====== LiftPort Space Elevator ======
The **LiftPort Space Elevator** is a proposed [[structure]] designed to transport [[cargo]] from the [[Earth]]'s [[surface]] into [[outer space|space]]. The goal of the LiftPort Space Elevator is to suppliment [[rocket propulsion]] with the traversal of a fixed ribbon structure via robotic [[lifter]] in order to move [[cargo]] into or beyond [[Planetary orbit|orbit]]. Space elevators have also sometimes been referred to as '''beanstalks''', '''space bridges''', '''space lifts''', '''space ladders''', '''skyhooks''', or '''orbital towers'''.
The LiftPort Space Elevator design calls for a [[tether]] in the form of a [[ribbon]] spanning from the surface to a point beyond [[geosynchronous orbit]]. As the planet rotates, the inertia at the end of the tether counteracts gravity, and also keeps the cable taut. [[Lifters]] can then climb the tether and escape the planet's [[gravity]] without the use of rocket propulsion. Such a structure could theoretically permit delivery of [[cargo]] and people to orbit with transportation costs a fraction of those of more traditional methods of launching a payload into orbit.
The LiftPort plan is to incorporate [[carbon nanotubes]] into the ribbon design, thus providing a link between space exploration and [[nanotechnology]].
==== Orbital Space Elevator ====
This concept is also called an '''orbital tether''', '''geosynchronous orbital tether''', or a '''beanstalk'''. 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 quite meet these requirements, although [[carbon nanotube]] technology shows 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.
==== Physics ====
The physics of the space elevator design work by [[centrifugal force]]. If you hold a string in your hand with a ball attached and swing the string in a circle over your head, the string is pulled straight and the ball lifts into the air, defying gravity. Take the same principal, but make the string really long and really strong and make it a ribbon. Instead of a ball you have a satellite at the end. Instead of your hand, the earth provides all the rotational energy that will be needed.
Once the string is pulled taut, [[lifters|robotic lifters]] will be able to drive up this ribbon like a roadway. [[Cargo]] placed on the lifter will be able to drive right up to geosynchronous orbit and be delivered.
[[Cargo]] may continue further along the ribbon. The effects of centrifugal force from this point on will exceed the pull of earth's gravity. If the cargo is released near the end of the ribbon, it will be flying off toward the Moon, Mars, anywhere else in the solar system with little more energy needed than azimuth correction. And it will be moving very fast.
==== Structure ===
The LiftPort Space Elevator design includes a floating platform in the Pacific ocean approximately 2000 miles west of Equador called the [[LiftPort Station]]. A [[ribbon]] - probably of carbon nanotubes - will stretch 62,000 miles (100000 km). The robotic [[lifters]] will carry the cargo to it's destination, and a [[counterweight]] will help balance the mass being pulled by the earth's gravity with [[centrifugal force]].
===Climbers===
[[Image:SpaceElevatorInClouds.jpg||thumb|300px|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 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 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.
Power is a significant obstacle for climbers. Energy and power storage densities, barring significant advances in compact nuclear power, do not yet provide the desired rate of climb performance. While the technology is current, no batteries of an adequate size have yet been constructed. Current Direct Energy Conversion radioisotopic batteries can deliver approximately 35 watts per kilogram continuous (based on Sr-90 fuel), allowing for a cargo to battery mass ratio of approximately 1 and an upward travel rate, making generous efficiency assumptions, of approximately 35 miles per hour (56 km/h). These devices do not require recharging. Some other potential solutions have involved [[laser]] or [[microwave]] [[power beaming]], and solar power.
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. Advancements in carbon nanotube production and manipulation would work directly into this; some carbon nanotube configurations exhibit photovoltaic properties, and some have exceptional thermal conduction properties.
Climbers must be paced at optimal timings so as to minimize cable stress and oscillations and to 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, an up-only elevator would require some other method to return people to Earth. Finally, only-up climbers (that do not return to Earth) must be disposable; if used, they should be modular so that their components can be used for other purposes in space. In any case, smaller climbers have the advantage over larger climbers of giving better options for how to timetable trips up the cable, but may impose technological limitations.
