high enough to escape the [[solar system]] entirely would be attained. This is accomplished by trading off overall angular momentum of the tower for velocity of the launched object, in much the same way one snaps a towel or throws a [[lacrosse]] ball. After such an operation a cable would be left with less angular momentum than required to keep its geostationary position. The rotation of the Earth would then pull on the cable increasing its angular velocity, leaving the cable swinging backwards and forwards about its starting point. For higher velocities, the cargo can be electromagnetically accelerated, or the cable could be extended, although that would require additional strength in the cable. ===Extraterrestrial elevators=== A space elevator could also be constructed on some of the other planets, asteroids and moons, although the benefit of such tethers is difficult to see. A [[Mars|Martian]] tether could be much shorter than one on Earth. Mars' surface [[gravity]] is 38% of Earth's, while it rotates around its axis in about the same time as Earth. <ref>[http://www.frc.ri.cmu.edu/~hpm/project.archive/1976.skyhook/1982.articles/elevate.800322 [[Hans Moravec]]: SPACE ELEVATORS (1980)]</ref>Because of this, Martian [[areostationary orbit]] is much closer to the surface, and hence the elevator would be much shorter. Exotic materials might not be required to construct such an elevator. However, building a Martian elevator would be a unique challenge because the Martian moon [[Phobos (moon)|Phobos]] is in a low orbit, and intersects the equator regularly (twice every orbital period of 11&nbsp;h 6&nbsp;min). A collision between the elevator and the 22.2&nbsp;km diameter moon would have to be avoided through active steering of the elevator.{{Fact|date=July 2008}}One simpler way to resolve the problem of Phobos (1.1 degree orbital inclination) or [[Deimos (moon)|Deimos]] (1.8 degree orbital inclination) interaction is to position the tether anchor perhaps five (5) degrees off the Martian equator.{{Fact|date=July 2008}} There would be a small payload penalty, but the tether would pass outside the orbital inclination of the two moons. A [[lunar space elevator]] can possibly be built with currently available technology about 50,000 kilometers long extending though the Earth-moon L1 point from an anchor point near the center of the visible part of Earth's moon. 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, can be made of existing engineering materials.<ref name="Pearson 2005">{{cite web| url=http://www.niac.usra.edu/files/studies/final_report/1032Pearson.pdf| last=Pearson| year= 2005| title=Lunar Space Elevators for Cislunar Space Development Phase I Final Technical Report| first=Jerome| coauthors= Eugene Levin, John Oldson and Harry Wykes| format=PDF}}</ref> Alternatively, due to the lack of atmosphere on the Moon, a rotating [[tether]] could be used with its center of weight in orbit around the Moon with a [[counterweight]] (e.g. a [[space station]]) at the short end and a [[Payload (air and space craft)|payload]] at the long end. The path of the payload would be an [[epicycloid]] around the Moon, touching down at some integer number of times per orbit.{{Fact|date=July 2008}} Thus, payloads are lifted off the surface of the Moon, and flung away at the high point of the orbit. Rapidly spinning asteroids or moons could use cables to eject materials in order to move the materials to convenient points, such as Earth orbits; or conversely, to eject materials in order to send the bulk of the mass of the asteroid or moon to Earth orbit or a [[Lagrangian point]]. This was suggested by [[Russell Johnston]] in the 1980s. [[Freeman Dyson]], a physicist and mathematician, has suggested using such smaller systems as power generators at points distant from the Sun where solar power is uneconomical. For the purpose of mass ejection, it is not necessary to rely on the asteroid or moon to be rapidly spinning. Instead of attaching the tether to the equator of a rotating body, it can be attached to a rotating hub on the surface. This was suggested in 1980 as a "Rotary Rocket" by Pearson<ref name="rotaryrocket">{{cite web |url=http://www.star-tech-inc.com/papers/asteroids/asteroids.pdf |title=Asteroid Retrieval by Rotary Rocket |publisher=NASA |accessdate=2007-06-12}}</ref> and described very succinctly on the Island One website as a "Tapered Sling"<ref name="taperedsling">{{cite web |url=http://www.islandone.org/LEOBiblio/SPBI1SL.HTM |title=Tapered Sling |publisher=Island One Society |accessdate=2007-06-12}}</ref> ==Construction== The construction of a space elevator would be a vast project, requiring advances in engineering, manufacture and physical technology. David Smitherman of [[NASA]] has published a paper that identifies "Five Key Technologies for Future Space Elevator Development":<ref>{{cite web | url = http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20060000015_2005248072.pdf | title = Critical Technologies for the Development of Future Space Elevator Systems | author = David V. Smitherman, Jr. | accessdate = 2007-11-03 }} NASA Tech Report IAC-05-D4.2.04</ref> # '''[[materials science|Material]]''' for ''cable'' (e.g. [[carbon nanotube]] and [[nanotechnology]]) and ''tower'' # '''[[tether propulsion|Tether]]''' deployment and control # '''[[world's tallest structures|Tall tower]]''' construction # '''[[Electromagnetic propulsion]]''' (e.g. [[magnetic levitation]]) # '''Space infrastructure''' and the development of [[space-based industry]] and economy Two different ways to deploy a space elevator have been proposed. ===Traditional way=== One early plan involved lifting the entire mass of the elevator into [[geostationary orbit]], and simultaneously lowering one cable downwards towards the Earth's surface while another cable is deployed upwards directly away from the Earth's surface. [[Tidal force]]s ([[Gravitational force|gravity]] and [[centrifugal force (fictitious)|centrifugal force]]) would naturally pull the cables directly towards and directly away from the Earth and keep the elevator balanced around geostationary orbit. As the cable is deployed, [[Coriolis force]]s would pull the upper portion of the cable somewhat to the West and the lower portion of the cable somewhat to the East; this effect can be controlled by varying the deployment speed. However, this approach requires lifting hundreds or even thousands of tons on conventional [[rocket]]s, an expensive proposition. Hypothetically, such a plan could make extensive use of [[In-Situ Resource Utilization|materials available in space]] to reduce costs, but this would require considerable [[space mining]] and space-based processing of materials, neither of which is currently practical using existing technology. ===Brad Edwards' proposal=== [[Bradley C. Edwards]], former Director of Research for the [[Institute for Scientific Research]] (ISR), based in [[Fairmont, West Virginia]] proposed that, if nanotubes with sufficient strength could be made in bulk, a space elevator could be built in little more than a decade, rather than the far future. He proposed that a single hair-like 18-[[tonne|metric ton]] (20 short [[ton]]) 'seed' cable be deployed in the traditional way, giving a very lightweight elevator with very little lifting capacity. Then, progressively heavier [[cable]]s would be pulled up from the ground along it, repeatedly strengthening it until the elevator reaches the required [[mass]] and [[Strength of materials|strength]]. This is much the same technique used to build [[suspension bridge]]s. Although 18 tonnes for a seed cable may sound like a lot, it would actually be very lightweight &mdash; the proposed average mass is about 200 grams per kilometer. In comparison, conventional [[copper]] telephone wires running to consumer homes weigh about 4 kg/km {{Fact|date=July 2008}}. === Loop elevator design === This is a less well developed design, but offers some other possibilities. If the cable provides a useful tensile strength of about 62.5 GPa or above, then it turns out that a constant width cable can reach beyond geostationary orbit without breaking under its own weight. The far end can then be turned around and passed back down to the Earth forming a constant width loop, which would be kept spinning to avoid tangling. The two sides of the loop are naturally kept apart by [[coriolis force]]s due to the rotation of the Earth and the loop. By increasing the thickness of the cable from the ground a very quick (exponential) build-up of a new elevator may be performed (it helps that no active climbers are needed, and power is applied mechanically.) However, because the loop runs at constant speed, joining and leaving the loop may be somewhat challenging, and the carrying capacity of such a loop is lower than a conventional tapered design.<ref>{{cite web |url=http://gassend.com/publications/ExponentialTethers.pdf |title=Exponential Tethers for Accelerated Space Elevator Deployment? |first=Blaise |last=Gassend |format=PDF |accessdate=2006-03-05}}</ref> ==Failure modes, safety issues and construction difficulties== As with any structure, there are a number of ways in which things could go wrong. A space elevator would present a considerable navigational hazard, both to aircraft and spacecraft. Aircraft could be dealt with by means of simple air-traffic control restrictions, but impacts by space objects (in particular, by meteoroids and micrometeorites) pose a more difficult problem. ===Cable strength=== The current strength/mass ratio of cables of any construction is inadequate to build a space elevator at the [[as of 2008|present time]]. Although [[carbon nanotubes]] embedded in the tether would give it enough strength to be practical, nanotubes of sufficient length have not yet been made. Theoretical objections have been raised to manufacturing bulk carbon nanotube structures with strengths approaching that which simple models and microscopic strengths suggest. H. K. D. H. Bhadeshia argues that the presence of defects would significantly reduce the strength actually attainable.<ref>[http://www.msm.cam.ac.uk/phase-trans/2005/chunk.html 52nd Hatfield Memorial Lecture: Large Chunks of Very Strong Steel<!