Pushing the space elevator closer to reality
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.
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 Carbon nanotubes do some bonding). 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.
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.
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."
Although tragedy strikes during Morgan's rescue mission, the book ends with an epilogue noting that many more space elevators followed and, with them, people colonized the solar system.
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Advocates of a "space elevator" seem to have forgotten about angular momentum. When an elevator ascends the ribbon, it must be accelerated eastward because the Earth's rotation represents a larger eastward velocity the higher you go. The required eastward force on the ascending elevator would have to be provided by a corresponding westward force on the ribbon. Does Bradley Edwards have an answer to this?
Robert H. Beeman
Coral Springs, Fla.
Bradley C. Edwards of the space-elevator design company Eureka Scientific in Berkeley, Calif., replies: "If you go through the math quantitatively, the angular momentum for the climbers requires a pound or so of force over the one-week travel time, and we do that easily with our many tons of material in the anchor and the counterweight."
The greatest concern for the space elevator appears to be impacts. Has someone forgotten that each and every satellite in Earth orbit (except for those in an exact geosynchronous orbit) crosses the equator twice during each and every orbit? Unless that elevator is really good at dodging, one of these satellites is going to run into it.
Huntington Beach, Calif.
Edwards replies: "We are good at dodging, and we will avoid the satellites. We are tracking them and will have days to weeks warning. We will move the anchor about a kilometer each day to avoid the debris."
The last space shuttle-tether experiment, which unspooled about 12 miles of cable, generated thousands of volts of electrical potential and kilowatts of power, burned through the insulation of the cable, and generated a tremendous explosive arc of electricity, that snapped the tether. Now imagine a 60,000-mile-long cable and its electrical-generating capacity and you begin to see the disastrous potential.
According to Edwards, the voltage builds up on tethers in space because they are traveling 11,000 mph relative to Earth's magnetic field. The space ribbon would be stationary relative to Earth's magnetic field and thus the voltage produced is essentially zero.–R. Cowen
Although the article mentions that "debris" would be a hazard to the cable, it neglects to note that the cable would be a major hazard to satellites. Almost any non-geosynchronous satellite would hit it eventually. For a different scheme using nanoengineered materials, have a look at http://discuss.foresight.org/~josh/tower/tower.html .
J. Storrs Hall
It seems to me a space elevator could be built without the development of novel carbon nanotube ribbons or some other super-strong material. More conventional materials could be used, by incorporating small propulsion systems at regular intervals along the 'elevator' to counter-act the forces that attempt to place stress on the system.
San Francisco, Calif.
Edwards replies: These engines would need a lot of fuel and servicing. "Imagine keeping 40 shuttles running continuously."
The elevator would exist at a constant angular velocity over its entire length, in a geosynchronous fashion. This means the linear velocity would be constantly increasing as you ascend. I see nothing addressing this in the article. Since the ribbon will be stressed to the limits by tension forces due to it's own weight, plus the weight of the climber, this seems to be less than optimal. I believe a far better design for a practical space elevator would be to construct it from tubes, not ribbons. A climber with a small fuel tank and plasma engine could tap electricity in the tube walls to ascend with no additional stress to the structure. The 'elevator' would simply be a conduit for shuttling earth-based resources to the altitudes they are needed at.
Edwards replies: "Any lateral force will only exist for limited times during deployment and brief times of movement, these forces will be very small compared with the other forces in the system. ...The mass of the tubular structure proposed would be at least 100,000 times that of our proposed ribbon. Constructing such a system is impossible with current technology."
It has been know for many years that there is a voltage difference of several hundred kilovolts between the Earth's surface and the ionosphere. The (presumably conducting) ribbon would at least provide a path to reduce the global potential and might even short it out, forcing the ribbon to carry a large current. Would this effect the construction of the highway? How might a shorting out of the global circuit effect the Earth's meteorology?
John T. Lynch
Edwards replies: "It is correct that there are substantial electrical potential in Earth's atmosphere. . . . However, doing a quantitative analysis of the situation, we found the currents that will flow through the ribbon will be minimal due to the resistance of the ribbon, cross-sectional area of the ribbon, the distance the potentials are separated by, and the poor coupling between the ionosphere and the ribbon . . . . The total effect of the ribbon on the potentials can be seen in conventional tethered balloons or kites."
It doesn't make sense to me. First of all, the article makes no argument for dealing with the change in angular momentum of the ascending elevators. Second of all, if saving rocket fuel is the motivation, it would take almost the same amount of energy to climb as used to power a rocket into orbit.
As elevators move up this ribbon, their angular momentum with respect to the earth's rotation would be conserved. This means that they would have a tendency to tug the ribbon in an opposite direction to the rotation of the earth. This will have the tendency to push the "spool" in that same direction. Over time the more objects making the ascent without descending, the faster the spool would move in the direction opposite to the earth's rotation. If the objects never made the trip back down to Earth, both spool and ribbon would eventually fall out of orbit.
In order to counteract this tug on the ribbon you would need a rocket with propellant attached to the ascending elevator, pushing in the direction of the Earth's rotation. While you're at it, you might as well use the rocket to push the elevator up the ribbon. You could do this by pointing the rocket upward a little. Now, you don't need the ribbon anymore since you have a rocket attached to the elevator. I can see how ascending slowly through the atmosphere would save energy lost due to wind drag. Beyond the atmosphere though, an elevator would require the same amount of energy to reach 100,000 km as a rocket would.
