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

[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.