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Space elevator

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A space elevator is a hypothetical elevator that connects a planet's surface with space. It is also called a geosynchronous orbital tether or a beanstalk (in reference to the fairy tale Jack and the Beanstalk). It is a variety of skyhook.

A space elevator on Earth would permit sending objects and astronauts to space at costs only a fraction of those associated with current means. Constructing one would, however, be a vast project, and the elevator would have to be built of a material that could endure tremendous stress while also being light-weight, cheap, and easy to manufacture. A considerable number of other novel engineering problems would also have to be solved to make a space elevator practical. Today's technology does not meet these requirements, but some optimists hold that the space elevator might become a reality in decades to come.

A conceptual drawing of a space elevator rising from its platform, courtesy of Liftport

History

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

Tsiolkovsky's tower would be able to launch objects into orbit without a rocket. Since the elevator would attain orbital velocity as it rode up the cable, an object released at the tower's top would also have the orbital velocity necessary to remain in geosynchronous orbit.

Building from the ground up, however, proved an impossible task; there was no material in existence with enough compressive strength to support its own weight under such conditions. It took until 1957 for another Russian scientist, Yuri N. Artsutanov, to conceive of a more feasible scheme for building a space tower. Artsutanov suggested using a geosynchronous satellite as the base from which to construct the tower. By using a counterweight, a cable would be lowered from geosynchronous orbit to the surface of Earth while the counterweight was extended from the satellite away from Earth, keeping the center of mass of the cable motionless relative to Earth. Artsutanov published his idea in the Sunday supplement of Komsomolskaya Pravda in 1960.

Making a cable over 35,000 kilometers long is a difficult task. In 1966, four American engineers decided to determine what type of material would be required to build a space elevator, assuming it would be a straight cable with no variations in its cross section. They found that the strength required would be twice that of any existing material including graphite, quartz and diamond.

In 1975 another American scientist, Jerome Pearson, designed a tapered cross section that would be better suited to building the tower. The completed cable would be thickest at its center of mass, where the tension was greatest, and would be narrowest at the tips to reduce the amount of weight that the middle would have to bear. He suggested using a counterweight that would be slowly extended out to 144,000 kilometers (almost half the distance to the Moon) as the lower section of the tower was built. Without a large counterweight, the upper portion of the tower would have to be longer than the lower due to the way gravitational and centrifugal forces change with distance from Earth. His analysis included disturbances such as the gravitation of the Moon, wind and moving payloads up and down the cable. The weight of the material needed to build the tower would have required thousands of Space Shuttle trips, although part of the material could be transported up the tower when a minimum strength strand reached the ground or be manufactured in space from asteroidal or lunar ore.

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

David Smitherman of NASA/Marshall's Advanced Projects Office has compiled plans for such an elevator that could turn science fiction into reality. His publication, "Space Elevators: An Advanced Earth-Space Infrastructure for the New Millennium" [1], is based on findings from a space infrastructure conference held at the Marshall Space Flight Center in 1999.

Another American scientist, Bradley Edwards, suggests creating a 100,000 km long paper-thin ribbon, which would stand a bigger chance of surviving impacts by meteors. The work of Edwards has expanded to cover: the deployment scenario, climber design, power delivery system, orbital debris avoidance, anchor system, surviving atomic oxygen, avoiding lightning and hurricanes by locating the anchor in the western equatorial pacific, construction costs, construction schedule and environmental hazards. Plans are currently being made to complete engineering developments, material development and begin construction of the first elevator. Funding to date has been through a grant from NASA Institute for Advanced Concepts. Future funding is sought through NASA, the United States Department of Defense, private and public sources. The largest holdup to Edwards' proposed design is the technological limits of the tether material. His calculations call for a fiber composed of epoxy-bonded carbon nanotubes with a minimal tensile strength of 130 GPa; however, tests in 2000 of individual single-walled carbon nanotubes (SWCNTs), which should be notably stronger than an epoxy-bonded rope, indicated the strongest measured as 63 GPa [2].

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 40% of Earth's, while it rotates around its axis in about the same time as Earth. Because of this, Martian geostationary orbit is much closer to the surface, and hence the elevator would be much shorter.

A Lunar 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 distance between the Earth and the Moon is 351,000 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.

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 (against the direction of 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 on Earth's surface, very slightly slowing it. The opposite process occurs for payloads descending the elevator, very slightly increasing Earth's rotation.

We can determine the orbital 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.

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

Components

Space elevators can require any number of components, depending on the design. Among those found in almost every design are a base station, a cable, climbers, and a counterweight.

Base station

A conceptual drawing of the ground "anchor station" of a space elevator, courtesy of Liftport

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).

Cable

The cable must be made of a material with extremely high tensile strength (the limit to which a material can be stretched without irreversibly deforming). A space elevator can be made relatively economically if a cable with a tensile strength of over 100 GPa can be produced in bulk at a reasonable price; below 50 to 60 GPa, the cost becomes astronomical and untenable. 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 that may approach 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 100GPa, 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. [3]. 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, 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 (March 2004).

Climbers

A conceptual drawing of a climber traveling through the clouds, courtesy of Liftport

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.

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).

Economics

With space elevators like this materials could be sent into orbit at a fraction of the current cost. Costs to geostationary orbit are from between $10,000 and $40,000 per kg today[4]. This does not repay capital costs, such as research and development of launch systems (turnaround and replacement costs for reusable systems, and construction costs for disposable systems). For a space elevator, using the equivalent accounting, the cost varies according to the design. Using the design specs developed by Dr. Bradley Edwards, "The first space elevator would reduce lift costs immediately to $100 per pound" ($220/kg) [5]. Development costs might be roughly equivalent, in modern dollars, to the cost of developing the shuttle system.

