It's particularly interesting I think because now that you've brought it up, it seems to me there is a similar issue with much Earthly colonization or frontier expansion, and I've never seen it mentioned.

For example, anyone who left England to start or join a New World colony early on, when settlements were far apart and it was a struggle for each to survive, was entering an environment where their children would have a much harder, much more constrained life than they could have had back in England.

Were there debates back then about the morality of moving somewhere where you descendants, possibly for generations, would have harder lives than if you stayed where you are?

In outer space, cosmic rays will make this process faster. Also, in outer space, it will need a constant source of light for biological reactions, and also just to keep from slowly cooling down to 0k through heat emission.

On the other hand, it is under two Mount Palomar observatory domes full.

I'm assuming anyone building a generation ship is going to build it in space, not build it on a planet's surface and then try to launch it, so size is not really going to be an issue.

The big engineering issue will be propulsion. We don't have anything now that could drive such a ship. My guess is that this is either something we'll have within the next couple hundred years or we'll never have it.

I'm assuming that by the time you want to build this kind of ship, you've gotten raising people from frozen embryos or frozen eggs and sperm perfected, and that is where most replacement crew would come from. That should greatly reduce the crew size needed.

I think we'll be able to handle the biological side of this within a hundred years, with a good chance of it being quite a bit sooner than that.