In order to keep the mass of the cable to a minimum, it should not carry any extra things like electrical cables, although electrical cables are appealing as they could provide power to the elevator. Multiple climbers are not likely as the most important loads are large pre-assembled pieces and the system must be designed around this capability rather than a number of small supply ships. In practice, the elevator will not just be an elevator, but a space ship as well, as it should be capable of going to any destination from low earth orbit to geosynchronous orbit or to the end of the cable. If the destination is low earth orbit, about 340km, then the elevator will detach itself at about 23,000 km, fall into an elliptical orbit, and use rocket power (or ion thrust) to change from elliptical to circular low earth orbit at the desired inclination and rendezvous with its destination space station. For safety and practical reasons this elevator/space ship should also be a tug. The basic ship should be small, contain rocket engines, life support, passenger space, parachutes (for emergency re-entry), but little cargo space. Instead it should have a cargo container hanging on a cable below it while ascending the main carbon nanotube cable. Once detached from the main cable the spaceship/tug would turn around, connect to the container and push it the remainder of the trip. This arrangement is for two reasons, the first, safety. In an emergency the cargo would be jettisoned so the passengers and crew would survive. Secondly, this allows different sizes and shapes of containers.
One major issue is transferring energy to the elevator to climb the cable. Once in space, a large solar panel could be used, but this would be difficult to build strong enough to withstand wind and rain and still be light weight, so leave it up there. The lower limit of this part would be about 100 km. This part would be a robotic module that is always attached to the cable and consist mostly of the solar panels, electric motors and climbing mechanism. This way the problem is divided into two parts; the first 100 km and the rest. With the issue divided into two, solutions that involve heavier per-unit length can be used at the earth end without that penalty being applied to the entire cable.
If the trip to geosynchronous is going to take a reasonable time, 2 days then, > 800 km/h average speed is required. The space shuttle (orbiter only) weighs 43000 kg empty. If the space elevator and cargo container weighed only 30000 kg then to go 100 km/h (near the earth) would require over 8000 kW or 11000 hp (do the math 1 watt = Newton-meter/sec). At higher points in space the same horsepower could drive the elevator much faster. State of the art motors like http://www.rasertech.com/tech_p-2.html can produce about 5.5 kW/kg or about 3.5 hp/lb. 8000 kW would require about 1500 kg of electric motor power. Berkley labs have designed new high-efficiency solar cells (up to theoretical 79% efficiency). This technology has been licensed by www.rosestreetlabs.com and they are coming out with 50% efficient solar cells in 2010, but even these will require a large area. There was some suggestion that power could be recovered 8000 kW of solar cells will require 8000 to 16000 square meters of solar cells. 10000 square meters would be 3 football fields; this would be arranged as 2 panels about 100 x 50 meters balanced on a horizontal beam on either side of the elevator. The beam and panels would be rotated to keep them facing the sun. If a large circular hoop has to layers of thin plastic sheet stretched across and the space inside is inflated, the the plastic will baloon outward in a parabolic shape. If one sheet was transparent and the other silvered on the inside, then this would form a cheap and light weight concentrator. This would allow a much smaller solar cell array and reduce the weight and cost of the system. Two concentrators at 5000 sqm each would be 80 meters or 260 feet in diameter. See example at http://www.coolearthsolar.com/ Anybody have any idea how much this panel array would weigh? It has to be strong enough to withstand near earth normal gravity.
There are other alternatives, of course. Some like Arthur Clarke suggest that the power consumed by going up can be recovered on the way back down. How? The basic premise that copper cables would be too heavy to be practical is a true statement. 8000 volts at 1000 amps = 8000 kW cable capable of carrying this kind of current would weigh > 3 kg/m and the resistance over several thousand kilometers would be a major loss. The only possible solution along these lines is high strength CNT super conductors; which may actually be possible some day. Maybe power produced on the down trip can be beamed some where. Storing that much power on board is out of the question. Getting rid of excess power on the way down is not going to be a trivial task, and must be accomplished.
