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Space Elevator? Build it on the Moon First.
Science fiction writer Arthur C. Clarke famously predicted that we'd see space elevators 50 years after people stopped laughing at the idea. Jerome Pearson has been thinking about space elevators since the early 1970s, and he's been watching the growing enthusiasm (and fading chuckles) with great interest. But he knows there are significant challenges in engineering and materials that still need to be overcome, so he's suggesting NASA build an elevator on the Moon first. And the agency is taking the idea seriously.
A speech by Arthur C. Clarke in the 1960s, explaining geostationary satellites gave Pearson the inspiration for the whole concept of space elevators while he was working at the NASA Ames Research Center in California during the days of the Apollo Moon landings.
"Clarke said that a good way to understand communications satellites in geostationary orbit was to imagine them at the top of a tall tower, perched 35,786 km (22,236 miles) above the Earth," Pearson recalls, "I figured, why not build an actual tower?"
He realized that it was theoretically possible to park a counterweight, like a small asteroid, in geostationary orbit and then extend a cable down and affix it at the Earth's equator. In theory, elevator cars could travel up the long cable, and transfer cargo out of the Earth's gravity well and into space at a fraction of the price delivered by chemical rockets.
... in theory. The problem then, and now, is that the material required to support even just the weight of the cable in the Earth's gravity doesn't exist. Only in the last few years, with the advent of carbon nanotubes - with a tensile strength in the ballpark - people have finally moved past the laughing stage, and begun investigating it seriously. And while carbon nanotubes have been manufactured in small quantities in the lab, engineers are still years away from weaving them together into a long cable that could provide the necessary strength.
Pearson knew the technical challenges were formidable, so he wondered, "why not build an elevator on the Moon?"
On the Moon, the force of gravity is one sixth of what we feel here on Earth, and a space elevator cable is well within our current manufacturing technology. Stretch a cable up from the surface of the Moon, and you'd have an inexpensive method of delivering minerals and supplies into Earth orbit.
A lunar space elevator would work differently than one based on Earth. Unlike our own planet, which rotates every 24 hours, the Moon only turns on its axis once every 29 days; the same amount of time it takes to complete one orbit around the Earth. This is why we can only ever see one side of the Moon. The concept of geostationary orbit doesn't really make sense around the Moon.
There are, however, five places in the Earth-Moon system where you could put an object of low mass - like a satellite... or a space elevator counterweight - and have them remain stable with very little energy: the Earth-Moon Lagrange points. The L1 point, a spot approximately 58,000 km above the surface of the Moon, will work perfectly.
Imaging that you're floating in space at a point between the Earth and the Moon where the force of gravity from both is perfectly balanced. Look to your left, and the Moon is approximately 58,000 km (37,000 miles) away; look to your right and the Earth is more than 5 times that distance. Without any kind of thrusters, you'll eventually drift out of this perfect balancing point, and then start accelerating towards either the Earth or the Moon. L1 is balanced, but unstable.
Pearson is proposing that NASA launch a spacecraft carrying a huge spool of cable to the L1 point. It would slowly back away from the L1 point as it unspooled its cable down to the surface of the Moon. Once the cable was anchored to the lunar surface, it would provide tension, and the entire cable would hang in perfect balance, like a pendulum pointed towards the ground. And like a pendulum, the elevator would always keep itself aligned perfectly towards the L1 point, as the Earth's gravity tugged away at it. The mission could even include a small solar powered climber which could climb up from the lunar surface to the top of the cable, and deliver samples of Moon rocks into a high Earth orbit. Further missions could deliver whole teams of climbers, and turn the concept into a mass production operation.
The advantage of connecting an elevator to the Moon instead of the Earth is the simple fact that the forces involved are much smaller - the Moon's gravity is 1/6th that of Earth's. Instead of exotic nanotubes with extreme tensile strengths, the cable could be built using high-strength commercially available materials, like Kevlar or Spectra. In fact, Pearson has zeroed in on a commercial fibre called M5, which he calculates would only weigh 6,800 kg for a full cable that would support a lifting capacity of 200 kg at the base. This is well within the capabilities of the most powerful rockets supplied by Boeing, Lockheed Martin and Arianespace. One launch is it takes to put an elevator on the Moon. And once the elevator was installed, you could start reinforcing it with additional materials, like glass and boron, which could be manufactured on the Moon
So, what would you do with a space elevator connected to the Moon? "Plenty," says Pearson, "there are all kinds of resources on the Moon which would be much easier to gather there and bring into orbit rather than launching them from the Earth. Lunar regolith (moon dirt) could be used as shielding for space stations; metals and other minerals could be mined from the surface and used for construction in space; and if ice is discovered at the Moon's south pole, you could supply water, oxygen and even fuel to spacecraft."
If water ice does turn up at the Moon's south pole, you could run a second cable there, and then connect it at the end to the first cable. This would allow a southern Moon base to deliver material into high-Earth orbit without having to travel along the ground to the base of the first elevator.
It'd be great for rocks, but not for people. Even if a climber moved up the cable at hundreds of kilometres an hour, astronauts would be travelling for weeks, and be exposed to the radiation of deep space. But when you're talking about cargo, slow and steady wins the race.
Pearson first published his idea of a lunar elevator back in 1979 and he's been pitching it ever since. This year, though, NASA's not laughing, they're listening. Pearson's company, Star Technology and Research, was recently awarded a $75,000 grant from NASA's Institute for Advanced Concepts (NIAC) for a six-month study to investigate the idea further. If the idea proves to be promising, Pearson could receive a larger grant to begin overcoming some of the engineering challenges, and look for partners inside and NASA and out to help in its development.
NIAC looks for ideas which are way outside NASA's normal comfort zone of technologies - for example... an elevator on the Moon - and helps develop them to the point that many of the risks and unknowns have been ironed out.
Pearson hopes this grant will help him make the case to NASA that a lunar elevator would be an invaluable contribution to the new Moon-Mars Space exploration vision, supporting future lunar bases and industries in space. And it would give engineers a way to understand the difficulties of building elevators into space without taking on the immense challenge of building it on Earth first.
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