We are at the cusp of a golden age in space science, as increasingly more entrepreneurs—Elon Musk, Richard Branson, Jeff Bezos—are seduced by the commercial potential of human access to space. But Beyond Earth does not offer another wide-eyed technology fantasy: instead, it is grounded not only in the human capacity for invention and the appeal of adventure, but also in the bureaucratic, political, and scientific realities that present obstacles to space travel—realities that have hampered NASA's efforts ever since the Challenger disaster.          In Beyond Earth, the authors offer groundbreaking research and argue persuasively that not Mars, but Titan—a moon of Saturn with a nitrogen atmosphere, a weather cycle, and an inexhaustible supply of cheap energy—offers the most realistic, and thrilling, prospect of life without support from Earth.