Zero Genie

Perhaps the biggest hurdle to humankind’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 jettisoning mass in one direction to generate thrust for a spacecraft in the other. And that requires huge volumes of propellant, which is ultimately discarded but also has to be accelerated along with the spacecraft.

The result is that placing a single kilogram into orbit costs in the region of tens of thousands of dollars. Getting to the moon and beyond is even more expensive. So there is considerable interest in finding cheaper ways into orbit.

One idea is to build a space elevator—a cable stretching from Earth to orbit that provides a way to climb into space. The big advantage is that the climbing process can be powered by solar energy and thus would require no onboard fuel.

But there is a big problem too. Such a cable would need to be incredibly strong. Carbon nanotubes are a potential material if they can ever be made long enough. But options available today are just too feeble.

Enter Penoyre and Sandford, who have revisited the idea with a twist. They say their version of a space elevator, which they call a spaceline, could be built with materials that are commercially available today.

First some background. A space elevator as conventionally conceived would consist of a cable anchored on the ground and extending beyond geosynchronous orbit, some 42,000 kilometers (26,098 miles) above Earth.

Such a cable would have considerable mass. So to stop it from falling, it would have to be balanced at the other end by a similar orbiting mass. The entire elevator would then be supported by centrifugal forces.

For many years, physicists, science fiction writers and visionaries have excitedly calculated the size of these forces, only to be sadly disheartened by the result. No known material is strong enough to cope with these forces—not spider silk, not Kevlar, not even the strongest modern carbon fiber polymers.

So Penoyre and Sandford have taken a different approach. Instead of anchoring the cable on Earth, they propose anchoring it on the moon and dangling it toward Earth.

The big difference comes from the centrifugal forces. A conventional space elevator would make a complete rotation every day, in line with Earth’s rotation. But the moon-based spaceline would orbit just once a month—a much slower rate with correspondingly lower forces.

What’s more, the forces are arranged differently. In extending from the moon to Earth, the spaceline would pass through a region of space where terrestrial and lunar gravity cancel each other out.

This region, known as a Lagrange point, becomes a central feature of a spaceline. Beneath 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.

Penoyre and Sandford quickly show that extending the cable from the moon all the way to Earth’s surface generates forces that are too great for today’s materials. But the cable need not stretch all the way to be useful.

The researchers’ main result is to show that today’s strongest materials—carbon polymers like Zylon—could comfortably support a cable stretching from the moon to geosynchronous orbit. They go on to suggest that a proof-of-principle device made from a cable about the thickness of a pencil lead could be dangled from the moon at a cost measured in billions of dollars.

That’s clearly ambitious but by no means excessive for modern space missions. “By extending a line, anchored on the moon, to deep within Earth’s gravity well, we can construct a stable, traversable cable allowing free movement from the vicinity of Earth to the Moon’s surface,” say Penoyre and Sandford.

The savings would be huge. “It would reduce the fuel needed to reach the surface of the moon to a third of the current value,” they say.

And it would open up an entirely new region of space to exploration—the Lagrange point. This is of interest because both gravity and the gravity gradient in this region is zero, making it much safer for construction projects. By contrast, the gravity gradient in low Earth orbit causes orbits to be much less stable.

“If you drop a tool from the International Space Station it will seem to rapidly accelerate away from you,” point out Penoyre and Sandford. “The Lagrange point has an almost negligible gradient in gravitational force; the dropped tool will stay close at hand for a much longer period.”

Neither is there any significant debris in this region. “The Lagrange point has been mostly untouched by previous missions, and orbits passing through here are chaotic, greatly reducing the amount of meteoroids,” they say.

For these reasons, Penoyre and Sandford say access to the Lagrange point is major advantage of the spaceline. “The Lagrange point base camp is the thing we believe to be most important and influential for the early use of the spaceline (and for human space exploration in general),” they say. “Such a base camp would allow construction and maintenance of a new generation of space-based experiments—one could imagine telescopes, particle accelerators, gravitational wave detectors, vivariums, power generation and launch points for missions to the rest of the solar system.”

That’s interesting work that invites a renewed focus on the idea of a space elevator. Cheap access to the Lagrange point, the moon, and points beyond may just have become considerably cheaper and more likely.

Ref: arxiv.org/abs/1908.09339 : The Spaceline: A Practical Space Elevator Alternative Achievable With Current Technology