Shell Worlds: "Man Caves: Humanity’s Next Home" by Ken Roy

[Paul451]

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.

[RotoSequence]

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

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.

[sanman]

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

(I'm talking to you, Netflix!)   

[edzieba]

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.

[RotoSequence]

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.

[edzieba]

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.

[RotoSequence]

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.

[RonM]

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

[Paul451]

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."

[Paul451]

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".

[RotoSequence]

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.

[envy887]

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.

[edzieba]

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.

[envy887]

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.

[Coastal Ron]

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.

[Mark K]

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.

[sanman]

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?

[Paul451]

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?

[darkenfast]

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.

[Coastal Ron]

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... 

[Mark K]

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... 

I agree completely!

Two points - to answer the earlier question first -
I meant the shell arrangement with no pressure gradient. It is not in equilibrium at all so the only posited forces to keep it in place would have to be the pressure gradient. Any little force must be counteracted by the pressure difference before the shell hits the center mass because nothing else is stopping it.

Second, having a pressure gradient strong enough to matter means that all the nice features related to not needing super strong structure in the shell go out the window. The shell needs to be strong enough to be pushed on by these pressure gradient forces which are not all normal to the shell so there are tension and shear forces. These forces - if the pressure gradient is really enough to stop the multi-megaton shell from moving in a short enough distance to not hit the central mass and then reversing its direction - will be astronomical in the bad sense. Plus since there is no equilibrium and no damping from it, any "bouncing" of the shell will not really go away - it might heat up the air inside over time, but that will be the cause of more forces on the shell - higher pressure which won't be evenly distributed. It could be slowly damped by moving the material of the shell - tearing it or bending it apart as well.  Aerodynamically inside you would get interesting air currents, pressure waves, i.e. winds from the back and forth...

Since there would be so little damping, lucky resonances would likely just build up... It could be quite spectacular quite quickly for such a huge object.

[Paul451]

The air between the shell and the surface can move freely to anywhere inside the interior

Ah, now I get it, that's the part you haven't understood. The air is gravitationally attracted by that "blob of matter", but not by the shell (which has a uniform internal gravity field). As long as the blob-of-matter has a gravitational potential sufficient to create an pressure gradient across the gap between surface and shell (8-10km or so), then that pressure difference will act as a counter-force to any tendency of the shell to drift towards the surface (or vice versa.)

If the contained mass is too small, then it won't have enough gravity to create a sufficient pressure difference. But Ceres does. And I suspect that most of the dozen largest main belt asteroids would.

[Paul451]

The shell needs to be strong enough to be pushed on by these pressure gradient forces which are not all normal to the shell so there are tension and shear forces.

No, all the force is perpendicular. There's only compression between the atmosphere and the mass of the shell.

Plus since there is no equilibrium and no damping from it

Not sure why you keep saying that. Why isn't there an equilibrium? Why isn't there damping?

[RotoSequence]

Gas floated structures will drift and settle with the ebb and flow of the atmosphere, which is highly compressible. Balancing the roof of an air supported structure requires consistent and equal pressure distribution.

[Mark K]

The shell needs to be strong enough to be pushed on by these pressure gradient forces which are not all normal to the shell so there are tension and shear forces.

No, all the force is perpendicular. There's only compression between the atmosphere and the mass of the shell.

Plus since there is no equilibrium and no damping from it

Not sure why you keep saying that. Why isn't there an equilibrium? Why isn't there damping?

There is no (i.e. negligible) damping because this is like an air spring not a shock absorber. There is very little loss of energy (i.e. damping) from anything - or the shell breaks. The pressure differential just gives restoring force, no damping. All the energy from tidal forces, from hot air in some place rather than others, from things landing on the top of the shell, from radiation pressure on one side of the outside and not the other and so on, is going to get translated into motion of the shell relative to the core.

The only only thing you have mentioned that is going to stop that motion from causing the shell to hit the center is the pressure -differential- between the bottom and the top of the inter shell zone. Not the pressure inside, say 1 bar average - that doesn't help at all, but only the difference between bottom and top - much less.

So as the shell moves the pressure on the inside of the shell moving toward the center increases slightly and the pressure on the other side decreases. Great we have a restoring force - an almost perfect spring. If this force is enough to stop the shell from hitting the center mass (unlikely over time in my opinion, but not calculated) then the shell is going to "bounce" back the way it came, and squish and stretch the shell. the forces will NOT be perpendicular, normal, to the shell.

If the shell is weak it will break. It has to be strong enough so support the whole bounce strains and stresses at all the angles. If it is that strong then little energy will be lost in the bounce - very little damping - so all the "bounces" will over time be added together. If there are any resonances, and there almost certainly will be, look out something will give.

[sanman]

Could you have towers or pillars linking the shells, in order to transmit force more effectively than atmospheric pressure? What complications would that introduce?

[Coastal Ron]

The air between the shell and the surface can move freely to anywhere inside the interior

The air is gravitationally attracted by that "blob of matter", but not by the shell (which has a uniform internal gravity field). As long as the blob-of-matter has a gravitational potential sufficient to create an pressure gradient across the gap between surface and shell (8-10km or so), then that pressure difference will act as a counter-force to any tendency of the shell to drift towards the surface (or vice versa.)

