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My colonizing business found a small planet with about 0.8G. They would like to build the colony in the giant caves under the crust of the planet, due to the conditions of the surface,(little atmosphere, intense radiation, very rocky geography, intense temperatures). Luckily for the colonists, there is some water in the caves. The caves are usually very big, a mile diameter, they slowly get smaller as you get deeper, (completely blocked off at 125 miles). The caves are connected to each other at some point.

So my question is:

Can they put airlocks on the entrances and keep a working colony with a atmosphere inside?

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  • $\begingroup$ What's the planet made of? $\endgroup$ – Ash Oct 31 '18 at 15:54
  • $\begingroup$ Things like, earth. Just more compacted, (makes it eaisier for colonists). $\endgroup$ – Aaron Oct 31 '18 at 15:55
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    $\begingroup$ A thick atmosphere with something toxic, like a lot of ammonia or chlorine, would be better; that way the lock is only for preventing atmospheric mixing and the pressure is negligible so you don't need strong doors or heavy anchours. $\endgroup$ – Ash Oct 31 '18 at 16:40
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    $\begingroup$ Now that answers all my questions. Thanks! :) $\endgroup$ – Aaron Oct 31 '18 at 16:42
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    $\begingroup$ Happy to help, there are a couple of other existing questions on the stack about sealed cave ecosystems that might interest you too like A water cycle in an underground setting and a couple of others I can't find right now. $\endgroup$ – Ash Oct 31 '18 at 16:49
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I'm assuming that by "little atmosphere" you mean vacuum or close enough; in which case no, not given entrance ways on the scale you've noted in the comments. The rock into which the locks would be anchoured simply doesn't have the tensile strength to withstand the structural pressures the lock would exert. The locks are going to be 14 million square feet on an end (assuming 4000x3500ft the door size doesn't change how big the plug needs to be); at one atmosphere internal pressure and an effective vacuum outside they'll exert a total force of 1.3x1011N. Even if the locks are a kilometre deep that's 23400Nm-2, 3.4 PSI, which doesn't sound like much but as an overpressure, which it will effectively be ever time you cycle the lock, it's enough to destroy most modern structures that aren't heavily reinforced. Even if you could build them and use them for a time the material fatigue of repeated cycles would destroy their anchourages in short order.

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