The Committee to Nuke Elysium

before actually sending out a generation starship, a necessary first step is to assemble the habitat section, populate and provision it, and boost it out to the Earth-Sun L2 point (a stable solar orbit in Earth’s shadow, about 1.5 million Km further out from the sun). L2 is a good proxy for interstellar space — it’s in perpetual shadow and very cold — and if we can run a self-contained and unreprovisioned habitat there for a century, then we can probably strap a propulsion system to it and run it for a century-long interstellar cruise. On the other hand, it’s close enough to home that if the biosphere crashes due to some obscure micronutrient cycle going nuts, it (and its inhabitants) can be brought back to Earth orbit.