While much of the first day of the Space Elevator Conference was dedicated to the problem of space trash, the concept also faces another significant challenge.
"We have one big problem -- everything else pales in comparison -- and that is materials," said Bryan Laubscher, president of Odysseus Technologies and a speaker at the annual conference in Redmond, Washington.
Scientists studying the possibility of building an elevator into space envision a ribbon made of carbon nanotubes stretching from the surface of the Earth up into geosynchronous orbit. Crafts, like elevator cars, could carry people and goods up the ribbon into space.
The problem is, it's not yet clear if strong enough nanotube ribbons can be made.
"Materials research of carbon nanotubes is dominated by people who are looking at electrical properties," Laubscher said. "That's the low-hanging fruit."
While that's important for the use of nanotubes in electronics, it has come at the expense of research into stronger carbon nanotubes, he said. Scientists who set out to build super-strong carbon nanotubes have given up in part due to lack of funding, he said.
The nanotube ribbon not only would have to stretch at least 22,000 miles (35,406 kilometers), it would also have to support an elevator car that might weigh 7 metric tons, plus a 13-ton payload, he said.
Space trash is another problem, since objects that collide with the ribbon could damage it. One proposal, presented at this year's conference, is to collect and remove trash from space using a giant net.
Another challenge is tying the elevator to a platform in the ocean that can be moved so the elevator can avoid collisions in space. That raises questions about how such a structure might react to being moved. No one knows exactly how the ribbon might oscillate when shifted at the base.
Scientists are researching space elevators as a way to dramatically decrease the cost of sending people and objects into space. It costs US$10,000 per kilogram to send a load into space using Delta and Atlas rockets, Laubscher said. A space elevator could transport loads at a cost of $3,000 per kg initially, with the cost quickly dropping to $300 per kg, he said.
He estimates that a space elevator will cost $1.5 billion in research and development and $18 billion to actually build. Subsequent elevators would cost less, with the second running around $7 billion.
If strong enough carbon nanotubes are developed, it could take around 15 years to build a space elevator.
In addition to the challenges of space junk and materials, elevator enthusiasts face another problem: Many people think the idea is ridiculous. Arthur C. Clarke, a science-fiction author who popularized the idea, is reported to have frequently said that he expects a space elevator to be built about 50 years after everyone quits laughing.
The idea has been around for longer than that by now. This year, Yuri Artsutanov, a Russian engineer born in 1929, attended the conference. While Clarke tends to be the first person people mention when talking about the origin of space elevators, Artsutanov published a paper outlining the concept in 1929. His work went unnoticed outside of Russia, however. Clarke described a space elevator in his 1979 book "Fountain of Paradise" after reading a paper about space elevators published in 1975 by another scientist, Jerome Pearson.