Actually, “scientists” aren’t contemplating a 1,000 year long mission to “save humanity.” Perhaps one or two crackpots who happen to have PhDs may be blogging nonsense, but the scientific consensus is nowhere near pretending that running off to some undetermined refuge (because at this point zero Earth-like planets have been found anywhere) is anything other than a child’s dream.
Let’s look at reality instead of Whizzo Sci-Fi. The Earth is in jeopardy, but not from meteors or 5-billion-year distant changes in the sun. It’s in jeopardy because humans are stupid and incapable of behaving rationally. That’s not our fault; there were never any selection pressures to make us evolve such capabilities. What this means, however, is that not only do we have no destination to aim for and no technology by means of getting there, but we also would self-destruct along the way.
So maybe, instead of pretending we’re living in a rubbish Hollywood TV series, we should think about cleaning up the mess we’re making of our only home, the only world we’re actually adapted to live on. That’s what scientists are actually saying, and thinking about, and trying to encourage.
Everything else is mindless escapist nonsense lacking even the tiniest shred of feasibility.