Ribbon to the Stars

If the laws of celestial mechanics make it possible for an object to stay fixed in the sky, might it not be possible to lower a cable down to the surface and so establish an elevator system linking earth to space?

—Arthur C. Clarke, 1978,

The Fountains of Paradise.

After a cruise through tropical waters, you arrive at a large, anchored platform in the middle of the Pacific Ocean. The sea is calm, the sky a picture-postcard blue. But you've come in search of an experience even more uplifting than floating on the balmy seas.

You board an elevator at the top of the platform and prepare for the ride of your life. After only a few minutes in the pressurized compartment, you leave Earth's atmosphere behind and the planet appears as a brilliant, ever-shrinking ball of blue. With Earth exerting less and less of a tug, you feel noticeably lighter. The sky gradually blackens and the heavens are aglow with more stars than you've ever seen before.

While you marvel at the crystal-clear view of the Milky Way, you try not to think about a harsher reality: For the next 7 days, your life will literally hang in the balance. All that will keep you aloft is a slender ribbon that stretches from the top of that mid-ocean platform to your destination 100,000 kilometers into space.

Welcome to the era of the space elevator. Without the roar of a rocket or its exorbitant cost, an elevator powered by a laser would quietly transport payloads and people to a space platform. And that may not be the end of the trip for some. The rotational energy of the platform's orbit could be used to fling a vehicle to the moon, Mars, or beyond.

A space elevator would transform the economics of space travel, making ventures ranging from space spas to exotic scientific exploration more possible.

Even a decade ago, an elevator to the heavens seemed like sheer fantasy, akin to the beanstalk Jack climbed in the fairy tale. There was no material strong enough to make the cables. But an advance in one of the tiniest of technologies—carbon nanotubes—has given a boost to this most lofty of schemes.

The space elevator "is no longer science fiction," says David Smitherman of NASA's Marshall Space Flight Center in Huntsville, Ala.

Physicist Bradley C. Edwards agrees. He left a job at Los Alamos (N.M.) National Laboratory to work full-time on the elevator design for a private company, Eureka Scientific in Berkeley, Calif. Edwards says that the elevator could be a reality in just 15 years. He presented his latest ideas in August at a workshop on the space elevator in Seattle.

Upwardly mobile

Discovered in 1991, carbon nanotubes are long molecular tubes of carbon atoms that resemble cylinders of minuscule chicken wire (SN: 12/16/00, p. 398). The bonds between carbon atoms in this configuration are so robust that, weight-for-weight, carbon nanotubes are at least 100 times as strong as steel. They are, in fact, the strongest material known. A carbon-nanotube string half the width of a pencil can support more than 40,000 kilograms, Edwards notes. That's equivalent to the weight of 20 full-size cars.

Strength is vital since the cables of a space elevator will have to withstand enormous tension. Because of gravity's action and the laws of motion, a cable stretching up to a stationary platform in orbit will simultaneously be pulled down and pushed up. The cable must remain intact despite this gargantuan tug-of-war.

With recent technology, making carbon nanotubes has become a cinch. The challenge now, says Edwards, is to incorporate nanotubes into fibers or ribbons that could be used for the space elevator. This requires inserting the nanotubes into a composite structure that causes them to align and aggregate (see "Carbon nanotubes do some bonding," in this week's issue, available to subscribers at http://www.sciencenews.org/20021005/note15.asp). For example, they might be encased in a material such as graphite.

The structure must be designed so that stresses on a ribbon are immediately transferred to the superstrong nanotubes rather than the much weaker composite surrounding it, says materials scientist Rodney Andrews of the University of Kentucky in Lexington. That feat will require 2 to 5 years of devoted research, he says.

"Although nanotubes are a hot topic," Andrews notes, "there's [currently] not as strong an interest in making the ribbons."

Building the space elevator faces other challenges, too. For one, the ribbons would act as lightning rods, the path of least resistance between a thundercloud and Earth. The heat generated by a lightning strike could sever a ribbon. One solution, says Edwards, is to place the ground station in a zone off the coast of Ecuador that receives few lightning strikes. The floating station could move the lower end of the cable out of the path of the rare storms that do occur in that region.

Micrometeors and humanmade space debris punching through a cable pose another hazard. Widening the cable in the region where space debris is most common—between 500 and 1,700 km above Earth—should make the elevator more tolerant of these random hits, Edwards says. Nonetheless, minor impacts from asteroid debris and damage from other hazards are inevitable.

"Think of the space elevator structure as a 100,000-km-long highway that will require ongoing maintenance and repair," says Smitherman. It will stretch 2.5 times Earth's circumference.

Bridging space

Edwards envisions building a space elevator one ribbon at a time, similar to the way bridges were once constructed. In building a bridge across a canyon, for instance, the first step was to catapult or shoot a string from one side of the chasm to the other. Then a larger string was attached to the first string and pulled across. The builder repeated this process until the entire supporting structure of the bridge was in place.