-- Bot generated title -->]</ref> ===Satellites=== If nothing were done, essentially all satellites with [[perigee]]s below the top of the elevator would eventually collide with the elevator cable. Twice per day, each orbital plane intersects the elevator, as the rotation of the Earth swings the cable around the equator. Usually the satellite and the cable will not line up. However, except for synchronized orbits, the elevator and satellite will eventually occupy the same place at the same time, almost certainly leading to structural failure of the space elevator and destruction of the satellite. Most active satellites are capable of some degree of orbital maneuvering and could avoid these predictable collisions, but inactive satellites and other orbiting debris would need to be either preemptively removed from orbit by "garbage collectors" or would need to be closely watched and nudged whenever their orbit approaches the elevator. The impulses required would be small, and need be applied only very infrequently; a [[laser broom]] system may be sufficient to this task. In addition, Brad Edward's design actually allows the elevator to move out of the way, because the fixing point is at sea and mobile. However, such movements would excite transverse oscillations of the cable. Edwards claims that these oscillations could be controlled so as to ensure that the cable avoids satellites on known paths. ===Meteoroids and micrometeorites=== [[Meteoroids]] present a more difficult problem, since they would not be predictable and much less time would be available to detect and track them as they approach Earth. It is likely that a space elevator would still suffer impacts of some kind, no matter how carefully it is guarded. However, most space elevator designs call for the use of multiple parallel cables separated from each other by [[strut]]s, with sufficient margin of safety that severing just one or two strands still allows the surviving strands to hold the elevator's entire weight while repairs are performed. If the strands are properly arranged, no single impact would be able to sever enough of them to overwhelm the surviving strands. Far worse than meteoroids are [[micrometeorites]]; tiny high-speed particles found in high concentrations at certain altitudes. Avoiding micrometeorites is essentially impossible, and they will ensure that strands of the elevator are continuously being cut. Most methods designed to deal with this involve a design similar to a [[hoytether]] or to a network of strands in a cylindrical or planar arrangement with two or more helical strands. Constructing the cable as a mesh instead of a ribbon helps prevent collateral damage from each micrometeorite impact. ===Failure cascade=== It is not enough that other fibers be able to take over the load of a failed strand &mdash; the system must also survive the immediate, dynamical effects of fiber failure, which generates projectiles aimed at the cable itself. For example, if the cable has a working stress of 50&nbsp;GPa and a [[Young's modulus]] of 1000&nbsp;GPa, its strain will be 0.05 and its stored elastic energy will be 1/2 &times; 0.05 &times; 50&nbsp;GPa = 1.25&times;10<sup>9</sup> joules per cubic meter. Breaking a fiber will result in a pair of de-tensioning waves moving apart at the speed of sound in the fiber, with the fiber segments behind each wave moving at over 1,000 m/s (more than the [[muzzle velocity]] of a standard [[.223]] [[caliber]] ([[5.56mm|5.56&nbsp;mm]]) round fired from an [[M16 rifle]]). Unless these fast-moving projectiles can be stopped safely, they will break yet other fibers, initiating a failure cascade capable of severing the cable. The challenge of preventing fiber breakage from initiating a catastrophic failure cascade seems to be unaddressed in the current (January, 2005) literature on terrestrial space elevators. Problems of this sort would be easier to solve in lower-tension applications (e.g., lunar elevators). ===Corrosion=== Corrosion is a major risk to any thinly built tether (which most designs call for). In the upper atmosphere, [[atomic oxygen]] steadily eats away at most materials. A tether will consequently need to either be made from a corrosion-resistant material or have a corrosion-resistant coating, adding to weight. [[Gold]] and [[platinum]] have been shown to be practically immune to atomic oxygen; several far more common materials such as [[aluminum]] are damaged very slowly and could be repaired as needed. Another potential solution to the corrosion problem is a continuous renewal of the tether surface (which could be done from standard, though possibly slower elevators). This process would depend on the tether composition and it could be done on the nanoscale (by replacing individual fibers) or in segments. ===Radiation=== The effectiveness of the magnetosphere to deflect radiation emanating from the sun decreases dramatically after rising several earth radii above the surface. This ionizing radiation may cause damage to materials within both the tether and climbers. ===Material defects=== Any structure as large as a space elevator will have massive numbers of tiny defects in the construction material. It has been suggested,<ref>[http://arxiv.org/abs/cond-mat/0601668 "ON THE STRENGTH OF THE CARBON NANOTUBE-BASED SPACE ELEVATOR CABLE: FROM NANO- TO MEGA-MECHANICS" Nicola M. Pugno]</ref><ref>[http://www.msm.cam.ac.uk/phase-trans/2005/SWpaper/index.html "Bulk Nanocrystalline Steel" H. K. D. H. Bhadeshia]</ref> that, because large structures have more defects than small structures, that large structures are inherently weaker than small, giving an estimated carbon nanotube strength of only 24 GPa down to only 1.7 GPa in millimetre-scale samples, the latter equivalent to many high-strength steels, which would be vastly less than that needed to build a space elevator for a reasonable cost. ===Weather=== In the