In 2004, the NRC released a workshop report about the future direction of the U.S. civil space program. At the same time, the Administration announced the Vision for Space Exploration, and in June 2004, it issued a report that articulated a balanced space program for human and robotic exploration and science. Subsequent NRC reports, however, have noted that NASA has not been given the resources to carry out this broad-based program. This challenge, along with others faced by the U.S. civil space program, stimulated the NRC to form an ad hoc committee to organize a second workshop, held in November 2007, to address the space program's future directions. The workshop's goal was to air a range of views and perspectives so as to inform discussions of these questions by policymakers and the public. This book presents a summary of the workshop.