# Space elevator

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"The Space Elevator" would consist of a cable attached to the surface and reaching outwards into space. By positioning it so that the total centrifugal force exceeds the total gravity, either by extending the cable or attaching a counterweight, the elevator would stay in place geosynchronously. Once sent far enough, climbers would be accelerated further by the planet's rotation. This diagram is not to scale.

A space elevator, also known as a space bridge, is a fixed structure from the Earth's surface into space for carrying payloads. Plausible techniques for building a space elevator include beanstalks or Space fountains or even certain very tall compressive structures, similar to those used for aerial masts. A Space fountain would use particles fired up from the ground to form a dynamic, quasi-compressive structure. However, space fountains and tall compressive structures, whilst possibly reaching the agreed altitude for space (100 km), are unlikely to reach orbit and would require additional rocket or other means to leave the Earth.

A beanstalk (see Jack and the Beanstalk), on the other hand, is an orbital space elevator that uses a cable that 'hangs down' to the surface from synchronous orbit. It is also called a geosynchronous orbital tether, and is one kind of skyhook. A beanstalk attached to the Earth could eventually permit delivery of great quantities of cargo and people to orbit, and at costs only a fraction of those associated with current means.

Construction would be a vast project: a beanstalk would have to be built of a material that could endure tremendous stress while also being light-weight, cost-effective, and manufacturable. Today's materials technology does not quite meet these requirements. 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, optimists say that we could develop the necessary technology by 2008 [1] (http://liftport.com/research2.php) and finish building the first space elevator by 2018 [2] (http://www.space.com/businesstechnology/technology/space_elevator_020327-1.html) [3] (http://www.isr.us/research_es_se.asp). It should be noted that early elevators would be restricted to cargo due to radiation shielding issues.

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## Physics and structure

One concept for the space elevator has it tethered to a mobile seagoing platform.

There are a variety of beanstalk 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—mobile and stationary. Mobile stations are typically large oceangoing vessels. Stationary platforms are generally located in high-altitude locations.

Mobile platforms have the advantage of being able to maneuver to avoid high winds and storms. While stationary platforms don't have this, they typically 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 width of the cable at the center (especially on materials with low tensile strength), and reduce the minimal length of cable reaching beyond geostationary orbit significantly.

### Cable

The cable must be made of a material with an extremely high tensile strength/density ratio (the limit to which a material can be stretched without irreversibly deforming divided by its density). A space elevator can be made relatively economically if a cable with a density similar to graphite, with a tensile strength of ~65–120 GPa can be produced in bulk at a reasonable price.

By comparison, most steel has a tensile strength of under 1 GPa, and the strongest steels no more than 5 GPa, but steel is heavy. The much lighter material Kevlar has a tensile strength of 2.6–4.1 GPa, while quartz fiber can reach upwards of 20 GPa; the tensile strength of diamond filaments would theoretically be minimally higher.

Carbon nanotubes have exceeded all other materials and appear to have a theoretical tensile strength and density that is well within the desired range for space elevator structures, but the technology to manufacture bulk quantities and fabricate them into a cable has not yet been developed. While theoretically carbon nanotubes can have tensile strengths beyond 120 GPa, in practice the highest tensile strength ever observed in a single-walled tube is 63 GPa, and such tubes averaged breaking between 30 and 50 GPa. Even the strongest fiber made of nanotubes is likely to have notably less strength than its components. Further research on purity and different types of nanotubes will hopefully improve this number.

A seagoing anchor station would incidentally act as a deep-water seaport.

Most designs call for single-walled carbon nanotubes. While multi-walled nanotubes may attain higher tensile strengths, they have notably higher mass and are consequently poor choices for building the cable. One potential material possibility is to take advantage of the high pressure interlinking properties of carbon nanotubes of a single variety. [4] (http://prola.aps.org/pdf/PRB/v62/i19/p12648_1). While this would cause the tubes to lose some tensile strength by the trading of sp2 bonds (graphite, nanotubes) for 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.

The technology to spin regular VdW-bonded yarn from carbon nanotubes is just in its infancy: the first success to spin a long yarn as opposed to pieces of only a few centimeters has been reported only very recently; but the strength/weight ratio was worse than Kevlar due to inconsistent type construction and short tubes being held together by VdW. (March 2004).