atmosphere, the risk factors of wind and lightning come into play. The basic mitigation is location. As long as the tether's anchor remains within two degrees of the equator, it will remain in the quiet zone between the Earth's [[Hadley cell]]s, where there is relatively little violent weather. Remaining storms could be avoided by moving a floating anchor platform. The lightning risk can be minimized by using a nonconductive fiber with a water-resistant coating to help prevent a conductive buildup from forming. The wind risk can be minimized by use of a fiber with a small cross-sectional area that can rotate with the wind to reduce resistance. Ice forming on the cable also presents a potential problem. It could add significantly to the cable's weight and affect the passage of elevator cars. Also, ice falling from the cable could damage elevator cars or the cable itself. To get rid of ice, special elevator cars could scrape the ice off. ===Vibrational harmonics=== A final risk of structural failure comes from the possibility of vibrational [[harmonic]]s within the cable. Like the shorter and more familiar strings of stringed musical instruments, the cable of a space elevator has a natural [[resonance|resonant]] frequency. If the cable is excited at this frequency, for example by the travel of elevators up and down it, the vibrational energy could build up to dangerous levels and exceed the cable's tensile strength. This can be avoided by the use of suitable damping systems within the cable, and by scheduling travel up and down the cable keeping its resonant frequency in mind. It may be possible to dampen the resonant frequency against the Earth's magnetosphere. ===In the event of failure=== If despite all these precautions the elevator is severed anyway, the resulting scenario depends on where exactly the break occurred: ====Cut near the anchor point==== If the elevator is cut at its anchor point on Earth's surface, the outward force exerted by the counterweight would cause the entire elevator to rise upward into an unstable orbit. The ultimate [[altitude]] of the severed lower end of the cable would depend on the details of the elevator's [[mass]] distribution. In theory, the loose end might be secured and fastened down again. This would be an extremely tricky operation, however, requiring careful adjustment of the cable's center of gravity to bring the cable back down to the surface again at just the right location. It may prove to be easier to build a new system in such a situation. ====Cut up to about 25,000 km==== If the break occurred at higher altitude, up to about 25,000&nbsp;km, the lower portion of the elevator would descend to Earth and drape itself along the equator east of the anchor point, while the now unbalanced upper portion would rise to a higher orbit. Some authors (such as science fiction writers [[David Gerrold]] in ''[[Jumping off the Planet]]'', [[Kim Stanley Robinson]] in ''[[Red Mars]]'', and [[Ben Bova]] in ''[[Mercury (book)|Mercury]]'') have suggested that such a failure would be catastrophic, with the thousands of kilometers of falling cable creating a swath of meteoric destruction along the planet's surface; however, in most cable designs, the upper portion of any cable that fell to Earth would burn up in the [[Earth's atmosphere|atmosphere]]. Additionally, because proposed initial cables have very low mass (roughly 1&nbsp;kg per kilometer) and are flat, the bottom portion would likely settle to Earth with less force than a sheet of paper due to [[Drag (physics)|air resistance]] on the way down.{{Fact|date=February 2008}} If the break occurred at the counterweight side of the elevator, the lower portion, now including the "central station" of the elevator, would entirely fall down if not prevented by an early self-destruct of the cable shortly below it. Depending on the size, however, it would burn up on re-entry anyway. Simulations have shown that as the descending portion of the space elevator "wraps around" Earth, the stress on the remaining length of cable increases, resulting in its upper sections breaking off and being flung away. The details of how these pieces break and the trajectories they take are highly sensitive to initial conditions.<ref>{{cite web| url=http://gassend.com/spaceelevator/breaks/index.html| title=Animation of a Broken Space Elevator| first=Blaise| last= Gassend| year=2004| accessdate=2007-01-14}}</ref> ====Elevator climbers==== Any climbers on the falling section would also reenter Earth's atmosphere, but it is likely that the climbers will already have been designed to withstand such an event as an emergency measure. It is almost inevitable that some objects &mdash; climbers, structural members, repair crews, etc. &mdash; will accidentally fall off the elevator at some point. Their subsequent fate would depend upon their initial altitude. Except at geostationary altitude, an object on a space elevator is not in a stable orbit and so its trajectory will not remain parallel to it. The object will instead enter an [[elliptical orbit]], the characteristics of which depend on where the object was on the elevator when it was released. If the initial height of the object falling off of the elevator is less than 23,000&nbsp;km, its [[orbit]] will have an [[apsis|apogee]] at the altitude where it was released from the elevator and a [[perigee]] within