There are a number of possibilities for the first 100 km. 1) Run power cables for this lower portion, this means that the electric motors become a seperate module from the solar panels. 2) Drums and a belt would allow the climber to be attached at the bottom, driven up by power applied to the lower drum/pulley and then ship and cargo would be attached to the solar powered climber mechanism. 3) The entire cable could be winched down 50 km, the ship and cargo attached, and winched up 100 km (while the solar climber maintains altitude by moving down the cable), and the cable is winched back to its normal 50 km resting place as the climber accends. Although at first attractive, this turns out to be a bad idea as the strain on the cable would be too much. 4) Laser (or other beam) power. This involves developing a powerful beam about 8000 kW and a reciever aboard the climber. Any beam with this power could cut the cable if an accident were to throw the cable in the path of the beam so safety is a major issue. There are currently contests being sponsered to develope this technology. This beam would easily burn a hole through clouds but may create an ionized path for lightning.
===Cable Construction===
{{:hoyt_weave_2.gif|:hoyt_weave_2.gif}}The cable must be designed to survive radiation, micrometeorites, and in the atmosphere, wind and rain. A popular construction method currently proposed for space tethers (made with kevlar) uses the Hoyt weaving pattern.
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===First 100 km===
This diagram depicts the top of the belt drive option.
{{:spaceelevator_toppulley.gif|:spaceelevator_toppulley.gif}}
This diagram depicts the bottom of the belt drive option.
{{:spaceelevator_beltdrive.gif|:spaceelevator_beltdrive.gif}}
In this option the bottom end of the cable is not solidly anchored, but attached to a series of powered rollers. These, of course would all be very large. I propose that the bottom end of the cable not be solidly anchored, but attached to a belt that goes around a large pulley at the top (100 km up) and the bottom. Actually I suggest that the bottom be a series of powered rollers. They are arranged in a series so there can be a low tension (even zero tension) section in the middle is to facilitate repairs. There are multiple rollers to give a large area for traction, if this is not required then fewer rollers could be used. The mode of operation would be to attach the elevator, then drive the belt, raising the elevator to the upper pulley. When it gets to the upper pulley an arm would hook up the elevator and, driven by the rotating pulley, lift the elevator to the waiting climber and attach to it. The climber then takes the elevator the remaining distance. The return trip would be the reverse of the above procedure.
The whole drive system would be anchored firmly below ground level in a safe area like a small island near the equator in the middle of the ocean. By safe, I mean that there would be minimal damage to property and people if the belt broke. Some have argued that a broken belt would land softly so this may not be a big issue. I do not agree with the concept of working from a boat because the boat would go up and down with waves and tides, applying high strains on the cable. I do not believe that the cable will have any significant amount of stretch, I am well aware that over the whole length of the cable even a small amount of stretch would add up to a lot, but the rate of strain propagation is the speed of sound in the cable so it would take more than a day for any stretching to be applied to the entire length of the cable.
Some people suggest that the portion of the cable exposed to the atmosphere will require the most frequent inspection and repair. With this system inspection and repair, for this portion of the cable, can be done on the ground at low tension.
===Counterweight===
There have been two dominant methods proposed for dealing with the counterweight need: a heavy object, such as a captured asteroid or a [[space station]], positioned past geosynchronous orbit, or extending the cable itself well past geosynchronous 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. The final decision will probably be economic, if the cost of the cable is very high, the counter weight will be the cheaper option.
===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 geosynchronous 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 3rd law.
Meanwhile, the overall effect of the 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 mass always stays above geosynchronous orbit[[http://www.mit.edu:8001/people/gassend/spaceelevator/center-of-mass/index.html]] 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.
