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Space elevator

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A space elevator is, in simplest terms, an elevator that rises above a planet's atmosphere and into space. It is also sometimes called a geosynchronous orbital tether or a Beanstalk (after the fairy tale Jack and the Beanstalk in which Jack climbs a magical beanstalk which has grown into the clouds). It is one of the varieties of a skyhook.


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). It would be similar to the fabled beanstalk in the children's story "Jack and the Beanstalk," except that on Tsiolkovsky's tower an electromagnetically driven elevator would ride up the cable to the "castle". Comments from Nikola Tesla are suggestive 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 the able to launch objects into orbit without a rocket. Since the elevator would attain orbit 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 anywhere 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 build 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 (Young Communist 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 tower, 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 narrow to its thinnest 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 equatorial island of Taprobane (closely based on 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.

Yet another American scientist, Bradley Edwards, suggests creating a 100,000 km long paperthin ribbon, which would stand a bigger chance of surviving impacts by meteors. The work of Edwards through Eureka Scientific [2] and HighLift Systems [3] 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, DoD, private and public sources.


Extraterrestrial elevators

A space elevator could also be constructed on the other planets, asteroids and moons.

A Martian tether could be much shorter than one on Earth. Mars' gravity is 30% (approximately 1/3) 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 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 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 Lagrange 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

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. For higher velocities, the cargo can be electromagnetically accelerated, or the cable could be extended, although that might necessitate counterweights below geosynchronus orbit in order to maintain the structure's center of gravity at geosynchronus 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 (eg. carbon nanotube and nanotechnology) and tower
  2. Tether deployment and control
  3. Tall tower construction
  4. Electromagnetic propulsion (eg. maglev)
  5. Space infrastructure and the development of space industry and economy


Materials

Carbon nanotubes have exceeded all other materials and appear to have a theoretical strength far above 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.

With Space Elevators like this, Humans can send materials into orbit at a fraction of the current cost (from around $30000 today to $3 per kg, a factor of 104!); the marginal cost of a trip would consist solely of the electricity required to lift the elevator payload, some of which could be recovered by using descending elevators to generate electricity as they brake, 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.) This means that hospitals, mining facilities, international trade, and travel could all be done in space with the help of these space elevators.


Other tethersystems

Another type of space elevator that doesn't 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 lattitude. Also unlike space elevators, space fountains require a continuous supply of power to remain aloft.

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


Cultural depictions



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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