Space

Inside-out asteroids: A practical method for creating space habitats

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You can't build space cities on the surface of asteroids, or even inside them, really, but you could turn them inside out to become the ground layers of spinning ring-type space stations, says a Rochester team, and it'd be a relatively cheap and easy way to build yourself a space habitat for humanity
University of Rochester
You can't build space cities on the surface of asteroids, or even inside them, really, but you could turn them inside out to become the ground layers of spinning ring-type space stations, says a Rochester team, and it'd be a relatively cheap and easy way to build yourself a space habitat for humanity
University of Rochester
As the asteroid spins faster and faster, its rubbly mass flings outward to fill and expand the nanofiber bag
University of Rochester

Some folk baked a lot of bread during lockdowns. Others assembled a team of scientists and figured out a practical way to turn asteroids into space habitats, by spinning them inside out and creating giant rotating, Manhattan-sized city-rings.

In the latter group are a team of University of Rochester scientists, who turned their attentions to a "wildly theoretical" paper now published in the journal Frontiers in Astronomy and Space. The challenge they set themselves: how to create a city-sized space habitat people could live on permanently, without the massive expense and challenge of launching all the materials into space from Earth.

Asteroids represent big piles of materials, decided the team: “all those flying mountains whirling around the sun might provide a faster, cheaper, and more effective path to space cities,” says co-author and Professor of Physics and Astronomy Adam Frank. The trouble is, they're nowhere near big enough to provide a useful amount of gravity. That's a big deal; extended periods in zero- or low-gravity cause a range of health issues in astronauts, so the team decided on a minimum of 0.3g – a third or so less than you'd get living on Mars.

To create some gravity, you could hollow a decent-sized asteroid out, potentially, and spin it up like a ring-station, using centrifugal force to create that 0.3g. Then, you could build your city entirely within the spinning asteroid; sure, it'd be dark in there, but the rock would protect people from harmful space radiation. That might have a chance of working if the asteroid was made of solid rock with high tensile strength throughout.

But most asteroids aren't solid rock – at least, not most of the ones in our solar system. The team looked into the composition of our local "flying mountains" and found that most are more or less giant piles of rubble, collections of big and small rocks held together weakly by their own mutual gravity. Hollow one of these things out and spin it up, and the "ground" you're trying to create inside the asteroid would just fling away into space and disappear.

So how could they practically build a human-friendly city on a pile of space rubble? By sticking it in a gigantic bag, they decided. A cylinder-shaped bag a fair bit bigger than the asteroid itself, made from a flexible, ultra-light, ultra-strong carbon nanofiber mesh. This, says lead author and Ph.D candidate Peter Miklavčič, "would be extremely light relative to the mass of the asteroid rubble and the habitat, yet strong enough to hold everything together. Even better, carbon nanotubes are being developed today, with much interest in scaling up their production for use in larger-scale applications.”

The team decided to model the process around a smallish asteroid, something like Bennu, with a radius of 300 m (984 ft). This would be wrapped in a nanofiber bag with a starting radius sized to wrap around the asteroid itself, but designed to expand, accordion-style, to a radius around 3 km (1.86 miles) with energy-absorbing expansion joints built into its structure.

Then, it'd be time to start spinning the asteroid to bits and turning it inside-out. This, the team decided, was feasible using solar-powered rubble cannons, attached to the outside surface of the containment net, which would grab bits of asteroid rubble using conveyor belts or Archimedes' screws, and fling them off tangentially into space, creating torque on the bulk of the asteroid within.

As the asteroid spins faster and faster, its rubbly mass flings outward to fill and expand the nanofiber bag
University of Rochester

Depending on how much solar power is available, how many rubble cannons you've got and how big the rubble chunks you're throwing are, the team created a formula to determine how long it'd take to spin the thing up to the kind of speed you'd need for useful artificial gravity. For the Bennu-sized example, the team found it'd be practically possible to get it up to speed within a few months.

At this point, the asteroid would be spinning fast enough that its surface would be straining against the nanofiber bag under centrifugal force – and the engineers could begin releasing the bag to allow it to expand outward in a controlled fashion, layers of rubble pushing out against the bag all the way around. Once it reached its full 3-km radius, the bag would snap taut, and voila: a ring spinning fast enough to create useful gravity, with a rocky ground layer about two meters (6.6 ft) deep – enough to act as a workable shield for space radiation.

“Based on our calculations," says Frank, "a 300-meter-diameter asteroid just a few football fields across could be expanded into a cylindrical space habitat with about 22 square miles (56.9 sq km) of living area. That’s roughly the size of Manhattan.”

The "hoop stress" placed on the carbon nanofiber net, the team found, would be "well within the range of many currently existing construction materials," even if you started with a 500-meter-radius (0.3-mile) asteroid and spun the thing up fast enough to give you a full 1g Earth-equivalent artificial gravity.

