Gravity Balloons: Colonizing the Asteroid Belt

The following article was submitted to us by an actual real-life scientist who asked us not to mention his full name for SEO reasons. Let’s call him Alan (because that’s his name). As well as doing lots of important scientist things, Alan writes a blog that explores his ideas about space colonization, specifically his ideas about how we might colonize asteroids. In this article, written specially for SciFi Ideas, he explains how and why the colonization of asteroids might actually be easier than the alternatives.

Two routes to space colonization are common in literature – planetary colonization and spacesteading via rotating space stations. The former has the advantage of some natural forces to help you, but the latter has easier access and unlimited scalability. Establishing colonies in the cores of asteroids is an under-utilized alternative that offers some benefits of both options. I argue that this is the most scientifically plausible future for moving into space. The concept also forces us to adopt a new, highly granular, perspective of solar system resources.

A space colony would have air, pseudo-gravity, and biological self-sufficiency, but life is more than basic needs. Writers must to go further into what defines human society, and what would lead future people to prefer space. For example, the book Colonies in Space writes about what there is to do on a Saturday night. That narrative, however, leaves me disappointed. To Earthlings, a space station is tremendously exciting, but if I put myself in the perspective of a teenager who was born in a space colony, the place sounds… obnoxiously boring.

Artificial habitats are limited in size because larger stations suffer from greater structural loading. A rule of thumb: the structure must be about as strong as a building on Earth of a height equal to its radius. We will strive to push the size envelope to decrease Coriolis forces, and also to increase the land area of these communities. Gerald O’Neil imagined an 5 miles diameter design, which would be difficult to build like a 2.5 mile tall building on Earth. Science fiction writers have trended toward the huge. Logically, if you have the materials to build a space elevator then you have the materials to build an Earth-sized rotating space station. Science fiction author Karl Schoeder saw an inefficiency of these massive space stations. A habitat with only the surface area of Earth is hardly more interesting than Earth. So he imagined smaller rotating rings *inside* a giant pressurized habitat, making a 3D megalopolis, which is the setting of the fictional Virga world.

Living on rotating rings in a weightless atmosphere is a cool idea, but physics-based revisions actually make it far more awesome. Adam Crowl correctly noted that with huge sizes, no hardened pressure vessel is needed. Gravity can do the work for you. If your walls are thick enough, they will simply “float” on your habitat’s air (which I call a “gravity balloon”). But you can go further. Virga is 5,000 miles across, but the same concept would apply for an asteroid 6 to 25 miles across. Hundreds of such bodies exist in the main asteroid belt alone. In their center, you could hollow out a cavity, line it with a minimalist plastic sheet, and fill it with air, entirely avoiding the need for a pressure vessel. This process of “inflating” an asteroid could get complicated. The end-state is physically sound, but the construction process is not obvious. Production of air must be balanced with mining activities which clear more space. Balancing all material movements and safety margins would be logistically complicated, but involve no magical technologies.

For a fast track to space colonies you may be able to do even better. Asteroids as large as Sylvia are known to be have lots of “missing” mass. It scientifically follows that this is evidence for large empty spaces, which are artifacts of how these rubble piles formed. We don’t know how big those spaces are, but some should be many kilometers in size. Additionally, the air could be shared between the entire network of empty spaces (or “caves”) that are about half the asteroid’s volume. How big? About 174 miles from end to end. The materials and the “box” for a massive habitat are essentially sitting there waiting for us. The evidence for this particular asteroid remains conflicting, which could still rule it out. Smaller objects would still work, and the resource of microgravity caves should be quite abundant given what we know now.

Artificial gravity by rotation, however, still has a problem with air resistance. In the style of Virga, rotating rings are free-floating in the atmosphere. Obtaining Earth gravity requires rotating at over 100 miles per hour (if under 2 rpm). This speed is relative to the surrounding air so that would feel like a hurricane. There is hope that a longer cylinder helps, but it will not fix the power consumption, which is prohibitive. My proposed solution is to surround a rotating cylinder with successive layers of flow dividing sheets. These do not need to be held firmly in place because it is in zero gravity. This method can be shown (with numbers) to eliminate the problem of air resistance. Acceptable power demands can be obtained with 10 to 20 layers of flow dividers, which may be flimsy plastic sheets. Turbulence also requires that the ends of the cylinder are tapered into a bottleneck that forms the mount / dismount points. At these points you can jump off into zero gravity or walk down into the neighborhood with gravity. Combining artificial gravity with natural asteroid central pressure is a special combination that is difficult to match. On any moon or asteroid you can build cave habitats or a shell world, requiring no material strength for pressurization. However, getting Earth gravity in those habitats would be challenging. You could use a rotating structure, but would need mechanical bearings to hold it in place. These would need to be maintained, and the weight they support would be extraordinarily large.

