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When astronaut Chris Hadfield performed a serenade on Earth with Bowie's Space Oddity, many of us have background calculations that attempt to calculate the cost of sending the guitar: probably about $ 75,000.
All in all, this seems to be money well spent – but the prohibitive cost of sending material into space is one of the many reasons why lunar bases and landings on Mars are being considered by writers of science fiction and by the 17th bishops of the century have not yet materialized.
Since it was devised by the Russian rocket specialist Konstantin Tsiolkovsky, supposedly inspired by the Eiffel Tower, then refined by another Soviet engineer in 1959, many people have evoked a space elevator as a solution. The most recent proposal, according to which the construction of a space elevator from the Moon to the Earth is theoretically feasible from the second moment, reaches us via a new article from researchers at the University of Cambridge and from Columbia University.
But first, a small space elevator background.
End the tyranny of the rocket equation
Why is access to space so expensive? The problem is that you have to carry your fuel with you and find a way to provide enough thrust to lift that fuel – as well as carry the payload that you actually want to carry in the space. As a result, most of the fuel and energy ends up being spent and just raising the fuel up in the Earth's atmosphere so it can be burned to speed up the payload. And the low Earth orbit – where Commander Hadfield's guitar was sent – is less expensive than sending objects to the moon or to a deeper space, where they must absolutely escape gravity.
The basic idea behind a space elevator is to remove the obligation to carry all that fuel with you … by building a giant cable and climbing it. It's not as ridiculous as it sounds.
Imagine such a cable, extending into space with a counterweight in orbit, which could be an asteroid or a space station, at the end of it. Just like in a captivating ball game, the (apparent) centrifugal force of this orbiting counterweight that revolves around the Earth pulls the rope taut. If the cable is long enough, this centrifugal force can be enough to support its weight, suspending it: a vast elevator in the sky.
Once you have this elevator in the space, robotic "climbers" on the outside go back up the rope. You can send payloads into low earth orbit, into geostationary orbit or further into space, simply by choosing the climb distance. If the tower is high enough, a simple laissez-aller at the top propels you into space, escaping entirely from Earth's orbit.
Regardless of their design, the economic aspect of the space elevator still seems glorious: the mass sending into low Earth orbit could be reduced from $ 10,000 a pound to $ 400 a pound. Some believe that an elevator could be built for only $ 6 billion. Compare that to the Space Shuttle program, which cost an estimated $ 209 billion.
It seems wonderful. But, of course, all the creative designs have been torpedoed so far by a single flaw: what are you making cable?
The cable must support a huge amount of tension without slamming. Part of this tension carrying the weight of the cable, the less dense the material, the less it feels the force. So you need a lightweight material that can be pulled without breaking. The steel, the titanium and almost anything you can imagine would break under the forces involved.
For a while, it was thought that carbon nanotubes could perhaps provide the solution: it is the first material designed to achieve the required strength. But problems abound here too.
Their manufacture to a sufficient purity is extremely difficult. A single fault can ruin the strength of the material. The cable could also be vulnerable to lightning and, if you are not sold yet, the longest cable ever made with a carbon nanotube was about half a meter, falling to 35,768 kilometers of the required length.
If a land elevator does not work, why not hang a cable from the moon?
The new design, which the authors have dubbed Spaceline, circumvents some of these abominable requirements by proposing that the cable be built on the moon and hang in the center of the Earth's orbit. This immediately removes the counterweight. The gravity of the Earth pulling on the cable is enough to keep it taut.
The major advantage is that the cable does not need to be as strong as it does not have to support large amounts of cable in the Earth's powerful gravitational field, but rather in the weaker lunar field. This means that you can actually manufacture such a cable with existing materials: the authors note that Kevlar, the same material used in bulletproof vests, could meet the challenge.
The major disadvantage is that the cable can only extend slightly closer than the geostationary orbit, which is still far away from the Earth's surface. So you will have to make this first leg of the trip and catch the rope by yourself. But at an estimated price of one billion dollars, this lunar lift could allow to regularly go to the surface of the moon with only one third of the fuel.
Although this does not solve the problem of the Earth's gravitational field escape – you will still need rockets and the miserable physics of space launch to reach the Spaceline in the first place – the authors envision some potential benefits, while reducing fuel costs once you reach the line.
For example, the cable would pass through the Lagrange point located between the Earth and the Moon, that is to say where the gravitational attraction of the Earth cancels the gravitational attraction of the Moon. This is one of the areas of space where you can actually dream of building a stable floating base. And Spaceline could transport materials from the Moon to the Earth for everything we want to build there – satellites, spacecraft, space stations, and so on.
The idea of using the Lagrange points as a starting point – beyond our current space outpost, the ISS – is an interesting idea that offers many benefits. In particular, this gives humanity a feasible project on which we can develop all the auxiliary technologies that will be needed to achieve all that is practical in space.
If building a Spaceline really costs a billion dollars, do not you think some billionaires could do it? We can all dream of a multiplanarian future in the abstract, but, as you can tell from Vikram's recent mission on Moon, managing space is extremely difficult. If we are ever to leave the cradle of our civilization, we will need all the ingenious ideas we can get.
Image credit: NASA / Bill Ingalls
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