Living Inside An Asteroid

Deriders of the new NASA direction have latched on to the announced human asteroid mission in the 2025 timeframe as something they "can't imagine" and therefore is not worth doing. Of course, the administration is talking up the "science" that can be done on an asteroid, and how this could better inform us should the need arise to divert or destroy one that threatens Earth. This is good politics as nothing motivates like fear, but for those of us who think the human spaceflight program is really about preparing us to live at the future homes of humanity, asteroids would seem to be just a stop on the way - I disagree.

As I've written previously, the new NASA direction isn't about asteroids - it isn't about destinations - it's about going and specifically, it's about going to Mars. I'm not sure NASA knows yet why they're going to Mars, but they're focusing on the technology to get there and get back safely, and some of the stepping stones along the way are asteroids. As such, although I will often advocate that I think asteroids are a much better future home for humanity, I recognize that in terms of the battle lines of this debate, asteroids are neutral or worse, disposable.

So how does one live on an asteroid? I've regularly heard this question asked by intelligent people. They point out the low gravity and how with just a misplaced step an astronaut could be hurtled into escape velocity and lost forever! NASA's mission to an asteroid will most likely be conducted on the surface, so this is a real risk, just as it is for astronauts conducting spacewalks on the International Space Station. However, the settlement of an asteroid would have little use for the surface, except perhaps as a place to lay solar panels, as all the interesting stuff happens below the surface.

The primary reason is radiation. Just like on the Moon or Mars, humans will need to live underground to provide passive protection from galactic cosmic rays and solar storms. On Earth (and Venus) the predominate protection from radiation is provided by the atmosphere, miles and miles of it. To achieve the same level of protection only a dozen feet or so of regolith is required.

Robotic probes will be sent ahead of NASA's human mission to an asteroid. More than likely, only an orbiter, but a much more capable robotic lander makes a lot of sense. For the long term settlement of an asteroid, it will carry essential drilling equipment which it will use to drill straight down. After digging down for a while, the robotic drill will turn some significant angle and keep drilling. The hole it produces need only be big enough to maneuver a crew module into without bumping the sides - once they arrive, weeks or months later. The right-hand-turn the drill makes is sufficient to protect the crew from radiation, which can only move in straight lines. If mirrors are installed on the turn the crew can enjoy natural sunlight and a view of the stars.

Having secured the safety of the crew from ionizing radiation, they are now free to get to work. Using drilling tools the astronauts can prospect deep into the core in search of the richest metals, or collect volatiles which can be purified into drinking water or oxygen for breathing.

Soon, they'll dig a long circular tunnel with a radius of at least 894 meters. The outside edge of the tunnel is lined with metal track. A simple electric train runs the length of it, completing a full circuit in just one minute. On a parallel track the astronauts enter an open carriage which accelerates them up to rendezvous with the ever moving train. As they speed up the astronauts feel the gentle pull of centripetal force as it builds to a full Earth-standard gravity.

As the astronauts step onto the train they cease being astronauts and become settlers. They now have access to resources, protection from radiation and a full Earth-standard gravity. The colony can now grow. The train can be extended compartment by compartment and deck by deck to accommodate the growing population. Excess metals and other materials can be exported to other settlements in the solar system.

Another one of the things the settlers may wish to do is to place airlocks on some of the tunnels that lead to the surface and place transparent plastic material or even glass over the ones dedicated to bringing in natural sunlight. That way the entire internal space of the asteroid can be pressurized and the settlers will be free to work and play in zero-g without spacesuits.

Here's an animated gif of humanity's future home inside the asteroids.

Imagination is a precious part of space advocacy. Yes, we must guard it with scientific skepticism but not so much that we're afraid to dream.


  1. Anonymous10:47 AM

    Heres a link to an article and images that I think you and your readers might like:
    Niven shamelessly stole these ideas for "Confinment" in his Known Space stories. Dandridge Cole is at the apex of my Belter Heros! (Along with Donald Cox of course.) The beauty is we start small, reforming rock and ice into small spin habs. Then work our way up to spin re-forming Ceres! Now thre's a project.

