First of all, the debate about the ethics of deep space travel was definitively concluded by Red Dwarf:
LISTER: Cat.
CAT: Mm?
LISTER: Did you ever see the Flintstones?
CAT: Of course.
LISTER: Do you think Wilma’s sexy?
CAT: Wilma Flintstone?
LISTER: Maybe we’ve been alone in deep space for too long but every time I see that show, her body drives me crazy. Is it just me?
CAT: I think in all probability, Wilma Flintstone is the most desirable woman who ever lived.
LISTER: That’s good, I thought I was goin’ strange.
CAT: She’s incredible!
LISTER: What do you think of Betty?
CAT: Betty Rubble? Well, I would go with Betty…but I’d be thinkin’ of Wilma.
LISTER: This is stupid. Why are we talking about going to bed with Wilma Flintstone?
CAT: You’re right. We’re nuts. This is an insane conversation.
LISTER: She’ll never leave Fred and we know it.
There’s no point talking about this. No-one’s going to space, man.
No-one.
NO-ONE.
You can’t even coherently talk about why not, because the obvious reasons seem ridiculous when set against the even more obvious reasons.
OK, but…
…but I disagree with the statement.
The ethical question here concerns freedom of movement. This is probably hard to appreciate for anyone living in the UK right now. But it matters.
Freedom of movement doesn’t mean the right to go anywhere you want. It doesn’t give you the right to move to Saturn, or even Rockall. Rather, it means the right to visit your friends wherever they live.
The people on a generation ship are denying their descendents freedom of movement, without any opportunity for those affected to give their consent. Kids will grow up without ever being able to visit New York City. That’s not fair.
The current generation of humans mostly does not have the opportunity to visit New York City. They do, however, technically have the right to cross international borders as refugees. This right is routinely denied. But the borders are routinely ignored too. The ability to migrate, potentially “illegally”, really is a choice available to most people today if they need it, albeit usually a terrible choice. At least you can say, there is no impossible-to-cross limit beyond which they cannot reach. Imposing such a limit would be keenly felt. Very few people today are as constrained as those kids on the generation ship would be.
Two sides
This is true regardless of how large the generation ship is. It’s easy to see that life on a generation ship with only a few thousand people could be stifling. But suppose that somehow there are billions of humans on the ship, big enough to have its own New York City. Surely everyone would be satisfied with that? But no, there would still be billions of members of the human race that children on the generation ship cannot be close to. Who knows, perhaps your perfect romantic match is back on Earth? This is an irreversible choice that parents would be imposing on their children. That is not ethical.
Or let’s go even further. Suppose 8 billion people are on the generation ship and only a few thousand left on Earth. Earth is big, but if it has only a few thousand people, those people will have as few opportunities to lead fulfilling lives according to their own criteria as if they were sealed in a tin can. For those people, the generation ship acted unethically by leaving without them.
The unethical choice is to erect an uncrossable barrier between two halves of the human race, indefinitely. Humans are not merely social animals, they are insatiably curious about other people even beyond their line of sight. For someone trapped on one side of a wall, it is irrelevant that their own side is pleasant, spatious, and filled with opportunity to meet with fascinating individuals. There is a wall, and something unreachable on the other side. That hurts. You might argue that this person’s life is not affected by the presence or absence of a space ship on the other side of Saturn. But you would be underestimating the social needs of humans.
Launching the generation ship effectively creates two prisons simultaneously. It doesn’t matter much which one you call the inside and which the outside.
Ethically
If we were going to take interstellar human settlement seriously, ignoring all the practical reasons it can’t happen, the only ethical way to approach it would be slowly and continuously.
There would need to be at least a continuous chain of station-keeping outposts strung out between the stars. Each one would be a stable healthy society, if not necessarily self-sufficient. It should be a good life for anyone who settles down anywhere. And that should include the opportunity to move house, in either direction along the chain. Perhaps it would still be impractical for anyone to travel from one star to another within a lifetime. But at least there would not be any single glaring boundary.
Then, of course, there are the various speculative options that would allow individuals to make the entire journey in one lifetime. Those don’t help this question though. If we had any of those, there would be no need for generation ships.
No
So, for anyone considering building and launching a generation ship, the answer is simple. Do not do that. Find another hobby to pursue this weekend.