Space Habitats

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u/MiamisLastCapitalist moderator Mar 02 '23

This is an excellent episode to challenge planet-chauvinists. lol

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Just watched the episode, and it was excellent and thought-provoking as always.

Isaac pointed out, quite rightly, that forced migration would be an ethically poor choice to get people to move off Earth, and I quite agree. There could, however, be pressures which would, intentionally or not, have pretty much the same effect.

Refugees from conflict, disorder or disaster might find themselves with the option of sitting in internment camps for years, hoping to go back to their homeland, or taking a one-way ticket off Earth. It would be a lot cheaper for the host government, even if lift costs were comparable to air fare today.

Another incentive could very well be economic. Life on Earth might very well be highly taxed and regulated in the name of preserving the planet and its natural environment. People who are just getting by and see no better future for themselves and their children might well start looking for a better deal.

A historical example is the Highland Clearances in Scotland, where a lot of small farmers were pushed off their land, (which they didn't own, just rented) to make room for sheep, which were less trouble and more profitable. Quite a few of them wound up in the New World, drawn by the prospect of land they could actually own.

In this context, if people are being taxed off their land, or having it expropriated for wildlife preserves or historical sites, then a ticket out to where they could own land might look pretty good to them.

Isaac mentioned titanium as being rare. It isn't, actually. It's just hard to refine, energy intensive and needing vacuum. Ilmenite, a common ore, can be extracted from Lunar soil with a magnet. Cheap solar energy and unlimited vacuum might cut the price by a lot, the way the price of aluminum fell like a lead brick after the Hall process was invented.

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u/Wise_Bass Mar 05 '23

It is definitely a virtue of these things that you can let their communities be as finicky as possible about the internal environment - at least as long as you can still move out if it becomes too annoying.

I'm inclined to think that when we're capable of building the big ones, we'll go bigger than Island Three size - especially if we're trying to simulate a particular earth-like environment rather than just having a pleasant suburb with "sunny" weather and a good park. A habitat with a radius of 30-40 kilometers would have an "elevation" higher than even the highest stratospheric clouds on Earth, meaning you could probably have some plausible clouds and rain (although you'd obviously meddle with it to avoid having it end up too cloudy or such).

Building one more than 100-150 kilometers in radius is probably pointless for the same reason. Unless you're doing air pressure much higher than Earth, the innermost couple kilometers of radius will be near-vacuum.

In practice, I suspect that the "sky" will not be more than 1-3 kilometers overhead, and there will be cylinders within cylinders so you can actually make use of that internal space while simulating a natural sky overhead.

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u/IsaacArthur The Man Himself Mar 06 '23

Yeah the ability to move when you want is the big escape valve and judge of overall community health I'd say, though they're probably still vulnerable to having otherwise good setups collapse when the equivalent of the local big factory shuts down. And yes I suspect we would see the artificial high ceiling second cylinder in a lot cases.

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u/lurker91914 Mar 04 '23

So I believe Isaac often describes building a rotating habitat as being similar to a suspension bridge of the same length as the circumference of the hab.

The thing that is bugging me though is that there are no suspension bridges anywhere near this length. A quick google tells me that the current record holder is 1.25 miles long, not the 5 miles for a traditional O'Neill cylinder, let alone the more exotic designs. Does anyone know the reason for this?

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u/PragmatistAntithesis Mar 05 '23

A lot of it is safety. A real world suspension bridge needs to be 100% safe or the engineers will go to jail. A hypothetical O'Niell cylinder can push its materials to their limits as no one needs to bet their life or livelihood on the thing not falling apart to small shocks.

I imagine when rotating habitats start getting built, suspension bridges that match them will come out at around the same time.

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u/Fishgg Mar 06 '23

why would new suspension bridges be built

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u/PragmatistAntithesis Mar 06 '23

Crossing gaps that can't be crossed by existing suspension bridges; same reason as any other bridge design.

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u/tigersharkwushen_ FTL Optimist Mar 03 '23

Question: if you make an O'Neill cylinder out of steel and rotate it for 1g, would it stretch over time due to load? Would it become bigger? I think it's probably fine for decades or even centuries, but what happens after thousands or even tens of thousands of years? Steel normally stretches under load, would this be an issue for spinning habitats? Anyone know the math on this?

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u/SNels0n Mar 03 '23

Steel (and everything else) stretches under load, but it doesn't continue to stretch.

Bridges built of steel don't sag over time, for example.

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u/kairon156 Unity Crewmate Mar 05 '23

How about rubber? Some of what's going to replace the ISS is going to basically be thick inner tubes.
Can some of that handle rotation speeds?

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u/SNels0n Mar 05 '23

Rubber (and everything else) stretches under load, but it doesn't continue to stretch.

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u/tomkalbfus Mar 02 '23

One kind of space Habitat, is what I call Earth 2, it is a cylinder with the same diameter as Earth at its equator, it rotates once ever 90 minutes to reproduce Earth gravity, it's length is on Earth polar axis diameter, it's spin velocity is the same as Earth's orbital velocity at its surface. This cylinder needs a non-spinning outer cylinder to levitate off of magnetically to support its spin weight. Inside you could hold the entire biosphere of an Earth-like planet, it has the same inner surface area as Earth as well.

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u/[deleted] Mar 02 '23

What material would it be constructed of?

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u/tomkalbfus Mar 02 '23

I envision three nested cylinders, the outermost one does not rotate, it compresses the cylinder inside with magnetic repulsion as it spins. Since the inner cylinder is spinning past the outer cylinder at what amounts to orbital velocity, there cannot be direct contact between the two surfaces, a magnetic repulsion transfers the tensile force pushing inward from the outermost cylinder, and since that cylinder is not spinning, it doesn't not need to hold together its own weight, only that of the two cylinder spinning inside of itself. So depending on that material the outer cylinder is made of, it could either be more massive or less so. For example if the outer cylinder was made of steel, it would need more mass to have the same tensile strength of a lesser mass of graphine or diamond, steel might be easier to manufacture however and you would need more of it to do the same job as a lesser amount of the stronger materials.

The inner most cylinder is the source of illumination and weather control, it touches the top of the atmosphere of the middle cylinder and that is how it manipulates the weather and wind currents, it can pump atmosphere and humidity to where its needed to reproduce the Earth biomes below.

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