2 Key technologies

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

3 Components

There are a variety of space elevator designs. Almost every design includes a base station, a cable, climbers, and a counterweight.

3.1 Base station

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

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).

3.2 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 1GPa, and the strongest steels no more than 5GPa. Kevlar has a tensile strength of 2.6-4.1 GPa, while quartz fiber can reach upwards of 20GPa; 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 120GPa, in practice the highest tensile strength ever observed in a single-walled tube is 63GPa, and such tubes averaged breaking between 30 and 50GPa. 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.

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. [6]. 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 centimetres 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 at present (March 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.

3.3 Climbers

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 return payloads/people that would 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.

3.4 Counterweight

There have been two dominant methods proposed for dealing with the counterweight need: a heavy object, such as a captured asteroid, positioned shortly 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).

4 Building an elevator

Two different ways to deploy a Space Elevator have been proposed.

4.1 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 whilst 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.

4.2 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 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 tons for a seed cable may sound a lot, it would actually be incredibly lightweight—the proposed average mass is about 0.2 kilogram per kilometer. (The pair of copper telephone wires running to your house weigh about 4 kg/km). Twenty tons is slightly less than a Russian geosynchronous communication satellite.

5 Prizes

Space elevator proponents are planning competitions for space elevator technologies [7], 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 [8] organizes climber-robot building competitions.

6 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.

7 Failure modes

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 spacecraft are a more difficult problem.

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 synchronised 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.

Meteoroids present a more difficult problem, since they would not be predictable and much less time would be available to detect and track them approaching 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.

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 aluminium are damaged very slowly and could be repaired as needed.

In the atmosphere, the risk factors of wind and lightning come into play. There are few good solutions to either of these problems, apart from avoiding storms (as has been suggested in some designs, to be done via 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 is a relatively unquantifiable problem. Elevators are probably less susceptible than suspension bridges carrying mass vehicular traffic, of which there are many worldwide, none of which have been destroyed. Nonetheless there are few more spectacular possible targets. This 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.

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.

7.1 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. 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 mass 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.

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 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.

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.

8 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 may be fatal 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. Also, a thicker elevator cable could carry passenger cars with heavy lead or other shielding.

9 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.

A lunar space elevator would not be so lucky. Since the Moon's rotation keeps the same face towards the Earth, the center of gravity of the elevator would need to be at the L₁ or L₂ Lagrangian points, which are special stable points that exist about any two orbiting bodies where the gravitational and rotational forces are balanced. The cable would point either to Earth for the L₁ point, or face away from Earth for the L₂ point. However, due to the lower gravity of the Moon, the total mass of a Lunar cable could be dramatically less than the mass of an Earth-based elevator, since less material would be needed in order to provide the necessary tensile strength to support itself against lunar gravity. Without a counterweight the 'L₁'-cable would have to be 291,901 kilometers long and the 'L₂'-cable would have to be 525,724 kilometers long. Considering that the average distance between the Earth and the Moon is 384,400 kilometers, that's a long cable. Far shorter cables, perhaps not more than twice the length of the ~60,000 km distance to the L₁ or L₂ points of the Earth-Moon system would suffice if a large counterweight, e.g. of lunar-derived materials, were placed at the end of the cable.

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.

10 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 tangental 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 might necessitate counterweights below geosynchronous orbit in order to maintain the structure's center of gravity at geosynchronous orbit, and would require additional strength in the cable.

11 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 $5,000,000,000 (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 that sort of money in the space industry.

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 defence 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.

Whilst there may be few ordinary citizens who might profit from Space Elevator applications unlike space agencies, commercial companies and the scientific community, it is highly likely that the general public will ultimately benefit from it through cheap solar power and a greener environment, enhanced satellite navigation and communication services, reduced risk from nuclear waste, and even through improved health, education and social services made possible because of the savings made by governments in accessing space.

12 Other types of elevator

Another type of space elevator that does not rely on materials with high tensile strength for support is the space fountain, a tower supported by interacting with a high-velocity stream of magnetic particles accelerated up and down through it by mass drivers. Since a space fountain is not in orbit, a space fountain can be of any height and placed at any latitude. Unlike space elevators, space fountains require a continuous supply of power to remain aloft.

13 Historical analogies

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"[9].

14 Fiction

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

• 2300 AD, role-playing game by Game Designers' Workshop
• Assassin Gambit , novel by William Forstchen
• Friday, novel by Robert A. Heinlein
• Gunnm, manga by Yukito Kishiro
• Hothouse , novel by Brian Aldiss
• Jack and the Beanstalk, fairy tale
• Jumping Off the Planet , novel by David Gerrold
• Mystery Science Theater 3000, television series "The Umbilicus"
• episode "Rise"
• Strata, one Terry Pratchett's two solely science fiction novels
• The End of the Empire , novel by Alexis A. Gilliland
• The Fountains of Paradise, novel by Arthur C. Clarke
• The Mars trilogy of novels by Kim Stanley Robinson
• The Web Between the Worlds , novel by Charles Sheffield
• Zavtra Nastupit Vechnost , novel by russian sci-fi writer Alexander Gromov

15 See also

• Space elevator economics discusses capital and maintenance costs of a space elevator.
• A space elevator is a type of skyhook.
• Skyhooks are a type of tether propulsion.
• Tether propulsion is a type of spacecraft propulsion.
• The space elevator passes through the Van Allen radiation belt.
• The space elevator is a geosynchronous satellite. That means it is in geostationary orbit.

16 Links and references

16.1 Animation

• View space elevator animation Windows Media Video (WMV) file - Institute for Scientific Research
• Download space elevator animation Windows Media Video (WMV) file - Institute for Scientific Research

16.2 Books

• Edwards BC, Westling EA. The Space Elevator: A Revolutionary Earth-to-Space Transportation System. San Francisco, USA: Spageo Inc.; 2002. BooksEnthsiast.com.

16.3 External links

• [10] The National Space Society Chapter for Space Elevator (NSECC).
• [11] The National Space Society Chapter for Space Elevator (NSECC) yahoo group.
• The Space Elevator: 3rd Annual International Conference June 28-30, 2004 in Washington, D.C.
• Space elevator, Institute for Scientific Research
• LiftPort Group - The Space Elevator Companies founded by Michael Laine
• Elevator:2010 Space elevator prize competitions
• The Space Elevator Reference
• The Space Elevator Reference - Books, Documents and References
• LiftWatch.org - Space Elevator News
• 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 Space Elevator Comes Closer to Reality. An overview by Leonard David of space.com, published 27 March 2002.
• Yahoo! Groups: The Space Elevator. A mailing list for space elevator discussion.
• http://www.spaceelevatorstore.com/
• http://www.robotstore.com/search.asp?keyword=space+elevator
• To the Moon in a Space Elevator? (February 4, 2003 Wired News)
• Liftoff (teenage education): Space Towers
• Audacious & Outrageous: Space Elevators
• Ziemelis K. "Going up". In New Scientist[12] 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 Political Economy of Very Large Space Projects" HTML PDF, John Hickman , Ph. D. Journal of Evolution and Technology Vol. 4 - November 1999.
• The Space Elevator NIAC report by Dr. Bradley C. Edwards
• http://www.cqe.nwu.edu/sk/C62/OrbitalTower_ME362.pdf
• Brief video (realmedia format) of the space elevator concept
• Various thoughts on space elevators posted by Blaise Gassend
• The major Russian site about Space Elevator, by Y. Artsutanov and D. Tarabanov

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