The base tower seems endless as it slides past the window. Then you're in open sky, at first a seemingly infinite expanse of blue, but gradually darkening until the Milky Way appears in all its glory. And throughout, the shimmering blue pool that is the Earth curves away beneath you, a sight that was once the preserve of a privileged few.

After what seems like forever -- but is actually little more than 10 minutes -- the acceleration eases. Now cruising at 2000 kilometres an hour, at an altitude of 150 kilometres and rising, you begin to feel uncomfortably buoyant in your seat.

Trying to keep calm, you avoid dwelling on the fact that for the next 18 hours the only thing stopping you from plummeting to Earth is little more than a glorified piece of rope. A cable some 47,000 kilometres long, yet no more than a few centimetres wide, stretching from the surface of the Earth into orbit. You are taking a trip on the space elevator. Get ready for the ride of your life.

The idea of an elevator to the heavens may sound preposterous, like an updated version of the Tower of Babel. But it's a serious proposition. Two independent NASA teams recently thrashed out the technological requirements for such a project and found them to be feasible. Extraordinarily demanding, yes, but feasible.

"You're looking at something we can seriously consider building by the end of this century," says David Smitherman of NASA's Marshall Space Flight Center in Huntsville, Alabama, who led one of the teams. The space elevator -- an idea long consigned to the wastebasket of pipe-dream technologies -- now looks like a real possibility. Just.

Why bother building one? Once such a structure is in place, it would allow cheap and cheerful access to space. Passengers and cargo could ride up and down the cable in a manner similar to a conventional elevator -- or, more accurately, a cable car -- travelling at a fraction of escape velocity.

That would cut the cost of putting payloads into orbit to as little as $1.48 a kilogram, compared with $22,000 a kilogram on a rocket. And you wouldn't have to be a super-fit astronaut to make the trip, which would open up space to the (modestly wealthy) masses.

The idea of the space elevator was first raised in 1960 by Russian engineer Yuri Artsutanov, and rehashed several times in the years that followed. But the idea went largely unnoticed until 1979, when Arthur C. Clarke used it as the centrepiece for his novel The Fountains of Paradise.

So how does it work? The best way to get a handle on the concept is to use that traditional tool of physics, the thought experiment. Start by imagining a satellite. The time it takes to orbit the Earth is determined by the strength of gravity, and this varies with distance: low-flying satellites orbit quickly, distant ones much more slowly.

In between is a special distance -- 35,786 kilometres -- at which a satellite takes exactly one day to orbit. If its orbit is aligned with the equator, a satellite at this distance will hover over the same point on the Earth's surface as the two turn in celestial tandem. Satellites parked in such an orbit are termed "geostationary".

To continue the thought experiment, imagine elongating the satellite inwards towards the Earth, and at the same time outwards into space, so that its centre of mass remains in geostationary orbit. Those parts of the satellite closer to Earth will be moving more slowly than necessary to maintain a stable orbit, and so will start to feel gravity's pull. In contrast, the parts further away will be moving too quickly for their distance and so, like a stone in a sling, will try to move further afield. The result: tension. The satellite becomes a taut cable in orbit.

Tower of power

It is then trivial to carry the thought experiment to its logical conclusion, where the satellite's innermost point strikes ground zero -- or, more likely, connects to a tall tower.

The result is a continuous structure stretching all the way from the equator into space. At the Earth end is the base station, a massive complex with all the trappings of a major international airport -- hotels, restaurants, duty-free shops and the like.

Looming above the complex is the launch structure, something like the Eiffel Tower but tens of kilometres tall. Then comes the cable: 47,000 kilometres long, uninterrupted except for a space station at the geostationary point. This would serve as the structure's centre of mass as well as housing labs, a business park and a zero-gravity resort.

Further out lies a counterweight, possibly a minor asteroid tethered to the end of the cable (see Diagram, p 27) [NOTE: Not available here - A.Y.]. So much for thought experiments. Could we actually build such a thing? The answer, according to NASA, is a cautious yes -- once we've overcome a few technological hurdles.

By far the greatest challenge is the cable itself. The sheer weight of the structure dangling from geostationary orbit would place extraordinary demands on the material used to make it. What sort of stuff has the tensile strength needed to support its own weight over such a length? Surprisingly, almost anything would work in principle, provided it was appropriately tapered: widest at geostationary orbit, where tension is highest, and narrowest at the extremities.

But possible is not the same as practical. A steel cable 1 millimetre across at ground level would have to be 40 billion kilometres in diameter at geostationary orbit -- equivalent to building an upside-down mountain bigger than the Solar System. Even Kevlar, which is stronger and lighter than steel, would need to widen to 16 metres, so you'd need 2 gigatonnes of the stuff.

To make matters worse, the cable would need a minimum diameter more like 10 centimetres, not 1 millimetre. For a cable of practical dimensions, you need a material with enormous tensile strength. NASA's estimates suggest a magic number of 62.5 gigapascals -- that's 30 times stronger than steel and 17 times stronger than Kevlar.

Until recently, the lack of such a material has denied the space elevator even a modicum of credence. Enthusiasts have been forced to make wildly exotic suggestions: fibres of crystalline hydrogen or even antimatter. But now it turns out that an element as down-to-earth as carbon might hold the key to the heavens.