Gravity would NOT create "pressure" within the shell, the air pressure would be created by how much air is maintained in the area between the "blob of matter" and the shell. And since the air can move freely, it does nothing to keep the "blob of matter" centered within the shell.

Gravity is the weakest of the four known fundamental forces, and not just a little, but by a lot. And since gravity is a force that applies in all directions, I don't see how there is a true centering force.

For instance, as the "blob of matter" moves off center, gravity will attract the portion of the shell that is closest stronger than it will attract the portions of the shell that are moving away. No doubt the part moving away is going to be of greater mass, but that means the centering force of the gravity is going to be even weaker.

If the contained mass is too small, then it won't have enough gravity to create a sufficient pressure difference. But Ceres does. And I suspect that most of the dozen largest main belt asteroids would.

Again, it is NOT gravity that determines the air pressure, but how much air has been pumped inside of the shell, and how air tight the shell is.

[Paul451]

Balancing the roof of an air supported structure requires consistent and equal pressure distribution.

Air supported structures have trivial amounts of internal pressure (relative to external pressure). Typically on the scale of a couple of hundred Pascal. Ie, less than half the pressure of Mars' atmosphere. Around 100 grams of force-equivalent per square metre. 1atm pressure gives around 10,000,000 grams per square metre.

[Paul451]

The only only thing you have mentioned that is going to stop that motion from causing the shell to hit the center is the pressure -differential- between the bottom and the top of the inter shell zone. Not the pressure inside, say 1 bar average - that doesn't help at all, but only the difference between bottom and top - much less.

Uh, dude, that's the whole point of shell worlds. It's not something that I've personally tacked on. If you don't understand why the pressure differential occurs, you don't understand the concept enough to criticise it.

If this force is enough to stop the shell from hitting the center mass (unlikely over time in my opinion, but not calculated)

It's not hard to calculate. It's the pressure difference between the ground and shell.

On Earth, as an example, a 5km shell height gives a pressure difference of over 40%, producing a difference in upward force between a pushed down shell section of 2 tonnes-equivalent per square metre.

For the input disturbances, let's use the tidal force due to the moon, which gives around 150 grams of force per square metre of shell.

A difference of four orders of magnitude more restorative force than input disturbance. The shell will barely budge.

Of course, on smaller bodies, the scale pressure difference is less, on Ceres a 10km shell height gives just 4% pressure difference, so 400kg per square metre. But the tidal force from the sun is a tiny fraction of that from the moon on Earth, although the mass of the shell (per square metre) is higher, so it's almost break even. But you still end up with three orders of magnitude difference between the two forces.

the forces will NOT be perpendicular, normal, to the shell.

Compression can't be anything but perpendicular to the surface, because it's ultimately due to gravity, regardless of the disturbing force.

[Paul451]

For instance, as the "blob of matter" moves off center, gravity will attract the portion of the shell that is closest stronger than it will attract the portions of the shell that are moving away. No doubt the part moving away is going to be of greater mass, but that means the centering force of the gravity is going to be even weaker.

The gravitational force inside a hollow shell is uniformly zero, regardless of your position within the shell. I think some guy from the Royal Mint worked it out awhile back. Isaac something. Google "gravitational force inside a hollow shell is".

it is NOT gravity that determines the air pressure

The pressure of the air at ground level is caused by the weight of the air column above it. That weight depends on the mass of the air times by the strength of gravity.

All the shell is doing is substituting for the air that would be above that height in an unconstrained system. It doesn't change the process that creates a pressure difference between the ground and the height of the shell. Hell, there's a pressure difference between the floor and ceiling of the room you're in.

[edzieba]

While the effective gravitational attraction of the shell is uniform inside it, you also have a load of atmosphere sloshing around inside it. e.g. for an earth-mass internal planetoid you have on the order of 5 petatons (5x10^18 kg) of atmosphere. As your internal planet displaces freely it will compress this springy mas and the shove it around inside your shell, causing all sorts of fun pressure differentials and oscillating masses. And by 'fun' I mean 'extremely high energy disruptions to your fragile eggshell pressure vessel'.

[rakaydos]

Instead of a free floating shell around an entire planet, 12 planetary scale pentagonal "domes" might be a better choice. The walls hold the shells in place, and the pressure in adjacent domes help hold up the walls.

[kenny008]

I've read thread several times, and I don't think I understand how this setup supposed to work.  If I'm reading this right:

- You put a shell around a planet
- You pressurize the atmosphere inside the shell to support the "weight" of the shell, keeping it thin and manageable.
- The shell is self-centering.  As it drifts, the air pressure will rise on one side as the shell approaches the planet, pushing it back into position.  This is due to the atmospheric pressure rising as the shell goes deeper into the atmosphere.
- You then propose additional shells with different pressure regimes to allow additional atmospheric pressure options.

     If this is all correct, I don't see at all how this works.  The self-centering mechanism just doesn't click with me.  Won't the atmosphere just move from the "high-pressure" side to the "low-pressure" side?  I can see this maybe generating a pretty significant wind on the planet, but I see no way that it would push against the shell. 

     The reason there is a CONSTANT pressure gradient from sea level to space is because gravity is pulling on the atmosphere, AND also because there is nothing pushing on the atmosphere from above.  Once you start bringing the shell down toward the planet, the atmosphere would have to push back UPWARDS on the shell to move it back into position.  This would cause the atmosphere to not just push up, but also sideways.  In effect, the atmosphere would just move around the planet to the other side, and the shell would not self-center.

