When I was a kid, I read an essay by Asimov describing an inside-out asteroid: (summary from Wikipedia)

The Bubbleworld or Inside/Outside concept was originated by Dandridge M. Cole in 1964. The concept calls for drilling a tunnel through the longest axis of a large asteroid of iron or nickel-iron composition and filling it with a volatile substance, possibly water. A very large solar reflector would be constructed nearby, focusing solar heat onto the asteroid, first to weld and seal the tunnel ends, then more diffusely to slowly heat the entire outer surface. As the metal softens, the water inside expands and inflates the mass, while rotational forces help shape it into a cylindrical form. Once expanded and allowed to cool, it can be spun to produce artificial gravity, and the interior filled with soil, air and water. By creating a slight bulge in the middle of the cylinder, a ring-shaped lake can be made to form. Reflectors will allow sunlight to enter and to be directed where needed.

inside out asteroid - external inside out asteroid - internal

The vintage illustrations may look like an O’Neill Cylinder, but actually are a decade earlier than that concept, from Dandridge Cole’s 1965 book Beyond Tomorrow: The Next 50 Years in Space. Well, it’s 50 years later now and still dreaming about space exploration.

I can’t find the Asimov essay that I recall and other websites allude to. If anyone knows, feel free to edit. Update: It seems to be There's No Place like Spome in this collection. Update²: No, it’s not. That essay mentions how they could spread across the galaxy, but doesn't go describe blowing up with mirrors.
Update³: It might be Larry Niven, not Asimov. Another post describes the blowing up with mirrors as being from Bigger Than Worlds. Nivin may have been relating the idea from the original paper by Cole; perhaps many popularizers related the idea. But this seems a likely candidate for the copy I may have actually read, as I do remember the collection it appeared in.

Besides building habitats by blowing up metal asteroids like balloons, Asimov described (related from the more esoteric stuff he was reading, to some extent) not just “a city” but a civilisation: many such bubble-worlds would both co-operate and do their own unique thing. They may even be cast away like dandelion seeds in the wind, colonizing other star systems by people who never really leave home nor feel great attachment to the home system: political differences or the urge for exploration? Just cast off.

I was fascinated by this idea, and used it as the setting of a number of high-school writing assignments. That was pre-computer, so I don’t have any surviving hand-written manuscripts, I'm sad to say.

So inspired by the fortnightly topic challenge to break my long streak at posting Answers but not new Questions, I’m bringing this to the attention of worldbuilding enthusiasts of the 21st century, exactly 50 years after it was initially imagined.

How would we design cities in bubble shells formed by blowing up miles-wide drops of molten iron/nickel?

We have general ideas in fiction about spinning cylinders with a ring-band lake and happy parkland wrapping around to the sky.

Maybe we’ll have parkland facing the large hollow area and houses and industry below (more toward the outside). But is that still the logical conclusion?

Such a unit might be small by human population standards: one city. But is it a Tokyo or Manhattan type city? Even so, it doesn’t need to be a self-sufficient nation because there will be many cooperating cities. Could they become very specialized, such as for a single “company town”?

I’m especially interested in approaches that have not been considered in well-known SF before (which seems dated), and practical considerations that have been ignored in stories that just postulate it as a setting.

