by Steve Price

"Yes, ladies and gentlemen, welcome aboard NASA's Millennium-Two Space Elevator. Your first stop will be the Lunar-level platform before we continue on to the New Frontier Space Colony development. The entire ride will take about 5 hours, so sit back and enjoy the trip. As we rise, be sure to watch outside the window as the curvature of the Earth becomes visible and the sky changes from deep blue to black, truly one of the most breathtaking views you will ever see!"

Does this sound like the Sci-Fi Channel or a chapter out of Arthur C. Clarke's, Fountains of Paradise? Well, it's not. It is a real possibility - a "space elevator" - that researchers are considering today as a far-out space transportation system for the next century.

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 last year. The workshop included scientists and engineers from government and industry representing various fields such as structures, space tethers, materials, and Earth/space environments.

"This is no longer science fiction," said Smitherman. "We came out of the workshop saying, 'We may very well be able to do this.'"

A space elevator is essentially a long cable extending from our planet's surface into space with its center of mass at geostationary Earth orbit (GEO), 35,786 km in altitude. Electromagnetic vehicles traveling along the cable could serve as a mass transportation system for moving people, payloads, and power between Earth and space.

Current plans call for a base tower approximately 50 km tall - the cable would be tethered to the top. To keep the cable structure from tumbling to Earth, it would be attached to a large counterbalance mass beyond geostationary orbit, perhaps an asteroid moved into place for that purpose.

"The system requires the center of mass be in geostationary orbit," said Smitherman. "The cable is basically in orbit around the Earth."

Four to six "elevator tracks" would extend up the sides of the tower and cable structure going to platforms at different levels. These tracks would allow electromagnetic vehicles to travel at speeds reaching thousands of kilometers-per-hour.

Conceptual designs place the tower construction at an equatorial site. The extreme height of the lower tower section makes it vulnerable to high winds. An equatorial location is ideal for a tower of such enormous height because the area is practically devoid of hurricanes and tornadoes and it aligns properly with geostationary orbits (which are directly overhead).

As early as 1895, a Russian scientist named Konstantin Tsiolkovsky looked at the Eiffel Tower in Paris and thought about such a tower. He wanted to put a "celestial castle" at the end of a spindle shaped cable, with the "castle" orbiting the earth in a geosynchronous orbit (i.e. the castle would remain over the same spot on the earth). 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". Another Russian, a Leningrad engineer by the name of Yuri Artsutanov, wrote some of the first modern ideas about space elevators in 1960. Published as a non-technical story in Pravda, his story never caught the attention of the West. Science magazine ran a short article in 1966 by John Isaacs, an American oceanographer, about a pair of whisker-thin wires extending to a geostationary satellite. The article ran basically unnoticed. The concept finally came to the attention of the space flight engineering community through a technical paper written in 1975 by Jerome Pearson of the Air Force Research Laboratory. This paper was the inspiration for Clarke's novel.

Building from the ground up, however, proved an impossible task. It took until 1960 for another Russian scientist, Y.N. Artsutanov, to propose another 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 the earth while the counterweight was extended from the satellite away from the earth.

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.

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.

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 to Saturn or Mars. If an object were allowed to slide freely along the upper part of the tower a velocity high enough to escape the solar system would be attained.

The space tower is a good concept for science fiction writers, but until new materials are developed and cheaper ways to reach orbit are found, the space tower will remain a dream.

The workshop's findings determined the energy required to move a payload by space elevator from the ground to geostationary orbit could remain relatively low. Using today's energy costs, researchers figured a 12,000-kg Space Shuttle payload would cost no more than $17,700 for an elevator trip to GEO. A passenger with baggage at 150 kg might cost only $222! "Compare that to today's cost of around $10,000 per pound ($22,000 per kg)," said Smitherman. "Potentially, we're talking about just a few dollars per kg with the elevator."

During the workshop, issues pertinent to transforming the concept from science fiction to reality were discussed in detail. "What the workshop found was there are real materials in laboratories today that may be strong enough to construct this type of system," said Smitherman.

Smitherman listed five primary technology thrusts as critical to the development of the elevator. First was the development of high-strength materials for both the cables (tethers) and the tower.

In a 1998 report, NASA applications of molecular nanotechnology, researchers noted that "maximum stress [on a space elevator cable] is at geosynchronous altitude so the cable must be thickest there and taper exponentially as it approaches Earth. Any potential material may be characterized by the taper factor - the ratio between the cable's radius at geosynchronous altitude and at the Earth's surface. For steel the taper factor is tens of thousands - clearly impossible. For diamond, the taper factor is 21.9 including a safety factor. Diamond is, however, brittle. Carbon nanotubes have a strength in tension similar to diamond, but bundles of these nanometer-scale radius tubes shouldn't propagate cracks nearly as well as the diamond tetrahedral lattice."

Fiber materials such as graphite, alumina, and quartz have exhibited tensile strengths greater than 20 GPa (Giga-Pascals, a unit of measurement for tensile strength) during laboratory testing for cable tethers. The desired 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. "The development of carbon nanotubes shows real promise," said Smitherman. "They're lightweight materials that are 100 times stronger than steel."

The second technology thrust was the continuation of tether technology development to gain experience in the deployment and control of such long structures in space.

Third was the introduction of lightweight, composite structural materials to the general construction industry for the development of taller towers and buildings. "Buildings and towers can be constructed many kilometers high today using conventional construction materials and methods," said Smitherman. "There simply has not been a demonstrated need to do this that justifies the expense." Better materials may reduce the costs and make larger structures economical.

Fourth was the development of high-speed, electromagnetic propulsion for mass-transportation systems, launch systems, launch assist systems and high-velocity launch rails. These are, basically, higher speed versions of the trams now used at airports to carry passengers between terminals. They would float above the track, propelled by magnets, using no moving parts. This feature would allow the space elevator to attain high vehicle speeds without the wear and tear that wheeled vehicles would put on the structure.

Fifth was the development of transportation, utility and facility infrastructures to support space construction and industrial development from Earth out to GEO. The high cost of constructing a space elevator can only be justified by high usage, by both passengers and payload, tourists and space dwellers.

During a speech he once gave, someone in the audience asked Arthur C. Clarke when the space elevator would become a reality.

"Clarke answered, 'Probably about 50 years after everybody quits laughing,'" related Pearson. "He's got a point. Once you stop dismissing something as unattainable, then you start working on its development. This is exciting!"