When scientists discovered the first ocean beyond Earth, they had two big questions: “Is it habitable?” and “How do we get there?” To answer the first, they had to answer the second, and so began a vivacious team’s twenty-year odyssey to mount a mission to Europa, the ocean moon of Jupiter. Standing in their way: NASA, fanatically consumed with landing robots on Mars; the White House, which never saw a science budget it couldn’t cut; Congress, fixated on going to the moon or Mars—anywhere, really, to give astronauts something to do; rivals in academia, who wanted instead to go to Saturn; and even Jupiter itself, which guards Europa in a pulsing, rippling, radiation belt—a halo of death whose conditions are like those that follow a detonated thermonuclear bomb. The Mission is the Homeric, never-before-told story of modern space exploration, and a magnificent portrait of the inner lives of scientists who study the solar system’s mysterious outer planets.