ALL EXISTING space vehicles, including the most advanced SDI launch systems and the renovated Space Shuttle, rely on a principle more than 800 years old. Whereas aviation has progressed over the centuries from hot-air balloons to propeller- and jet-driven engines, and now stands on the verge of further breakthroughs, spacecraft are all driven by rocketry, embodying techniques that, although refined, date from the invention by the Chinese of powder-propelled fire arrows, in the twelfth century.
Although the principle of rocketry—essentially Newton's third law, on action and reaction—is elegant in its simplicity, it sets an upper limit on the efficiency and performance of rocket engines. A rocket can neither derive its motive power from the medium it moves through, like a clipper ship, nor refuel during its journey, like a bomber. The fuel that a rocket-powered-spacecraft requires for the final part of its ascent must be carried throughout the journey. The Space Shuttle is more than 90 percent propellant by weight as it sits on the launch pad; the percentage for vehicles capable of reaching higher orbits or other planets is even greater.
Although propellant is not a major factor in the cost of the space program—the four million pounds burned in a typical shuttle mission account for less than two percent of the launch's cost—its enormous consumption demonstrates that the physics of rocketry relies on brute force. Aircraft employ the earth's atmosphere, using it for lift and taking advantage of its winds; rockets simply punch through it. Greater efficiency comes from more dangerous propellants or higher burn temperatures, which require highly advanced engines with relatively short working lifetimes.
Thousands of engineers and technicians are required to maintain the complex shuttle, an expensive burden that would only increase with an expansion of the shuttle program. And, as the Challenger disaster showed, a rocket, however complex its computerized regulatory systems, is essentially a space capsule sitting atop an enormous bomb, which is merely burned slowly rather than detonated all at once.
The exhaust from solid-fuel rockets leads to the formation of acid rain, while some liquid-fuel rockets, like the Space Shuttle, contribute to global warming. Although the warming effected by the shuttle on its current restricted launch schedule is minuscule compared with, say, rain-forest burning in Brazil, an ambitious space program, like the one NASA envisioned in the mid-1960s, might eventually pose significant problems, especially if other nations began similar programs.
All these factors have persuaded growing number of scientists that if humanity is going to establish vacuum and zero-gravity industries in space, or build large-scale facilities in orbit, a revolution in the means for lifting payloads into space must be achieved.
IN 1960 A SOVIET engineer named Y. N. Artsutanov published an article detailing how emerging technology could produce a previously unconsidered means of lifting objects into orbit. Artsutanov noted that at a distance of 22,400 miles a satellite takes exactly twenty-four hours to circle Earth. But because its "geostationary" orbit corresponds to Earth's own period of rotation, an object orbiting over the Equator at this height would remain fixed over one spot. Artsutanov boldly imagined such a satellite lowering a cable to Earth, where it would be anchored in place. If a counterweight was extruded in the opposite direction, such a system would be dynamically stable, maintaining itself without expenditure of energy. The cable would serve as a "space elevator" climbing into the sky. Artsutanov imagined such a cable system—or "heavenly funicular," as he called it—hauling 12,000 tons of payload a day into orbit. Once the cable system was completed, the cost of lifting payloads would be measurable in terms of electricity expended, or about a dollar per pound. Such a structure would be so huge at its base that it would be invulnerable to natural or human assault. An airplane or other manmade object would, however, be demolished if it crashed into the structure.
Artsutanov published his speculations in a Communist youth magazine rather than in a scientific journal, and his concept went largely unseen by scientists. In the following fifteen years it was independently reinvented several times, once by a team of oceanographers engaged in lowering long cables to the bottom of the sea. By the late seventies the concept of a space elevator—called variously a skyhook, a beanstalk, an orbital tower, and an anchored satellite—had appeared in science-fiction novels, and was widely disseminated among research scientists, university brainstormers, and advocates for the development of the space industry and permanent orbital habitats. Although NASA engineers have expressed varying degrees of skepticism, a number of scientists now believe that only technical hurdles stand in the way of the construction of a space elevator in the next fifty years, and that a working model, though very expensive to install, would soon render Earth-to-orbit rockets obsolete.
