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 Beanstalk (after the fairy tale Jack and the Beanstalk in which Jack climbs a magical beanstalk that has grown into the clouds), a Skyhook, or a geosynchronous orbital tether.
The concept of the space elevator originated 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 to an altitude of 35,800 kilometers. It would be similar to the fabled beanstalk in the children's story "Jack and the Beanstalk," except that on Tsiolkovsky's tower an elevator would ride up the cable to the "castle".
One "spinoff" use of Tsiolkovsky's tower would be the ability 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 near 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, the 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 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 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 110,000 kilometers (half the distance to the moon) as the lower section of the tower was built. The upper portion of the tower would 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 24,000 Space Shuttle trips, although part of the material could be transported up the tower when a minimum strength strand reached the ground or manufactured in space from asteroidal or lunar ore.
Later, Pearson thought about building a tower on the Moon. He determined that the center of gravity needed to be at the L1 or L2 Lagrangian points, which are special stable points that exist about any two orbiting bodies where the gravitational forces are balanced. The cable would have to be 291,901 kilometers long for the L1 point and 525,724 kilometers long for the L2 point. Compared to the 351,000 kilometers from the Earth to the Moon, that's a long cable, and the material would have to be gathered and manufactured on the Moon. However, due to the lower gravity of the Moon, the total mass of the cable would be less since less material would be needed to provide strength.
We can determine the orbital velocities that might be attained at the end of Pearson's 144,000 km tower. At the end of the tower, the tangential velocity is 10.93 kilometers per second which is high 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.
Arthur C. Clarke introduced the concept 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 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", is based on findings from a space infrastructure conference held at the Marshall Space Flight Center in 1999. It can be found at:
http://flightprojects.msfc.nasa.gov/fd02_elev.html
The desired tensile strength for the space elevator is about 62 GPa. 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.
A space elevator would be much easier to construct on Mars due to its lower gravity. A space elevator for the Moon could theoretically be constructed with kevlar using current technologies.
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. This means that hospitals, mining facilities, international trade, and travel could all be done in space with the help of these space elevators.
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.