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    $\begingroup$ my humble opinion is that asteroid usually is found way further from the star, this is due to the planets orbiting around the star will literally throw their weights around (bullying these poor kids) hence any sizable habitation will need to find a reliable and abundant source of power since solar power is extremely feeble. A gentle reminder: it cold out there in space no water nor air to sustain basic human needs. $\endgroup$
    – user6760
    Apr 11, 2015 at 3:04
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    $\begingroup$ SEP (ion drive) still throws matter out the back, in this case Xenon gas. It worked great for Deep Space 1 a.k.a. "the little engine that could" and is now used for stationkeeping in some Earth sattelites. $\endgroup$
    – JDługosz
    Apr 11, 2015 at 6:11
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    $\begingroup$ @JDługosz - You might want to do a little math. Iron melts at ~1800 K. At that temperature, assuming iron has an emissivity of 0.5, the surface of the asteroid will be radiating about 300 kW / m^2. So the minimum power required for a 1 km asteroid is (in round numbers) 2 TW (emissivity of 0.5 implies reflectivity of 0.5). At earth orbit, this will require a mirror area of ~1400 km^2. Cheap or not, that's an awful lot of mirror. A more practical mirror size (for reasonable heating time) is on the order of 3 billion square meters. $\endgroup$ Apr 12, 2015 at 2:11
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    $\begingroup$ Greg Bear's Eon has an interesting description of both cities built within the Juno asteroid, with the more modern one, Thistledown, having structures shaped like golf tees, getting larger as they got taller (due to the reduced gravity near the axis of rotation). $\endgroup$ Apr 20, 2015 at 9:08
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    $\begingroup$ I think you are remembering Larry Niven's description of "Confinement Asteroid" from his Known Space series, and mistakenly thinking it was an Asimov essay. See everything2.com/title/Confinement+Asteroid. $\endgroup$ Jul 6, 2018 at 3:24

6 Answers 6


Let's start with a size. Just for grins, assume the interior is a cylinder 1 km in diameter with a 10 km length (that's 6 miles long). It will have to rotate at a bit over 1 rpm to provide 1 g at the inner surface. Total surface area is about 30 km², or about 12 square miles.

Construction. Let's assume a final wall thickness of 100 meters. The total wall volume is about 3.6 km³, so the original asteroid had to be about 2 km in diameter. Absolute minimum power level required to melt this chunk of iron will be ~ 8 TW, since at melting point it will be radiating about 300 kW per square meter in blackbody radiation. At earth orbit, that will require a bit more than 5 million square kilometers of mirror. That's a circular mirror about 2500 km in diameter.

Lighting: To provide normal daylight levels (1 kW / m²) over the entire surface, the total power will be about 30 GW. Pushing this through a port in the end cap will be hazardous. If you assume a 100 m diameter window, the power levels at the window will be just about 1 MW / m², or 1000 times the brightness the sun. This beam will have to be sent down the axis with a series of mirrors all the way down the axis to spread the light out to the surface. So the axis will be very definitely off-limits. And frankly, I don't know quite how to specify a high-quality window 100 meters in diameter which will contain 10 to 15 psi. It will have to be one piece, I suspect, since if you make it out of panels the supporting structure will have to take high temperatures (due to the power flux) and still be strong enough to hold together under the pressure. Titanium/sapphire, perhaps? I'm not sure about cost for this project. Since the colony fabrication required a truly humongous mirror array, producing the necessary light levels (even at Outer System distances) shouldn't be a real problem.

Pressure compartments. This would seem to be a good idea, as Thucydides has pointed out, but the problem of light transmission remains. The more the compartmentalization, the more windows you need, and the weaker the structure becomes.

Population. It's probably a good idea to assume that a colony should be self-sufficient in terms of food, since food production is essentially a zero-sum game over the total community of colonies. So how much area do you need to feed people? Let's assume a semi (but not completely) vegetarian lifestyle, and go with 1 acre per person. I'd rather use 2 acres, but let's say the long daylight cycle will increase productivity. Meat is rabbits, fish and chickens, but no cows. 1 acre is roughly 4000 m², so the total internal area will support about 7500 people. That's a pretty small town, so something like representative democracy ought to work. Note that you can't get around this by assuming multi-level farms: it's light levels that are the limiting factor. Also to be considered is the need to recycle water and extract nitrates and phosphates. Since the system is a closed one, you can't keep adding fertilizer to keep up crop yields without poisoning the ecosystem. Also to be considered is the capital cost of soil. 30 million m² of dirt 1 foot deep will total about 10 km³. Nickel-iron asteroids have about 30% impurities such as silicates that conceivably could provide the basis for rock/sand/dirt, but it seems to take a lot of hand-waving to explain exactly how that would be separated from what was originally a molten blob. Again, I'm wondering about cost.