     This seems to be equivalent to putting a small ball inside a larger ball.  I can shake the large ball, and the small ball just impacts the sides.  It doesn't build up a pressure on one side and float to the center, where it is stable.  Even performing this experiment in a zero-g environment wouldn't change the results.  The inner ball will just bounce around the outer ball.
     I know in this experiment there is not already a gravitationally-induced pressure gradient, but Pascal seems to indicate that this pressure gradient would shift around the planet.

     Sorry if I'm missing something here.  Just trying to picture this system.

[Stan-1967]

The primary assumption for the system stability is that you are dealing with completely uniform gravitational attraction of the outer shell by the mass contained inside the shell.  This is a poor assumption for an even extremely advanced civilization.  If you wanted to build it around an earth sized & shaped mass, the earth is an oblate spheroid, and has a non uniform gravitational field on the order of milli g's.  That should be plenty to tidally lock some part of your inner sphere to the shell. 

I also think the concept has problems with tidal forces from moons & even the central star.  The outer shell will decouple  its rotation rate from the inner planet over time, as well as it center of rotation.  Similar to how satellites have to periodically correct for lunar induced precession.  There is also the problem of Lagrange points & the system barycenter. The whole system will have a barycenter, however the much larger mass of the inner planet will be moving around this barycenter while the outer shell is decoupled from that barycenter.  How will the system stay stable when the inner planet is not actually rotating around it's center point, but some point far off center.  In the case of the earth, this point is some 1700km from the center of the earth.  This means the inner planet will slam into the outer shell without some magical means of aligning the center of rotation.

So many bad assumptions.  Thermodynamics is also a problem.  When a shell is covering a planets atmosphere ( like the Venus example ) it will cool & collapse the atmosphere because it will have much less solar irradiance.  Now you have a shell not supported by the needed atmospheric pressure.

 

[jee_c2]

Reflecting on the problems with relative movements of the shell and the planet/inner body induced either by gravitational forces or by pressure variencies: theoretically could the shell be anchored to the core with some sort of cables? I think the forces could be really big, so perhaps pillars would be needed as well to stabilize the construct. Still I assume, it would be really hard to achieve a stable system.

Anyway, interesting idea.

[Paul451]

you also have a load of atmosphere sloshing around inside it.

"Sloshing". If the windspeed is, for eg, 100mph/160kmh, the pressure difference against the under-surface of the shell is around 0.17PSI. The pressure difference due to altitude, on Ceres, is more than 0.5PSI, around 3.5 times higher. And if you have enough thermal differences to create 100mph winds, you've probably done something wrong. (Indeed, an issue will be the lack of thermal variation under the thick shell.)

The primary assumption for the system stability is that you are dealing with completely uniform gravitational attraction of the outer shell by the mass contained inside the shell.

No, it's just easy to explain the concept. Your own example of non-uniform gravity in the milli-g's shows how trivial a difference it makes.

The whole system will have a barycenter, however the much larger mass of the inner planet will be moving around this barycenter while the outer shell is decoupled from that barycenter.

The barycentre of the shell and core will barely differ from the CoM of the core, because the shell is fairly uniform.

So you presumably meant the barycentre of the core mass and another mass (such as the Earth and its moon)? Externally, the shell acts as a point mass. Being uniform around the core, its CoM will be trivially the same as the core's. Therefore both would move around the same barycentre, their motion will be similarly coupled to the external mass. That is, if Earth is orbiting around a point 1700km away from the CoM due to the mass of the moon, then the shell is orbiting around a point 1700km away from its CoM (which it shares with Earth) due to the mass of the moon. And because they are orbiting about the barycentre of the same mass (the moon) they will even be locked in the same period and phase.

Similarly to their orbit around the sun. They will share an orbit. Here they aren't locked together and can drift apart, but the system is self-stabilising because of the atmosphere. You'd have to have sufficient external forces to overcome the pressure difference caused by the atmosphere in order to make the shell touch the surface, let alone collapse. As I showed in previous examples, when it comes to tidal forces, you have orders of magnitude margin.

When a shell is covering a planets atmosphere ( like the Venus example ) it will cool & collapse the atmosphere because it will have much less solar irradiance.

That's why it's proposed for Venus. Precisely to help freeze out the atmosphere, while keeping the excess carbon under the shell.

Now you have a shell not supported by the needed atmospheric pressure.

Over the thousand or so years it will take to lose the existing thermal energy, the shell's size will need to be reduced about 1/6th of 1%.

[Paul451]

I've read thread several times, and I don't think I understand how this setup supposed to work.  If I'm reading this right:
- You put a shell around a planet
- You pressurize the atmosphere inside the shell to support the "weight" of the shell, keeping it thin and manageable.
- The shell is self-centering.  As it drifts, the air pressure will rise on one side as the shell approaches the planet, pushing it back into position.  This is due to the atmospheric pressure rising as the shell goes deeper into the atmosphere.
- You then propose additional shells with different pressure regimes to allow additional atmospheric pressure options.