Note that as of 2004, carbon nanotubes have an approximate price higher than gold at $100/gram, and 20 million grams would be necessary to form even a seed elevator. This price is decreasing rapidly, and large-scale production would reduce it further, but the price of suitable carbon nanotube cable is anyone's guess at this time. The cable material is an area of fierce worldwide research, the applications of successful material go much further than space elevators; this is good for space elevators because it is likely to push down the price of the cable material further. Other suggested application areas include suspension bridges, new composite materials, better rockets, lighter aircraft etc. etc. #### 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. In an ideal cable the stress would be constant throughout the whole length, which means at each point tapering the cable in proportion to the total weight of the cable below. Using a model that takes into account the Earth's gravitational and centrifugal forces (and neglecting the smaller Sun and Lunar effects), it is possible to show that the cross-sectional area of the cable as a function of height looks like this: [itex] A(r) = A_{0} \ \exp \left[  \frac{\rho}{s} \left[ \begin{matrix}\frac{1}{2}\end{matrix} \omega^{2} (r_{0}^{2} - r^2) + g_{0}r_{0} (1 - \frac{r_{0}}{r}) \right]  \right] [itex] Where [itex] A(r) [itex] is the cross-sectional area as a function of distance [itex] r [itex] from the earth's center. The constants in the equation are: • [itex] A_{0} [itex] is the cross-sectional area of the cable on the earth's surface. • [itex] \rho [itex] is the density of the material the cable is made out of. • [itex] s [itex] is the tensile strength of the material. • [itex] \omega [itex] is the rotational frequency of the earth about its axis, 7.292 × 10-5 radian per second). • [itex] r_{0} [itex] is the distance between the earth's center and the base of the cable. It is approximately the earth's equatorial radius, 6378 km. • [itex] g_{0} [itex] is the acceleration due to gravity at the cable's base, 9.780 m/s². 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 geosynchronous orbit. The cable thickness then decreases again out from geosynchronous orbit. Thus the taper of the cable from base to GEO (r = 42,164 km), [itex] \frac{A(r_{\mathrm{GEO}})}{A_0} = \exp \left[ \frac{\rho}{s} \times 5.294 \times 10^{10} \, \mathrm{ \frac{m^2}{s^2} } \right] [itex] Using the density and tensile strength of steel, and assuming a diameter of 1 cm at ground level yields a diameter of several hundred kilometers (!) at geostationary orbit height, showing that steel, and indeed most materials used in present day 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 and 5000 kg/m³ • 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. The exponential relationship means a small increase in base height results in a large decrease in thickness at geostationary level. Towers of up to 100 km high have been proposed. Not only would a tower of such height reduce the cable mass, it would also avoid exposure of the cable to atmospheric processes. • 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 nanotube would typically be just a millimeter wide at the base. ### Climbers Missing image SpaceElevatorInClouds.jpg 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 at all times. While designs employing smaller, segmented moving cables along the length of the main cable have been proposed, most cable designs call for the "elevator" to climb up the cable. Climbers cover a wide range of designs. On elevator designs whose cables are planar ribbons, some have proposed to use pairs of rollers to hold the cable with friction. Other climber designs involve moving arms containing pads of hooks, rollers with retracting hooks, magnetic levitation (unlikely due to the bulky track required on the cable), and numerous other possibilities. Power is a significant obstacle for climbers. Energy storage densities, barring significant advances in compact nuclear power, are unlikely to ever be able to store the energy for an entire climb in a single climber without making it weigh too much. Some solutions have involved laser or microwave power beaming. Others have gained part of their energy through regenerative braking of down-climbers passing energy to up-climbers as they pass, magnetospheric braking of the cable to dampen oscillations, tropospheric heat differentials in the cable, ionospheric discharge through the cable, and other concepts. The primary power methods (laser and microwave power beaming) have significant problems with both efficiency and heat dissipation on both sides, although with optimistic numbers for future technologies, they are feasible. Climbers must be paced at optimal timings so as to minimize cable stress, oscillations, and maximize throughput. The weakest point of the cable is near its planetary connection; new climbers can typically be launched so long as there are not multiple climbers in this area at once. An only-up elevator can handle a higher throughput, but has the disadvantage of not allowing energy recapture through regenerative down-climbers. Additionally, as one cannot "leap out of orbit", an only-up elevator would require another method to let payloads/people get rid of their orbital energy, such as conventional rockets. Finally, only-up climbers that don't return to earth must be disposable; if used, they should be modular so that their components can be used for other purposes in geosynchronous orbit. In any case, smaller climbers have the advantage over larger climbers of giving better options for how to pace trips up the cable, but may impose technological limitations. ### Counterweight There have been two dominant methods proposed for dealing with the counterweight need: a heavy object, such as a captured asteroid, positioned past geosynchronous orbit; and extending the cable itself well past geosynchronous orbit. The latter idea has gained more support in recent years due to the simplicity of the task and the ability of a