Author Topic: Shell Worlds: "Man Caves: Humanity’s Next Home" by Ken Roy  (Read 10842 times)

Offline Paul451

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Article about shell worlds: "Man Caves: Humanity’s Next Home" by Ken Roy



An extreme form of paraterraforming. Shell worlds are based on the idea of using the weight of any dumb mass to compress an atmosphere to any arbitrary pressure (such as Earth SL pressure) on any arbitrary body, creating a bubble of breathable air of whatever thickness you wish, around an entire world. The shell holds in the air, the air supports the weight of the shell. The atmosphere under the shell can be thick enough (8-10km) to have "normal" weather.

Because the dumb mass of the shell is being supported by the air, there's no real tensile or compression force. The author talks of a couple of metres of steel, topped with regolith (or ice and regolith). Therefore it only needs existing technology, it's the scale that is... ahem... advanced.

The shell is thick enough that the occupants are better protected from radiation than Earth. Not just ordinary solar and cosmic rays, but extinction level events like nearby GRBs.

Interesting, the mass of the shell is almost the same, around 10^18 kg, regardless of the size of the object or planet. So for Ceres, it's roughly 1/10th of 1% of its mass. The author thinks Ceres is a small as you can go, but I believe any of the largest dozen main-belt asteroids should be practical (along with a crap-tonne of moons.)



The author misses that you can have multiple shells, with an atmosphere of decreasing pressure between each layer, not just a single shell. That means part of the dumb mass can be usable/habitable/farmable. For example, have a 1atm layer with a shell that has a a dozen metres of regolith plus 50-100m deep ocean on top, plus a half-pressure atmosphere, then another shell on top of that. Multiple shells also gives you safety/redundancy.

Also, you can build a surface around a gas giant using two layers (although we're pushing that "existing technology" thing...) The first shell rests on top of the hydrogen atmosphere of the gas giant, then you add a few kilometres of breathable air, held by the outer shell. Saturn would give you 1g surface gravity, which is nice, Neptune and Uranus slightly less. This would also be a way to terraform Venus. No need to find a way to lock up that extra carbon, just hide it under the rug.
« Last Edit: 12/31/2019 02:12 am by Paul451 »

Offline RotoSequence

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Maybe this is what Elon Musk was referring to when he tweeted about building his own secret underground city "like in Neon Genesis Evangelion."  :o ;D

https://twitter.com/elonmusk/status/1210777492027363328

Digging up, digging out, and back-filling a canyon in Mariner Valley is probably a quicker and easier approach than bootstrapping an atmosphere on the entire planet.
« Last Edit: 12/31/2019 04:34 am by RotoSequence »

Offline sanman

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This could certainly be a cool setting for some kind of sci-fi/fantasy story.

(I'm talking to you, Netflix!)   :P


Online edzieba

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Same physics issue as the 'old' meaning of shell-worlds (wrap a solid surface around an arbitrary point-mass at such a separation to get a 1g surface gravity on the outer surface of the shell): the setup is unstable, the 'contained' mass will drift into the shell wall (or the shell wall will drift into the mass, depending on your point of view) without constant active control and manipulation to keep it centred.
On top of that, the ~2x10^18 kg mass makes it uncompetitive in mass-per-unit-habitable-area compared to a rotating station (e.g. Island 3 on the order of 4.5x10^12) until you want to build more than a million stations.

Offline RotoSequence

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Same physics issue as the 'old' meaning of shell-worlds (wrap a solid surface around an arbitrary point-mass at such a separation to get a 1g surface gravity on the outer surface of the shell): the setup is unstable, the 'contained' mass will drift into the shell wall (or the shell wall will drift into the mass, depending on your point of view) without constant active control and manipulation to keep it centred.
On top of that, the ~2x10^18 kg mass makes it uncompetitive in mass-per-unit-habitable-area compared to a rotating station (e.g. Island 3 on the order of 4.5x10^12) until you want to build more than a million stations.

Mass efficiency is probably not a major concern if you're working on the surface of a planetary body. A more modest air supported structure would be more readily assembled, and is a proven, if out of favor, technology.

Online edzieba

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Same physics issue as the 'old' meaning of shell-worlds (wrap a solid surface around an arbitrary point-mass at such a separation to get a 1g surface gravity on the outer surface of the shell): the setup is unstable, the 'contained' mass will drift into the shell wall (or the shell wall will drift into the mass, depending on your point of view) without constant active control and manipulation to keep it centred.
On top of that, the ~2x10^18 kg mass makes it uncompetitive in mass-per-unit-habitable-area compared to a rotating station (e.g. Island 3 on the order of 4.5x10^12) until you want to build more than a million stations.

