Is it possible to have a structurally self-supporting hollow Earth where the weight of the shell is balanced against the atmospheric pressure inside?

  • It must be human habitable (gravity is nice but optional)
  • It must be able to keep its occupants safe from radiation from space....
  • It must be able to survive a long long time floating around the solar system without becoming uninhabitable.
  • The designers couldn't afford to use hand-wavium so they had to make do with more traditional construction materials (which, thankfully were not in short supply)

Is such a structure actually possible, and if so, how big can it get before it isn't?

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    $\begingroup$ Self supporting against which kind of load? In space there is no "bottom" $\endgroup$ – L.Dutch - Reinstate Monica Apr 28 '18 at 4:53
  • $\begingroup$ To live on the outside or the inside (because you don't mention an internal source of light)? $\endgroup$ – RonJohn Apr 28 '18 at 13:29
  • $\begingroup$ "more traditional construction materials". Since we don't know what existing Hollow Earths are made of, there are no "more traditional construction materials". $\endgroup$ – RonJohn Apr 28 '18 at 13:31
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    $\begingroup$ @L.Dutch against it's own gravity trying to crush it under its own weight $\endgroup$ – Samwise Apr 28 '18 at 22:55
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    $\begingroup$ @RonJohn as in materials that exist so no carbon-neutronium alloys $\endgroup$ – Samwise Apr 28 '18 at 22:56

Can we build it?

Sure. We can always depend on Clarkean Magic:

Any sufficiently advanced technology is indistinguishable from magic. (Arthur C. Clarke)

From the point of view of simple physics, we can make inflatable balls out of rubber and buoys out of metal. Therefore, such a shell is only a head scratcher from the point of view of where all the materials came from (but you said that wasn't a problem).

Can we inhabit it?

Again, sure. Fill with air, add atmospheric recyclers, build habitats, fill void with floating islands of fresh produce because...

No gravity

The Shell Theorem demonstrates why there is no gravity inside a sphere. Basically, it's because the gravity pulling toward any point on the inside of the sphere is perfectly balanced by all the gravity pulling toward all other points on the sphere. No gravity.

Well... you could have "gravity" along the equatorial region if you set the sphere to spinning, but only there. You'll need big honking thrusters to keep the planet both spinning and where it belongs... You should read Niven's "Ringworld" books, you'd like them.

Radiation free

Possibly. If you have the muscle and the dough to build a basket ball this big, you can afford to coat it in a couple of yards of lead. Earth survives due to both its atmospher and its magnetosphere. You're missing a boatload of mass, a liquid core, and that core probably needs to be rotating. So Clarkean defensive shields would be helpful. If you can't have shields, that layer of lead becomes really important.

Don't get too close to a star and don't let any meteors hit you. That's the real problem. Planets generally take care of this automatically. You'll need to have some defensive system in place that deals with things more tangible than radiation. But, you can build the sphere, so you can build those, too.

Survival is paramount

This depends solely on how you build the interior. You have plenty of volume to work with and the capacity to get some gravity if that matters. You need broad spectrum light, water, atmosphere, something akin to soil (Orchids use bark...), and a way to hold it all together. Maybe nets. If you can build the sphere, you obviously can build all the equipment to keep the interior habitable. The light's the biggest problem because unless you have a bazillion glo-globes floating around you'll need to dedicate some of your limited surface space to light panels (a lot of it).

Of course, as you work through these details, it'll beg the question why you're simply not living on a planet orbiting a sun that does all this for you.

And I want it WHOMPING big!

I'll be honest with you, you can make it as big as you want. But there comes a point where people need to ask why you're bothering? Think of it this way: Let's say you want one so large that it takes the utilization of all planetary mass within your solar system to do it. You now have a big honking sphere, let's say 10X the volume of Jupiter (I've not done the math, but just for fun, 10X), you've filled it with air, water, pizza, and cream soda. And you have a lifetime subscription to Netflix and more than enough interior surface to project the latest episode of The Walking Dead for societal enjoyment.

Now what? Even if you could build such a creature (oh, let's call it a Dyson Sphere) what would be the point? Economically, it's simpler to just inhabit other planets. Heck, it would be cheaper to terraform other planets. With the tech level needed to do this, it would be cheaper to convert your population to androids that no longer need food, water, or atmosphere.1

So, yes. As I said before, with enough Clarkean Magic you can do it.

Can you launch an ICBM horizontally?

Sure! Why would you want to? (The Hunt for Red October)

1Which really makes me wonder what those astronomers who think they've found evidence of mega-structures actually found. They really don't make any sense. But, people bought into the idea that NASA found a face on Mars. Until someone got a better look at it.

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  • $\begingroup$ I’m not sure we could make it arbitrarily large: the mass of the atmosphere inside the shell will mean that to support larger shells we need denser atmospheres etc, leading to the point where any atmosphere dense enough to support the shell would start sublimating/crystallising/doing all sorts of weirdness, or possibly just collecting on its own into a star. $\endgroup$ – Joe Bloggs Apr 28 '18 at 7:52
  • $\begingroup$ The roman arch shows how to construct the sphere so it won't collapse. I.e. atmospheric pressure isn't needed to preserve the spheres structural integrity. $\endgroup$ – pojo-guy Apr 28 '18 at 12:33
  • $\begingroup$ @pojo-guy the sphere will still have a gravitational force trying to crush it under its own weight, this is why pressure is higher at the center of the Earth, it's just in this case, the core is made of air and not rock and iron but the same problem applies. $\endgroup$ – Samwise Apr 28 '18 at 23:03
  • $\begingroup$ @samwise The Roman arch depended on the force of gravity trying to collapseit to maintain its structural integrity. $\endgroup$ – pojo-guy Apr 29 '18 at 0:00
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    $\begingroup$ @pojo-guy An arch relies upon the compression strength of the materials to take the force. At planetary scales even the hardest materials known to man would not survive - your arch would need to be built of the purest handwavium. $\endgroup$ – pluckedkiwi May 22 '18 at 19:09

Is it possible to have a structurally self-supporting hollow Earth where the weight of the shell is balanced against the atmospheric pressure inside ?