payload that travels to the end of the counterweight-cable to be flung off as far as Saturn (and farther using gravitational assists from planets). ### Launching into outer space As a payload is lifted up a space elevator, it gains not only altitude but angular momentum as well. This angular momentum is taken from Earth's own rotation. As the payload climbs it "drags" on the cable, causing it to tilt very slightly to the west (lagging behind slightly on the Earth's rotation). The horizontal component of the tension in the cable applies a tangential pull on the payload, accelerating it eastward. Conversely, the cable pulls westward on Earth's surface, insignificantly slowing it. The opposite process occurs for payloads descending the elevator, tilting the cable eastwards and very slightly increasing Earth's rotation speed. In both cases the centrifugal force acting on the cable's counterweight causes it to return to a vertical orientation, transferring momentum between Earth and payload in the process. We can determine the velocities that might be attained at the end of Pearson's 144,000 km tower (or cable). At the end of the tower, the tangential velocity is 10.93 kilometers per second which is more than enough to escape Earth's gravitational field and send probes as far out as 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 (and the Earth) for velocity of the launched object, in much the same way one snaps a towel or throws a lacrosse ball. 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. A Martian tether could be much shorter than one on Earth. Mars' gravity is 38% of Earth's, while it rotates around its axis in about the same time as Earth. 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 is in a low orbit, and intersects the equator regularly (twice every orbital period of 11 h 6 min). A collision between the elevator and the 22.2 km diameter moon would have to be avoided through active steering. 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. Alternatively, due to the lack of atmosphere on the moon, a rotating tether could be used with its center of mass in orbit around the moon with a counterweight at the short end and a 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. 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 has suggested using such smaller systems as power generators at points distant from the Sun where solar power is uneconomical. ## Construction The construction of a space elevator would be a vast project, requiring advances in engineering and physical technology. NASA has identified "Five Key Technologies for Future Space Elevator Development": 1. Material for cable (e.g. carbon nanotube and nanotechnology) and tower 2. Tether deployment and control 3. Tall tower construction 4. Electromagnetic propulsion (e.g. magnetic levitation) 5. Space infrastructure and the development of space 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 geosynchronous 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 forces (gravity and centrifugal force) would naturally pull the cables directly towards and directly away from the Earth and keeps the elevator balanced around geosynchronous orbit. However, this approach requires lifting hundreds or even thousands of tons on conventional rockets. This would be very expensive. ### Brad Edwards' proposal Brad Edwards, Director of Research for the Institute for Scientific Research (ISR), based in Fairmont, West Virginia, is a leading authority on the space elevator concept. He proposes that a single hairlike 20 short ton (18 metric ton) 'seed' cable be deployed in the traditional way, giving a very lightweight elevator with very little lifting capacity. Then, progressively heavier cables would be pulled up from the ground along it, repeatedly strengthening it until the elevator reaches the required mass and strength. This is much the same technique used to build suspension bridges. Although 20 short tons for a seed cable may sound like a lot, it would actually be very lightweight — the proposed average mass is about 0.2 kilogram per kilometer. Conventional copper telephone wires running to consumer homes weigh about 4 kg/km. Twenty tons is slightly less than a Russian geosynchronous communication satellite. ### Other designs These are far less well developed, and will be mentioned here only in passing. 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 Geosynchronous 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. The two sides of the loop are naturally kept apart by coriolis forces due to the rotation of the earth and the cable. By exponentially increasing the thickness of the cable from the ground a very quick buildup 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 strength of the loop is lower than a conventional tapered design, reducing the maximum payload that can be carried without snapping the cable [5] (http://www.mit.edu/people/gassend/publications/ExponentialTethers.pdf) Other structures such as mechanically-linked multiple looped designs hanging off of a central exponential tether might also be practical, and would seem to avoid the laser power beaming; this design has higher capacity than a single loop, but still requires perhaps twice as much tether material. ## Failure modes and safety issues 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. ### Satellites If nothing were done, essentially all satellites with perigees below the top of the elevator will eventually collide. 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, eventually, except for synchronized orbits, the elevator and satellite will be in the same place at the same time and there will be a disaster. 