Mass efficiency is probably not a major concern if you're working on the surface of a planetary body. A more modest air supported structure would be more readily assembled, and is a proven, if out of favor, technology.
You will need to acquire that mass from somewhere & refine it, as well as loft it above your target body. As the 'pressure supported' structure only works once the structure is complete and pressurised, lifting it to altitude either requires enormous amounts of scaffolding structure (to support a planetoid-wide peta-tonne roof several kilometres high), or launching your shell into orbit in sections before joining and despinning once the atmosphere is pressurised. Either are enormous energy and mass expenditures that could also be served launching a fraction of that mass into orbit and assembling into stations instead. This is true even if you;re harvesting the host body for mass, but much moreso of you need to import your mas from another body.

Offline RotoSequence

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You will need to acquire that mass from somewhere & refine it, as well as loft it above your target body. As the 'pressure supported' structure only works once the structure is complete and pressurised, lifting it to altitude either requires enormous amounts of scaffolding structure (to support a planetoid-wide peta-tonne roof several kilometres high), or launching your shell into orbit in sections before joining and despinning once the atmosphere is pressurised. Either are enormous energy and mass expenditures that could also be served launching a fraction of that mass into orbit and assembling into stations instead. This is true even if you;re harvesting the host body for mass, but much moreso of you need to import your mas from another body.

A shell-"tent" only requires an atmosphere supporting bag, a protective layer on top, and a bunch of junk mass that can come from whatever dirt you scrape off the surface, particularly on the Moon and Mars. Mass efficient or not, I don't think any large scale space habitation will be cost competitive with this approach without staggering reductions in the cost of producing goods in space at Earth's terrestrial scale.

Offline RonM

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Issac Arthur has a video on shell worlds. As always, he takes it to the extreme.



Offline Paul451

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the setup is unstable, the 'contained' mass will drift into the shell wall (or the shell wall will drift into the mass, depending on your point of view) without constant active control and manipulation to keep it centred.

Incorrect. If the gap between the gravitational mass (in this case, acting as the surface) and the shell is large enough to permit a pressure gradient, then there will be more pressure on low points of the shell than on high points. Given that the gravity on a sphere is uniform, even a slight difference in the pressure will continually centralise the shell (or the contained mass, depending on your point of view.)

You might want to have a frame of some kind to assist with uneven masses on the shell, and to stop (or perhaps encourage) its rotation, but the loads will be slight thanks to that self-centring effect.

On top of that, the ~2x10^18 kg mass makes it uncompetitive in mass-per-unit-habitable-area compared to a rotating station (e.g. Island 3 on the order of 4.5x10^12) until you want to build more than a million stations.

True. But unlike an Island 3, that mass can be almost anything. The actual "shell" can be much, much lighter, due to the lack of loads. The bulk weight isn't structural. It can be low value regolith, or ice, or stores of useful bulk materials. It can be the mass of industry, it can be solar panels, it can be radiators, it can be tanks of useful volatiles, or tanks of waste. It can be multiple individual shells, each with their own unique atmosphere and levels of radiation protection, for agriculture, and aquaculture, and aeroculture, and industry, and storage, and....

The "wasted" mass of a shell world, compared with a pure rotating habitat, is the contained mass. It suffers from the inefficiency of all planets. But presumably you are building the shell-world either to exploit the material inside it, or because you aren't yet technologically capable of utilising the material (as in the case of Venus or the gas giants.) Whereas a rotating habitat can only use material that has already been processed.



IMO, the issue with a shell world is the construction. Unlike other types of paraterraforming, or building individual rotating habitats, it doesn't lend itself to natural expansion, it's all or nothing.

Unless someone can come up with a better construction technique than either "cover the whole thing with a deflated balloon, inflate the balloon" or "build thousands of orbital rings until you cover the whole surface, then string a balloon between them, then inflate the balloon."

Offline Paul451

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Digging up, digging out, and back-filling a canyon in Mariner Valley is probably a quicker and easier approach than bootstrapping an atmosphere on the entire planet.

It doesn't make sense to do it on a planet (unless the planet doesn't have a usable surface.) You only save part of the required mass of atmosphere, and nothing on the required water, etc.

Shell-worlds just show that you can create better worlds than even a fully terraformed Mars. It breaks some of the planetary monomania that space advocates have; the choice isn't between "living in tin cans" and "living on a second Earth".

Offline RotoSequence

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Digging up, digging out, and back-filling a canyon in Mariner Valley is probably a quicker and easier approach than bootstrapping an atmosphere on the entire planet.

It doesn't make sense to do it on a planet (unless the planet doesn't have a usable surface.) You only save part of the required mass of atmosphere, and nothing on the required water, etc.

Yes you do, if you don't try to build a garden world out of the entire planet.

Offline envy887

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IMO, the issue with a shell world is the construction. Unlike other types of paraterraforming, or building individual rotating habitats, it doesn't lend itself to natural expansion, it's all or nothing.

