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This question already has an answer here:

Would it be possible for a planet to have a geometry that allows relatively stable orbits beneath the surface? I understand an artificial object could achieve this just fine, given the technology, but could a naturally formed planet either have the structure to allow for this or be carved out in such a way that it remains stable while allowing this?

By "relatively stable orbit" I don't mean a circular one nor one that doesn't require periodic corrections, simply one that could count as stable enough that a modern satellite could use its thrusters to correct it for its operational lifetime (10-20 years).

Obviously this would require the planet to lack an atmosphere. However, I'm wondering if specifically, an atmosphere-less planet with a vast subterranean cave that spans its underground would allow for this. Would it require rocky columns? If there was no physical connection between the underground and surface would it hold together?

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marked as duplicate by a CVn Dec 21 '15 at 15:09

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • $\begingroup$ I'm pretty sure we have discussed this earlier, but the punchline is that no, it can't work; there's too much mass involved, so you can't make the sphere hollow (the sphere will collapse in on itself), and thus there's no way to have anything on the "inside" of the surface simply because even if it could form in the first place, there won't be any "inside" for long enough for it to matter. $\endgroup$ – a CVn Dec 21 '15 at 14:27
  • $\begingroup$ @MichaelKjörling Hmm, do you mean this? Seems like it's pretty conclusive. Should I just delete this or flag it as a duplicate? $\endgroup$ – mechalynx Dec 21 '15 at 14:32
  • $\begingroup$ @ivy_lynx, in this case we're talking about an artificial satellite of a natural world. Different question. $\endgroup$ – o.m. Dec 21 '15 at 14:35
  • $\begingroup$ This seems strangely appropriate. $\endgroup$ – Joe Bloggs Dec 21 '15 at 14:59
  • $\begingroup$ @ivy_lynx Yes, that looks like the question I had in mind. I can close this as a duplicate if you want? I think having this as a sign post serves a useful purpose, since the terms used are reasonable in both but different between the two; it helps searching. (The fact that one asks about an artificial satellite and the other asks about a natural satellite is IMO not a large enough difference to warrant having two questions that cover essentially the same ground; no pun intended.) $\endgroup$ – a CVn Dec 21 '15 at 15:07
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You could have an artifician canyon or artificial cavern. Something like that features in one of the Culture stories by Iain M. Banks. A natural canyon or cavern with the same geometry would be extremely unlikely.

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