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. Further, transverse oscillations of the cable could be controlled so as to ensure that the cable avoids satellites on known paths -- the required amplitudes are modest, relative to the cable length. ### 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 struts, 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. Creating the cable as a mesh instead of a ribbon helps prevent collateral damage from each micrometeorite impact. It is not enough, however, that other fibers be able to take over the load of a failed strand — 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 GPa and a Young's modulus of 1000 GPa, its strain will be 0.05 and its stored elastic energy will be 1/2 × 0.05 × 50 GPa = 1.25×109 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 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. ### 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 cells, 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. ### Sabotage Sabotage is a relatively unquantifiable problem. Elevators are probably less susceptible than suspension bridges carrying mass vehicular traffic, of which there are many worldwide. Nonetheless there are few more spectacular possible targets: no terrorist act in history has approached the potential destruction caused by the carefully-targeted sabotage of a space elevator. Concern over sabotage may have an effect on location, since what would be required would be not only an equatorial site but also one outside the range of unstable territories. ### Vibrational harmonics A final risk of structural failure comes from the possibility of vibrational harmonics within the cable. Like the shorter and more familiar strings of stringed instruments, the cable of a space elevator has a natural resonance 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 intelligent 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 do damping against Earth's magnetosphere, which would additionally generate electricity that could be passed to the climbers. Oscillations can be either linear or rotational. ### 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 a stable orbit. This is because a space elevator must be kept in tension, with greater centrifugal force pulling outward than gravitational force pulling inward, or any additional payload added at the elevator's bottom end would pull the entire structure down. 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 at about 25,000 km If the break occurred at higher altitude, up to about 25,000 km, the lower portion of the elevator would descend to Earth and drape itself along the equator eastward from the anchor point, while the now unbalanced upper portion would rise to a higher orbit. Some authors have suggested that such a failure would be catastrophic, with the thousands of kilometers of falling cable creating a swath of meteoric destruction along Earth's surface, but such damage is not likely considering the relatively low density the cable as a whole would have. The risk can be further reduced by triggering some sort of destruct mechanism in the falling cable, breaking it into smaller pieces. In most cable designs, the upper portion of the cable that fell to earth would burn up in the atmosphere. Because proposed initial cables (the only ones likely to be broken) are very light and flat, the bottom portion would likely settle to Earth with less force than a sheet of paper due to air resistance on the way down. 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 reentry anyway. #### Elevator pods Any elevator pods on the falling section would also reenter Earth's atmosphere, but it is likely that the elevator pods will already have been designed to withstand such an event as an emergency measure anyway. It is almost inevitable that some objects - elevator pods, structural members, repair crews, etc.—will accidentally fall off the elevator at some point. Their subsequent fate would depend upon their initial altitude. Except at geosynchronous 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 km, its orbit will have an apogee at the altitude where it was released from the elevator and a perigee within Earth's atmosphere—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 wind up 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 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 may prove impossible to rescue them. All of these altitudes are given for an Earth-based space elevator; a space elevator serving a different planet or moon would have different critical altitudes where each of these scenarios would occur. ### Van Allen Belts The space elevator runs through the 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 sickness to any unshielded human or other living things. Some people speculate that passengers and other living things will continue to travel by high-speed rocket, while the space elevator hauls bulk cargo. Research into lightweight shielding and techniques for clearing out the belts is underway. An elevator could carry passenger cars with heavy lead or other shielding, however for the thin cable of an initial elevator that would reduce its overall capacity; this becomes less of a problem later, when the cable has been thickened. However, the shielding itself can in some cases consist of useful payload- for example food, water, supplies, fuel or construction/maintenance materials, and no additional shielding costs are then incurred on the way up. More conventional and faster reentry techniques such as aerobraking might be employed on the way down to minimize radiation exposure. Deorbit burns use relatively little fuel, and so can be cheap. ## Economics Main article: space elevator economics With a space elevator, materials could be sent into orbit at a fraction of the current cost. Modern rocketry gives prices that are on the order of thousands of U.S. dollars per kilogram for transfer to low earth orbit, and roughly 20 thousand dollars per kilogram for transfer to geosynchronous orbit. For a space elevator, the price could be on the order of a few hundreds of dollars per kilogram. 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. 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. ## 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 US$5 billion for a very primitive tether), and it could take at least a decade to recoup such expenses. At present, only governments are able to spend in the space industry at that magnitude.

