Building a space station, in which astronauts would live for a considerable time, is essential even if the space station project were to contribute no new knowledge about space or Earth that could not otherwise be obtained. For future missions to explore Mars, we will need the medical knowledge that the space station project will give us about the limits of human capacities to live in spacecraft for an extended time. The argument makes the assumption that ['the capacities of astronauts are typical of those of ordinary human beings', 'the exploration of Mars will be carried out by people traveling in spacecraft and not by robots alone', 'living in a spaceship for an extended time presents insurmountable medical problems', 'a mission to Mars will be the first of many missions that will explore the solar system']