The space elevator: going up? (part 2)
Debris and other hazards
Besides addressing the technological barriers to building a space elevator, conference attendees also looked at the various hazards an elevator would face once built. Paramount among them was the issue of orbital debris, the small (and not so small) inert objects orbiting the Earth that could damage, or even sever, a space elevator. “The major problem is from orbital debris and satellites,” said noted author and space elevator proponent Arthur C. Clarke, appearing via a satellite link from Sri Lanka.
“Debris is a serious concern,” Edwards concurred. In his original proposal, Edwards argued that putting the elevator anchor at sea, on a mobile platform, would allow it to move the elevator ribbon around, allowing it to dodge large debris. Minor damage to the ribbon caused by small pieces of debris as well as micrometeoroids could be fixed by special repair climbers.
David Smitherman of NASA’s Marshall Space Flight Center, who led a look at the space elevator concept in the late 1990s, offered some data on the risk orbital debris poses to the elevator. Using current estimates of the amount of debris in orbit 10 centimeters across and larger, his model found that, on average, 52.5 objects a month would pass within one kilometer of the elevator. Of those, 12.5 per month would pass within 500 meters. The major peaks in the distribution of debris are at altitude of 910 and 1525 kilometers, which Smitherman said were from upper stages of launch vehicles.
Besides debris, another issue addressed at the conference was radiation from the belts of charged particles orbiting the Earth. While not so much an issue for unmanned spacecraft, which can be easily shielded from the particles, the radiation becomes an issue for future human transit on the elevator. Anders Jorgensen of Los Alamos noted that the risk is severe because the elevator would be traveling slowly—about 200 kmph—and thus could expose passengers to the belts for about 90 hours.
To demonstrate the effects of the radiation, Jorgensen looked at radiation exposure data from the Apollo missions. The missions that traveled through the radiation belts to the Moon received, on average, about 0.5 rads, with Apollo 14 getting the most at 1.14 rads. However, he noted, those missions passed through the radiation belts in only about a half hour. This means that the passengers on a space elevator could be exposed to as much as 100 rads during their passage through the belts, enough to induce acute radiation sickness.
During questions, Jorgensen acknowledged that not all of the Apollo crews’ radiation exposure came from the belts, with perhaps as little as half caused by the belts themselves. “Still, if you make this 50 rads instead of 100, I’m still not going to go!” he quipped. Humans could be protected through shielding, he said, although he noted that “there is no standard way of protecting humans from the radiation belts.” Any shielding would also add mass to the crawler, reducing the size of its payload significantly.
Economic and social issues
The payoff for the space elevator, and the reason why a number of people are so interested in it, is its potential to dramatically reduce the cost of space access. Space elevator proponents argued that a space elevator could be constructed relatively cheaply—Edwards said the “technical” cost of the elevator would be on the order of $7-10 billion, with a second that could be built for an additional $2 billion—and push the cost of space access down to as little as $100 a pound. Those price points became an issue of debate at the conference.
Eric Westling, who coauthored the book The Space Elevator with Edwards, reminded attendees that money was the eventual name of the game. “Are we in it for the money or the profit?” he asked. “At some point we have to cross over to the profit.” He then presented a case for gradually reducing the costs of the elevator, starting with the initial 20-ton elevator that could carry seven-ton payloads into space for about $1150 per kilogram. That elevator could, in turn, be used to build more and larger elevators, pushing down the per-kilogram cost to as little as $13.50 in the case of multiple, higher-capacity space elevators, each carrying several climbers at any time.
Those numbers were the subject of some heated debate among some attendees, who argued that they were too optimistic. One of those people, consultant Jordin Kare, gave an impromptu presentation the next day with his own figures for the per-kilogram cost of the elevator. He estimated a far higher per-kilogram cost for the first elevator: $4,100. By comparison, if one assumes that a new RLV could be developed for about the same cost, $5-10 billion, he calculates that a single such vehicle could put payloads into low Earth orbit for somewhat less, $3,435 per kilogram. (He acknowledged that the payload cost would be higher since some kind of upper stage or transfer stage would be needed to bring payloads to GEO, although the elevator would need a similar transfer stage to place payloads in orbits other than GEO.) Laser launch, Kare’s own proposal for launching small cargos into orbit, could cost as little as $567 per kilogram. “They’re all in the same ballpark,” he concluded.
Some people noted that the $5-10 billion figure for developing an RLV seems too low, given recent developments, but Kare noted that the cost to develop a space elevator might also be too low. “Whose figures are more optimistic, I don’t know,” he said. He cautioned space elevator proponents to be aware that RLVs and other launch technologies will not stand still while the space elevator is developed. “The space elevator’s advantage is not that it is cheap,” he said, “but that it is an elevator,” with the environmental and reliability advantages that it offers over other technologies.
Other sessions of the conference looked at legal and regulatory issues involving a space elevator, with the conclusion that many aspects of the Outer Space Treaty that deal with launches would also apply to the elevator. William Press, deputy director for science and technology at Los Alamos, reminded people of another issue: terrorism. “The space elevator is a physically fragile thing,” he noted. “We may need a stable world society to support the space elevator.” This, he concluded, may mean that while the elevator could be technically feasible in a decade, it might be closer to a century before the world is stable enough to permit its construction.
The highlight of the conference was arguably at the very beginning, when Arthur C. Clarke spoke to attendees via a video link from Sri Lanka. Clarke, looking very fit, gave a few prepared remarks (including taking time to promote his latest book, The Last Theorem), before engaging in an extended question-and-answer session with the audience in Santa Fe.
In his comments, Clarke did note that that there are issues like orbital debris that pose challenges to the space elevator, but he was optimistic that they can be overcome in the near future. “There are solutions to all these problems,” he said. Clarke said he saw similarities between the space elevator and the first efforts to lay a transatlantic telegraph cable. “The first transatlantic cable was the Victorian equivalent of the space elevator,” he said.
Clarke emerged as one of the leading proponents of the elevator by featuring it in his 1978 novel The Fountains of Paradise. That book was set well into the future, but now Clarke thinks he has an outside chance of seeing work on an elevator begin in his lifetime. “I don’t know if I will live to see construction begin,” he noted. “But, in 20 years I will be 106, so maybe I will be around to see it!”