Ignore the last part. Until you understand the basic concept, making it more complex doesn't help. But to clarify, it's not "adding additional shells", it's splitting the one shell into several layers. Same overall mass, but we get more use out of it.

- You pressurize the atmosphere inside the shell to support the "weight" of the shell, keeping it thin and manageable.

Not especially thin. The mass has to equal the pressure you want for the atmosphere. So to approximate SL Earth, you need 10 tonne of force per square metre (minus a bit for the scale height, to allow, say 10km of gap between the shell and the planet or asteroid.)

So for Ceres, you're looking over three hundred tonnes of mass per square metre in order to keep the atmosphere at SL Earth pressure. That means several hundred metres thickness.

The lower the mass of the shell, the lower the air pressure under it.

If this is all correct, I don't see at all how this works.  The self-centering mechanism just doesn't click with me.  Won't the atmosphere just move from the "high-pressure" side to the "low-pressure" side?  I can see this maybe generating a pretty significant wind on the planet, but I see no way that it would push against the shell.

In order to do so, it would have to in-effect raise the atmosphere on the other side up 10km. Or compress it an equivalent amount. That increases the pressure on that side (effectively doubling it), which exceeds the pressure difference you've created on the "high-pressure side", so that obviously can't happen. Instead, the pressure is raised slightly everywhere, which creates a counter-force against the shell. Provided the scale pressure difference exceeds the force acting on the shell.

Remember, we're not talking about a small object floating inside a space-station. The core mass (planet/asteroid) has enough mass to pull the atmosphere and shell sufficiently to create the situation in the first place. That gravity means you can't arbitrarily double the height of the atmosphere on the other side, with exerting an enormous force to counter that gravity.

The reason there is a CONSTANT pressure gradient from sea level to space is because gravity is pulling on the atmosphere, AND also because there is nothing pushing on the atmosphere from above.

Not quite. The pressure at sea-level is caused by the weigh of the air on top. The pressure at any altitude is caused by the weight of the column of air above it.

We are replacing the weight of the air above (for example) 10km in height, with the weight of the shell. Nothing else changes. Including the fact that the air just below the shell is only supporting the weight of the shell, while the air at the surface is supporting the weight of the shell and the 10km column of air between.

Once you start bringing the shell down toward the planet, the atmosphere would have to push back UPWARDS on the shell to move it back into position.

And it does. Look, forget the mass-balancing thing. Imagine the shell was inflated, under tension. Like the classic space-domes that everyone obsesses over. Do you accept that the skin is being stretched by nearly 15psi? Do you accept that a small force, much less than this pressure, will not suddenly cause the skin to collapse against the ground?

This seems to be equivalent to putting a small ball inside a larger ball.  I can shake the large ball, and the small ball just impacts the sides.  It doesn't build up a pressure on one side and float to the center, where it is stable.  Even performing this experiment in a zero-g environment wouldn't change the results.  The inner ball will just bounce around the outer ball.
I know in this experiment there is not already a gravitationally-induced pressure gradient, [...]
Sorry if I'm missing something here.  Just trying to picture this system.

Because you are picturing a small (effectively non-gravitational) mass inside a larger volume. The central mass is the source of gravity for the system. Everything is drawn to it. It is the centre. It's not that the system re-centres the inner ball, the inner-ball forces everything else to move with it. Imagine the inner ball was connected to the outer by springs (representing the atmosphere). We are holding the inner ball (somehow), and moving it around, the outer shell is going to follow, yes? You are seeing the atmospheric effect as a small afterthought (I think so are the others) rather than the whole central mechanism.

I think the better starting point is to picture the pressured skin under tension. Once you are comfortable with that, start imagining the skin was heavier, able to counter some of the outward force of the atmosphere. Ask yourself if anything's changed in how the system works except that the skin doesn't experience as much tension.

If you get stuck with that, picture a gas-filled tube on the surface of the planet/asteroid. (The tube is magically frictionless, naturally.) If we have a column of gas high enough, the weight will put the gas at the bottom under a particular pressure. If we instead have a solid mass sitting on top of the column of gas, equalling the mass of gas we've replaced, the situation will be the same. Provided we keep the gas from sneaking past the solid mass.

[edzieba]

And it does. Look, forget the mass-balancing thing. Imagine the shell was inflated, under tension. Like the classic space-domes that everyone obsesses over. Do you accept that the skin is being stretched by nearly 15psi? Do you accept that a small force, much less than this pressure, will not suddenly cause the skin to collapse against the ground?

Here's the problem: with a gravitationally significant inner mass, that is exactly what will happen. A small offset in skin position will end up with the skin drifting and impacting the interior object. The atmosphere is not a helper here: the shifting skin pushes atmosphere to the other side of the volume, but there is no damping here. The atmosphere is free to oscillate back and forth and has a rather enormous mass of its own. As it is pushed to the other side of the volume, it compresses (and increases the perturbation force for this time), then springs back and produces a pressure spike at the antipode as the atmosphere rushes back around the internal mass to that point.

Because you are picturing a small (effectively non-gravitational) mass inside a larger volume. The central mass is the source of gravity for the system. Everything is drawn to it. It is the centre. It's not that the system re-centres the inner ball, the inner-ball forces everything else to move with it. Imagine the inner ball was connected to the outer by springs (representing the atmosphere). We are holding the inner ball (somehow), and moving it around, the outer shell is going to follow, yes? You are seeing the atmospheric effect as a small afterthought (I think so are the others) rather than the whole central mechanism.

