In the folktale, Jack climbs a magic beanstalk up into the clouds. Today, a surprisingly large community of scientists and engineers is hard at work on an almost equally improbable quest: to build a beanstalk that stretches far, far above the clouds. The proposed space elevator would stretch from the Earth's surface to beyond geostationary orbit, 35,000 km high, and allow payloads (and people!) to be carried into space for a tiny fraction of the cost of a conventional rocket launch. Such a thing may at first seem as plausible as a giant's castle in the sky, but there are a number of new technologies promising to maybe—just maybe—make this particular vision a reality.
A brief history of an audacious idea
The basic idea of a space elevator is simple: objects in geosynchronous orbit, such as most communications and weather satellites, are effectively motionless relative to the surface of the Earth, 35,000 km below. Hundreds of such satellites ring the planet today. Imagine simply lowering a long, strong cable from such a satellite down to the Earth's surface. By climbing up this cable, one can get into space without any of that expensive and dangerous messing about with rockets. Current cost estimates by space elevator enthusiasts are that, for a fully developed space elevator, the cost to orbit should be between $100 and $400 per pound. That's about a hundred times cheaper than today's going rate, which is around $10,000 per pound on the space shuttle, and only slightly cheaper on a Russian Progress or Soyuz. Imagine being able to fly a hundred times more space missions for the same budget we have today, or being able to easily build orbiting structures that dwarf the International Space Station, and you may come to understand why space elevators have attracted a dedicated group of advocates. Even better, most space elevator designs require a counterweight cable stretching outwards from geosynchronous orbit to balance the weight of the downward cable. The top end of the counterweight will be whipping around the Earth like a rock in a sling, moving much faster than orbital velocity at that height. Drop something off the end of this gigantic slingshot at the right moment, and you've just got a free launch to nearly anywhere in the solar system.
The engineering reality, on the other hand, is anything but simple: a space elevator is in essence a kind of suspension bridge between two points tens of thousands of kilometers apart. Tremendously strong materials will be needed to keep the cable from snapping under the incredibly high tension. Even though the top end of a space elevator is in free fall, effectively weightless, it still has to support the weight of the entire rest of the cable dangling downwards. There is a cruel logic to cable design: in order to support the weight of a given width of hanging cable, the next-higher section of cable must be stronger and thicker—and the cable segment above that must be thicker still. The optimal design for a space elevator cable tapers gracefully, growing from a thin base on the Earth's surface to a massive, strong trunk around geosychronous orbit before tapering back down again on the outflung counterweight arm. But "gracefully" is a relative term, and in fact the precise mathematical relationship is exponential: a cable one centimeter across at ground level made of regular steel would need to be several hundred kilometers across at its midpoint to support its own weight. Without exotic materials that have very high ratios of strength to mass (specifically, high tensile strength to density), space elevators will remain forever purely science fiction.
Arthur C. Clarke is often credited with inventing the idea of a space elevator; his 1978 novel The Fountains of Paradise told of the construction of a space elevator in a loosely fictionalized Sri Lanka, conveniently relocated to the equator. But in fact Clarke only popularized the space elevator concept, and never claimed to have invented it (unlike the geosynchronous communications satellite, which Clarke really did invent). The idea of a space elevator as a serious matter worthy of real-world engineering concern dates back to the Russian space pioneer Konstantin Tsiolkovsky, who conceived in the late 1800s of a tower built upwards from the ground into space. His fellow countryman Yuri Artsunov suggested in 1957 instead lowering the cable down from the top. That suggestion spawned many other studies of space elevators, resulting in a steady stream of new ideas throughout the '60s and '70s: recognition of the tapered shape as the optimum design, the need for advanced materials, the potential use of a captured asteroid as a counterweight to reduce the need for extra cable, plans for bootstrapping construction by using a tiny initial cable to lift a larger version of itself, even a proliferation into related but non-synchronous designs such as rotating skyhooks. But the need for tremendously strong yet very lightweight materials seemed an insurmountable hurdle, and progress slowed.
NASA gets involved
Space elevator research took off again in the late 1990s, buoyed by two related events. The first was excitement that carbon nanotubes, first widely studied starting in 1991, might represent the holy grail of cable materials. Carbon nanotubes, essentially long cylindrical molecules of diamond, derive tremendous strength from their network of carbon bonds. The highest tensile strength measured in lab tests for nanotubes is some fifty times stronger than steel. Because of the exponential dependence of cable size on tensile strength, a 50x increase in strength decreases the required maximum cable diameter by tens of thousands of times, resulting in a far more managable few meters across. Actually creating nanotube ropes thousands of kilometers long with this level of performance remains a challenging goal, however, and all large cables to date fall far below the strength demonstrated by single, microscopic nanotubes.