  2. I think it would be a lot easier just to build a large rotating space habitat next to the asteroid taking advantage of its materials. Eg. with a dense material, e.g. iron, from the regolith, you only need a meter or two for earth surface (or at least high mountain) level rad protection. Depending how close the habitat is to the asteroid, it also shields some percentage of the solid angle as well. Since there are no transport costs involve, you can build up the shielding as high as desired over time. (O'Neill and students showed way back when that rotating a large massive habitat in free space can be done without any magic engineering or structural materials required.)

  3. Clark, absolutely, but what's the duration from starting construction to when your bones stop melting from zero-g exposure? How's that compare to building a train and a track? You could certainly do both but it's nice to have a low tech solution to start with.

    That's the major flaw I find in O'Neill's thinking.. it was all predicated on huge infrastructure to get raw materials off the Moon, catch it at L2, transport it to L5, process it in orbit, build a giant metal structure, wrap it all in glass.. how long is that supposed to take? Decades? How many crew flights is that? Hundreds?

    A human mission to a significant sized NEA will likely take 2 to 3 months.. maybe just the one month with some technology improvements. So, at worst, they have 5 to 6 months before the nagging flight surgeons will be demanding they get some gravity.

    Assuming the vehicle they travel there in can't provide both radiation protection and artificial gravity, (and if it could, that would significantly raise the cost of the mission and delay it's launch) then it seems to me that you either need productive robotics that can build a complete rotating habitat before the humans arrive (again, making the mission more expensive and delaying its launch, if it's possible at all with current robotics tech) or you need a quick and simple way to keep the crew safe and healthy while they found the colony.

    With the resources of an entire asteroid at your disposal you can build O'Neill colonies and fly them to anywhere in the solar system.

  4. Jonathon1:07 AM

    Fascinating read. I do tend to think the complexities of a circular train inside the asteroid might be more complicated than that of a rotating station, however there is a point of compromise. We don't know how much partial gravity it would take to stop the onset of bone loss.

    If low gravity is enough, it might be possible simply to have a small track operating enough to give people shifts of excersize on the tracks for a limited period of time per day, while the main station was built.

    The low grav train would still have future use once the main station was built as an ore delivery stystem, brig, or storage area.

  5. Trent, I believe a compromise is in order between your scheme and that of Clark Lindsay's. This is useful for 2 reasons. One, the probable thickness of an acceptable radiation shield is more like 6-10 meters, because of primary cosmic rays generating secondaries. Those secondary particles turn a cosmic ray rifle bullet into a shotgun blast of radiation, doing more damage than the original bullet. So, we really do need enough protection to stop the secondaries, ..about 6-10meters. radiation protection.

    Two, the costs of tunneling are huge here on Earth. The only thing that *might* decrease those on an asteroid, (or a Mars moon)is the chance that they are the "orbiting gravel piles" that some measurements of their tumbling rates seem to make probable.

    Absent that, as on Eros, the set-up for a sufficiently large tunnel project is simply astronomical when moved to an NEO. So, put the track *on*the*surface*, and cover an enclosure for that track with at least 10 meters of asteriod regolith. Then, run the trains on that track through the artificial cave created there, canted at 90 degrees to the flattened surface track bed. Lots less capital expended per cubic meter of living space that way.

    Of course, if outgassing of ice deposits has left *natural*voids* within the asteroid large enough for your scheme, then we may be looking at an even easier way to get the job done. Then we could just run your train through the cave, with a little smoothing of track bed surface needed about the same as would be needed on the surface.

  6. One of the things I happen to like about asteroids is that there are so many of them. Once you get the technology set up to go to one of them, it can be a "wash, rinse, repeat" type strategy when you move on to the next one. All of the infrastructure needed for one of those destinations is also essentially the same, with only modest modifications as you encounter different types of asteroids. A fair bit is already known about quite a few of them due to spectral analysis, but I'm sure there would be some fairly standard practices once it gets going.