Space distribution. An obvious approach would be to build living quarters up the sides of the end caps. With a total cap area of about 1.5 million square meters, that's about 200 square meters per person. Note that that's not floor space, but window space. If the living space extends 100 meters along the axis at both ends, you only lose 200 meters out of 10 km, or 5% of your farmland, and total living space is about 20,000 m³ meters per person. Even allowing for common space and infrastructure, it seems adequate. It's a tossup as to which area would be more desirable — up towards the axis or down towards the surface. The natural first tendency to go for the axis has two things going for it — exclusivity (there's less area available) and low g luxury. The less-obvious drawback to this is that living at low g's is bad for one's health. The next obvious approach is the make the cylinder walls in multiple layers, with agriculture on the "roof", and industry "underneath". Since the materials involved are presumably nickel-steel, this ought to be straightforward, but rust due to groundwater might be a real long-term structural integrity problem. Industrial areas can obviously use artificial light.

Self-suffiency. As I mentioned in the farming section, I don't think that specialized "farm-habitats" make a lot of sense. In addition to the basic question of light levels needed, transportation would be a problem. Assume a habitat has specialized in something (industry, let's say) and has a population of 100,000, which isn't much by terrestrial standards. It will need something like 5 pounds of supplies per person per day, or 250 tons per day. Moving that sort of tonnage by spaceship is a bit iffy. Half of it (food) is fairly perishable, which means that transport needs a fairly high average speed over relatively low distances, which in turn implies high thrust. Worse, any technology has to be reactantless, since reaction mass is lost and has to be imported, and this is not likely to be developed Real Soon Now. (Photon drives are reactantless, it's true. At high thrust they make really excellent death rays, and traffic control gets very touchy.)

Specialization. Although each colony would be close to self-sufficient, there are intellectual economies of scale in technological activities, which might or might not be offset by ready communication. (Some companies have found, for instance, that outsourcing creates as many problems as it solves. Face-to-face meetings can be a very good thing.) This could lead to specialization among colonies, but would always be constrained by the cost of transportation of finished goods. Particularly, if local fabrication becomes simple (think of 3D printers on steroids), colonies might well become niche designers, with designs being sold and distributed rather than finished products or parts. Dissemination of artwork would be simple enough for writers and visual artists, and craft art (pottery, sculpture, paintings, custom artifacts, etc) would probably be cheap enough to ship to allow artist colonies to be established, although the need to provide food and basic services would keep the artistic population in check. Also, it might become standard in viable colonies (that don't tear themselves apart with internal conflicts) to require everyone to put in a certain amount of agricultural labor. To do otherwise would encourage a social split between the farmers and the (artists/engineers/bureaucrats/etc) which would have the potential to end very badly.

  • $\begingroup$ Very interesting. I recall that O'Neil (later) did a more rigorous analysis with real math, and he had farms on plates separate from the main living cylinder. Natural planets have a lot going for them. Even if natural farms looks like steampunk to the future generation, the real limit is energy. If little boxes consume power and rearrange the atoms in whatever garbage you shovel in to produce replicated food, each person's metabolism consumes a minimum power budget. $\endgroup$
    – JDługosz
    Apr 12, 2015 at 5:17
  • $\begingroup$ You're assuming the boxes work on a low power/food mass ratio. If the process is inefficient, and there is no reason to guarantee it isn't, the replicated food starts to get expensive. Furthermore, since food requires physical structure if it's not going to be flavored slime or mush, the boxes may take a long time to construct an item - it has to be built up molecule by molecule in the right order. If it takes a month to create a steak, you need a LOT of replicators per person - and that impacts cost and feasibility. $\endgroup$ Apr 12, 2015 at 14:43
  • $\begingroup$ Yes, I was nothing that even if replicaters were perfect in other ways, it's still limited by energy. If the technology was not better than the old fashond way, it would not be helpful. $\endgroup$
    – JDługosz
    Apr 12, 2015 at 17:45
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    $\begingroup$ Your light level calculation appears to be based on maintaining midday peak continuously. If you maintain day-night cycles and even seasonal cycles adjusted for a temperate rather than equatorial climate you should be able to reduce the window power level massively. $\endgroup$
    – Separatrix
    Jan 30, 2017 at 8:54
  • $\begingroup$ The animal protein could be provided by crickets. They are much more cost-efficient food out into raising them vs how much food you get out of it than contemporary meat-animals. $\endgroup$ Sep 27, 2020 at 22:28

Creating a space inside an asteroid is probably the simple part (you could simply dig out the interior), but your question is more about what happens afterwards.

