The first of these, a cross-section view (at the top of this page), starts with the comet 30 kilometers above the surface. The comet produces a brilliant bow shock in the atmosphere as it speeds downward. Seven tenths of a second later it hits the ocean, unleashing an impact energy of 300 gigatons of TNT--about 10 times the explosive power of all the nuclear weapons in existence during the height of the cold war in the 1960s. The torrent of energy excavates a large transient cavity in the ocean and creates a dent in the ocean floor below.

The second animation--an aerial view simulating the perspective of a (very well protected) outside observer--shows that the comet itself is almost instantaneously vaporized on impact, along with 300 to 500 cubic kilometers of ocean. A high-pressure steam explosion swiftly rises into the atmosphere. Vaporized debris from the comet and water vapor are ejected into ballistic trajectories that will take them around the globe, with some of the material achieving escape velocity.

An impact such as the one depicted here could spell the end of modern civilization; a more powerful one, such as occurred 65 million years ago, might well mark the end of the human line. A handful of scientists have been tracking threatening objects in space and thinking about ways to head off these comet and asteroid impact hazards long before they approach Earth.