The Walker1812 Files

Going Up: Self-assembling nanotubes bring the impossible dream of space elevators down to Earth.

BY JIM WILSON
Illustrations by Paul DiMare

Magnetically levitated elevator cars that never touch the tether would cruise at 1200 mph as they carry people and cargo to and from the zero-gravity lab. Energy recovered from cars heading down would help power those going up.

In the not-too-distant future, riding into space in a rocket may seem as impractical as crossing the continent in a Conestoga wagon. Rockets will still be used for the long trips to the outer planets and stars, but for short hops--jobs in zero-gravity laboratories and vacations on the moon--we will simply take the elevator. The first stop will be a massive zero-gravity platform. The "roof" will be a captured asteroid that anchors the tower in place. You'll have to hang on tight as you near the end of the ride. If you don't, the same laws of physics that carried David's rock into Goliath's head will propel you as far as Saturn.

It is a historically apt analogy. "The idea of building a tower into space dates back to some of the earliest known manuscripts," says David Smitherman. NASA's point man for advanced spaceflight concepts, he reminds us that the original space tower, better known as the Tower of Babel, was already an old story when told in the Bible's book of Genesis. "It has been dreamed, invented and reinvented many times throughout modern civilization." Two decades before Arthur C. Clarke's 1979 novel, The Fountains Of Paradise, introduced the space elevator to the English-speaking world, Russian engineer Yuri Artsutanov described it in a Sunday supplement article in the old Soviet newspaper Pravda.

The idea remained suspended between science fact and science fiction until three years ago. A group of researchers from several top engineering schools and aerospace companies, along with NASA scientists, met at the Marshall Space Flight Center in Huntsville, Ala., to consider the technological hurdles to building a space elevator. Their conclusion: "In the later part of the 21st century it has the potential to provide mass transportation to space in the same way highways, railroads, powerlines and pipelines provide mass transportation across the Earth's surface." Since that 1999 conference, improvements in materials have occurred with rocket-sled speed. Some readers of this article may live to follow POPULAR MECHANICS's coverage of the elevator's construction and the report on our first ride.

An asteroid whose position could be slightly adjusted would anchor the zero-gravity station, which would be in a geostationary Earth orbit. Use of a tall tower lightens the overall weight of the space elevator. A materials-transfer pipe (inset) would be surrounded by rails for passenger, cargo and service cars.

"Theoretically, you could build a tower to GEO [geostationary Earth orbit] out of bubble gum, but the base would probably cover half the sphere of the Earth," says Smitherman. A practical choice of material for the elevator is a major difficulty because of the immense distances involved--22,236 miles to the zero-gravity platform in GEO, and 29,204 miles to the asteroid that would anchor the entire structure in place. Dani Eder of Boeing did a material comparison for the Huntsville conference and found that a column made of structural steel would collapse under its own weight when it reached a height of about 3 miles. The maximum height climbs to 9 miles for aluminum, 71 miles for a carbon/epoxy composite and about 76 miles for boron/epoxy. More exotic high-strength composite materials can soar higher still, but they all top out a long way from GEO.

One of the fundamental and frustrating problems of designing with high-strength materials is that they are usually stronger in tension than in compression, Smitherman explains. "The strongest tensile material readily available today is Polybenzbisoxazole (PBO) graphite epoxy, a high-strength polyaramid fiber capable of supporting its own length up to 114 miles," he says. "What is needed is a way to convert tensile strength into compressive strength."

Pressurized Towers
Geoffrey Landis of NASA's Glenn Research Center in Cleveland suggests that one solution would be to pressurize structural members as if they were balloons, thus converting the tower from a compression structure to a tension structure. He calculates that a pressurized structure made of PBO fiber could rise to the astounding height of 1864 miles.

Impressive as this design might seem, a pressurized structure would be subject to the same weakness of all tall towers: buckling. "So, the ideal structure will likely be a combination of a tall tower in compression connected to a tension structure," predicts Smitherman. And this is where science fact dovetails with science fiction.

Futurists and novelists traditionally envision the space elevator as moving along a slender, high-strength tether stretched between the Earth and an orbiting asteroid. Engineers realize that the tethers would need to taper to support their weight. The filament that would connect to a point on Earth might be so narrow that it could easily slip between your fingers. But, its diameter as it approaches the zero-gravity platform at GEO could measure yards or even miles, depending upon the strength of the material. "Finding the right material in combination with the right construction method is the key to success," says Smitherman.

NASA engineers currently favor a strategy in which the tether connects to the top of a 15-mile-high tower. Nothing even approaching this height has ever been built. Yet, there is confidence that using existing composites and robot-building equipment, constructing a tower of this height is a doable, albeit difficult task. "We came out of the workshop saying, 'We may very well be able to do this,'" Smitherman says.

The Right Material
At the time of the Huntsville conference, the greatest unknown was whether a promising new material made of tubes constructed of sheets of hexagonally arranged carbon atoms would ever be more than a laboratory curiosity. On a weight basis, these nanotubes would be 100 times stronger than steel. This exceeded by a factor of two the tensile strength NASA estimated would be necessary for the tether. There were, however, problems of scaling up production. Nanotubes had been produced only under carefully controlled laboratory conditions, and had been grown to the length of just a few molecules. And, no one was certain individual tubes could be connected. Since then, nanotechnology researchers at Harvard, Purdue and Rice universities, to name a few, have found ways of not only making nanotubes in large quantities, but of teaching them to "self-assemble" into larger structures. What is more, they have found that it might not be necessary to "grow" a continuous tether. Instead, they may be able to place short segments of high-strength nanotubes in a matrix of high-purity fiberglass.

While answers to questions about materials and construction methods are years from being resolved, Rob Suggs, another NASA researcher in Huntsville, has located possible building sites. The two current frontrunners are the Indian Ocean island of Gan in the Maldives chain southwest of India, and the Galápagos Islands in the Pacific. Both locations offer stable geology, mild winds, and calm, hurricane-free, weather.

The surrounding ocean also offers a "safety" feature. Although the tether would have sufficient flexibility to be moved out of the path of meteoroids and space debris, a catastrophic failure would result in gravity causing everything below a 15,534 mile orbital altitude to crash back to Earth.

NASA is working out preliminary plans for a simplified startup system, literally a string hanging from the sky. In his 2002 Presidential Budget Request, President Bush approved $11 million for perfecting manufacturing techniques for single-walled carbon nanotubes.

Critics, including many at NASA, caution that future propulsion technologies might render the elevator obsolete before it is complete. Yet, the enthusiasts remain undeterred.

"It is indeed very complex," says Smitherman, "but it is less massive than the Great Pyramids of Gîza and short in comparison to our interstate highway system."