As a sphere this is a terrible idea and the most likely way you'd build a habitation in space would be as a cylinder ( e.g. an O'Neill cylinder ) which has huge advantages, not the least of which is pseudo gravity.

A single large structure is also a very bad idea. Let's say we build a single world like this and we have say four billion people on it. A single large asteroid can wipe it out because structurally it would be a disaster for a major impact to occur. It would, at a minimum, be ripped apart by an explosive decompression of the internal atmosphere. But build lots of smaller cylinders and one impact just damages one of them. A tragic loss, but not a catastrophic one.

So the all-your-eggs-in-one-basket strategy is a dreadful idea.

If you insist on spheres, you may as well build lots of them.

It must be human habitable (gravity is nice but optional)

Humans need gravity for safe long-term biomechanical function. Gravity (or something equivalent) is not optional for long term human habitation. And for humans it needs to be be as close to one gee as possible. We are "designed" for one gee.

We can live and work in zero gee for extended periods (months), but don't confuse that with living our lives out in microgravity.

Here's a brief article from Scientific American on the subject, although there are plenty more out there.

As your hollow shell will have microgravity at best, this simply kills the idea dead for humans living there in the sense of a permanent home. It's still viable for short term (months) stays (followed by a lot of hard work to recover fully from the experience).

A rotating sphere can provide some pseudo gravity of course, but it will vary with position and this means you're effectively making large areas effectively useless for habitation. The cylinder remains the ideal shape.

It must be able to keep its occupants safe from radiation from space....

This is fairly easy and can probably be considered trivial.

It must be able to survive a long long time floating around the solar system without becoming uninhabitable.

Define "a long time". Years ? Decades ? Centuries ? Thousands of years or millions ?

Let's say centuries.

You need airlocks for leaving the sphere (if you want to).

In principle you could prevent most outgassing on those timescales. Note that if an asteroid impacts it could create a leak. It's going to depend on the thickness of your shell, but at best you'd need ongoing maintenance.

This leads to another issue with a single large (Earth-sized) sphere. If you have lots of O'Neill cylinders and you think there's an asteroid on the way, it's likely you can move far enough out of the way to avoid the problem at all. Such a cylinder has a huge mass, but it's movable and you can design in enough structure to make it safe for small impulse engines to do this. Your sphere has a huge mass and it's a bigger target. You have to move it further and you need much more energy to move it (and of course return it to it's original orbit). This makes it more vulnerable to asteroids, IMO.

To afford extra protection you need a thicker shell. But that means you're making an even more inefficient use of resources than a fleet of O'Neill cylinders would. So it's a poor choice.

The designers couldn't afford to use hand-wavium so they had to make do with more traditional construction materials (which, thankfully were not in short supply)

But we can make a more flexible "O'Neill fleet" from the same (if not fewer) resources. We gain safety and flexibility and a nice healthy pseudo-gravity and we loose nothing by building the fleet. The sphere just wastes resources.

Is such a structure actually possible, and if so, how big can it get before it isn't?

Probably you can build as large as you like if you have the resources. It's inefficient and it doesn't provide a safe one-gee environment for your inhabitants.

You might be able to prevent the atmosphere from becoming a dangerously gravitationally compressed core that destabilizes the system by carefully heating the core (e.g. solar energy gathered from the shell - itself a complication with lots of downsides) to keep it hot enough to prevent gravitational concentration. You could in principle combine this with a system to provide light on the interior in a day night cycle, but you would need vast transparent areas or equally vast light banks focused on the core. All very complex and with lots of potential problems.

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Actually, yes. Not sure if passive support is a hard requirement, but with active support it's not to big a challenge. The idea is similar to that of an orbital ring.

Build a ring of desirable size and make it spin with orbital velocity which will reduce strain of self-support to pretty much zero. Build another, hollow, ring around the first one which doesn't spin using the spinning one as support. The outer ring will experience gravity as it constantly "falls" towards the center of mass of both rings.

Since you can build this orbital rings at any size you can "stack" them, displacing them radially around a joint center of mass. Since the outer rings don't rotate you can build a support structure between them and even "level" that structure. (Giving you a flat surface to build upon).

Now this might sound a bit ridiculous, but you could use something heavy that produces energy, e.g. a "small", spinning neutron star to build something like a dynamo and provide the active support with energy and further provide earth like gravity, if you place it into the joint center of gravity.

There are some caveats like the rotating magnetic field of the star, maintenance or the fact that lateral rotation of the orbital ring (assuming you want rotation of the planet) might require some tricks, but I'll let you solve those details.

Also flat earthers behold, you could construct two such orbital rings and have a relatively small lateral displacement between them in a cylindrical shape and build a support structure between.

In one direction you can walk around it, in the other you can literally fall of the edge of "earth" :D

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