The limiting factor for the space elevator is cable strength. In 1960 the strongest existing industrial material was capable of supporting about a 120 mile length of itself hanging in Earth's gravity. Because gravity is slightly reduced 120 miles above Earth, though, a greater length of such a cable could hang suspended. Tapering the cable, so that the center of mass (which would bear the weight of the full extent of the cable) was far thicker, would allow still longer lengths. Materials developed since have strength-to-weight ratios that are closer to those necessary to sustain a heavily tapered cable for the full 22,400 miles. Charles Sheffield, a former president of the American Astronautical Society, believes that a small working model—with a cable perhaps microscopically thin by the time it reached Earth's surface—could be in place within thirty years.
Robert L. Forward, formerly of Hughes Research Laboratories and now a consultant and writer, envisions a space cable made of graphite "whisker," which has a higher ratio of tensile strength to density than any substance existing outside laboratory conditions. A whisker is a thin filament of crystal several millimeters long, which can possess exceptional tensile strength. Industrial graphite fiber, used in tennis rackets and fishing rods, has a higher strength for its weight than any metal, but graphite whisker cannot yet be used in greater lengths while maintaining its full strength. However, brand-new materials that use filaments of carbon atoms arranged like a spiral staircase foreshadow greatly increased material strength in coming years.
A skyhook made of graphite whisker would need to be at least a hundred times as thick at the center of mass as at the Earthward end, which in Forward's design would measure one millimeter across. This would be strong enough to lift two tons. Its lifting capacity would first be used to hoist more cable into place, allowing it to grow progressively larger and more durable, like a thickening stalactite. Ultimately the skyhook would comprise either a series of independent cables, on which trains would run like vertical monorails, or a rim of cables forming a hollow structure, inside which electrified tracks could be built.
But thousands of tons of highly engineered graphite whisker would have to be put into orbit 22,400 miles above Earth, far higher than any shuttle has gone. Unless builders are willing to consider making thousands of shuttle launches, enough raw material to provide such a tonnage of refined carbon would have to be found in space.
One plan involves diverting a small asteroid into high orbit around Earth. Space-industry enthusiasts speak of entrepreneurs venturing out to locate a carbon-rich asteroid the size of a small mountain and nudging it into the desired orbit with controlled nuclear explosions. Forward speculates that "a new breed of wildcatter" might come into being—high-risk companies that would supply orbital engineering projects with millions of tons of nickel-iron or carbon from diverted asteroids. (The history of companies doing business with the government suggests much less romantic prospects, but NASA might contract with some aerospace giant, with the usual guarantees for cost overruns, to provide this service.)
VARIANTS OF the skyhook could be built more easily. One design, developed by Hans Moravec, of Carnegie Mellon University, has been called a nonsynchronous skyhook. It is essentially a tether 5,300 miles long, set spinning about its midpoint, which would orbit Earth at a height of 2,600 miles once every three hours. Such a device can be envisioned as two opposite spokes of an imaginary ferris wheel rolling over the surface of the earth.
The tether would spin at a rate of one revolution every two hours, so that three times during each orbit one of its ends would sweep down into Earth's atmosphere. At the lowest point of the swing—perhaps a few miles above the surface—supersonic aircraft could dock with the end in the way that jets dock to refuel in flight. The payload would be transferred to the tether in a minute or two, and it would then be swept up into space like the end of a bola.
NASA is already planning to use a short tether to lift a satellite to a new orbit. A shuttle mission scheduled for this summer will deploy a satellite upward—that is, away from Earth—on a tether twelve miles long and then reel it back in like a fishing lure. On a subsequent mission astronauts will lower a satellite on a sixty-two-mile tether to probe regions of the atmosphere that are too high for aircraft but too low to sustain an object in orbit.
Ivan Bekey, of the NASA Office of Space Flight, sees this system as the forerunner of free-spinning tethers that could lift satellites or spacecraft from low orbits into higher ones or vice versa, and even fling them toward the Moon or Mars. "Once you start dipping into the atmosphere, things get difficult very rapidly," he warns. "But rotating tethers in orbits outside the atmosphere make an enormous amount of sense."
Forward and Bekey both note that a skyhook could be built in orbit around the Moon using currently available materials. In the lower lunar gravity Kevlar, Du Pont's superfiber, would suffice for either a spinning tether, which could raise and lower loads of several tons every twenty minutes, or a tower reaching thousands of miles above the lunar surface. Bekey believes that a spinning tether in orbit around the Moon, combined with one spinning in Earth's orbit, could eventually become a "highway" for payloads to travel to and from the Moon. Such a system would become economically attractive if NASA were to construct a permanent lunar base. "Studies indicate that several missions per year, carrying payloads of five to ten tons, might make the system pay off" Bekey says.