Earth's atmosphere &mdash; it will intersect the atmosphere within a few hours, and not complete an entire orbit. Above this critical altitude, the perigee is above the atmosphere and the object will be able to complete a full orbit to return to the altitude it started from. By then the elevator would be somewhere else, but a [[spacecraft]] could be dispatched to retrieve the object or otherwise remove it. The lower the altitude at which the object falls off, the greater the eccentricity of its orbit. If the object falls off at the geostationary altitude itself, it will remain nearly motionless relative to the elevator just as in conventional orbital flight. At higher altitudes the object would again be in an elliptical orbit, this time with a perigee at the altitude the object was released from and an apogee somewhere higher than that. The eccentricity of the orbit would increase with the altitude from which the object is released. Above 47,000&nbsp;km, however, an object that falls off of the elevator would have a velocity greater than the local [[escape velocity]] of Earth. The object would head out into interplanetary space, and if there were any people present on board it might prove impossible to rescue them. ===Van Allen Belts=== [[Image:Van Allen radiation belt.svg|thumb|Van Allen radiation belts]] The space elevator would run through the [[Van Allen radiation belt|Van Allen belts]]. This is not a problem for most freight, but the amount of time a climber spends in this region would cause [[radiation poisoning]] to any unshielded human or other living things.<ref>{{cite news |title=Space elevators: "First floor, deadly radiation!" |date=[[2006-11-13]] |work=[[New Scientist]] |author=Kelly Young |url=http://space.newscientist.com/article/dn10520-space-elevators-first-floor-deadly-radiation.html}}</ref><ref>{{cite journal |journal=Acta Astronautica |volume=60 |issue=3 |date=February 2007 |pages=189&ndash;209 |doi=10.1016/j.actaastro.2006.07.014 |publisher=Elsevier Ltd. |title=Passive radiation shielding considerations for the proposed space elevator |author=A.M. Jorgensena, S.E. Patamiab, and B. Gassendc}}</ref> Some speculate that passengers would continue to travel by high-speed rocket, while space elevators haul bulk cargo. Research into lightweight [[radiation shielding|shielding]] and techniques for clearing out the belts is underway. More conventional and faster [[atmospheric reentry]] techniques such as [[aerobraking]] might be employed on the way down to minimize radiation exposure. De-orbit burns use relatively little fuel and are cheap. An obvious option would be for the elevator to carry shielding to protect passengers, though this would reduce its overall capacity. The best radiation shielding is very mass-intensive for physical reasons. Alternatively, the shielding itself could in some cases consist of useful payload, for example food, water, fuel or construction/maintenance materials, and no additional shielding costs are incurred during ascent. To shield passengers from the radiation in the Van Allen belt, perhaps counter-intuitively, material composed of light elements should be used, as opposed to lead shielding. In fact, high energy [[electron]]s in the Van Allen belts produce dangerous [[X-ray]]s when they strike [[atom]]s of [[heavy element]]s. This is known as [[bremsstrahlung]] ("braking") radiation, and is the way X-rays are created for medical use (for example, dentistry). Materials containing great amounts of [[hydrogen]], such as [[water]] or (lightweight) [[plastic]]s such as [[polyethylene]] and lighter metals such as [[aluminium]] are better than heavier ones such as [[lead]] for preventing this secondary radiation. Such light-element shielding, if it were strong enough to protect against the Van Allen particle radiation, would also provide adequate protection against X-ray radiation coming from the sun during [[solar flare]]s and [[coronal mass ejection]] events. Nevertheless the total mass required for radiation shielding is very high. ==Economics== {{main|Space elevator economics}} With a space elevator, materials might be sent into orbit at a fraction of the current cost. Conventional rocket designs give prices on the order of thousands of [[U.S. dollar]]s per [[kilogram]] for transfer to [[low earth orbit]],<ref>{{cite news |url=http://www.domain-b.com/companies/companies_f/futron_corporation/20021018_countdown.html |title=Delayed countdown |accessdate=2008-03-16 |author=Chennai |date=2002-08-18 |work=Fultron Corporatoin |publisher=The Information Company Pvt Ltd}}</ref> and roughly twenty thousand dollars per kilogram for transfer to geostationary orbit.{{Fact|date=February 2008}} Even optimistic rocket proposals (such as the [[DH-1]]) only claim to bring prices down to about $400 per kilo.{{Fact|date=February 2008}} For the first space elevator, the price could be as low as $220 per kilogram and would decrease as time went on.<ref>{{cite web |url=http://www.spaceward.org/elevator2010-faq?primer |title=The Space Elevator FAQ |accessdate=2008-03-16 |author=The Spaceward Foundation}}</ref> Space elevators have high capital cost but low operating expenses, so they make the most economic sense in a situation where it would be used over a long period of time to handle very large amounts of payload. The current launch market may not be large enough to make a compelling case for a space elevator, but a dramatic drop in the price of launching material to orbit would likely result in new types of space activities becoming economically feasible. In this regard they share similarities with other transportation infrastructure projects such as highways or railroads.