In 1957, a small group of scientists, supported by the U.S. government, launched an attempt to build a four-thousand-ton spaceship propelled by nuclear bombs. The initial plan called for missions to Mars by 1965 and Saturn by 1970. After seven years of work, political obstacles brought the effort to a halt. The Orion team, led by the American bomb-designer Theodore B. Taylor, included the physicist Freeman Dyson, whose son George was five years old when the existence of the project was first announced. In Project Orion, George Dyson has synthesized hundreds of hours of interviews and thousands of pages of newly excavated documents, still only partially declassified, to piece together one of the most tantalizing "what if" stories of the twentieth century.