The barycentre problem is because your ball-and-shell is not in isolation. It's (one would assume) orbiting a host body (shell around a planet orbiting a star), and may even have that host body orbiting a host body (shell around a moon orbiting a planet orbiting a star). The barycentre offset means that the centre of mass of the planet is not the barycentre of the system as a whole. Even in the basic case of a star with a single planet (and no other bodies) that means your shell will be pulled slightly sunwards.

[Paul451]

And it does. Look, forget the mass-balancing thing. Imagine the shell was inflated, under tension. Like the classic space-domes that everyone obsesses over. Do you accept that the skin is being stretched by nearly 15psi? Do you accept that a small force, much less than this pressure, will not suddenly cause the skin to collapse against the ground?

Here's the problem: with a gravitationally significant inner mass, that is exactly what will happen. A small offset in skin position will end up with the skin drifting and impacting the interior object. The atmosphere is not a helper here: the shifting skin pushes atmosphere to the other side of the volume, but there is no damping here.

In order to push the skin (or shell) down on one side, you have to raise the local air pressure. That takes energy/work.

Yes, the air will migrate out sideways because of the new pressure difference between that half of the atmosphere and the rest. That is not frictionless nor perfectly elastic however, you aren't going to squeeze air (using Ceres) around ~1500km through a gap 10km wide without serious energy losses. Additionally, the local air doesn't just squeeze out the sides, it doesn't "know" anything but pressure. In order to move sideways, you have to increase the pressure, and if you increase the pressure, you increase the force pushing up on the shell. That acts against whatever force is pushing the skin/shell down to the surface.

How much force are we talking about? I did the maths for both Earth and Ceres. Tidal force (moon and sun respectively) is orders of magnitude too small to overcome the atmospheric pressure gradient and force the shell onto the surface. Orders of magnitude is a decent engineering margin, IMO.

And any disturbance large enough to create a force across a wide area of the shell of more than a tonne per square metre is sufficient to destroy anything on or under the surface of that body. Indeed, probably enough to break up many objects, crack the crust of planets, etc.

The barycentre offset means that the centre of mass of the planet is not the barycentre of the system as a whole.

I've already addressed this in more detail.

The CoM of the shell will be the same as the CoM of the planet. Therefore, WRT any external mass, both are functionally point-masses at the same spot. Therefore both will, independently, have the same barycentre offset. Any small variations between the two will be countered by the atmosphere.

Even in the basic case of a star with a single planet (and no other bodies) that means your shell will be pulled slightly sunwards.

Why would there be a higher gravitational force on the shell than on the planet, that doesn't make any sense at all.

[Stan-1967]

Therefore it only needs existing technology, it's the scale that is... ahem... advanced.

This will require extremely advanced technology that does not exist today.  My BOE calculations for the work required to lift a 1 meter thick steel shell to a height of 1km would take Kardashev I type civilization to dedicate it's full power output ( 10^16 W) for 45 days in an earth equivalent gravitational field. 

If the shell was 8-10 km high per the OP, that would make it over a full year of power output from a K1 civilization.  If it is several meters thick of equivalent mass, then multiply that number accordingly.   K1 civilizations need to have harnessed H-H fusion, as well as possibly antimatter production.  Again, this is just the dedicated power needed to lift the steel or equivalent regolith mass.  It does not include the energy needed to mine & smelt the steel, nor does it account that the construction energy budget would be some fraction of the civilizations total energy production.   

Also what technology allows you to get the shell into place for the initial construction without it falling down?  It seems like you need something not invented yet to magically place all that steel, fully connected , welded, and structurally sound, into it position in the sky as a shell.  Existing technology would require something along the lines of 1st generation space elevator technology for the scaffolding to raise it & place it together.  ( might as well leave the scaffolding in place when done, since that would potentially solve the problem with dynamic motion.

The arguments back & forth about stability remind me of the difference between static & dynamic stability as it relates to aircraft & other systems of motion.  A shell around a central world has a mathematical solution for static stability that seems sound.  However the dynamic stability still seems to have problems.  Hand waiving away & assuming that all the dynamic forces are miniscule or orders of magnitude too small to matter are not good assumptions for long timescale ( decades, century's, millenia, etc. )

[Coastal Ron]

For instance, as the "blob of matter" moves off center, gravity will attract the portion of the shell that is closest stronger than it will attract the portions of the shell that are moving away. No doubt the part moving away is going to be of greater mass, but that means the centering force of the gravity is going to be even weaker.

The gravitational force inside a hollow shell is uniformly zero, regardless of your position within the shell.

While gravity is the weakest of the four forces, it's effects are reliant on mass and distance. And again, the force of gravity is too weak to keep the "blob of matter" centered within the shell.

it is NOT gravity that determines the air pressure

The pressure of the air at ground level is caused by the weight of the air column above it. That weight depends on the mass of the air times by the strength of gravity.

While that is true for a body like Earth, which has no shell surrounding it, it is NOT true when you have a shell surrounding your planetoid. You could actually pressurize the shell, or have a partial vacuum.

And as many have pointed out, there is nothing keeping the "blob of matter" inside of the shell from bouncing around inside of the shell, which as it moves around it will squish the air around in ways that would likely result in forces greater than any hurricane here on Earth.