So yes, the team concluded, turning asteroids inside out in nanofiber mesh bags does look like a feasible way of laying the foundations for a space city – and a much cheaper and simpler one, it would appear, than if you tried to launch all your materials from Earth. Mind you, you'd still need to launch the materials to populate this barren, rocky ring with buildings, life support systems, space pubs, and fences to stop people floating off into inky blackness after getting lost on the way home from the space pub.

“Obviously," says Frank, "no one will be building asteroid cities anytime soon, but the technologies required to accomplish this kind of engineering don’t break any laws of physics. The idea of asteroid cities might seem too distant until you realize that in 1900 no one had ever flown in an airplane, yet right this minute thousands of people are sitting comfortably in chairs as they hurtle at hundreds of miles an hour, miles above the ground. Space cities might seem like a fantasy now, but history shows that a century or so of technological progress can make impossible things possible.”

The paper is open access in the journal Frontiers in Astronomy and Space Sciences.

Source: University of Rochester

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18 comments
martinwinlow
Why on Earth (no pun intended) bother with the 'rubble'?
Brian M
Do wonder whether spinning and the use of centrifugal force is a real long-term substitute for real gravity, which is a completely different and fundemental force?

i.e. Gravity acts in the opposite direction of centrifugal force, for example objects are attracted towards the center of the earth by gravitational forces, whereas centrifugal force acts in a radial outwards direction forcing objects away!

They do feel similar in that both make your body feel heavy and both keep your feet planted on a surface, but whether the long-term biological impact is the same is a question.
Plus centrifugal 'gravity' allows you to walk on a ceiling - can't do that with real gravity unless you are spiderman!
yawood
Brian M, the floor would be to the outside and the ceiling to the centre. Under centrifugal force you would be walking on the floor.
michael_dowling
O'Neill Cylinder is the term for spinning space habitats,and they would be much,much bigger than this idea. AND they would be constructed using material from the asteroid belt. https://space.nss.org/o-neill-cylinder-space-settlement/
Rick O
"Halo did it" Where's my Cortana?
rgbatduke
Ringworld! And IIRC several other SF authors have already had similar ideas. Personally, I'm less inclined towards spinning a bag and like the idea of starting with a nickel-iron asteroid, using solar mirrors to smelt it, cast ring segments, and weld the segments into a continuous ring. Then sure, by all means line the inside with rocky-asteroid rock, well crushed. It still leaves you with the problem of atmosphere -- a second solution is to start with nickel-iron asteroid, heat the whole thing to molten, and blow it up like a balloon. Cool it, balance it, spin it, and you've got a complete sealed environment, small "Death Star" in size. Solar cells on the outside, locks and access hangers at the poles, lights on an axis rod stretching from pole to pole. You'd still have to keep it balanced (something nobody ever seems to worry about, but unbalanced rotors are unlikely to have the principle axis you think they should have and will wobble nastily about the principle axis you ACTUALLY have instead).

This latter bit is a chunk of physics that seems to have been ignored in the article above. The odds of the interior rock settling out uniformly to produce something that neatly spins on its true cylindrical axis are close to zero, and there will be "interesting" stresses in their bag and the new "ground" as the hole thing wobbles away initially. Sure, if you spin it up VERY slowly and use a space-tractor of some sort to hand balance it as it settles, well, maybe you can end up neat. But probably not, and certainly it is a major engineering feat to make it so in spite of this.
TechGazer
To answer Martinwinlow, they are using the rubble as a readily available mass to tension the structure. Nickel-iron asteroids aren't common, and using other materials for structures might be more expensive. Rubble makes good radiation shielding too.

Let the battle begin, between the Mars colonization fans and the O'Neill fans.
Nathan
Or use a reflective mylar sheet to pump solar energy into the asteroid and melt it. Pump gas into the mass to blow it up like a balloon. Viola! Instant spherical habitat
clay
I like the idea John Ringo imaginates in his Troy Rising series. Just take a lot of mirror, a good dose of time..and stir.

While his story heats and spins M Type asteroids for his Troy-Class battle stations, It seems C-Types could yield impressively capable basalt-esc shells of adequate tensile strength. C-class asteroids could be harvested to make basalt fiber ...lots of it. This could be used as a high tensile material for the bag/net described in the article.
Cryptonoetic
@Brian M: The equivalency principle from Einstein's theory of relativity states that there is no difference between a force experienced in a gravitational framework and a force experienced in an inertial framework. Also, gravity is not a "real" force, it is an apparent force (like the centrifugal or the Coriolis forces). Gravity as a "force" is what we experience from being in curved space-time. Biologically there is no difference between experiencing "gravity" as a consequence of space-time curvature or as a consequence of Newton's third law.