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