It comes as no real surprise that carbon has been elevated to the material of choice. In the form of diamond, it shows record-breaking mechanical properties. Diamond can't be spun into filaments, but there is a form of carbon that combines strength with length: nanotubes. These tiny, hollow cylinders made from sheets of hexagonally arranged carbon atoms exceed the tensile strength of steel by at least a factor of 100. Even conservative estimates place their strength at 130 gigapascals, which surpasses the magic number by a comfortable margin.

So what's the catch? (And there's always a catch ...) For a start, they're extremely expensive, clocking in at a cool $500 per gram. They're also a little short at present, with even the best synthesis methods yielding tubes no longer than a few micrometres. Bradley Edwards of Los Alamos National Laboratory in New Mexico, who led the other NASA team, has worked out how long nanotubes would need to be to form a viable composite material. The figure he has come up with is 4 millimetres.

But there is hope. According to Dan Colbert of Carbon Nanotechnologies, a spin-off from Rice University in Houston, Texas, the cost of making nanotubes is set to tumble. At the moment they are produced by laser vaporisation of graphite, a process that yields small batches of pure product perfect for laboratory use but far too expensive for the construction industry -- let alone anyone building a space elevator.

But Carbon Nanotechnologies has a new production process called "high pressure carbon monoxide deposition", or HiPCO, which promises to be scalable, so production plants could be as big as you like -- and bigger means cheaper. Colbert reckons that within seven years HiPCO will have cut the cost of nanotubes to just a few cents a gram, though he won't give details of how it works.

What about the problem of length? Things might not be too bad as they stand. Nanotubes have a tendency to "rope up", or stick together side by side, and the cohesive forces between them seem strong. Good news. But on the downside, roped-up nanotubes also slip and slide erratically against one another in a way we don't fully understand. Nobody has yet measured the strength of a nanotube rope, but early indications are that the tensile strength is reduced by at least a factor of 3, putting it "right on the ragged edge" of what is needed for an elevator, Colbert says.

And when a multibillion-dollar project is at stake, what engineer would work on the ragged edge? Perhaps the simplest solution is to find a way of incorporating nanotubes into a composite material like fibreglass. The downside of this approach is that whatever material is used to bind the nanotubes together will dilute their strength.

The most elegant solution would be to produce continuous nanotubes extending the full length of the cable. There's no doubt that such a material would be strong enough, but is it a realistic prospect? At present no one knows how to join individual nanotubes together to make longer molecules. But researchers are working on the problem, and Colbert believes there's a very good chance of success.

Getting attached

So now that we have a cable dangling from a distant point in space, we need something to attach it to. We could, of course, extend it all the way down to sea level and tie it in place. But recall the taper problem: the cable needs to widen as it gets higher in order to support its own weight. And the lowest section must have a certain minimum thickness which, in turn, determines the cable's girth at geostationary orbit -- and hence the mass and cost of the structure as a whole. Raise the bottom of the cable and you'll save an awful lot of material at the top. Ideally we need to attach it to something very tall.

A well-placed mountain near the equator would be a good start, but there are safety concerns with this. Should the unthinkable happen and the cable snap, a large amount of debris would fall on land. Little wonder, then, that the preferred option is a gigantic tower built on a platform out at sea. The tower would have to be tens of kilometres tall, but compared with dangling a cable from orbit, building one would be child's play.

The tallest self-supporting building in the world today is the 553-metre CN Tower in Toronto, nowhere near the theoretical limit. With existing construction methods you could raise a tower 20 kilometres tall, more than enough for the base station. With the cable and tower in place, we have the skeleton of a space elevator.

All that is lacking is a means of climbing it. Traditional mechanical means -- cables, wheels and pulleys -- wouldn't do. Given the stupendous distances involved, a viable transport system must satisfy two basic requirements: very low maintenance and extremely high speeds. Magnetic levitation and propulsion holds the key to both.

By using repulsive magnetic forces to keep the vehicle out of direct contact with the cable, maglev eliminates the wear and tear that plagues most transport systems. And in the absence of friction, the vehicle can rapidly accelerate to several thousand kilometres an hour. Another advantage of the system is that you can use the braking and descending phases of the journey to generate electricity. This makes running the elevator very energy efficient.

Is that everything covered? Not quite: space is a hazardous place. The near-Earth environment is fizzing with energetic particles, all waiting to etch, sputter and generally erode any material they come across. Then there are potentially cable-cutting projectiles, including meteoroids and space debris. But such hurdles are surmountable. Just look at the success of the Moon shots, interplanetary probes and, most recently, the International Space Station, all of which had to contend with similar problems.

There's also the small matter of economics. There's no doubt that an elevator would slash the cost of getting into space, but would this justify the phenomenal expense of building one in the first place? On this point Smitherman is optimistic. He says the trick is to start generating revenues early on, perhaps by using the first nanotube strands to deliver solar power from space. Then the project becomes comparable in scale to building a road or rail network.

Some four decades after the space elevator was first dreamed up, there are still plenty of reasons to be sceptical about it, even allowing for the tremendous technological advances that have been made during this period.

What if nanotubes prove too weak or can't be made long enough? What if the near-Earth environment is too hostile for such a structure? What if it's too expensive after all? Well, as Mr Wonka said in Roald Dahl's Charlie and the Great Glass Elevator, "Bunkum and tummyrot! You'll never get anywhere if you go about what-iffing like that."

So if all goes well, when can we expect such a structure to be built? Arthur C. Clarke was once asked this question and came up with the answer: "The space elevator will be built about 50 years after everyone stops laughing". They just stopped.

Karl Ziemelis is physical sciences editor at Nature,a nd this article was first published in the May 5, 2001 edition of New Scientist

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