A space elevator must, of course, span a much wider gap. The initial string would consist of a flat carbon-nanotube ribbon 100,000 km long, Edwards says. A conventional rocket would carry a spool of the ribbon to an orbit some 35,000 km above Earth's surface. The scientists have chosen this orbit because it keeps the elevator above the same point on Earth as the planet rotates. Otherwise, the elevator ribbon would drift east or west relative to a fixed point on Earth, and tension on the ribbon would vary.

As the ribbon begins to unspool, the spacecraft, which acts like a counterweight, is moved outward. This ensures that the ribbon falls toward Earth. Ultimately, the craft carrying the end of the ribbon will be parked in an orbit 100,000 km from Earth. Edwards calculates that this ribbon, several micrometers thick and 20 to 40 centimeters wide, could support a load of 1,800 kilograms.

In the next stage, robotic climbers would ascend the ribbon, epoxying additional carbon-nanotube ribbons to the mother line as the robots shimmy spaceward. The devices would be powered by an Earth-based laser shining on photocells attached to their limbs. Each time another climber completed its journey, the ribbon would become 1.3 percent stronger, Edwards says.

After 230 climbers make the trip—a task expected to take about 2.5 years and cost about $10 billion—the ribbon would be strong enough to support a 20-ton climber carrying a 13-ton payload. At this point, Edwards says, people or other cargo could be transported to any Earth orbit.

Not that it would be quick. The space elevator would take about a week to reach geosynchronous orbit and would require another 5 to 10 days to reach the end of the ribbon, 100,000 km from Earth. Using the enormous centrifugal force there, spacecraft could be inexpensively flung toward Venus and Mars. The Red Planet might even be fitted with its own space elevator.

The Red Planet's elevator, which would travel between the surface of Mars and a Mars synchronous orbit, could be constructed in Earth's orbit. Because Mars is less massive than Earth, the ribbons for that elevator need only be half as long and one-twentieth the mass of the terrestrial device. Further, lightning and micrometeor impacts would be far rarer. However, Mars' global dust storms would pose a new hazard.

The Mars structure could be built up alongside the terrestrial elevator. A spacecraft would then carry to a Mars orbit the final ribbons on spools. Once the Martian device is in place, the journey from Earth to Mars would require but a single rocket. For instance, someone wanting to explore the Martian surface would first ascend the terrestrial elevator. A spacecraft at the top of this elevator would be released at just the right time to head toward Mars. When the craft approached the Martian elevator, it would attach to it and descend to the Red Planet.

One thing at a time, of course. With just a few terrestrial elevators in place, making the journey to the station in Earth orbit every few days, the potential for space tourism and other commercial ventures would be enormous, says Edwards. Although teen idol Lance Bass of the band N Sync apparently had difficulty in delivering the $20 million that the Russian Space Agency said it required for a ride into space, he and a host of other, less wealthy individuals would probably consider a ride on the space elevator. Once it's been running for several years, a round-trip ticket might cost only $20,000.

Elevator History

Charting its ups and downs

Scientists and science fiction writers have thought about space elevators for more than a century. In 1895, inspired by the brand new Eiffel Tower, the self-taught Russian scientist Konstantin Tsiolkovsky envisioned a "celestial castle" attached to Earth by a spindle-shaped cable. The castle would move in synchrony with Earth.

Five decades later, another Russian engineer, Yuri Artutanov, penned some of the first modern ideas about space elevators. He suggested that a cable could be lowered to Earth from a geosynchronous satellite. But his 1960 report appeared only in the Soviet newspaper Pravda; people outside the Soviet Union never heard about it.

Writing in Science in 1966, American oceanographer John D. Isaacs and his collaborators briefly described ultrathin wires that might extend from Earth to a geostationary satellite. But that article also garnered little publicity.

Eleven years later, Jerome Pearson of the Air Force Research Laboratory wrote an article in Acta Astronautica that finally brought the notion of space elevators to the attention of engineers. Jazzed by Pearson's article, science fiction writer Arthur C. Clarke wrote Fountains of Paradise. That 1978 novel first described a potential space elevator to the U.S. public

In the story, set in the 22nd century, engineers build an elevator atop a mountain on Taprobane, an island that resembles Sri Lanka, the country where Clarke now lives. In pitching the space-elevator idea to the leaders of Taprobane, fictional engineer Vannevar Morgan quotes Artutanov: "And then, for the first time in history, we will have a stairway to heaven—a bridge to the stars. A simple elevator system, driven by cheap electricity, will replace the noisy and expensive rocket."

To construct the ribbons, the engineers rely on materials they call hyperfilaments made of carbon monomers, presaging the real-life nanotubes that wouldn't be discovered until 1991.

The resulting elevator carries people to stations thousands of kilometers into space. At one of these platforms, a professor and his six students have begun making observations of the solar system, but sunspot activity traps them there. Morgan, the engineer behind the project, travels up alone to save them.

"It was hard to think of a greater contrast to an old-time rocket launch, with its elaborate countdown, its split-second timing, its sound and fury. Morgan merely waited until the last two digits on the clock became zeroes, then switched on power at the lowest setting.

"Smoothly, silently, the floodlit mountaintop fell away beneath him. Not even a balloon ascent could have been quieter."

The Original Story from: sciencenews.org