The marginal cost of a trip would consist solely of the electricity required to lift the elevator payload, maintenance, and in one-way designs (such as Edwards'), the cost of the elevators. The base cost of payload transfer, given the current power grid rate of about 10 cents per kWh, is around $1.74 per kg. To this, you factor in the requisite transmission losses (discussed later in more detail). Assuming current laser efficiencies of 2%, solar cell efficiencies of 30%, and atmospheric losses of about 20%, this works out to about 0.5% overall efficiency, or $350/kg. In Edwards' design, he assumes 2% energy efficiency based on the predicted efficiencies of lasers and solar panels currently under development, and the rest of his calculated cost per kilogram is due to the need for the climbers to be disposable.

It may additionally be possible to recover some of the $1.74 energy transferred to each lifted kilogram by using descending elevators to generate electricity as they brake (suggested in some proposals), or generated by masses braking as they travel outward from geosynchronous orbit (a suggestion by Freeman Dyson in a private communication to Russell Johnston in the 1980s).

While it is hard to define the lower limit on rocket technologies precisely, a few serious proposals are being seriously considered that could cost just a few thousand dollars per kilogram, but payload lift costs have so far remained almost unchanged since 1960 [6]. Other non-rocket technologies have also been proposed that offer more encouraging results for low-cost payload launch (see spacecraft propulsion), although few have even a theoretical lift cost as low as that of a space elevator.

For the space elevator, the efficiency of power transfer is often a limiting issue. In most designs, the concept of a superconducting cable for transferring power - even if incredibly light - will add hundreds of tons of weight to the cable, easily breaking it. Consequently, power beaming is often viewed as the only efficient mechanism for power transfer. The most efficient power beaming in the present-day is a laser beaming system with photovoltaic panels on the climber optimized to the wavelength of the laser. With the best (and most expensive) current technology that could be used, the efficiency is around 0.5%. If climbers are to be disposable, the most expensive photoelectric panels may not be an option.

Losses due to atmospheric spreading can be reduced by the use of adaptive optics, and losses due to absorption can be reduced by a properly chosen laser wavelength. But although laser and photovoltaic technologies have been rapidly advancing, it is unknown how far beyond the current transmission efficiency can be achieved. Further optimization of photovoltaics, for example, typically rely on enhancing the absorption of particular wavelengths, which may not match up with the wavelengths of more efficient lasers. The most efficient lasers - diode laser arrays, which can surpass 50% efficiency - currently have poor coherence, and could not be used, leaving as available options standard chemical lasers with efficiencies of a few percent or less. Only with the advent of high-coherence diode laser arrays or a similar technology could a space elevator achieve good energy efficiency in its power usage.

The cost of the power provided to the laser is also a limiting issue. While a land-based anchor point in most places can use power at the grid rate, this is not an option for a mobile oceangoing platform.

Finally, up-only climber designs must replace each climber in its entirety or carry up enough fuel to get it out of orbit - a potentially costly venture.

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 feasable. In this regard they share similarities with other transportation infrastructure projects such as highways or railroads.

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. Over a long period of time all satellites with perigees below geostationary altitude would eventually collide with the space elevator, as their orbits precess around Earth. Most active satellites are capable of some degree of orbital maneuvering and could avoid these 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.

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, elemental 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.

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.

A final risk of structural failure comes from the possibility of vibrational harmonics within the cable. Like the shorter and more familiar strings of stringed instruments, the cable of a space elevator has a natural resonance frequency. If the cable is excited at this frequency, for example by the travel of elevators up and down it, the vibrational energy could build up to dangerous levels and exceed the cable's tensile strength. This can be avoided by the use of intelligent damping systems within the cable, and by scheduling travel up and down the cable keeping its resonant frequency in mind. It may be possible to do damping against Earth's magnetosphere, which would additionally generate electricity that could be passed to the climbers. Oscillations can be either linear or rotational.

In the event of failure

If despite all these precautions the elevator is severed anyway, the resulting scenario depends on where exactly the break occurred. 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 centripetal 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 any 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.

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 or even minutes, 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.

Political will

One of the potential problems with a space elevator would be one of who would own/control it? 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 put into creating such a device, there would then be the problems of who would use it and how often, as well as 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, so a space elevator could potentially cause numerous rifts between states as to who may and may not deploy military satellites via it. 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.

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 whatever monopoly they may have gained on space access. However, once the technologies are in place, there is nothing short of an international ban backed up by serious consequences that would stop other nations or companies from developing their own elevators in the same method that the original nation or company built theirs.

As space elevators (regardless of the design) are inherently highly fragile but militarily valuable structures, they would likely be targeted immediately in any major conflict with a state that controls one. Consequently, conventional rockets (or other similar launch technologies) would most likely continue to be utilized by militaries to provide backup methods to access space.

Other elevator and tether systems

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, unlike a space elevator, it can be of any height and placed at any latitude. Also unlike space elevators, space fountains require a continuous supply of power to remain aloft.

Still smaller-scale tether propulsion is a possible method for launching spacecraft into planetary orbit.

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"[7].

Fiction

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

Animation

Books

  • Edwards BC, Westling EA. The Space Elevator: A Revolutionary Earth-to-Space Transportation System. San Francisco, USA: Spageo Inc.; 2002. ISBN 0972604502.
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