===Geosynchronous Space Station===
It is not pratical to have a destination space station attached to the cable at geosynchronous orbit. The cable will move around a few degrees for various reasons. A space station parked within a few degrees away is most desirable. This will make it a short trip for the climbing ship go from the cable to the station. Also a space station that spins to produce artifical gravity can only have ships dock on its axis. If this spun around the cable then the cable would interfere with the docking space ships.
===Launching into outer space===
The velocities that might be attained at the end of Pearson's 144,000 km cable can be determined. The tangential velocity is 10.93 kilometres 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.
===Docking with the Space Elevator===
Interplanetary space ships from say the Moon or Mars can dock with a Space Elevator at the Geostationary Orbit (GEO) Point. Permitting crews, passengers and cargo to use climbers to descend to the Earth's surface. Using the ribbon as a vertical pier means that the space ships do not have to carry heavy Earth re-entry capsules nor risk motors and photovoltaic panels being damaged by the atmosphere. New people, cargo, food, water and fuel can be loaded into the space ships directly from the cable or via possible GEO point space stations. Reusable climber tugs can lift the interplanetary space ships to the launch height for their new destination.
In theory the transfer station can be docked at the counter weight but in practice this is very dangerous. The space ship has to come in a very high speed, has a window of about 1 second a day and if it misses will fly off into space. Rescue may not be possible.
Space craft in Low Earth Orbit (LEO) can attach to and release from the ribbon cable at about 24000 km up. At this point it will take minimal rocket power to change from eliptical to circular orbits. This is called a drop point and can be calculated for each target circular orbit. It would take more energy to dock by flying to GEO Point. Unlike interplanetary space craft LEO space craft will likely use this drop point even if the destination is out to GEO. The space ship will need to carry a mechanism to catch and hold onto the ribbon, or power climber.
===Extraterrestrial elevators===
A space elevator could also be constructed on some of the other planets, asteroids and moons.
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. 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 h 6 min). A collision between the elevator and the 22.2 km diameter moon would have to be avoided through active steering of the elevator, or perhaps by moving the moon itself out of the area. 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. There would be a small payload penalty, but the tether would pass outside the orbital inclination of the two moons. Also, the tether would depart the Martian anchor at 5–10 degrees from vertical.
Conversely, a [[Venus]]ian space elevator would need to be much longer. Although a tether placed at the stationary orbit of the slowly rotating Venus would intersect the Sun, one could be constructed that rotated with the fast-moving cloud decks of the planet which take only four Earth days to make a complete cycle. The cable would need to exceed 100,000 kilometers long but, counter-intuitively, would experience less stress due to the slightly smaller gravity exerted on the cable. Such an elevator could service [[aerostat]]s or [[Floating cities (science fiction)|floating cities]] in the benign regions of the [[atmosphere of Venus|atmosphere]].
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]] (e.g. a [[space station]]) 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]], a physicist and mathematician, has suggested using such smaller systems as power generators at points distant from the Sun where solar power is uneconomical.
It may also be possible to construct space elevators at the three smaller [[gas giant]]s, [[Saturn]], [[Uranus]] and [[Neptune]]. These would all involve tapering several times greater than those of the inner solar system, and would need to be approximately 50–60 thousand kilometers long, yet are still within the limits of advanced nano-tubes.{{Fact|date=April 2007}} These outer space elevators could facilitate the exchange of supplies and [[helium-3]] between floating mining colonies in the atmospheres and local moon settlements. However, difficulties such as the equatorially orbiting lower [[planetary ring|rings]] and moons of these giant planets would first need to be overcome.
==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"{{Fact|date=May 2007}}:
# '''[[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 [[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 force]]s ([[Gravitational force|gravity]] and [[centrifugal force]]) would naturally pull the cables directly towards and directly away from the Earth and keep the elevator balanced around geosynchronous 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]] has presented a plausible scheme showing how a space elevator could be built in little more than a decade, rather than the far future.
