Since the dawn of the space era more than six decades ago, there’s been just one way to get to the moon and back: rockets. But a pair of graduate students say we should now be able to ferry humans and cargo between Earth and our natural satellite via a sort of high-tech elevator.
The idea of space elevators isn’t new; spaceflight visionaries have been talking about them at least since 1895. But Zephyr Penoyre and Emily Sandford envision a system that would be used not to ferry humans and cargo from Earth’s surface to Earth orbit — the goal of so-called classical space elevator concepts — but to provide transportation to and from the moon.
In a study published Aug. 25 on the online research archive arXiv, the students contend that it’s technologically and financially feasible to build such a "lunar space elevator," which was first publicly detailed by Jerome Pearson at a conference in 1977 and by Yuri Arsutanov in a separate paper published in 1979.
“It shocks me how cheap it could be,” says study co-author Penoyre, a graduate student in astronomy at the University of Cambridge, adding that the $1 billion it might take to build such an elevator “is within the whim of one particularly motivated billionaire.”
A very long cable
Penoyre and Sandford, a graduate student in astronomy at Columbia University and a co-author of the study, call their lunar space elevator concept Spaceline. Its central element is a cable that would be anchored to the moon and span more than 200,000 miles to a point above Earth's surface — perhaps an orbit about 27,000 miles from our planet. (The cable of a lunar space elevator couldn’t be anchored to Earth’s surface because the relative motions of the moon and our planet wouldn't permit it.)
As explained in the paper, the simplest version of the Spaceline cable might be barely thicker than the lead in a pencil and might weigh about 88,000 pounds — within the payload capacity of a next-generation NASA or SpaceX rocket. It could be made from Kevlar or other existing materials rather than the exotic and hard-to-make carbon-based materials that have long been seen as the key to building a classical space elevator.
Future space travelers would use a spacecraft to fly from Earth to the end of the dangling cable, which would be held taut by Earth's gravity, and then transfer to solar-powered robotic vehicles that would climb up the cable to the moon. The voyage might take days or weeks. Return trips would simply reverse the process.
Why spend hundreds of millions or even billions of dollars erecting a lunar space elevator instead of relying on proven rocket technology? Penoyre and Sandford say in their paper that the former might ultimately be more economical, especially for bringing raw materials back to Earth from moon-based mines.
A paper published by the American Institute of Aeronautics and Astronautics estimated that a lunar space elevator system might pay for itself within 53 trips by transporting lunar materials to a space station between the moon and the Earth.
Mining the moon
For years, experts have been eyeing the moon as a potential source of valuable raw materials ranging from helium-3, a heavy version of the familiar gas that could find possible use in fusion reactors, to rare earth minerals like neodymium and gadolinium, which are used to make cellphones, medical scanners and other high-tech devices.
“A space elevator is like a railroad — you don’t build it unless you expect a lot of railroad traffic,” says physicist Marshall Eubanks, chief scientist at Space Initiatives, a satellite technology company in Palm Bay, Florida. He calls the calculations used in the Spaceline paper sound but cautions that Earth-orbiting satellites could collide with the colossal cable — a potential problem that could be mitigated by keeping the cable outside Earth's orbital space lanes.
Despite their potential advantages over rocket transport, neither lunar space elevators nor classical space elevators have gotten much attention from space agencies or aerospace manufacturers. NASA has funded occasional studies on classical space elevator concepts since the late 1970s. But as of now there is no SpaceX for space elevators, even though companies in China and Japan have floated proposals for building classical space elevators by 2045 and 2050, respectively.
“One thing that’s frustrating is the lunar space elevator idea doesn’t have much traction, and yet it’s a feasible idea and economically a game-changer,” says Space Initiatives CEO Charles Radley.
Some experts say a classical space elevator might make more sense than a lunar space elevator, at least initially, because it could help facilitate exploration. For example, a classical space elevator might be used to assemble a huge spacecraft in Earth orbit and then launch it from there. Experts say that would likely be easier than launching the spacecraft from Earth, where gravity is so intense.
But the classical space elevator would require a much stronger cable to withstand the higher forces exerted on the cable — and it’s unclear when materials strong enough and of sufficient length to make a suitable cable will be available.
"The classical space elevator is a really tough problem, because the Earth's gravity field is so great that you need such strong materials that we don’t have right now," says Jerome Pearson, an aerospace engineer and president of Star, Inc., an aerospace company in Mount Pleasant, South Carolina, and the author of several papers on space elevators. "On the other hand, you could build a lunar space elevator with existing materials right now."
Given that concern, Pearson endorses the idea of moving forward with a lunar space elevator as a precursor to building a classical space elevator. "There are a lot of advantages with a lunar space elevator," Pearson says. "And with this new NASA program to return to the moon, there may be additional interest."
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