But spurred on by the possibility that nanotubes might be the answer, the NASA Institute for Advanced Concepts (NIAC) began cautiously pouring some money into the area. NIAC is NASA's wild-and-crazy ideas incubator, perhaps better known for funding studies of weather control, cities on the moon protected by electromagnetic force fields, and insect-like flying robots for Mars (all real projects, by the way!). NIAC funds research intended to lead to real-world payoffs within 10-50 years, and the development of nanotubes brought space elevators into that realm of possibility. NASA funded several studies of space elevator design and construction through NIAC, ultimately leading to the founding of several companies focused solely on space elevator research. One such company, the Liftport Group of Bremerton, WA, in addition to publishing increasingly detailed elevator designs, has recently opened a commercial nanotube factory in New Jersey, selling to the construction and high-tech industries that need ultra-strong materials. Liftport is hard at work on increasing cable strengths, and has tested cables as long as a mile held aloft by a balloon.
But NASA's interest in the space elevator is not limited to NIAC. The success of the Ansari X Prize for suborbital spaceflight (won by Scaled Composite's White Knight/SpaceShipOne in 2004) has led NASA to establish a series of prize competitions in a wide range of space-related technologies. The second annual Space Elevator Games were held last month in Las Cruces, NM. A dozen teams—hailing from universities, engineering firms, and gigantic aerospace conglomerates—competed in two events: the Strong Tether Challenge (to build a tether at least 50% stronger than the current state-of-the-art) and the Beam Power Challenge (to build a robot capable of climbing up a tether, using only power transmitted wirelessly from the ground via laser or microwaves).
The Strong Tether Challenge this year was somewhat of a disappointment: three of the four teams competing were disqualified for narrowly missing the 2 meter minimum cable length requirement (longer cables are harder to manufacture, hence the desire to compete with as short a cable as possible, right down to the millimeter). On the bright side, the remaining entrant, AstroAraneae, produced a cable strong enough (with a breaking strength of 1331 gigapascals) to have won last year's competition. But that wasn't strong enough to win the contest's $200,000 purse; it couldn't beat this year's state-of-the-art industry supplied cable, which had still not broken by 1660 gigapascals, at which point the contest stopped because the tension was so high the cable-stretching machine's arms had begun to bend dangerously.
More exciting was the Beam Power Challenge. A space elevator cannot be an elevator in the classical sense, with cables pulling boxes of cargo up and down; instead robot climbers must grip the cable between their wheels and drive themselves up and down. Powering such robot climbers without resorting to thirty-thousand-kilometer extension cords is a tricky issue, and hence the Beam Power Challenge. Twelve teams competed, using technologies ranging from high-powered lasers to microwave transmission to huge arrays of mirrors to focus vast amounts of sunlight onto a climber's solar cells. The robot from the leading contender, USST, climbed a 55 meter cable in 57 seconds, falling just short of the 1 meter/second threshold to win the $400,000 prize.
So how much closer does this really bring the space elevator to reality? Neither of the prizes were awarded this year, but that doesn't seem to have discouraged the teams involved from planning renewed efforts for next year's competition. But the prize money being offered is only the merest drop in the bucket compared to the full cost of such an audacious project. Space elevator proponents believe that the construction cost for a space elevator capable of launching hundreds of tons of cargo daily would be somewhere between $15 and $20 billion dollars, most of which goes into developing sufficiently strong nanotube cables and factories for producing them by the thousands of kilometers. All such estimates are highly uncertain at best; predicting development costs for new technologies is a subtle and difficult art, and humongous cost overruns are not uncommon in the aerospace world. But compare the space elevator to projects like the proposed $ 20 billion Gibraltar Bridge, which would use towers 2000 feet high and a million miles of steel wire to hold up a suspension bridge from Europe to Africa, and suddenly the elevator doesn't seem quite so crazy any more. If current estimates are in fact correct, the cost of a space elevator is only two or three times greater than the planned development of the Orion spacecraft to replace the space shuttle. Once operational, a space elevator would offer far greater capacities at a far lower cost, yet it would require billions of dollars more in up-front research costs to develop the necessary materials.
Perhaps the true success of the Space Elevator Games will not be the development of any particular new tether, or the demonstration of any specific robot cable-climber. Instead, if the Space Elevator Games can serve as an incubator for scientists and engineers, if it can get the public excited about investing in these technologies (an audience of 20,000 spectators watched the Beam Power Challenge!), then the Space Elevator Games may—if we're lucky—lead to the increasingly large investments needed to solve the incredibly difficult array of technical challenges. In this article I haven't even touched on many of the other engineering concerns, like base station design, durability against meteorite impact and corrosive atomic oxygen in the upper atmosphere, and active avoidance of existing satellites—to say nothing of sociological concerns like management and ownership, the risks of terrorism, and the international geopolitical implications of a tower three times taller than the planet is wide.
A space elevator truly is a revolutionary technology, one with the potential to completely change the way we inhabit the solar system. Sure, today it seems like a crazy, phenomenally improbable technology, but space is big. Vastly, hugely, mindbogglingly big, as Douglas Adams said. It might just take an equally mindbogglingly big idea to get us there.
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