@futurebird It depends on the scale, both spatial and temporal.
We have an existence proof (exactly one) that you can build a self-sustaining ecosystem that flies through space and survives for a few billion years. So that's obviously possible. The questions are:
How stable can you make it? You probably want fewer mass-extinction events for an artificial one.How small can you make it. I'm willing to claim that 'smaller than a planet' is almost certainly possible, but how much smaller than a planet is an open question.And these are not distinct concerns. If you consider a generation ship that's going to another place where it has a ready supply of raw materials, it maybe needs to be stable for a thousand years or so. That's hard, but orders of magnitude simpler than the ecosystem we currently live in.
We've also got several examples of islands where, even if you're getting a regular injection of fresh oxygen, sea water, and sea wildlife (plant and animal), the ecosystem isn't sufficiently stable. So possibly 'somewhere between the size of an island and the size of the planet' is the scale where we know it can be done. Especially if 'and humans can avoid all dying' is one of the constraints.
https://infosec.exchange/@david_chisnall/113605590825007809
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