    Going to Mars, on the other hand, is something that requires some significant architecture development and isn't replicated for anywhere else in the Solar System. Indeed, I would argue that learning how to cope with asteroids can even help with the settlement of Mars as Phobos and Deimos would be excellent "jumping off" points in terms of building infrastructure (fuel depots, "storage" facilities for interplanetary components, etc.) and it would be much easier to do a manned landing on Phobos than it would be on Mars. Build up the supplies and materials on Phobos with knowledge gained by working on asteroids, and then getting to Mars would be relatively trivial by comparison with a "resupply" depot handy that could support missions on the surface of Mars. Doing stuff on Mars is going to need some substantial logistical support, and using the asteroids first can make a big difference.

    In terms of mining on the Earth, one of the huge problems facing most miners is trying to keep the mountain above the mine from crushing down on top of you. While tons of rock still is some substantial mass, you don't have to deal with the crushing pull of 10 m/s^2 constant acceleration of all that mass as well such as happens on the Earth. All in all, it would seem as though most mining operations on an asteroid would be akin more to tunneling operations for a subway rather than the deep underground mining such as is done for coal and diamonds on the Earth.

    Compared to the price of some of the spacecraft currently being proposed, a machine like the Chunnel digger would at least be comparable in price. Getting something like that set up for use in space would be a bit trickier but I don't think it would be substantially more complicated.

    BTW, I love the train concept, as it is a simple technology that already has most of the theoretical work done for it. None of this requires new physics (space elevators) or really even much in terms of new engineering other than applying current technology to a slightly new environment. The technical hurdles here are mainly in terms of how to get to the asteroid in the first place and how to keep the miners/settlers sustained until the permanent facilities are finished.

  7. Thanks for the comments everyone.

  8. .
    maybe, something like this?

  9. Is Gaetano Marano an old troll? I consistently see him on space sites hawking his bizarre website. The link he posted mentions asteroids and robots, nothing about living in them. Why?

  10. Trent I'm not sure if I got the point being made but if it was that using an asteroid would be fast and simple:

    1. Drilling on a relatively large scale is a challenge in the space environment, it's hard enough to do on the Moon that no one has done anything at all except extremely small and limited core sampling (most of it manually). It seems sure to be even more difficult on and in asteroids.

    2. As good as all asteroids rotate and tumble in ways that impact the purpose of a simple internal habitat unless one changes their movement, a task which is also extremely challenging and massive. While slow there is also rotation/torque continually created by differences in albedo and blackbody radiation.

    There's much more but I think I've already shot down any notion of this being fast and simple.

    I don't remember where I first saw the concept of using asteroids for internal habitation but I've always liked it so don't get me wrong.

  11. The average density of all the asteroids which have had probes sent to them is less than the density of the lunar regolith. We're not even talking about drilling here, it's more like sand than dirt.

    To support this we have meteorite data:

    C2-type: 3.3 g/cm^3
    C1-type: 2.0-2.8 g/cm^3
    S-type: 3.5-3.8 g/cm^3
    M-type: 7.0-7.8 g/cm^3
    Lunar regolith: 1.5-1.9 g/cm^3

    But the lesson of the Galileo spacecraft was that meteorite data lies to us, because it has been exposed to atmospheric entry which burns up all the loose material leaving only the more solid core. 951 Gaspra's density was estimated at 2.7 g/cm^3, almost 22% less than the S-type meteorite data would suggest. 243 Ida's mass was estimated at 2.6 g/cm^3, backing up the finding. NEAR Shoemaker provided more evidence with a 2.67 g/cm^3 mean density for 433 Eros. Hayabusa has since provided yet more evidence that the meteorite data is completely wrong, with an estimated mean density for 25143 Itokawa of 1.9 g/cm^3.

    Compare the density of S-class asteroids to our common experience:

    Talcum powder: 0.88 g/cm^3
    Clay: 0.8-1.12 g/cm^3
    Earth: 1.17-1.22 g/cm^3
    Sand: 1.59-2.08 g/cm^3
    Iron ore: 2.59 g/cm^3
    Steel grit: 3.60 g/cm^3
    Tungsten Carbide: 4.00 g/cm^3

    Now, M-types, that's a different story.

