Trading Rockets for Space Elevators
for National Geographic News
|August 25, 2005|
Blasting a space shuttle away from Earth's gravity and through atmospheric friction at 15,000 miles an hour (24,140 kilometers an hour) is the most dangerous and costly part of every mission.
Why not just take an elevator instead? Thanks to a new development in the manufacture of molecule-size cylinders known as carbon nanotubes, that may one day be a viable option.
In theory, space elevators need a fixed line, or cord, that stretches from an anchor on Earth to a station out in space. The station acts like a counterweight, forever "held" above the planet by the centrifugal force from Earth's rotation.
A tram-like vehicle equipped with electric motors could climb this tether from Earth's surface into space at a safer speed than rocket alternatives.
In theory, space elevators would need far less energy than conventional space launches. As a result, the cost of transporting matter could drop from U.S. $20,000 a kilogram (the going rate for the space shuttle) to as little as $250 a kilogram.
"This is a trillion-dollar moneymaker for a ten billion dollar investment," said Bradley Edwards, whose work with Los Alamos National Laboratory and the NASA Institute for Advanced Concepts has made him a go-to expert on space elevators. "Some of the largest companies in the world are just waiting for the word that this is possible."
The word on cheap space freight rates won't be sounded until at least one major obstacle is overcome: developing a strong enough material for the tether. Scientists believe they found the fibers for that materialcarbon nanotubesin 1991.
The tubes are molecules of carbon linked together in a shape that resembles a Chinese finger trap. Carbon is among the stickiest elements, and these tubes hold together hundreds of times better than Kevlar, the material used to make bullet-proof vests.
In theory, a space elevator tether made of carbon nanotubes would look and feel like a span of three-foot-wide (one-meter-wide) plastic wrap that stretched skyward as far as the eye could see. It could be rolled up and simply dropped to the ground from space-based orbit.
The problem with this scenario is that individual carbon nanotubes are only millimeters tall and nanometers wide. (A nanometer is a billionth of a meter.) No one has successfully woven the tubes together in a way to make sheets that are as strong as their individual fibers.
But Ray Baughman, who directs the Nanotech Institute at the University of Texas at Dallas, and his team have developed what could be a first step.
The scientists have produced yards of woven, pure nanotubes in sheets two inches (five centimeters) wide. The method is easily scalable to produce sheets of any dimension and can roll them out as fast as 33 feet (7 meters) a minute.
The technique, which is described in the August 19 issue of the journal Science, begins by building nanotubes. Gaseous carbon atoms are coaxed to deposit themselves onto a specially prepared surface in an oven. The surface "catches" the atoms in such a way that they cling to each other and form into tubes.
Baughman likens the result to a tiny bamboo forest. The researcher catches the row of tubes from the "forest edge" with an adhesive strip. As the first row is pulled away, the second row clings to its neighbor and is also pulled away. Eventually, enough of a sheet is pulled away that it can be reeled up with a plastic roller.
But for now, the resulting "fabric" isn't nearly as strong as Kevlar.
Other scientists have experimented with producing nanotube-based materials by embedding the tubes in polymerslarge, chain-like molecules made from repeating links of smaller molecules. Rodney Andrews from the University of Kentucky has produced fibers this way that are five times stronger than Baughman's.
"What's good about Ray's [product] is that it's made entirely out of nanotubes," Edwards said. "And the final material that will make up the [space elevator] tether will probably have to be at least 50 percent nanotubes."
Making the new carbon nanotube sheets stronger is a matter of finding better pulling methods and more advanced surfaces. Still, research won't produce a viable tether for some time, probably a few decades, Baughman said.
Edwards's own company, Carbon Designs Inc., also develops nanotube-based materials. He and other experts are slightly more optimistic, putting the estimated time of arrival of a space elevator tether at less than 20 years.
So don't pack your bags for the first space elevator trip anytime soon. But in the near future you might see carbon nanotubes coming to more commonplace products.
Carbon nanotubes have other interesting properties besides their strength, and Baughman has already demonstrated that their conductivity can make his sheets glow, providing whole surfaces that give light.
He's also shown that a sheet placed between pieces of glass will heat up when electrified, and may make for the perfect in-glass radio antenna.
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