[Aug 5, 2022: Emerging Technology from the arXiv]
One idea is to build a space elevator – a cable that extends from Earth to orbit that provides a way to ascend into space. (credit: creative commons)
Perhaps the biggest obstacle to mankind’s expansion throughout the Solar System is the prohibitive cost of escaping Earth’s gravitational pull. So say Zephyr Penoyre from the University of Cambridge in the UK and Emily Sandford at Columbia University in New York.
The problem is that rocket engines work by moving mass in one direction to generate thrust for a spacecraft in the other direction. And that requires a huge amount of propellant, which is eventually discarded, but also has to be accelerated along with the spacecraft.
The result is that it costs thousands of dollars to put one kilogram into orbit. Going to the moon and beyond is even more expensive. That’s why there’s a lot of interest in finding cheap avenues in the classroom.
One idea is to build a space elevator – a cable that extends from Earth to orbit that provides a way to ascend into space. The major advantage is that the ascent process can be powered by solar power and thus will not require onboard fuel.
But there is also a big problem. Such a cable would need to be incredibly strong. Carbon nanotubes are a potential material if they can ever be made long enough. But the options available today are very weak.
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Enter Pennoyre and Sandford, who have revisited the idea with a twist. They say that their version of a space elevator, which they call a spaceline, could be built from materials that are commercially available today.
First some background. A space elevator, as traditionally envisioned, would have a cable attached to the ground and extend beyond geosynchronous orbit, about 42,000 kilometers (26,098 mi) above Earth.
Such a cable would have considerable mass. So to prevent it from falling, it has to be balanced by the same orbiting mass at the other end. The entire lift would then be supported by centrifugal forces.
For many years, physicists, science fiction writers and visionaries have enthusiastically calculated the size of these forces, only to be saddened by the result. No known material is strong enough to withstand these forces—not spider silk, not Kevlar, not even the strongest modern carbon fiber polymers.
So Pennoyre and Sandford have taken a different approach. Instead of anchoring the cable to Earth, they propose anchoring it to the Moon and hanging it toward Earth.
The big difference comes from centrifugal forces. A conventional space elevator would make one complete rotation every day, corresponding to the rotation of the Earth. But the Moon-based space line orbits just once a month—that is, at a much slower rate with less forces.
What’s more, the forces are arranged differently. As it extends from the Moon to Earth, the space line will pass through a region of space where terrestrial and lunar gravity cancel each other out.
This region, known as the Lagrange point, becomes the central feature of a space line. Below it, closer to Earth, gravity pulls the cable toward the planet. But above it, closer to the Moon, gravity pulls the cable toward the lunar surface.
Pennoyre and Sandford quickly show that extending the cable from the Moon to Earth’s surface produces forces that are too great for today’s materials. But the cable doesn’t need to stretch all the way to be useful.
The researchers’ main result is to show that today’s strongest materials—carbon polymers such as xylon—can comfortably support cables extending from the Moon to geosynchronous orbit. They suggest that a proof-of-principle device made from a cable about the thickness of a pencil lead could be hung from the Moon at a cost measured in the billions of dollars.
This would reduce the fuel needed to reach the surface of the Moon. (credit: Liftport)
This would reduce the fuel needed to reach the surface of the Moon. (credit: Liftport)This is clearly ambitious but by no means excessive for modern space missions. “Anchored on the Moon, deep within Earth’s gravity, we can build a stable, traversable cable that allows free movement from around Earth to the surface of the Moon,” say Pennoyre and Sandford.
The savings would be huge. “This would reduce the fuel needed to reach the surface of the Moon to a third of the current value,” he says.
And it will open up an entirely new area of space for exploration – the Lagrange point. This is of interest because both the gravity and the gravity gradient in this area are zero, making it much safer for construction projects. In contrast, the gravitational gradient in low Earth orbit makes the orbits much less stable.
“If you drop an instrument from the International Space Station it will appear to be moving away from you rapidly,” Pennoyre and Sandford explain. “There is an almost negligible gradient in the force of gravity at the Lagrange point; the dropped device will remain near the hand for a longer period of time.”
Nor is there any significant debris in the area. “The Lagrange point has remained mostly untouched by previous missions, and the orbits passing here are chaotic, greatly reducing the amount of meteorites,” he says.
For these reasons, Penoyre and Sandford state that access to the Lagrange point is a major advantage of the spaceline. “Lagrange Point Base Camp is what we consider to be most important and influential for early use of the spaceline (and for human space exploration in general),” he says. “Such a base camp would allow the construction and maintenance of a new generation of space-based experiments—one could imagine a telescope, particle accelerator, gravitational wave detector, vivarium, power generation, and launch points for missions to the rest of the Solar System.” could.”
This is interesting work that brings a renewed focus to the idea of the space elevator. Affordable access to the Lagrange point, the Moon, and beyond has probably become significantly cheaper and more likely.
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Note: Materials provided above by Emerging Technology from arXiv. Content can be edited for style and length.
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