The past quarter century of space science has been extraordinarily productive. The United States has held the lead in space science during most of these years, exploring new worlds, discovering new phenomena in space, and providing new ways to observe and predict changes in the global environment. The national space science program has amply fulfilled the objective of the National Aeronautics and Space Act of 1958 to extend "human knowledge of the Earth and of phenomena in the atmosphere and in space." Moreover, it has contributed substantially to other objectives set forth in the act,...