He proposes 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. All of these cables would have to be tapered, thicker and stronger at the middle.
First even thinner and lighter, 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 — the proposed average mass is about 200 gram per kilometer. In comparison, conventional [[copper]] telephone wires running to consumer homes weigh about 4 kg/km.
===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 force]]s 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.[{{cite web
|url=http://www.mit.edu/people/gassend/publications/ExponentialTethers.pdf
|title=Exponential Tethers for Accelerated Space Elevator Deployment?
|first=Blaise
|last=Gassend
|format=PDF
|accessdate=2006-03-05}}]
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, 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.
===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. 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 [[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 — 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 a standard [[.223]] [[caliber]] ([[5.56mm]]) 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 in a 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,[http://xxx.lanl.gov/ftp/cond-mat/papers/0601/0601668.pdf][http://www.msm.cam.ac.uk/phase-trans/2005/SWpaper/index.html] 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.
===Sabotage===
Sabotage is a relatively unquantifiable problem. A space elevator might prove an attractive target for a terrorist or other politically motivated attack. Concern over sabotage may have an effect on location, adding the constraint of avoiding unstable territories to the existing requirement of an equatorial site.
===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 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 Earth'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 (the only ones likely to be broken) have very low mass (roughly 1 kg per kilometer) and are 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 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.[{{cite web| url=http://www.mit.edu/people/gassend/spaceelevator/breaks/index.html| title=Animation of a Broken Space Elevator| first=Blaise| last= Gassend| year=2004| accessdate=2007-01-14}}]
====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 — climbers, 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 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 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.[{{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}}][{{cite journal|journal=Acta Astronautica|volume=60|issue=3|date=February 2007|pages=189–209||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}}] 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, of course. 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 then incurred on the way up.
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]], or braking radiation. 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 flares]] and [[coronal mass ejection]] events.
==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 gives prices that are on the order of thousands of [[U.S. dollar]]s per [[kilogram]] for transfer to [[low earth orbit]],and roughly twenty thousand dollars per kilogram for transfer to geosynchronous orbit. Even optimistic rocket proposals (such as the [[DH-1]]) only claim to bring prices down to $200 per kilo. For a space elevator, the price could be on the order of a few hundred dollars per kilogram, or possibly much less. Once the financial costs of construction are paid off, launch costs might mostly consist of electricity to climb to geosynchronous orbit, in which case the cost could drop to dollars per kilo.
Space elevators have high capital cost but low operating expenses, so they make the most economic sense in a situation where it would be used over a long period of time to handle very large amounts of payload. The current launch market may not be large enough to make a compelling case for a space elevator, but a dramatic drop in the price of launching material to orbit would likely result in new types of space activities becoming economically feasible. In this regard they share similarities with other transportation infrastructure projects such as highways or railroads.
Development costs might be roughly equivalent, in modern dollars, to the cost of developing the shuttle system. A question subject to speculation is whether a space elevator would return the investment, or if it would be more beneficial to instead spend the money on developing rocketry further. If the elevator did indeed cost roughly the same as the shuttle program, recovering the development costs would take less than about a hundred thousand tons launched to low earth orbit or five thousand tons launched to geosynchronous orbit.
==Political issues ==
One potential problem with a space elevator would be the issue of ownership and control. Such an elevator would require significant investment (estimates ''start'' at about [[United States dollar|US$]]5 billion for a very primitive tether), and it could take at least a decade to recoup such expenses. At present, few entities 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 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 knowledge of the positions and paths of all existing satellites in Earth orbit and their removal if they cannot adequately avoid the elevator (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. 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, projects on the scale of a space elevator need to be either joint public-private partnership ventures or government ventures, and they involve multiple partners.
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 elevators at various locations around the globe, since subsequent elevators would be significantly cheaper, thus allowing general access to space and consequently eliminating the instabilities a single system might cause.
