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Remember Journey to the Center of the Earth? The premise is that there's some kind of cave in the center of the Earth, which has an ecosystem inside it. Well, of course you can't do such a silly cartoonish thing with a planet like Earth. Firstly, it couldn't form naturally, and you couldn't create it artificially. Long before you would get to the center of the Earth, your drilling equipment would melt, and your tunnels would collapse.

But what about other smaller worlds? The concept of hollowing out asteroids to make a habitat is already a thing. How far can you take this? What is the largest object that can theoretically support a cave or void in the center? My basic assumption would be that dwarf-planets are a reasonable cut-off point, since they are compressed into spherical objects by their own gravity. This should squash any voids out of the center, and these can no longer be supported by using the material of the asteroid. Then again, dwarf-planets probably take some time to do this, and it's possible that you could create a void that would slowly slump under self-gravity, giving you time to reinforce it by bringing in stronger materials than the natural rock.

I checked up on what the pressure at the core of Ceres is calculated to be, and based off this paper, it seems to be between 140-200MPa. The paper suggests that Ceres is quite porous. Still the compressive strength of concrete might be 70MPa at the very most, so as I would have thought, digging into Ceres' core to create a cave would not work. I'm not sure how to calculate the core pressure of an object, but if it varies linearly then perhaps an object half the mass of Ceres is about on the limit where you could create a small void in its very center.

I'm not sure about temperature. Buried within this paper, I find that the core temperature of Ceres is thought to be ~350K/76 degrees Celsius, a bit too hot but not immensely so. Half that temperature is a very hot summer day but manageable with cooling systems, but I don't think temperature varies linearly in this way, so an object with half the mass and similar other parameters might not be expected to be twice as cool inside. Given we are dealing with volume, there's probably a cube root in whatever formula would apply.

My guess based on my own limited research is that creating and holding up a void within a body like Vesta might be possible, as it is 27% the mass of Ceres, and probably substantially cooler in its core. It isn't even spherical, but is still a fairly massive object as objects go (equatorial escape velocity is 360m/s). Any actual calculations are welcome.

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  • $\begingroup$ It might be just as easy to build your own death-star-type object. $\endgroup$
    – FlaStorm32
    Nov 16, 2022 at 3:45
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    $\begingroup$ 1) Using current materials? 2) With infinite budget? $\endgroup$
    – user86462
    Nov 16, 2022 at 4:25
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    $\begingroup$ It depends on the size of the void and how stable of an equilibrium you need the system to be in, which makes the question unanswerable as asked. Any sufficiently low-density spherical shell can have as big dimensions as you like, especially if you pressurize it a little bit. Any spherical shell will collapse under its own gravity if you smack it with a sufficiently large cosmic baseball bat. For a given mass, the thinner the shell, the less energy the cosmic baseball bat needs to have to make it collapse under its own weight. $\endgroup$
    – g s
    Nov 16, 2022 at 4:28
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    $\begingroup$ would a dyson sphere count? $\endgroup$
    – mgh42
    Nov 16, 2022 at 6:04
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    $\begingroup$ From the question it looks like you're asking about voids in naturally forming astronomical bodies, rather than manufactured things, but it would be helpful if you could clarify this. $\endgroup$ Nov 16, 2022 at 9:05

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It's not easy to calculate the core pressure because heavy elements tend to sink to the core, the density we can estimate is just an average. It will happen even in objects smaller than Ceres, and here is the big problem, the outer layers are rubble piles, pieces of rock held together more by the gravity at the centre than by sticking to the neighbouring rocks. Beyond taking into account the risk of a collapse you have also to take into account the risk of big chunks of rock falling off and eventually the whole thing falling apart. It is not worth the pain, and entirely artificial object would require only a small amount of extra work and would be a lot more stable.

You could try to dig a big cave in the outer layer that does not touch the core, but even in this case you risk to have pieces falling inside because they did not stick to the other rocks and pieces falling off at the lightest touch like the gas jet from a piece of ice sublimating after being heated by Sun ray, or like a vibration from an asteroid impact on the other side.

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