  12. While the following is just my layman take on the issues and ideas discussed I have to say that if that solves the need for any actual drilling it replaces it with the problem of scooping ^_^ or even blowing or melting it away in a very precise manner. All the methods share many of the same core difficulties like equipment traction and all other Newtonian "counter-forces" that have to be mitigated and controlled in order to get functional tools --major difficulties even in a Lunar 6th of a g. It also adds different depth requirements due to less massive cover (could possibly be trivial depending on the asteroid size) as well as increased/additional structural requirements to take and disperse the loads from what's essentially a very large internal flywheel. It could all easily boil down to such an asteroid becoming more of a materials source than anything else and possibly not a cost-effective one compared to using material from the Moon.

    All that said it can be done, I'm absolutely convinced it can, I also think it's a great idea (although likely not the best one) and something that in my opinion should be very high up on the list of priorities. Maybe 5th after (not in order) smaller scale/smaller mass in-space human-inhabitable/usable artificial gravity research and testing, lower launch costs, ISRU, and a range of tech development (the usual stuff really) that includes the tools to do this or somewhat equivalent concepts (of course I have my own private hobby horses in that regard but I won't post any of that unless I can get it further than its present state ^_^).

    However (sadly) it still won't be fast and simple (nor will my own alternative), I don't say that to discourage, only for perspective and to avoid overselling the concept and it's "simplicity" (including the "simplicity" of artificial gravity by rotation) as has happened with the O'Neill cylinder (and the Bernal sphere and Stanford torus as well to a more limited extent).

  13. Richard Dowling9:06 PM


    I found this post because I was researching asteroid mining--I love the idea of a train circuling the interior!

    I'm a layman so forgive me for asking what may be stupid questions but maybe you wouldn't mind answering a few points:-

    How would it feel for the miners inside the train? Where would up and down be in reference to the train compartment? Would the up and down be constant or would they change?

    Would the rotation of the asteroid also impact on the notions of up and down?

    What would be the power source for the base?

    I'm working on a novel set on an asteroid so I'm particularly interested in the reality of living and working in such an environment.

    Many thanks,

    Richard Dowling

  14. Richard Dowling9:07 PM

    Sorry, that should be "circling" not "circuling"!

  15. On the train, "down" would be radially outwards from the center of rotation, so it would be a normal feeling. This could only be done in a very large asteroid, over a few km in diameter, and such asteroids don't rotate very fast otherwise they would fly apart, so the effect from rotation should be minimal. For power, I was thinking solar panels either on the surface or co-orbiting with the asteroid, depending on what technology is available when you go. If a government was doing it, some small nuclear power plant would be great, but eventually switching to solar power or some day fusion would be necessary for self-sufficiency.

  16. Richard Dowling8:57 PM

    Many thanks, I really appreciate that! If I understand it right the train is moving at roughly 60km per hour, correct?

  17. Anonymous12:16 PM

    To save lots of shovel work I wonder if it would be practical to just drill a hole, shove a balloon down it, and inflate to create a habitat volume.

    Andrew W

  18. I've expanded (no pun intended) on the balloon idea here:

  19. okay I could be very wrong but wouldn't a train running circles inside the asteriod make the asteroid start to spin in the opposite direction cuasing the train to technically spin slower and create less g's?

    Also if the asteriod has less than one g holding it together wouldnt a mass push out from the inside at one g just push the asteriod apart? espacially if its not very dense?

    I do like the idea of useing asteriods as a stepping stone or even for colonizing. but to me it seems much more important to focus on life support vs where we will be. personally i would never trade earth for an asteriod if it meant living on just barely enough oxygen and very limted food and water.especially if i was to bring my family. and a couple of scientist/astronuats hardly make for a colony. So we need to be able to live comfortably. until we can build a proper working biosphere life any where other than earth will be very difficult and limited.

  20. The asteroid would counter-rotate, yes. Much much slower than the train as it has much much more mass. Even if the mass was identical, that wouldn't change the "gravitational" force generated.

    The assumption here is a solid asteroid, not a rubble pile, however even in that situation, the load is on the track, not the mass of the asteroid, so the binding to the counter-mass can still be managed.

    I think you're misunderstanding the scale of the near earth asteroids. One the size described in this post is many millions of tons of material. Enough to support millions of inhabitants for their lifetimes, depending on the energy sources.

  21. I included mentions to this writing about a different idea. Your proposal was one of the most similar things out there.


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