{{Fact|date=August 2007}} Development costs might be roughly equivalent, in modern dollars, to the cost of developing the shuttle system. A question subject to speculation is whether a space elevator would return the investment, or if it would be more beneficial to instead spend the money on developing rocketry further. If the elevator did indeed cost roughly the same as the shuttle program, recovering the development costs would take less than about a hundred thousand tons launched to low earth orbit or five thousand tons launched to geostationary orbit; but construction costs are predicted, to the extent that they can be predicted, to be much higher than the development costs.{{Fact|date=August 2007}} ==Political issues == One potential problem with a space elevator would be the issue of ownership and control. Such an elevator would require significant investment (estimates ''start'' at about [[United States dollar|US$]]5 billion for a very primitive tether), and it could take at least a decade to recoup such expenses. At present, few entities in the space industry are able to spend at that magnitude. Assuming a multi-national governmental effort was able to produce a working space elevator, many political issues would remain to be solved. Which countries would use the elevator and how often? Who would be responsible for its defense from [[terrorism|terrorists]] or enemy states? A space elevator could potentially cause rifts between states over the military applications of the elevator. Furthermore, establishment of a space elevator would require removal of existing satellites if their orbit intersects the cable (unless the base station itself can move in order to make the elevator avoid satellites, as proposed by Edwards). An initial elevator could be used in relatively short order to lift the materials to build more such elevators, but the owners of the first elevator might refuse to carry such materials in order to maintain their [[monopoly]]. As space elevators (regardless of the design) are inherently fragile but militarily valuable structures, they would likely be targeted immediately in any major conflict with a state that controls one.{{Fact|date=July 2008}} Consequently, most militaries would elect to continue development of conventional rockets (or other similar launch technologies) to provide effective backup methods to access space.{{Fact|date=July 2008}} [[Arthur C. Clarke]] compared the space elevator project to [[Cyrus West Field]]'s efforts to build the first [[transatlantic telegraph cable]], "the Apollo Project of its age".<ref>{{cite web |url=http://www.spaceelevator.com/docs/acclarke.092079.se.2.html |title=The Space Elevator: 'Thought Experiment', or Key to the Universe? (Part 2) |last=Clarke |first=Arthur C. |accessdate=2006-03-05 |year=2003 }}</ref> ==Alternatives to geostationary tether concepts== Many different types of structures ("[[outer space|space]] [[elevator]]s") for accessing space have been suggested; However, [[As of 2004]], concepts using geostationary tethers seem to be the only space elevator concept that is the subject of active research and [[Commercialization of space|commercial interest in space]].<ref name="iac2004">{{cite web |url=http://www.spaceelevator.com/docs/521Edwards.pdf |title=THE SPACE ELEVATOR AND NASA’S NEW SPACE INITIATIVE |author=Bradley C. Edwards, Ben Shelef |publisher=55th International Astronautical Congress 2004 - Vancouver, Canada |year=2004 |accessdate=2007-07-28}} 'At this time the space elevator is not included in the NASA space exploration program or funded in any form by NASA except through a congressional appropriation ($1.9M to ISR/MSFC)'</ref> ===Compressive structures=== The original concept envisioned by Tsiolkovski was a compression structure. The compressive concept is similar to an [[Radio masts and towers|aerial mast]]. While such structures might reach the agreed [[Karman line|altitude for space]] (100&nbsp;km), they are unlikely to reach [[geostationary orbit]] (35,786&nbsp;km). Due to the difference between [[sub-orbital spaceflight|sub-orbital]] and [[orbital spaceflight]]s, a means of propulsion (such as a rocket) would be necessary to achieve orbital speed. [[Arthur C. Clarke]] proposed a compressive space tower made of [[diamond]] in his novel ''[[2061: Odyssey Three]]''. The towers have actually been built by ''[[3001: The Final Odyssey]]''. It has been proposed that the concept of a Tsiolkovski tower could be combined with that of a classic space elevator cable, so that the tower reaches upward from Earth and meets a cable extended downward.