I always considered generational ships to be a horrible idea. For a big enough population to avoid problems with inbreeding, you need massive amounts of resources, ability to produce resources, and, most "optimistically" to me at least, an AI to oversee everything. Why do I suggest an AI instead of a captain? People die, people are greedy selfish shiny happy people, and people are emotional beings. Computer systems, built with redundancy, are essentially immortal, and can (could) be programmed with logic systems to avoid the problems of human chain of command.
I think, haven't looked into it in a long time so guessing on advancements in that time frame, our artificial womb technologically is almost where it would need to be for a "seed drone", but we currently, to my knowledge, lack a way to suspend the growth of the zygote for years on end. I do think seed drones are our best chance of leaving the solar system and eventually the galaxy though, if they are used properly. Properly, in this case, means launching every 20-50 years. In theory, and Moors law tends to support this, our tech gets better, faster, smaller, over time. In theory, this means later launched seed drones could reach the target before earlier ones.
I take that to mean we should start launching them as soon as possible. "Well why bother if the tech is just going to keep getting better?" Because we would wait around forever instead of actually doing it otherwise.
While the seed drones are traveling we can also expand locally. Terraforming Mars will mostly take money, cubic cubic amounts of dollars, but is feasible even with existing technology. I personally think we'll have human boots on Mars by 2040, with a permanent colony by the end of the century. As for actually making it livable without habitats or space suits, that time frame really depends on how much money could be thrown at it.
Venus is another ball of wax. Closer, but requiring the exact opposite type of work Mars would. In fact we may fix our planet and atmosphere on the way to figuring out how to deal with Venus and it's runaway greenhouse effect.
Along with either of our planetary neighbors, what would greatly help in making big changes as well as providing funding would be to collect an asteroid for local mining. We're nearing that reality. If the "DARPA projects are 20 years ahead of current reality" line of thinking holds true in this like it has with computers, agriculture, the Internet, and military weaponry, we're probably already there technology, just not publicly.
I think the biggest hurdle we really have is how pathetically short the human life is on a galactic time scale. 80 years isn't even the blink of an eye, hell the 200,000 years we've been out of caves and genetically human is barely a blip on a galactic time scale. Overcoming either the biological limits of our bodies and selves, OR, finding a way to make the species as a whole give a berkeley about the future instead of their immediate selfish gratification or quarterly profits is probably the biggest hurdle in the whole program.
Although if we could divert the world's military budget for a single year, we would be well on the way. Stretch it to a decade and there's no stopping us. When humans want, and can financially back, something they do amazing work. Unfortunately, war is way to profitable to give up for the betterment of the race. Putting that money to a good use instead would allow everyone on the planet to be fed, housed, and educated, leaving us no major roadblocks on our way to the stars. But that's just wishful thinking on my part.
In closing, I berkeleying love space, I think about it all the time, I'm generally reading hard science fiction, and I hate that I won't live long enough to leave the planet let alone the solar system barring a miraculous breakthrough.