Most designs call for massive open space inside, but for safety I suspect it would make a lot more sense to have the interior full of "bubbles" so any issues like a puncture or disease don't spread or cause cascade failure. This also allows you to isolate activities like agriculture, industry and other things so they don't interfere with each other.

Illumination could come from a mirror of any arbitrary size (even out into the Kruiper belt). Having the spin axis pointed at the sun allows the mirrors to focus the light into the asteroid, while the pole opposite the mirror can be clear to allow for spacecraft to dock without impediment.

The central axis would probably have a cable for transportation between poles, and more bubbles could be hanging from the cable to make a free fall "suburb" inside the asteroid.

Since the asteroid will be full of ice and other valuable raw materials, a large part of it will be a mining site, although probably covered from view for most of the people who live inside the asteroid.

As for being a "nation", it has the potential to be a very large city state due to the three dimensional nature of the interior volume, and millions of people could be living inside. Because the actual spacing of asteroids is so far apart, they will be largely isolated and independent of each other.

If they are city states, then the future might be a reprise of the ancient past, with the asteroids becoming much like the city states of classical Greece.

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    $\begingroup$ Not a big open space: that's what I'm getting at. City-states: nice. $\endgroup$
    – JDługosz
    Apr 11, 2015 at 6:05

The main problem I see here is retaining your atmosphere. Rock is all very good for building on but it is far from air-tight.

Your spinning cylinder forces air against the rocky walls, acting as a centrifuge for diffusing your oxygen in the rock and, eventually, having it escape out the other side and into space. Here is quite a neat experiment showing the diffusion of gasses through rock in a centrifuge. If you want to use this as a seed ship spinning at 1g and still retain your oxygen between stars you need something to combat this.

Either design a nonporous coating of outside of the city with to retain the oxygen. Even melted rock would still absorb some oxygen and result in a loss.

Or we fill the rock with another, denser (but non-reactive) gas first. You fill the balloon of nickle/iron with this gas (lets say argon) and then the same centrifugal forces push it out into the gaps in the rock where it takes longer to pass through and provides a barrier against your oxygen doing the same thing.

One problem, however, would be that if the asteroid was made to slow down its spinning then the heavy gas would rise back up through the rock and suffocate everyone at ground level...not pretty but perhaps a plot device...

  • $\begingroup$ On your last paragraph, there's not a lot of events that will significantly decrease the angular momentum of something that large without killing everyone anyway, so I'm fairly sure that's not going to be a major problem. $\endgroup$
    – Gryphon
    Nov 17, 2017 at 1:52

First question that I think needs to be asked is whether there are in fact such things as nickel-iron asteroids of that size. We do have half a century more science to build on, and spacecraft have visited several asteroids. My impression from casual reading of the findings is that most smaller asteroids are in fact rather weakly-consolidated "rubble piles". which means you'd have to do a lot more material processing to turn them into a working habitat.

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    $\begingroup$ Maybe less: burrow in, inflate a balloon and let the gravel coat the surface, without fracuring solid rocks to do so. Then, sinter the shell together, which does not require melting the whole thing at once. As for whether most asteroids are loose rubble piles, the first one we inspected was not, so there are at least populations of both kinds and probably a continuum. Gravel coating a core of several large chunks. $\endgroup$
    – JDługosz
    Apr 12, 2015 at 5:23
  • $\begingroup$ But the sintered shell has to be a) leakproof; and b) strong enough to keep the shell from flying apart under a substantial fraction of 1 g outward acceleration. $\endgroup$
    – jamesqf
    Apr 12, 2015 at 18:01
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    $\begingroup$ Right. They can go over the whole thing carefully, rearrange material and add stuff, even re-melt a spot. It takes far less energy than melting the whole thing and you don't do, it all at once. $\endgroup$
    – JDługosz
    Apr 12, 2015 at 22:15

You don't inflate the asteroid, you separate it out, then inflate/spin the iron to make a shell.