All these structures would be able to sustain their great mass against stresses because they would be constructed under tension rather than compression—that is, built from the center of mass out. Around 1980 a group of research scientists, including Forward, Moravec, John McCarthy, and Marvin Minsky, discussed various means of sustaining a skyhook's center of mass at a height less daunting than 22,400 miles. Out of this discussion emerged the first design for a dynamic "beanstalk," which could be built from the ground up but would require a constant expenditure of energy to hold it in place. Partisans have given it, like other models, a number of colorful names: Charles Sheffield calls it "the Indian rope trick," while Forward favors "the space fountain," noting that it supports itself the way a water fountain sustains a ball bobbing at the top of its column.
The heart of the structure would be a hollow tower through which particles were fired upward at hypersonic speeds. These particles would be slowed by electromagnetic drag devices, creating a braking force that exerted an upward lift on the structure. This lift would support the tower in the same way that pressurized air can stiffen and hold up the walls of large ribbed tents. When the particles reached the top of the tower, they would be turned around by an electromagnetic device, fired downward, and accelerated more than gravity accelerates them naturally. In the process they would exert still more upward force, and would reach the ground at nearly the same velocity at which they had left. They would then be diverted by more magnets through an underground tunnel, where electromagnetic accelerators would push them back up to launch velocity and send them on their way again.
As long as enough energy was pumped into the system to keep the particles at running speed, the tower could rise to any height. Because the slowing and speeding of particles would be adjusted to counter the forces of gravity, no superstrength building materials would be needed. Although lavish amounts of energy would be required to get the system going, the power gained from braking the particles on their upward journey could be used to accelerate them back downward, which means that the tower would require additional power only to offset losses due to inefficiencies in operating its various motors—just as a pressurized tent will begin to sag only as the air leaks out.
If its electromagnetic drivers were to cease operating for any reason, the entire tower would slowly collapse. Forward notes that the space fountain would have to be built "with a good deal of redundancy in it." A prudently designed space fountain would have several energy streams, each with an independent power supply. Each system would suffice to support the tower, and all of them together would sustain the lifting of great loads. Should all systems fail simultaneously, the space fountain would come apart, its top sections going into orbit around Earth and its lower sections burning up in the atmosphere or crashing to the ground in a line stretching a few hundred miles to the east. Thus it would be sited on the western edge of a desert or an ocean.
Proponents of dynamic beanstalks see no need to wait for improved graphite material to be developed. Rod Hyde, a scientist at Lawrence Livermore National Laboratory who has done considerable work on engineering details for a space fountain, insists, "This is a skyhook that can be built using only engineering development rather than requiring scientific breakthroughs."
THESE SYSTEMS sound impressive, but none of them has ever been tested, and some NASA engineers warn of the pitfalls that await any attempt to bring untried technologies into use. Jesco von Puttkamer, a program manager for the shuttle, who has been with NASA for thirty years, sees the space elevator as "a beautiful, idealistic concept that looks great on paper and would work in principle." But, he says, "the practicality of translating it into the real world is insurmountable." Von Puttkamer speaks of the engineer's natural preference for proven technologies, and of the "real-world constraints"—the need for congressional and societal approval and the unpredictable behavior of long, flexible materials under exotic conditions. He allows that "perhaps thirty to sixty, maybe up to a hundred, years from now, a future generation may have the technology to do it," but regards claims for a skyhook we can build now as detracting from NASA's hard-won achievements.
In the dozen years since it was popularized, the concept of the space elevator has led to a variety of new ideas, and its value may finally reside in its tendency to provoke such speculation. Emerging technologies and new materials have prompted smaller, less grandiose designs, which will end up competing with one another—and, Von Puttkamer notes, with future generations of rocket launch vehicles.
Such devices can easily be dismissed as science-fiction extravagances whose realization lies beyond our lifetime, but technology—especially when it has military applications—can develop extremely fast once the engineering principles have been well defined. Many of the young engineers who began working on rocketry immediately before the Second World War imagined that manned space stations would be orbiting Earth during their working lifetimes, though few others foresaw such progress. The U.S. President whose Administration will oversee the first launch system to supersede rocketry in sending cargoes into space may today be serving in some state legislature.
In the meantime, an increasing number of scientists as well as space buffs are designing plans for tethers and space elevators. They, at least, are ready to start now.