Assuming a multi-national governmental effort was able to produce a working space elevator, many delicate political issues would remain to be solved. Which countries would use the elevator and how often? Who would be responsible for its defense from terrorists or enemy states? A space elevator would allow for easy deployment of satellites into orbit, and it is becoming ever more obvious that space is a significant military resource. A space elevator could potentially cause numerous rifts between states over the military applications of the elevator. Furthermore, establishment of a space elevator would require knowledge of the positions and paths of all existing satellites in Earth orbit and their removal if they cannot adequately avoid the elevator.

The U.S. military may covertly oppose a space elevator. By granting inexpensive access to space, a space elevator permits less-wealthy opponents of the U.S. to gain military access to space—or to challenge U.S. control of space. An important U.S. military doctrine is to maintain space and air superiority during a conflict. In the current political climate, concerns over terrorism and homeland security could be possible grounds for more overt opposition to such a project by the U.S. government.

An initial elevator could be used in relatively short order to lift the materials to build more such elevators, but whether this is done and in what fashion the resulting additional elevators are utilized depends on whether the owners of the first elevator are willing to give up any monopoly they may have gained on space access. However, once the technologies are in place, any country with the appropriate resources would most likely be able to create their own elevator.

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. Consequently, most militaries would elect to continue development of conventional rockets (or other similar launch technologies) to provide effective backup methods to access space.

The cost of the space elevator is not excessive compared to other projects and it is conceivable that several countries or an international consortium could pursue the space elevator. Indeed, there are companies and agencies in a number of countries that have expressed interest in the concept. Generally, megaprojects need to be either joint public-private partnership ventures or government ventures and they also need multiple partners. It is also possible that a private entity (risks notwithstanding) could provide the financing — several large investment firms have stated interest in construction of the space elevator as a private endeavor. However, from a political standpoint there is a case to be made that the space elevator should be an international effort like the International Space Station with the inevitable rules for use and access.

The political motivation for a collaborative effort comes from the potential destabilizing nature of the space elevator. The space elevator clearly has military applications, but more critically it would give a strong economic advantage for the controlling entity. Information flowing through satellites, future energy from space, planets full of real estate and associated minerals, and basic military advantage could all potentially be controlled by the entity that controls access to space through the space elevator. An international collaboration could result in multiple ribbons at various locations around the globe, since subsequent ribbons would be significantly cheaper, thus allowing general access to space and consequently eliminating any instabilities a single system might cause. The Epilogue of Arthur C. Clarke's Fountains of Paradise shows an Earth with several space elevators leading to a giant, circumterran, space station. The analogy with a wheel is evident: the space station itself is the wheel rim, Earth is the axle, and the six equidistant space elevators the spokes.

While few ordinary citizens might profit directly from space elevator applications, the general public would probably reap benefits through cheap, environmentally-friendly solar power, enhanced satellite navigation and communication services, and even through improved health, education and social services made possible by the savings made by governments in accessing space.

Clarke compared the space elevator project to Cyrus Field's efforts to build the first transatlantic telegraph cable, "the Apollo Project of its age" [6] (http://www.spaceelevator.com/docs/acclarke.092079.se.2.html).

## History

The concept of the space elevator first appeared in 1895 when a Russian scientist Konstantin Tsiolkovsky was inspired by the Eiffel Tower in Paris to consider a tower that reached all the way into space. He imagined placing a "celestial castle" at the end of a spindle-shaped cable, with the "castle" orbiting Earth in a geosynchronous orbit (i.e. the castle would remain over the same spot on Earth's surface). The tower would be built from the ground up to an altitude of 35,800 kilometers (geostationary orbit). Comments from Nikola Tesla suggest that he may have also conceived such a tower. Tsiolkovsky's notes were sent behind the Iron Curtain after his death.

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 geosynchronous orbit.