<ref name="JBIS1999">{{cite journal | author = Geoffrey A. Landis and Christopher Cafarelli | year = 1999 | title = The Tsiolkovski Tower Reexamined | journal = Journal of the British Interplanetary Society | volume = 52 | pages = pp. 175–180 | other = Presented as paper IAF-95-V.4.07, 46th International Astronautics Federation Congress, Oslo Norway, 2-6 October 1995 }} </ref> ===Orbital ring=== {{main|Orbital ring}} An orbital ring would be a circular cable spinning in low earth orbit around the Earth, with stationary spokes hanging down to the ground, resting on superconducting magnetic bearings. Due to the short length of the cables, the materials issues are greatly eased, and it is thought that it could be built with today's materials. Vehicles could climb the spokes and then accelerate up to orbital speed along the cable or otherwise. ===Space fountains=== {{main|Space fountain}} The space fountain concept fires pellets, with a [[mass driver]], up from the ground through the center of a tower. These pellets then impart their kinetic energy to the tower structure via electromagnetic drag as they travel up and again as their direction was reversed by a magnetic field at the top. Thus the structure would not be supported by the compressive strength of its materials, and could be hundreds of kilometers high. Unlike tethered space elevators (which have to be placed near the equator), a space fountain could be located at any [[latitude]]. Space fountains would require a continuous supply of power to remain aloft. Some mechanism is needed at the top to launch objects into orbit. ===Launch loops=== {{main|Launch loop}} [[Image:LaunchLoop.svg|thumb|right|A launch loop (Keith Lofstrom 1985)]] A launch loop would be an iron ribbon, carried on magnetic bearings, running at 14 km/s within a stationary vacuum sheath around a very long (~2000 km) track. It would be constructed so that as the structure speeds up, the middle part of the loop raises up in an arc up to 80 km altitude and forms an acceleration track. Vehicles are electromagnetically accelerated along the structure using [[maglev]] with an acceleration of around 3 [[g-force|''g'']]. In this way, they are launched into an elliptic orbit, using only electrical power, at very low cost. It is thought that launch loops could be built with today's materials and technology. Many dozens of launches per hour could be achieved, more than with a geostationary space elevator.<ref name="LLISDC2002">[http://www.launchloop.com/isdc2002loop.pdf Launch Loop slides for the ISDC2002 conference]</ref> Unlike a geostationary space elevator, launch loops would be intended for and suitable for launching human cargo.<ref name="LLISDC2002"/> === Skyhooks ===<!-- This section is linked from [[Robert L. Forward]] --> {{main|Skyhook (structure)}} A tidal stabilized tether is called a "[[Skyhook (structure)|skyhook]]" since it appears to be "hooked onto the sky". This term was introduced by the Italian scientist [[Giuseppe Colombo]]. Skyhooks rotate precisely once per orbit and hence are always oriented the same way to the parent body. They are also called "hypersonic tethers" because the tip nearest the earth travels about [[mach number|Mach-12]] in typical designs. Longer tethers would travel more slowly. An aircraft or sub-orbital vehicle transports cargo to one end of the skyhook. Skyhook designs typically require climbers to transport the cargo to the other end. Robert Raymond Boyd and Dimitri David Thomas (with Lockheed Martin Corporation) patented the Skyhook idea in [[2000]] in a patent titled "Space elevator".<ref>http://www.google.com/patents?vid=USPAT6491258</ref> The company [[Tethers Unlimited Inc]] (founded by Dr. [[Robert Forward]] and Dr. [[Robert P. Hoyt]]) has called this approach "Tether Launch Assist".<ref>[http://www.tethers.com/LaunchAssist.html Tethers Unlimited Inc, "Tether Launch Assist"]</ref> ==See also== {{commonscat|Space elevators}} {{Portal | Spaceflight | RocketSunIcon.svg}} * [[Lunar space elevator]] for the moon variant * [[Space elevator in fiction]] * [[Space elevator economics]] discusses capital and maintenance costs of a space elevator. * [[Space fountain]] * [[Tether propulsion]] * [[Launch loop]] * [[Space gun]] ==References== === Specific === {{reflist|2}} [Isaa66] Isaacs, J. D., A. C. Vine, H. Bradner & G. E. Bachus (1966) ‘Satellite Elongation into a True “Sky-Hook”' Science 151: 682-683. === General === <div class="references-small"> * Edwards BC, Ragan P. "Leaving The Planet By Space Elevator" Seattle, USA: Lulu; 2006. ISBN 978-1-4303-0006-9 [http://www.leavingtheplanet.com/ See Leaving The Planet] * 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://flightprojects.msfc.nasa.gov/pdf_files/elevator.pdf Space Elevators - An Advanced Earth-Space Infrastructure for the New Millennium] <nowiki>[PDF]</nowiki>. A conference publication based on findings from the Advanced Space Infrastructure Workshop on Geostationary Orbiting Tether "Space Elevator" Concepts, 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://isr.us/Downloads/niac_pdf/contents.html The Space Elevator] [[NIAC]] report by Dr. Bradley C. Edwards *[http://www.spectrum.ieee.org/aug05/1690 A Hoist to the Heavens] By Bradley Carl Edwards *Ziemelis K. "Going up". In [[New Scientist]] 2001-05-05, no.2289, p.