That people will travel to Mars, and soon, is a widely accepted conviction within NASA. Rachel McCauley, until recently the acting deputy director of NASA’s Mars campaign, had, as of July, a punch list of 800 problems that must be solved before the first human mission launches. Many of these concern the mechanical difficulties of transporting people to a planet that is never closer than 33.9 million miles away; keeping them alive on poisonous soil in unbreathable air, bombarded by solar radiation and galactic cosmic rays, without access to immediate communication; and returning them safely to Earth, more than a year and half later. But McCauley does not doubt that NASA will overcome these challenges. What NASA does not yet know — what nobody can know — is whether humanity can overcome the psychological torment of Martian life. A mission known as CHAPEA, an experiment in which four ordinary people would enact, as closely as possible, the lives of Martian colonists for 378 days, sets out to answer that question.