Humans not only can live in space—we already do live in space! Since 1971, humans have been maintaining space stations in low Earth orbit, where people can stay in outer space for extended periods of time. Human beings have now lived continuously in space for the entire twenty-first century—and, except for a brief fifteen-month interruption, continuously since 1989. Humans have been living in space for so long, in fact, that most people on Earth do not even give it a second thought any more. The challenge now for humanity is to live in outer space beyond low Earth orbit, such as on the Moon, or Mars, or in interplanetary or interstellar spacecraft.