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." *[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 [[27 March]] [[2002]]. * Krishnaswamy, Sridhar. Stress Analysis &mdash; [http://www.cqe.northwestern.edu/sk/C62/OrbitalTower_ME362.pdf The Orbital Tower] (PDF) * [[LiftPort]]'s Roadmap for Elevator To Space [http://www.liftport.com/papers/SE_Roadmap_v1beta.pdf SE Roadmap] (PDF) </div> * [http://space.newscientist.com/article/dn13552-space-elevators-face-wobble-problem.html?feedId=online-news_rss20 Space Elevators Face Wobble Problem]: New Scientist ==External links== {{Spoken Wikipedia|Space_elevator.ogg|2006-05-29}} * [http://www.google.com/patents?vid=USPAT6491258 Space elevator - Google Patents] * [http://www.socialtext.net/wikinomics/index.cgi?elevator_to_space Wikinomics Elevator to Space Article] ===Organizations=== * [http://seattlewebcrafters.com/nsecc/ The National Space Society Special Interest Chapter for the Space Elevator (NSECC)] * [http://www.elevator2010.org/ Elevator:2010] Space elevator prize competitions * [http://www.isr.us/SEHome.asp?m=1 Space elevator, Institute for Scientific Research] Last news item on web site dated July, 2004. * [http://www.isr.us/Spaceelevatorconference/ The Space Elevator: 3rd Annual International Conference] [[28 June]]-30, 2004 in Washington, D.C. * [http://www.isr.us/Spaceelevatorconference/2003presentations.html 3rd Annual International Conference Presentations] * [http://www.isr.us/Spaceelevatorconference/2004presentations.html 4th Annual International Conference Presentations] * [http://liftwatch.org/ LiftWatch.org - Space Elevator News] * [http://www.liftport.com/ Liftport Group] - The Space Elevator Companies ===Animations=== * [http://www.isr.us/SEanimation.asp View space elevator animation] [[Windows Media Video]] (WMV) file - Institute for Scientific Research * [http://www.isr.us/video/SE-INTRO_Final-1stream-384.wmv Download space elevator animation] [[Windows Media Video]] (WMV) file - Institute for Scientific Research * [http://wid.ap.org/video/video/elevator.rm Brief video (realmedia format) of the space elevator concept] ===Miscellaneous links=== * [http://www.frc.ri.cmu.edu/~hpm/project.archive/1976.skyhook/papers/skyhook.bib Skyhook Bibliography] * [http://www.leavingtheplanet.com/ Leaving The Planet By Space Elevator] * [http://kcspacepirates.com/ Kansas City Space Pirates] * [http://www.warr.de/projekte.php?projekt=space_elevator Project of the Scientific Workgroup for Rocketry and Spaceflight](WARR) (German) * [http://www.spaceelevator.com/ The Space Elevator Reference] * [http://www.gizmonicsinc.com/elevator/ California Engineering Company's Site Regarding Improvements to Current Designs] * [http://groups.yahoo.com/group/space-elevator/ Space Elevator Yahoo Group] A discussion list for space elevator related topics * [http://spacelift.gondor.ru/ A major Russian site about space elevators, by Y. Artsutanov and D. Tarabanov] * [http://gizmonicsinc.com/elevator/ Some technical papers and a numerical/graphical tool for calculating ribbon properties and deployment scenarios.] * [http://science.howstuffworks.com/space-elevator.htm HowStuffWorks article on the space elevator] * [http://www.liftport.com/files/521Edwards.pdf The Space Elevator: A Brief Overview] * [http://bbs.keyhole.com/ubb/showthreaded.php?Cat=0&Board=EarthTransportation&Number=387676&fpart=1&PHPSESSID= Space Elevator in 3D] for [[Google Earth]] * [http://www.pbs.org/wgbh/nova/sciencenow/3401/02.html Space Elevator] [[NOVA scienceNOW]] segment * [http://www.geocities.com/jcsherwood/ACClinks2.htm Arthur C. Clarke links & image archive] * [http://spacelf8r.blogspot.com Space Elevator Journal] * [http://google.com/coop/cse?cx=007554473059150349008:_oqnsivumzk Space Elevator Search Engine] * [http://keithcu.com/wordpress/?p=17 Interview with Brad Edwards] * [http://www.spaceelevatorblog.com/ The Space Elevator Blog] ===Articles/Interviews=== * [http://www.spectrum.ieee.org/oct07/5584 October 2007 IEEE Spectrum interview with Arthur C Clarke mentions the space elevator] * [http://www.wired.com/news/technology/0,1282,57536,00.html To the Moon in a Space Elevator? ([[4 February]] [[2003]] Wired News)] * [http://liftoff.msfc.nasa.gov/academy/TETHER/spacetowers.html Liftoff (teenage education): Space Towers] * [http://science.nasa.gov/headlines/y2000/ast07sep_1.htm Audacious & Outrageous: Space Elevators] * [http://gassend.com/elevator/ Various thoughts on space elevators posted by Blaise Gassend] * [http://www.msnbc.msn.com/id/9454786/ Space elevator robot passes 1,000-foot mark] * [http://economist.com/science/tq/displayStory.cfm?story_id=7001786 The Economist: Waiting For The Space Elevator] ([[June 8]], [[2006]] - subscription required) * [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://spacemonitor.blogspot.com/2006/12/space-elevators-future-i-can-invision.html Space Elevators: A Future I can Envision...Part 1] ([[December 23]], [[2006]]) and [http://spacemonitor.blogspot.com/2006/12/space-elevators-future-i-can-invision_24.html Part 2] ([[January 15]], [[2007]]) from [http://www.spacemonitor.blogspot.com The Space Monitor] [[Category:Exploratory engineering]] [[Category:Megastructures]] [[Category:Space colonization]] [[Category:Spacecraft propulsion]] [[Category:Space technology]] [[Category:Vertical transportation devices]] [[Category:Space access]] {{Link FA|it}} [[ar:مصعد الفضاء]] [[bs:Svemirski lift]] [[ca:Ascensor espacial]] [[cs:Orbitální výtah]] [[da:Rumelevator]] [[de:Weltraumlift]] [[es:Ascensor espacial]] [[eo:Kosmolifto]] [[fr:Ascenseur spatial]] [[ko:우주 엘리베이터]] [[it:Ascensore spaziale]] [[he:מעלית חלל]] [[ka:კოსმოსური ლიფტი]] [[lv:Kosmiskais lifts]] [[hu:Űrlift]] [[nl:Ruimtelift]] [[ja:軌道エレベータ]] [[no:Romelevator]] [[pl:Winda kosmiczna]] [[pt:Elevador espacial]] [[ru:Космический лифт]] [[fi:Avaruushissi]] [[sv:Rymdhiss]] [[th:ลิฟต์อวกาศ]] [[vi:Thang máy vũ trụ]] [[zh:天梯]]