This design ethos has a natural hierarchical organization. A rotating cylinder’s dimensions would be limited to a few miles, similar to Manhattan. From the dismount point, getting to other cylinders would be easy. People would live in one cylinder and work in another. Since the environment is normal air and zero-gravity, you could use wingless planes. Similar details of Virga have been illustrated beautifully. You could have a billion person society within an hour or two of transit. If this is a natural asteroid cave network, there might be some structure that further divides the habitat into sub-regions. From a hypothetical Sylvian space megalopolis, the universe has these layers of organization:

Geography of Colonization of the Asteroid Belt

This forms a good network topology for a thriving economy. Quick access to zero gravity and to the rest of the solar system also help. The natural rotation of the asteroids themselves can be used as a slingshot for spaceflight. Not every asteroid could host a habitat like Sylvia, because they are a diverse bunch. The amount of volume that an asteroid can contain is limited by its mass, so you must have the end-state in mind when selecting a candidate. Because of that limitation, I divide the asteroids into several categories in terms of their characteristics as a gravity balloon.

Types of Gravity Balloon Habitats

One reason to be excited about living inside asteroids is that a new geography of the solar system follows from it. The inner solar system lacks useful candidates aside from some “small” sizes in Mars orbit. Requirements for a rocket to get there are significant, since the velocity change for a trip to the asteroid belt from Earth is 10 km/s and up (about what it takes to get into Low Earth Orbit, repeated over again). This is beyond current technology for manned missions, so either the moon or Mars would have to be base camp, supplying propellant for voyage. Plus, a decent picture of the empty spaces in asteroids is still developing. Maybe a civilization in the center of Phobos will be more pivotal than one on the surface of Mars. Perhaps 52 Europa will prove to be more valuable to future humans than Europa. Planets are interesting anthropologically, but asteroids are likely more useful for industrial society.

This weird collection of bubble-like worlds full of mysteries seems like the type of place people might prefer to live versus Earth. Stories of living inside asteroids exist, but it would be great to read more diverse and specific visions. So far, I’ve only seen extremely scarce mentions of concepts that are technically a gravity balloon, and nothing that approximates the flow divider solution to drag I’ve described. The concepts have evolved somewhat organically on the internet, and there’s obviously plenty of room for anyone to manifest the details of such a world however they see fit.

Interested in using this concept as a setting for a science fiction story? Visit for a full details of the science behind the idea.

Image credit: Danielle Futselaar / SETI Institute.

  • James Pailly

    I vaguely remember another post here on Sci Fi Ideas about people living out in the Oort Cloud with a whole clan based structure to their society. When we talk about the colonization of space, we tend to only think about the planets of the Solar System or planets in other star systems, but there’s so much more potential for the expansion of human civilization if we start talking about all those smaller objects like asteroids and comets as well.

  • AlanSE

    The main belt is most obvious for near-term, and my main focus has been on the convincingness of basic viability. One possible exception, however, is the Jupiter Trojans. Those might be equally easy to reach with a Jovian gravity assist, plus there have more volatiles.

    If we switch gears to long-term, I see three motivations that would push for outward expansion.
    – Thermodynamic
    – More candidates
    – Volatiles

    Fusion power adds a twist. If you can produce your own power, then it becomes quickly apparent that coldness is more valuable than hotness (because you can produce hot). The colder you can get your radiator, the greater “multiplier” you can get on the fusion power you produce, which follows directly from the 2nd law. The further from the sun you go, the more space there is, so the more heat you can dump at a lower temperature. But it’s also a collective problem. More neighbors means you can’t get as low temperatures. So this is the population outward pressure.

    Number of objects is obviously much greater if you include the outer reaches of the solar system. For the sweet spot of a gravity balloon, we know it’s at least around 30 times greater than the main belt. So the outer solar system could host a much larger population. However, while the number of candidates is greater by 30x, the distances are greater by 100,000x. That would leave you very unconnected, which has several problems. But then again, they might want more space considering the thermodynamic issue.

    Then you have the obvious fact that these bodies have frozen compounds, and a different composition than the inner solar system. Inner solar system is “rocky”, so you want to get heavy elements there. Outer solar system has a mix of everything.