  1. asteroids that size are rarely solid, instead they are rubble piles, so you first burn off all your volatiles (and capture them) then you have to melt your metals in a container that you can inflate along with it, upside by spinning it along with inflating it you can get decent control of the thickness.

  2. then you build your internal struts and compartments. it needs to be reinforced if you plan to spin it for gravity.

  3. once you have your airtight shell, you cover it with layers of solidified rock. The rock is both your radiation shield and your impact shield. this is also when you cut holes in it. 2 and 3 can be done at the same time.

  4. now you put some of your gasses back in and build your habitat.

the key to this is using rock/mining slag to coat the outside, either as bound rubble or remolded coating. this will look like an asteroid, and provides cheap durable radiation shielding.

Or you just drill a hole.

the other option is find the rare solid asteroid of the right size, drill a big hole in it, sort of hollow it out, maybe 30–40% of its volume at most. then you melts some iron and coat the inside with it to make it air tight. This is less structurally sound but easier to make. This can only make small constructs however.


This is my favourite topic of thought.

I absolutely love O'Neill Cylinder concepts and I think conceivably they are the best solution for future humans to exist in.

So here are my thoughts:

First let's get something cleared up about Farming - it actually makes little sense for any farms to be in an O'Neill Cylinder. In terms of resources, it would be far easier to have hydroponic, or aeroponic, automated satellite farms in zero gravity nearby. Optimised in environment for maximum yield, able to re-use water and injected nutrients, controlled in space, these would produce the most food per cubic unit of volume.

Similarly, meat production would likely be cultured meat - it makes no sense to have cows or chickens using valuable space inefficiently when robots can make these in zero-g environments in ultra dense satellites attached to the cylinder.

And industry - in reality, it is much easier for robots and computers to actually do production, which would happen in space. Humans can still programme or coordinate, but the role of work would definitely be less primary and secondary industries and more tertiary or entertainment.

So what is left? Commerce and Lifestyle.

So living in an O'Neill cylinders, now removed of the need to be food generators, can now concentrate on Commerce and Lifestyle - and what possibilities!

A true 3-Dimensional entertainment and commerce city With the advantage of normal gravity at the edge, and zero-g in the centre, you could really have a major entertainment focus:

  • Imagine an enormous cruise ship, with slides, cinemas, rides, gyms, function halls, restaurants in an amazing 3-dimensional matrix of varying fun experience. Your kids would have a ball of a time! (While you do your serious discussions with other adults on the edge at 1g)
  • Zero-g sports arena in the middle for the best Olympics or competitive sports ever
  • A continuous looping ocean Cylinder, for those sailing enthusiasts, with controlled weather.
  • A jungle Cylinder, with plants linking one end to the other, people living in low-g tree houses, having a ball of a time, or intently studying ecosystems
  • A marketplace Cylinder. Imagine the Grand Bazaar of Istanbul, but in a huge 3-dimensional matrix with different gravity areas. Goods and services can be transported and distributed throughout easily via arteries, and commerce could occur in all sorts of environments. A low-g haircut, or beauty therapy, zero-g boardrooms and university lecture theatres, or even high-g fitness centres!

The possibilities are just mind-bogglingly numerous, and exciting beyond belief. It may actually be that with all of the above, and your ability to visit many different types of cities and cylinders, that other possibilities may yet reveal themselves that we haven't even dreamt of...

  • $\begingroup$ I suppose any farms in the habitat cylinder are along the lines of a Heritage Museum and concert venue, producing a small quantity of boutique products at high prices, but are not the source of the vast production of food. $\endgroup$
    – JDługosz
    Sep 30, 2021 at 15:24
  • $\begingroup$ I don't know about crops being good in zero-g though, in general. You may need Earth-normal gravity for sprouting, and even if established plants are adapted for no gravity, hydroponics works better if the water stays on one side of the chamber in one contiguous shape, rather than floating around as globules. $\endgroup$
    – JDługosz
    Sep 30, 2021 at 15:26

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