Not sure why you want to ignore these shortcomings... 

[Paul451]

The arguments back & forth about stability [...] Hand waiving away & assuming that all the dynamic forces are miniscule or orders of magnitude too small to matter are not good assumptions

No. Absolutely not. You do not get to accuse me of being the one "hand waiving away & assuming" the size of effects. I'm the only one who worked out the actual forces involved, I did not "handwave" them away as minuscule, I showed them to be so.

Others just pulled an idea ("tidal force", "atmosphere sloshing") out of their butts and claimed without any proof that it must be sufficient to overcome the scale pressure difference. That the atmosphere must be able to magically relocate frictionlessly to the other side of the body. That the shell must be able to drift freely. That there must be no dampening effect. Without making a single effort to show any actual values.

You, at least for one thing, actually did work out a number. And yes, the scale is huge. That's patently obvious. It's mega-scale engineering. Did I say otherwise? You are essentially building an artificial planet. However, unlike many other mega-scale proposals, it requires no magic "scrith" super material, not even room temp superconductors. It doesn't even require the best stuff we have available today, like CNT. It works with dumb bulk mass.

[Paul451]

While gravity is the weakest of the four forces

I don't know why you think you're saying something useful in repeating that. We're dealing with a gravitational system. That's the force involved.

And again, the force of gravity is too weak to keep the "blob of matter" centered within the shell.

Prove it. Show me a single number that even hints you've actually done the maths.

While that is true for a body like Earth, which has no shell surrounding it, it is NOT true when you have a shell surrounding your planetoid.

Incorrect, as I explained in detail in the comment you quoted.

And as many have pointed out, there is nothing keeping the "blob of matter" inside of the shell from bouncing around inside of the shell, which as it moves around it will squish the air around in ways that would likely result in forces greater than any hurricane here on Earth.

"Blob" "bounce" "squish" "likely". Just stop it. Do the freakin' maths. It's not hard. It's basic algebra using equations that are nicely detailed on multiple physics sites, for values of mass/distance/radius you can pull off of Wikipedia.

Stop just pulling this stuff out of your ass and insisting it's gold. Look at the actual numbers. You can work out the pressure difference due to scale height (I've shown it elsewhere), you can work out the pressure created by high winds (I picked 100mph, but choose your own value). You can work out the air pressure pushing up on the shell, you can work out the volume (and hence pressure) changes that would work against the shell being pushed down to the surface, and therefore will resist such pushing. (As I've also showed.) You can work out the static case, to show that the system is self-levelling, self-centring. (As I did.) You can show the scale of the dynamic external forces on the shell/core and their scale relative to that self-stabilising force. (As I also have.)

Personal incredulity is not an argument.

[Stan-1967]

No. Absolutely not. You do not get to accuse me of being the one "hand waiving away & assuming" the size of effects. I'm the only one who worked out the actual forces involved, I did not "handwave" them away as minuscule, I showed them to be so.
 it requires no magic "scrith" super material, not even room temp superconductors. It doesn't even require the best stuff we have available today, like CNT. It works with dumb bulk mass.

I may have missed your calculations of dynamic stability. Citation please? 

You did post your results of static forces on the shell, you did not show your work, but I do not doubt there is a stable static solution.  A solution for the dynamic motion of the planet around a star, & with a moon type body was not shown.  The math is actually not easy.  I think the burden is on you to show solutions whether derived or numerically simulate that. 

The concept is interesting, & try to dial back the butthurt when we challenge your ideas.  It's not personal.  I really like the idea as a sci-fi trope, but as an advanced concept, I'm going to look at it through my engineering eyes, and what I know of physics.  I think it has some serious problems with that.  It does take "magic" to make it work.  Show me how existing technology can generate the energy levels needed just to lift the shell?   What % of the earth's crust will you have to mine to get that much steel?  I worked in a gold mine once upon a time.  A primary input metric was tons of diesel fuel to extract 1 Toz of gold.  As I see it, the magic is assuming that this shell gets built at all vs. other better & more efficient competing needs for energy a K1 civilization may have a need for. 

[Paul451]

I do not doubt there is a stable static solution.

Then you appear to be alone in that.

I think the burden is on you to show solutions whether derived or numerically simulate that.

At this point, I just don't see any value unless someone can show there is a disruptive force that is within at least an order of magnitude of the corrective force.

try to dial back the butthurt when we challenge your ideas.

You accused me of "handwaving and assuming" that the disturbing forces were minuscule, rather than calculating it, as I have. Everyone else who has made factual claims about what "will" and "must" happen has not shown even a pretence that they've worked out the forces involved. Yours was the straw that broke the camel's back.

I have no problem with people presenting their personal incredulity as personal incredulity ("I don't understand how this works, because...") as kenny008 did, but not people who just invent issues and claim them as facts. The latter is not valid criticism.

Also,

when we challenge your ideas.

It's not my idea, I just posted an article that seemed interesting.

The original idea goes back decades, at least to the late Paul Birch (though I'm not sure if he invented it, he certainly took it too an extreme with Birchworlds and Matryoshka-worlds.) There are other, related concepts, such as bubble-worlds (aka gravitational balloons or Eder-worlds (after Daniel Eder, who popularised an extreme version some years back)) which don't have a central mass, just a contained atmosphere and a gravitationally compressed shell. Same physics at work, but allows much smaller "worlds".

