On March 12, 1998, newspaper headlines warned of impending doom from the skies in 2028.  The next day it was called off.  Two movies that summer had killer asteroids or comets as their theme, with the Earth being saved at the last moment by heroic space farers.  In response to this and earlier publicity, the U.S. Congress mandated NASA to conduct a survey to find and catalog at least 90% of Earth-crossing asteroids larger than 1 km in diameter in the next ten years.  Since that time much progress has been made.

The impact hazard is the most extreme natural disaster:  by far the least frequent, but potentially the most deadly.  We will examine the effects of past impacts on the Earth, including the giant impact 65 million years ago that led to the extinction of the dinosaurs, and smaller more recent impacts like the Barringer Meteor Crater in Arizona.  We'll examine our current knowledge of how often impacts of various sizes occur, and their likely consequences.

The first step response to the impact hazard is to simply look for and catalog as many Earth-crossing bodies as possible, as mandated by Congress.  The surveys in progress have thus far established that the number of Near-Earth asteroids larger than 1 km in diameter is about 1,200.  To date, almost half of them have been discovered.  Nevertheless, since the remaining ones are harder to find, the present level of survey effort will not quite meet the 10-year goal specified.

We will conclude by examining briefly the societal issues of how to communicate the hazard to the public, in particular means of avoiding "false alarms," and the risk of mitigation itself, as called by Carl Sagan, "the deflection dilemma."