A pair of astrophysicists from Cambridge University and Columbia University believe the answer to the problem of getting stuff to the Moon cheaply, might be to 'dangle' a Spaceline from the Moon rather than invest in space elevators.
They believe this solution will be more practical, easier to build, and cheaper than the oft-cited alternative.
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The study's authors Zephyr Penoyre and Emily Sandford believe we may be thinking in the wrong direction when it comes to finding alternatives for current Earth to Moon logistics.
At present, getting stuff into Earth's orbit is very costly because of the need to overcome the planet's gravity. Space elevators have long been cited as a potential solution.
If achievable, they could dramatically cut the cost in the long run and allow payloads to be carried up the elevator extending from Earth's surface into orbit. But this introduces new problems altogether.
The modern materials we have on hand don't seem to fit this purpose. They are simply not strong enough to support the weight of such an ambitious project, or so the authors claim.
In their view, it might be better to 'suspend' a Spaceline from the moon to deep within the Earth's gravity well.
"In this work, we present an alternative to the classic space elevator, within [the] reach of modern technology: The Spaceline. 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," Penoyre and Sandford note in their paper.
They believe such a project should be achievable given our modern technology. It should, they argue, be possible to extend a cable from the Moon, close to the height of geostationary orbit.
This, they believe, would enable easy traversal and construction between mother Earth and the Moon.
Are either space elevators or this new Spaceline proposal necessary?
As already mentioned, it is currently very expensive to get stuff into space from the Earth. This is because we currently rely on rocket technology to achieve this.
This is very impressive, but it is pretty inefficient. Currently, many payload missions to space require the use of large amounts of propellents to "brute-force" their way into space.
The vast majority of this is simply jettisoned to push the rocket upwards. This approach is very wasteful, and it needs to overcome not only the weight of the payload and rocket but the large reservoir of fuel that it carries.
Because of this, it currently costs tens of thousands of dollars for every kg of payload to get it into space. If you wanted to get the payload to the Moon, the costs could be even more.
For this reason, there is a great incentive to reduce costs as much as possible. For all the great work that private enterprises, like SpaceX, are making, it is still a pretty costly affair.
The answer, many believe, is to provide a fixed piece of infrastructure that can transport material up a cable, into space and beyond.
Why is the Spaceline a better idea than a Space Elevator?
The idea of a space elevator sounds intuitively great, but there is a problem. Such a thing would have to be very strong, indeed.
A space elevator would need to be very long, around 42,000 km long to put the end-most part of the cable in a stable geosynchronous orbit. It would have an enormous mass and so would require a similarly-massed object at the 'business end' in space to prevent it from falling back to Earth over time.
This, in theory, would keep the elevator suspended by virtue of centrifugal force (centripetal effect).
Even with some of our latest in high-strength materials, like carbon nanotubes, they may not be able to 'cut the mustard.'
For this reason, Penoyre and Sandford, suggests we may be looking in the wrong direction (for Moon missions, at least). The reasoning is relatively simple.
A space elevator would orbit Earth every 24 hours. A Moon-based Spaceline, on the other hand, would only orbit the Earth every month or so.
This slower rate of orbit would dramatically reduce the forces imposed on the Spaceline when compared to a space elevator. The distribution of such forces would also be very different on a Spaceline.
"In extending from the Moon to Earth, the Spaceline would pass through a region of space where terrestrial and lunar gravity cancels 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," according to MIT Technology Review.
This would mean that current materials, like Zylon, could conceivably be used for its construction. Such an approach would also make Lunar exploration a lot easier.
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,” the authors believe.
It would also open up opportunities for us to explore, and exploit, the Lagrange point between the Earth and Moon.
The original study was published on Emerging Technology from the arXIV.org.