To be sure, Earth is facing some big problems, including climate change, overpopulation, epidemics, and asteroid strikes. But before we flee this planet like an action hero jumping out of an explosion, let's think about this for a second. Sure, it'd be great to have a backup civilization somewhere in case asteroids wipe out all life on Earth. And it would be one of the most exciting things humankind has ever done. But what would it actually require?