麻豆传媒

THEY say the first 100 kilometres are the best. Moments after the door slides
shut with a reassuring 鈥渒er-chunk鈥�, the acceleration takes hold, pushing you
gently but firmly into your seat. Terra firma drops precipitously from view, and
your internal organs groan in sympathy. The base tower seems endless as it
slides past the window. Then you鈥檙e 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鈥攂ut is actually little more than 10
minutes鈥攖he 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鈥檚 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. 鈥淵ou鈥檙e looking at something we can seriously consider building by
the end of this century,鈥� says David Smitherman of NASA鈥檚 Marshall Space Flight
Center in Huntsville, Alabama, who led one of the teams. The space
elevator鈥攁n idea long consigned to the wastebasket of pipe-dream
technologies鈥攏ow 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鈥攐r, more
accurately, a cable car鈥攖ravelling 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鈥檛
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鈥�35,786
kilometres鈥攁t 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鈥檚 surface as the two turn in celestial tandem.
Satellites parked in such an orbit are termed 鈥済eostationary鈥�.

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鈥檚 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鈥檚 innermost point strikes ground zero鈥攐r, 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鈥攈otels, 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鈥檚 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).

So much for thought experiments. Could we actually build such a thing? The
answer, according to NASA, is a cautious yes鈥攐nce we鈥檝e 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鈥攅quivalent 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鈥檇 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鈥檚 estimates suggest a magic number of 62.5
gigapascals鈥攖hat鈥檚 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鈥檛 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鈥檚 the catch? (And there鈥檚 always a catch鈥�) For a start, they鈥檙e
extremely expensive, clocking in at a cool $500 per gram. They鈥檙e 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鈥攍et
alone anyone building a space elevator. But Carbon Nanotechnologies has a new
production process called 鈥渉igh pressure carbon monoxide deposition鈥�, or HiPCO,
which promises to be scalable, so production plants could be as big as you
like鈥攁nd 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鈥檛 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 鈥渞ope up鈥�, 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鈥檛 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 鈥渞ight on the ragged edge鈥� 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鈥檚 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鈥檚 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鈥檚 girth at geostationary orbit鈥攁nd hence the mass and cost of the
structure as a whole. Raise the bottom of the cable and you鈥檒l 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鈥檚 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鈥攃ables, wheels and pulleys鈥攚ouldn鈥檛 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鈥檚 also the small matter of economics. There鈥檚 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鈥檛 be made long enough? What if the near-Earth
environment is too hostile for such a structure? What if it鈥檚 too expensive
after all? Well, as Mr Wonka said in Roald Dahl鈥檚 Charlie and the Great
Glass Elevator, 鈥淏unkum and tummyrot! You鈥檒l never get anywhere if you go
about what-iffing like that.鈥�

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: 鈥淭he space
elevator will be built about 50 years after everyone stops laughing鈥�. They just
stopped.