By the way, ascending elevators wouldn't be the only forces tugging on this ribbon. Winds and even hurricanes would do quite a bit of tugging. Even in good weather the moon would tug at it as it does the on the earth which results in ocean tides. I don't think this Ribbon to the Stars would last too long after it was unfurled.
Edwards replies: "Yes, the ascending climber will need angular momentum and the ribbon will be pulled very slightly off of vertical. However, the ribbon recovers for the same reason that it stays up in the first place. Centripetal acceleration is acting on the upper two-thirds pulling it outward, and the lost angular momentum is replaced very quickly (essentially as fast as it is lost). The ribbon will never loose enough angular momentum to even deflect a single degree let alone fall."
Unless I've forgotten my physics, there were a couple of errors in this article. First, when the writer says, "All that will keep you aloft is a slender ribbon . . . ." I believe that should be something like, "All that will keep you attached to Earth is a slender ribbon." In other words, if the ribbon broke, you would be flung into space. Second, there is no such thing as "centrifugal force," this is a very common misconception, what keeps the top of the ribbon going around Earth is centripetal acceleration imparted by the ribbon. If the ribbon breaks, the top will not fly straight out, it will continue in a line tangent to its orbit at the time of breakage. Of course, after the breakage, the line of flight will not be straight; it will be slightly bent by Earth's gravity.
The article didn't say anything about protecting the elevator against traffic near the surface of the earth, like airplanes. Would the ladder be cordoned off somehow from such traffic? Also, wouldn't the ladder be fairly easy to sabotage near ground level, if a lightning strike is enough to split it? The lightning wouldn't have to be natural.
David M. Brown
Edwards replies: "The anchor is located in the equatorial Pacific 400 miles from any air or shipping lanes. The ribbon would also have restricted airspace around it. The ribbon and anchor would be protected like any other valuable piece of property, in this case probably by the U.S. military."
I wonder how so many people working on this project can continue to overlook one big issue. That is answering an obvious question: What kind of engine and control system will hold the space-end of this ribbon in place? Any payload climbing this ribbon must be accelerated to geosynchronous orbital velocity by the time it reaches 35,000 km high. Since there isn't any such thing as a free lunch in physics, something must do the work, which is in a direction perpendicular to the ribbon. This is in addition to the job of overcoming gravity. Even with rockets in the anchor point in space, the ribbon will need to be bent a lot to pull the payload up to orbital velocity, unless the ribbon is very stiff indeed. Some effort at conserving energy and momentum is called for at least. For example, the ribbon could be made into a belt, with pulleys on each end. It could be built up by applying the carbon filament Earthside on a continuous basis. It could be arranged that for each weight going up, another goes down, evenly distributed along the belt, so the anchor and belt control systems have minimal dynamic force to overcome. Take the money saved and apply it to building a system for providing the required dynamic energy and momentum.
Edwards replies: "Earth's rotation creates the centripetal acceleration to keep the upper end in place. The acceleration for the tangential velocity comes from the ribbon and eventually from the rotation of Earth. All the required power for getting to orbit is sent up from Earth by laser."
The en route time from the Earth's equator to docking at the geosynchronous orbit could be much shorter than most people realize. If the elevator leaves Earth with only 0.03 g acceleration (a 100-pound person would weigh only 103 lbs), after less than 14 minutes the craft would be legally in space, 100 km (62 miles) above liftoff. At that altitude, the person would weigh 3 percent lighter, weighing the normal 100 lbs. Then the craft holds an apparent 1g, gradually increasing its acceleration rate to hold the 1 inside the craft, while outside gravity drops off due to distance from Earth. Although the occupant would feel normal, the speed could build to over 10 km/sec. Then the acceleration stops and is in free fall (zero gravity) for a few minutes. Then deceleration at 1g for long enough to arrive at docking with no more speed. Total time is about one and a half hours. And the passengers never get over 1.03 gs inside the craft! The math is simple. The trip would take less time than a jet from NYC to Chicago.
Edwards replies: "In the future, our travel time will decrease. At the moment, we have limited speeds to 120 mile per hour, which is within current technology and would imply a travel time of hour to low orbit . . . and one week to geosynchronous orbit."
In the article, you write: "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." The newspaper was not Pravda, but Komsomolskaya Pravda, Sunday addendum. It's a very common error. It actually makes a lot of difference: While something printed in Pravda was considered to be 100 percent official, the Komsomolskaya Pravda was for the younger audience and could afford printing an idea as weird (for the time) as a space elevator. Its "Sunday addendum" edition, especially, wasn't considered to be very serious.
Eugene Chuck Bogorad
In your article, you have a glaring error. On page 220, second paragraph, you state, "Using the enormous centrifugal force there . . . ." As every freshman physics student should know, there is no such thing as a centrifugal force. It is a fictional force.
Bruce W. Liby
This article states that after reaching the end of the 100,000 km ribbon, "Using the enormous centrifugal force there, spacecraft could be inexpensively flung toward Venus and Mars." Centrifugal force is a fictitious force, and doesn't "fling" anything. The only force on the spaceship would be the tension of the ribbon pulling the spaceship toward earth. When released, the spaceship would move tangentially to its orbit, and not be "flung" away as the article suggests.
I think that your article was very interesting and eye-catching. I have to present a science related article every week to my Earth Science class and I saw this article and I just couldn't stop reading it. The idea that someday just ordinary people like me, with no training of in-space living could go up in space whenever seems far-fetched. But with all the new discoveries every day I know that things like this can't be too far away. I hope that in my lifetime I get a chance to ride on one of these space elevators.
Bradley C. Edwards
Eureka Scientific/HighLift Systems
1904 3rd Avenue
Seattle, WA 98101
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