[envy887]

This would cause the atmosphere to not just push up, but also sideways.  In effect, the atmosphere would just move around the planet to the other side, and the shell would not self-center.

     This seems to be equivalent to putting a small ball inside a larger ball.  I can shake the large ball, and the small ball just impacts the sides.  It doesn't build up a pressure on one side and float to the center, where it is stable.

It's like an air bearing, but with a closed cycle where all the air is trapped under the shell.

Air bearings can lift incredibly heavy loads:
 

[edzieba]

Air bearings distribute the load around the bearing body. In the case of the shell, that bearing body is like a half-thickness gold leaf, but flimsier.

[rakaydos]

Air bearings distribute the load around the bearing body. In the case of the shell, that bearing body is like a half-thickness gold leaf, but flimsier.

Is there supposed to be an argument in here somewhere?

[edzieba]

That moving air masses exert force, and that the entire point of this shell is for it to not need any significant tensile strength. By solving the issues introduced by (a), you obviate (b).

[Coastal Ron]

This would cause the atmosphere to not just push up, but also sideways.  In effect, the atmosphere would just move around the planet to the other side, and the shell would not self-center.

     This seems to be equivalent to putting a small ball inside a larger ball.  I can shake the large ball, and the small ball just impacts the sides.  It doesn't build up a pressure on one side and float to the center, where it is stable.

It's like an air bearing, but with a closed cycle where all the air is trapped under the shell.

Air bearings don't rely on trapped air, they rely on a constant flow of pressurized air to be forced between the air bearing surface and the floor.

That effect is not possible in a closed sphere.

Other than some temporary cushioning as the "blob of matter" sloshes around inside of the sphere and the air rushes out of the way, the air inside of the sphere does not provide any centering forces.

[rakaydos]

Can someone expain to me what is wrong with 12 or 20 centering ropes equally spaced around the shell, for stability? What is the problem with this concept that isnt Trivially solved?

[Paul451]

That moving air masses exert force

How much moving air (ie, at what speed) and hence how much force?

From what I can find, research into atmospheric tides gives a pressure variation on Earth due to combined moon/solar tides of around 300 pascals at the equator, closer to 100 pascals at mid-latitudes. The shell is held up by a roughly 100,000 pascal force, and (on Ceres) stabilised by a roughly 4000 pascal scale-height pressure difference. So the stabilising effect is an order of magnitude higher than  the disturbing effect. (And I'm ignoring that the tidal effects on Ceres will be vastly less than those on Earth, since tidal force falls with the inverse-cube of distance. I'm matching the worst-case disturbance with the worst-case stabilising effect.)

[Coastal Ron]

The shell is held up by a roughly 100,000 pascal force, and (on Ceres) stabilised by a roughly 4000 pascal scale-height pressure difference.

Assuming the shell is impermeable then that means the atmosphere inside of the shell (i.e. the amount of molecules of "air") determines whether the shell experiences a vacuum, neutral force, or pressure from the air inside.

From a strength standpoint you'd actually want a vacuum since that provides additional strength to the shell, whereas having a positive pressure pushing from the inside of the shell - seeking to create or find leaks - is the least favorable situation.

The "blob of matter" within the shell has no bearing on the air pressure within the shell, unless it will be absorbing air or outgassing and contributing gas pressure inside of the shell.

But as a free-floating mass in space, the air inside of the shell is not "holding the shell up". The shell is holding the air in.

[Paul451]

The shell is held up by a roughly 100,000 pascal force, and (on Ceres) stabilised by a roughly 4000 pascal scale-height pressure difference.

The "blob of matter" within the shell has no bearing on the air pressure within the shell

Incorrect. The central mass, whether Ceres, the moon, Mars or Jupiter, is a gravitational source and therefore a part of the forces on the mass of the atmosphere.

But as a free-floating mass in space, the air inside of the shell

The atmosphere is not free-floating. It is around a gravitational source.

the air inside of the shell is not "holding the shell up". The shell is holding the air in.

Incorrect, the shell is being pulled to the surface of the central mass by gravity, the air provides counter-pressure. Balance the two and only compressive force remains.

[Coastal Ron]

The shell is held up by a roughly 100,000 pascal force, and (on Ceres) stabilised by a roughly 4000 pascal scale-height pressure difference.

The "blob of matter" within the shell has no bearing on the air pressure within the shell

Incorrect. The central mass, whether Ceres, the moon, Mars or Jupiter, is a gravitational source and therefore a part of the forces on the mass of the atmosphere.

There is BIG difference in how such a construct would exist depending if it is around Ceres, the Moon, Mars or Jupiter, so for the sake of debate I'll stick with Ceres, since if we can't enclose Ceres we certainly won't be able to enclose Jupiter.

But as a free-floating mass in space, the air inside of the shell

The atmosphere is not free-floating. It is around a gravitational source.

The mass of Ceres generates 0.029 g at its surface, and according to Wikipedia:

Any atmosphere, however, would be the minimal kind known as an exosphere.

From Wikipedia concerning an exosphere:

The exosphere is a thin, atmosphere-like volume surrounding a planet or natural satellite where molecules are gravitationally bound to that body, but where the density is too low for them to behave as a gas by colliding with each other.

