When space shuttle Atlantis rolled to a stop in 2011, it did not mark, as some worried, the end of human spaceflight. Rather, as the extinction of the dinosaurs allowed early mammals to flourish, retiring the shuttle signals the opening of far grander opportunities for space exploration. Led by ambitious private companies, we are entering the early stages of the migration of our species away from Earth and our adaptation to entire new worlds. Mars is the stated goal of Elon Musk of PayPal fortune; polar explorers Tom and Tina Sjogren, who are designing a private venture to Mars; and Europe’s privately funded MarsOne project, which would establish a human colony by 2023. The colonization of space is beginning now.