It's basically a dome with gravity containing air pressure, made large enough to cover a world. Domes can be arbitrarily small, so expansion is easy, at least up until the point where you have covered the entire world.

Start with a 10 m dome, then build a 100 m dome, then 1 km, then 10 km, then 100 km, and so forth until the world is covered. All of these can be connected with airlocks for redundancy. If you run out of space, cover over one or all of the earlier smaller domes.

Online edzieba

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That doesn't work this this technique: it relies on the mass of the shell to counter the internal pressure. If you have a dome, its own mass cannot do this (as the dome surface normal is only aligned to local gravity at the peak) so the dome must be built to mechanically resist the pressure, defeating the entire point of the concept and leaving you with a regular old dome.

Offline envy887

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That doesn't work this this technique: it relies on the mass of the shell to counter the internal pressure. If you have a dome, its own mass cannot do this (as the dome surface normal is only aligned to local gravity at the peak) so the dome must be built to mechanically resist the pressure, defeating the entire point of the concept and leaving you with a regular old dome.

If you have a high aspect ratio dome (that is, much larger in diameter than in height), then gravity can supply the vast majority of the containment force. The radial force does need to be mechanically contained.

Offline Coastal Ron

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That doesn't work this this technique: it relies on the mass of the shell to counter the internal pressure. If you have a dome, its own mass cannot do this (as the dome surface normal is only aligned to local gravity at the peak) so the dome must be built to mechanically resist the pressure, defeating the entire point of the concept and leaving you with a regular old dome.

If you have a high aspect ratio dome (that is, much larger in diameter than in height), then gravity can supply the vast majority of the containment force. The radial force does need to be mechanically contained.

ISTM there is nothing keeping the internal mass from bouncing around inside of the shell.
If we don't continuously lower the cost to access space, how are we ever going to afford to expand humanity out into space?

Offline Mark K

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the setup is unstable, the 'contained' mass will drift into the shell wall (or the shell wall will drift into the mass, depending on your point of view) without constant active control and manipulation to keep it centred.

Incorrect. If the gap between the gravitational mass (in this case, acting as the surface) and the shell is large enough to permit a pressure gradient, then there will be more pressure on low points of the shell than on high points. Given that the gravity on a sphere is uniform, even a slight difference in the pressure will continually centralise the shell (or the contained mass, depending on your point of view.)


If you are using pressure differential as stabilization all the points about the shell not being under stress just went out the window and you are trying to build a structure that will have to contain resonant pressure gradients that would tear up regular material to shreds. The shell would bounce around the planet like on springs and and would create shear stresses with huge component forces.

These things are not even in dynamic equilibrium without restoring force so any little thing will start the shell moving.
Once moving the pressure forces you mentioned will act like a spring. There will then be a stress gradient from the lowest point of the bounce to the point highest that will have a component not normal to the surface of the sphere.
Lots of things will cause these - differential heating in different areas for example, tidal forces, etc.

It would probably be very exciting while it lasted.

Offline sanman

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What maintains the strength of the shell? Is a wonder-material required, like graphene or nanotubes? Or Unobtainium?

Or can it be done with more conventional materials?

Offline Paul451

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The shell would bounce around the planet like on springs and and would create shear stresses with huge component forces.
These things are not even in dynamic equilibrium without restoring force

How is a spring not in dynamic equilibrium, providing a restoring force?
« Last Edit: 01/04/2020 05:39 am by Paul451 »

Offline darkenfast

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A slight aside: Brian Stableford's "Journey to the Center" (1982) is set on an ancient alien world or artifact like this.  No one can agree on whether the whole world was built, or built over an existing world or an existing world was tunneled into.  Archeologists and scavengers at the beginning of the story have only made it down through four layers (of perhaps thousands).  Fun read.  Just found it in a used books store a few days ago.  Not quite a shell-world, at least on the outer layers.
Writer of Book and Lyrics for musicals "SCAR", "Cinderella!", and "Aladdin!". Retired Naval Security Group. "I think SCAR is a winner. Great score, [and] the writing is up there with the very best!"
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Offline Coastal Ron

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The shell would bounce around the planet like on springs and and would create shear stresses with huge component forces.
These things are not even in dynamic equilibrium without restoring force

How is a spring not in dynamic equilibrium, providing a restoring force?

Some analogies don't work. In this case the blob of matter inside the shell is not held in place by any strong force. The air between the shell and the surface can move freely to anywhere inside the interior, which means that it won't act as a spring to keep the blob of matter in the shell centered.

While the concept is interesting, the laws of physics won't allow it to exist without active forces (i.e. LOTS of energy being expended) to keep the blob of matter centered in the shell. Wouldn't be a safe place to live...  :o
If we don't continuously lower the cost to access space, how are we ever going to afford to expand humanity out into space?

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