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, set...