Imagine this scenario: You have been chosen as one of seven astronauts on the first human mission to Mars. You are four months into the three-year, round-trip mission. You share a small spacecraft with six people of different cultures who you do not know very well; one of them does not like you, and there is no place to escape from this person. The spacecraft is noisy and the lighting is poor. You have not been sleeping well because your internal clock has been thrown off by the lack of a normal day/night cycle. The last time you spoke to your loved ones on Earth was a month ago. Though you cannot feel it, your bones are becoming weaker due to calcium loss. Your heart is shrinking too. You have a toothache, but there is no dentist onboard—one of your crewmates will have to...