Proxima b, our nearest neighboring exoplanet, is almost 25 trillion miles away. Even one of our fastest spaceships—the 31,600-mile-per-hour New Horizons—would take hundreds of thousands of years to get there. Assuming we can’t figure out how to warp space-time (seems unlikely, but fingers crossed), we’re still looking at a couple-hundred-year trip in the best-case scenario, which leads to the real problem: No human crew could survive the entire ride. Science-fiction writers have long floated so-called generation ships as a solution. Designers would outfit these interplanetary cruise vessels to support a community of adults and their children, and their children’s children, and their children’s children’s children…until humanity finally reaches a new celestial shore. Here’s our best guess for what it would take to sow the seeds of an extrasolar species.