After years of hard physical and mental training, they take their first steps to the shuttle, waving goodbye to all the spectators and Earth. These astronauts, originally scientists, teachers, pilots, and engineers, each endured at least three years of rigorous professional training before even applying to an astronaut program. Earning a bachelor’s degree is the minimum requirement for NASA positions, and the astronauts train beyond the classroom by swimming laps in a space suit to experience zero-gravity. A day in an astronaut’s life might start by climbing aboard the “vomit comet,” an aircraft that flies a parabolic path to simulate microgravity conditions. Astronauts also accustom themselves to move and work in weightlessness at the Neut...