So Ceres is unable to keep an atmosphere today, meaning an impermeable shell would be the primary force keeping an atmosphere in. It also means that an atmosphere will have to be imported, and that if it is to be usable for humans the atmosphere will need to be pressurized inside of the shell.

the air inside of the shell is not "holding the shell up". The shell is holding the air in.

Incorrect, the shell is being pulled to the surface of the central mass by gravity, the air provides counter-pressure. Balance the two and only compressive force remains.

Ceres can barely keep molecules from escaping its surface, so gravity effects on the shell are going to be minuscule.

So for a shell around Ceres, yes, the shell would be holding the air in. Maybe that would be different for Mars or Jupiter, but not Ceres.

[lamontagne]

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.

I wonder how this compares to dynamic structures and rotating habitats, as far as constructability goes?  It does seem a lot safer than dynamic structures, but I'm not certain we're getting our money's worth compared to rotating habitats.  Not to say that those don't have a great number of difficulties that usually get hand waved away....
I wonder how many responders actually read the article?  It seems to cover most objections nicely and clearly.  And proposes a construction method.

[Coastal Ron]

I wonder how this compares to dynamic structures and rotating habitats, as far as constructability goes?

Rotating habitats have their own issues concerning strength and cost, but a shell around Ceres is well beyond the ability of humanity today, or likely in the near future. At least until we create true self-replicating space machinery.

It does seem a lot safer than dynamic structures, but I'm not certain we're getting our money's worth compared to rotating habitats.

Sidestepping that debate, I think it's safe to say that our evolution of space hardware will start with rotating space stations well before we move onto enclosing dwarf planets. For a number of reasons, including being able to locate rotating habitats wherever we need them, cost, amount of material needed, etc.

I wonder how many responders actually read the article?  It seems to cover most objections nicely and clearly.  And proposes a construction method.

I did not read the article, but just from viewing the portion you excerpted above, I would not agree that they "..cover most objections nicely and clearly." For instance, first they say:

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.

So they are talking SPECIFICALLY about having the atmosphere being pressurized within the shell. And based on my research on Ceres (noted above) we know that Ceres is not able to contain an atmosphere using only its mere gravity well. But then they say:

Because the dumb mass of the shell is being supported by the air, there's no real tensile or compression force.

First they admit that the shell will be like a balloon, having to contain the atmosphere to a usable pressure well above what Ceres would be able to provide, so if anything the surface of the shell would NOT be in compression, but would be under tensional stress.

And the phrase "...the dumb mass of the shell is being supported by the air" is nonsensical, since the shell is a sphere, and spheres don't care what is on the inside, they only react to the forces on the inside - either pressure or a lack of pressure (i.e. vacuum).

So I'm not thinking their paper has passed a peer review, because otherwise someone would have pointed that out.

And I still haven't heard a valid idea for how Ceres will stay centered within the shell, especially since Ceres has such a weak gravity, so any force on the shell could easily send it bumping into Ceres.

[Barley]

The mass of Ceres generates 0.029 g at its surface, and according to Wikipedia:

...

Ceres can barely keep molecules from escaping its surface, so gravity effects on the shell are going to be minuscule.

Gravities effect on the shell may be small, but it is calculable.  You should do so.  Try a 200m thick shell of gypsum.

[ppnl]

  And proposes a construction method.

Step 1: Construct vast underground habitat to house workers.

Step 2: Enlarge habitat to house family of workers. They are going to be here for a long time.

Step 3: Built giant casino and other attractions to attract tourists and immigrants to grow local economy. Dedicate most of the proceeds from that economy to enlarging habitat.

Step 4: Wait... why did we all come here? Never mind LETS PARTY!

[Paul451]

according to Wikipedia:

And based on my research (noted above)

{laughs}

I did not read the article

No kidding.

[lamontagne]

The two shell world papers were published in the Journal of the British Interplanetary Society, that has a peer review process.
The second paper specifically addresses the question of shell world stability, since it is such an obvious question.
Unfortunately, I don't have the specific paper so I can't comment on the math beyond the first page.  If anyone has the paper we could check the math?

[ppnl]

according to Wikipedia:

And based on my research (noted above)

{laughs}

I did not read the article

No kidding.

I did read the article. It is a short fluff piece with few details published by a science fiction publisher. Did not really impress me. The science fiction equivalent of a cute cat video.

It is true that for a large enough planet there will be a pressure gradient that will help stabilize the shell's gravitational instability. But such a large thin structure is likely to be unstable for a vast number of other reasons. And for small bodies the gravitational instability is still there.

The economic and technological power to make such a structure would also make such a structure obsolete. Imagine Tarzan trying to build a continental transportation system composed of a network of very tall towers and swinging ropes. Once you have the economic and technological power to construct on this scale you have much better options.

If you are serious about a shell world the best way to proceed is to plate the surface with with a meters thick metal and then dig out underneath. But once you commit to digging you don't need the metal shell any time soon if ever. You can proceed at your own pace for generations and if you eventual create a shell world then great. If not then you have billions of people in underground settlements.

[Lampyridae]

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

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.

St. Lowell in the anime Armitage III was built pretty much the same way, prior to terraforming.