"When I first looked back at the Earth, standing on the Moon, I cried." Those are the words of astronaut Alan Shepard, who in 1971 became the fifth person to walk on the Moon, 10 years after becoming the first American to travel into space. He's one of the select few who've seen Earth as a globe from space — an experience that's often described by astronauts as giving them a sudden awareness of the fragility of our planet. Psychologists have dubbed this 'the overview effect'. One day, we might all experience this feeling. As temperatures rise, populations grow and ecosystems strain under our influence, the Earth is looking increasingly fragile. Indeed, last year Prof Stephen Hawking warned that we only have 100 years left to find a new home planet. It might sound like pie in the sky, but the idea of leaving Earth for good is one that scientists are taking seriously. From 3D-printed buildings to asteroid mining, and from space agriculture to food made from poo, a raft of new technologies will make it easier for our descendants to set up base elsewhere in the cosmos. Meanwhile, recent experiments on the International Space Station are revealing more about how we'll stay healthy on long-term missions, and how space travel affects the human reproductive system — something that's crucial to understand if we're ever going to go forth and procreate on another planet. Our special report starts on page 40.