Aurora - a critique by Stephen Baxter and others

[Rynn]

The point raised by Baxter about predators is a good one. If you are trying to take entire chunks of a biosphere along with you on an interstellar trip, then to take pack predators like wolves you would need to take many, many square kilometres, which woul require very large ships indeed. Any large predators or other animals with extensive ranges would need to be taken as DNA or gametes/zygotes, along with gestation technology.

Problem is predators serve a very important role in ecosystems. If you don't have them then you're going to find yourself over-run with animals whose populations are normally controlled and that could lead to ecosystem collapse. Hmm...mission failure due to explosive rabbit population :p never thought of that as a failure mode for space travel before.

Ways round this would involve increasing technological intervention of the ecosystem. You could replace the predators with culling bots that do little more than kill the prey and deposit the bodies appropriately (don't want to leave too many lying around or you'll have to spend resources culling scavengers and dealing with the problem of excessive rotten meat). I imagine that most of the small ecosystems in OA have a substantial mechology supporting them.

If you have access to gestation technology, then you don't need a massive generation ship; a moderately small one would do, even if cryogenic tech is not available. I call this the hybrid generation ship concept- you can still have a multigeneration ship, but with only a fraction of the number of living sophonts on board.

I disagree, slightly. To check I understand do you mean gestation technology for growing colonists at the other end? If so I disagree that this allows you to have a small ship. Automation is what shrinks ship size by decreasing the amount of colonists you need to maintain a ship.

Space habs (and I'd class generation ships as this, they're just habs going places) designed to be closed ecosystems are going to require a large amount of technological infrastructure to maintain. As I said above you'll need bots to manage animal/plant/etc populations, you'll need smart dust of some sort to monitor species levels, O2 levels, soil health, track any emerging diseases etc. You'll need chemical plants, preservation like areas to safely breed up population numbers, transport, utilities, recycling plants, factories to maintain the bots and infrastructure etc. All of that is going to require a labour force. With low levels of automation the number of people it would require to maintain a technological economy sufficient enough to maintain the ecosystem that supports them likely ranges in the millions.

The more automation you add, the lower this number becomes. That's the only real way you're going to get smaller interstellar colony ships. Without that you need to take a small city's worth of people just to keep everything ticking over.