Building from the ground up, however, proved an impossible task; there was no material in existence with enough compressive strength to support its own weight under such conditions. It took until 1957 for another Russian scientist, Yuri N. Artsutanov, to conceive of a more feasible scheme for building a space tower. Artsutanov suggested using a geosynchronous satellite as the base from which to construct the tower. By using a counterweight, a cable would be lowered from geosynchronous 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 published his idea in the Sunday supplement of Komsomolskaya Pravda in 1960. He also proposed tapering the cable thickness so that the tension in the cable was constant—this gives a thin cable at ground level, thickening up towards GEO.[7] (http://www.liftport.com/files/Artsutanov_Pravda_SE.pdf)

Making a cable over 35,000 kilometers long is a difficult task. In 1966, four American engineers 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. They 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, designed a tapered cross section that would be better suited to building the tower. The completed cable would be thickest at the geosynchronous orbit, where the tension was greatest, and would be narrowest at the tips to reduce the amount of weight that the middle 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 tower was built. Without a large counterweight, the upper portion of the tower would have to be longer than the lower due to the way 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 tower would have required thousands of Space Shuttle trips, although part of the material could be transported up the tower when a minimum strength strand reached the ground or be manufactured in space from asteroidal or lunar ore.

Arthur C. Clarke introduced the concept of a space elevator to a broader audience in his 1978 novel, The Fountains of Paradise, in which engineers construct a space elevator on top of a mountain peak (Adam's Peak in Sri Lanka) in the equatorial island of Taprobane (the Discoveries era name for Sri Lanka) .

David Smitherman of NASA/Marshall's Advanced Projects Office has compiled plans for such an elevator that could turn science fiction into reality. His publication, "Space Elevators: An Advanced Earth-Space Infrastructure for the New Millennium" [8] (http://flightprojects.msfc.nasa.gov/fd02_elev.html), is based on findings from a space infrastructure conference held at the Marshall Space Flight Center in 1999.

Another American scientist, Bradley Edwards, suggests creating a 100,000 km long paper-thin ribbon, which would stand a greater chance of surviving impacts by meteors. The work of Edwards has expanded to cover: the deployment scenario, climber design, power delivery system, 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. Plans are currently being made to complete engineering developments, material development and begin construction of the first elevator. Funding to date has been through a grant from NASA Institute for Advanced Concepts. Future funding is sought through NASA, the United States Department of Defense, private, and public sources. 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 nanotubes with a minimal tensile strength of 130 GPa; 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 63 GPa [9] (http://bucky-central.mech.nwu.edu/RuoffsPDFs/91.pdf).

Space elevator proponents are planning competitions for space elevator technologies [10] (http://msnbc.msn.com/id/5792719/), similar to the Ansari X Prize. Elevator:2010 (http://www.elevator2010.org/) will organize annual competitions for climbers, ribbons and power-beaming systems. The Robolympics Space Elevator Ribbon Climbing [11] (http://robolympics.net/rules/climbing.shtml) organizes climber-robot building competitions. In March of 2005 NASA's Centennial Challenges program announced a partnership with the Spaceward Foundation (the operator of Elevator:2010), raising the total value of prizes to US\$400,000 [12] (http://www.nasa.gov/home/hqnews/2005/mar/HQ_m05083_Centennial_prizes.html)[13] (http://www.space.com/news/050323_centennial_challenge.html).

On April 27, 2005 "the Liftport Group of space elevator companies has announced that it will be building a carbon nanotubes 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." [14] (http://www.universetoday.com/am/publish/liftport_manufacture_nanotubes.html?2742005)

## Fiction

Note: Some depictions were made before the space elevator concept became known.

## References

• Edwards BC, Westling EA. The Space Elevator: A Revolutionary Earth-to-Space Transportation System. San Francisco, USA: Spageo Inc.; 2002. ISBN 0972604502.
• Space Elevators - An Advanced Earth-Space Infrastructure for the New Millennium (http://flightprojects.msfc.nasa.gov/pdf_files/elevator.pdf) [PDF]. 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" HTML (http://www.jetpress.org/volume4/space.htm) PDF (http://www.jetpress.org/volume4/space.pdf), John Hickman, Ph. D. Journal of Evolution and Technology Vol. 4 - November 1999.
• Ziemelis K. "Going up". In New Scientist 2001-05-05, no.2289, p.24-27. Republished in SpaceRef (http://www.spaceref.com/news/viewnews.html?id=337). Title page: "The great space elevator: the dream machine that will turn us all into astronauts."
• The Space Elevator Comes Closer to Reality (http://www.space.com/businesstechnology/technology/space_elevator_020327-1.html). An overview by Leonard David of space.com, published 27 March 2002.
• Krishnaswamy, Sridhar. Stress Analysis — The Orbital Tower (http://www.cqe.nwu.edu/sk/C62/OrbitalTower_ME362.pdf) (PDF)

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