Even with innovations like SpaceX’s reusable robotic boosters, chemical rockets remain an expensive, dangerous and unreliable way to reach orbit. How much easier it would be if astronauts could simply step into an elevator, press O for Orbit, and ascend gracefully to outer space.
This is the dream of the collection of scientists, engineers, and entrepreneurs in the International Space Elevator Consortium (ISEC) who got together for its annual conference last week in Seattle.
The idea of a space elevator has been around for over a century. The basic concept is simple: a tether descends from a spacecraft in geostationary orbit to a floating platform at the equator, probably in the eastern Pacific Ocean. Because of a counterweight that would extend far into space, the space elevator’s tether would be gravitationally stable, allowing electric elevator cars to make the week-long climb to orbit powered by solar panels and ground-based lasers.
Such a system, ISEC researchers believe, could eventually slash the cost of raising a kilogram of payload into geosynchronous orbit from roughly US $25,000 to $300 or less. The key word, of course, is “eventually.” Technical challenges are legion, including building the aircraft carrier–size floating platform, designing safe, speedy climbers, and avoiding space debris and other satellites. But the truly fundamental obstacle is the lack of a material strong and resilient enough to form the elevator’s tether.
In current designs, the space elevator’s tether is not a thick round cable as originally proposed, but a paper-thin ribbon, a meter wide and 100,000 kilometers long. Even with such a slimmed-down approach, the strain of simply keeping its own mass aloft would instantly shred any tether made from steel, Kevlar, carbon composites, or even the best carbon nanotubes we can currently make.
In the ISEC conference’s keynote address, Mark Haase, a materials engineer from the University of Cincinnati, talked about how a tether at least ten times stronger than anything existing today might be made. His idea started with carbon nanotubes, which still hold tremendous promise for the manufacturing of superstrong materials. Discovered in 1991, carbon nanotubes are cylindrical structures formed by sheets of single carbon atoms. They are already being manufactured in bulk, mostly as an additive to other materials in order to boost their strength and thermal or electrical conductivity.
But while individual carbon nanotubes can be immensely strong, they are awkward to tease into macroscopic-scale objects. They can’t be melted and extruded like Kevlar, nor sorted and aligned like natural fibers. The longest carbon nanotube made so far, if stood on its end, would barely reach a child’s knee, let alone one-quarter of the way to the Moon. Haase believes that the way forward is to cross-link nanotube molecules using minuscule amounts of polymer glue.
ISEC is not putting all of its eggs in a basket made from carbon nanotubes, however. Graphene, a single-atom-thick sheet of carbon, is also a promising material, although it does not respond as well as nanotubes to the kind of twisting and bending that a 100,000-km-long tether moving back and forth through the atmosphere would certainly experience.
More exotic still are boron nitride nanotubes. Similar in form to carbon nanotubes but made up of alternating boron and nitrogen atoms, this ceramic is incredibly chemically stable. That quality should help it survive long periods situated high in Earth’s atmosphere where highly reactive atomic oxygen would likely degrade a carbon nanotube tether. (Engineers on the nanotube track say they have devised a solution to this cosmic erosion: a gold coating for vulnerable sections of the tether.) Boron nitride nanotubes could also cope well with solar and cosmic radiation beyond the magnetosphere.
“As we gain more knowledge about these materials, we have a real chance to improve strengths,” says Haase. He predicts that the most promising candidates, nanotube-polymer composites, will reach the minimum strength needed for a space elevator tether “in about 20 years.”
Bryan Laubscher, a director at ISEC, believes that the search for a tether material will have an impact long before then. Laubscher, who left his job as an engineer at Lockheed Martin in 2010 to form a company developing high strength materials for use in aviation and space applications, says, “Imagine a Boeing 797 made from carbon nanotubes. It would have one-tenth of the mass of today’s aircraft, and an airframe that won’t come apart in a crash.”
In fact, ISEC is relying on the private sector for every dime of the space elevator’s estimated $18 billion price-tag. In a position paper published earlier this year, ISEC noted, “To this point, we have found no needed capability within the government that must be incorporated in the space elevator architecture.”
That might be a swipe at NASA, which in 2012 abandoned a $2 million competition aimed at creating ultra-strong tether materials. But when even the world’s richest and most visionary space agency can’t help with your moonshot, you might want to at least consider that your lofty ambitions for a space elevator seem destined to stay firmly on the ground floor.