In recent years, impacts from Earth-crossing asteroids and comets have been identified as a threat that is real and credible. Although infrequent in relation to a human lifetime, devastating impacts have occurred countless times during Earth's history and will occur again in the future. In 1990, the United States House of Representatives directed the National Aeronautics and Space Administration (NASA) to study the impact problem. NASA organized an international conference and conducted two workshop studies. The first workshop, comprising three formal meetings in 1991, defined a program for dramatically increasing the detection rate of large Earth-orbit-crossing asteroids, and established the requirements for determining the orbits of such bodies. The second workshop, in January of 1992, focused on defining systems and technologies to alter the orbits of Earth threatening asteroids, or obliterate them. The major result of these efforts was the establishment of a coordinated ground-based effort, known as the Spaceguard Survey, whose primary goal is to search for large asteroids capable of global devastation. This logical first step has significantly reduced the chances that Earth will be impacted by a large near- Earth asteroid without significant warning time. Just over 2 years later, from July 16 to 22, 1994, the inhabitants of the Earth witnessed the colossal impacts on Jupiter caused by more than 20 fragments of the Shoemaker-Levy 9 comet. Each impact zone was approximately the size of the entire Earth and provided a timely and vivid illustration of the impact hazard.