[[Arthur C. Clarke]] compared the space elevator project to [[Cyrus Field]]'s efforts to build the first [[transatlantic telegraph cable]], "the Apollo Project of its age".[{{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
}}]
==History==
===Early concepts===
The 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. 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 [[1 E7 m|35,790 kilometers]] above mean sea level ([[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.
===Twentieth century===
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 [[geostationary orbit|GEO]].[{{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}}]
Making a cable over 35,000 [[kilometer]]s long is a difficult task. In 1966, four [[United States|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[http://flightprojects.msfc.nasa.gov/fd02_elev.html - 404 error as of 2006-03-05] is based on findings from a space infrastructure conference held at the Marshall Space Flight Center in 1999.
Another American scientist, [[Bradley C. 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, [[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. 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 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.[
{{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
}}
]
Space elevator proponents are planning competitions for space elevator technologies,[{{cite web
|url=http://msnbc.msn.com/id/5792719/
|title=Space elevator contest proposed
|first=Alan
|last=Boyle
|publisher=MSNBC
|accessdate=2006-03-05}}][{{cite web
|url=http://www.elevator2010.org/
|title=The Space Elevator - Elevator:2010
|accessdate=2006-03-05}}] similar to the [[Ansari X Prize]]. [[Elevator:2010]] will organize annual competitions for climbers, ribbons and power-beaming systems. The Robolympics Space Elevator Ribbon Climbing[{{cite web
|url=http://robolympics.net/rules/climbing.shtml
|title=Space Elevator Ribbon Climbing Robot Competition Rules
|accessdate=2006-03-05}}] 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.[{{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}}][{{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}}]
On [[April 27]], [[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."[{{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}}] On [[September 9]] the group announced that they had obtained permission from the [[Federal Aviation Administration]] to use airspace to conduct preliminary tests of its high altitude robotic lifters.[{{cite web | title=Space Elevator Gets FAA Lift | work=Space.com | url=http://www.space.com/astronotes/astronotes.html | accessmonthday=September 19 | accessyear=2005}}] The experiment was successful.
On [[February 13]], [[2006]] the LiftPort Group announced that, earlier the same month, they had tested a mile of 'space elevator tether' (sic) made of carbon-fibre composite strings and fibreglass tape measuring 5 centimetres wide and 1 mm (approx. 6 sheets of paper) thick, lifted with balloons.[{{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}}]
The x-Tech Projects company has also been founded to pursue the prospect of a commercial Space Elevator.
==See also==
{{Portal|Spaceflight}}
* [[Space elevator in fiction]]
* [[Space elevator economics]] discusses capital and maintenance costs of a space elevator.
* [[Lunar space elevator]] for the moon variant
* [[Space fountain]]
* The [[Aresian Well]] is a proposal to use a beanstalk (space elevator) to export water mined from Mars's north polar cap.
==References==
=== Specific ===
{{reflist|2}}
=== General ===
* 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] [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" [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 — [http://www.cqe.northwestern.edu/sk/C62/OrbitalTower_ME362.pdf The Orbital Tower] (PDF)
==External links==
{{Spoken Wikipedia|Space_elevator.ogg|2006-05-29}}
*[http://www.google.com/patents?vid=USPAT6491258 Space elevator - Google Patents]
===Organizations===
*[http://seattlewebcrafters.com/nsecc/ The National Space Society Special Interest Chapter for the Space Elevator (NSECC)]
*[http://www.ing-math.net/ Ing-Math.Net (Germany)] - Ing-Math.Net (German Max-Born Space Elevator Team 2006)
*[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
*[http://www.usst.ca/ University of Saskatchewan Space Design Team]
===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.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] NOVA:Science Now 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]
===Articles===
*[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://www.mit.edu/people/gassend/elevator/ Various thoughts on space elevators posted by Blaise Gassend]
* There have been accusations of corruption in NIAC's award process see: [http://english.pravda.ru/mailbox/22/98/396/14417_NASA.html] and [http://www.inauka.ru/mifs/article51866.html].
*[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]