IT DOES HAPPEN HERE. A meteorite apparently dug this hole near Winslow, Arizona, just 25,000 years ago.

Space may be silent, but it is not always peaceful. The stray comets and asteroids that wander the solar system sometimes cross paths with planets, to devastating effect. Comet Shoemaker-Levy 9 provided an uncomfortable reminder of that fact when it walloped Jupiter in 1994. Each large fragment of the disintegrating comet created a huge black spot in the giant planet's atmosphere that was clearly visible from Earth-based telescopes. And Earth is not immune. The great pockmarks that dot Earth's crust--such as the Barringer Crater near Winslow, Arizona--present hard evidence that the planet has been struck many times in the past.

BRUISED JUPITER. Impacts from Comet Shoemaker-Levy were clearly visible with Earth-based telescopes.

Giant impacts could have grave consequences on life on Earth; they almost surely have in the past. Most scientists now accept the theory (advanced in 1980 by Luis Alvarez and his colleagues at the University of California at Berkeley) that an asteroid about 10 kilometers in diameter crashed into Earth 65 million years ago, killing off more than half the species of life, including the dinosaurs. Recent geologic and geophysical evidence suggests that it was an exceptionally severe blow. The associated crater, called Chicxulub and located in the Yucatn peninsula, may be the largest impact basin to form on Earth in the past billion years.

No human eyes were around to witness that awesome blast. But researchers got a chilling vision of what the last--or next--major impact would look like during a test run of what will soon be the world's most powerful supercomputer. That machine, located at Sandia National Laboratories, is being developed for defense uses. When completed, it will be capable of 1.8 teraflops, or trillion calculations per second.

To explore the capability of the new supercomputer, Sandia researchers David Crawford and Mark Boslough simulated the impact of a comet hitting the ocean rather than the complex detonation of a nuclear weapon. The calculation assumed a one-kilometer-diameter comet weighing about a billion tons, traveling 60 kilometers per second and striking Earth's atmosphere at a 45-degree angle. This simulated body is actually rather small as comets go; Comet Hale-Bopp weighs about 10 trillion tons. Planetary scientists studying the probability of impacts estimate that one-kilometer objects strike Earth every 300,000 years or so.

Crawford and Boslough tackled the problem using a shock-physics code developed by Sandia and informally known as "bang and splat." Even on the ultrafast supercomputer the simulation required 48 hours to complete. But the effort paid off with two animated movies that dramatically illustrate the effects of a cometary collision.

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.

AERIAL VIEW shows the 300 megaton explosion of the simulated comet.

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.

The new results graphically confirm previous ideas about the horrendous environmental effects of such a collision. "What's unique about this is we can now do three-dimensional simulations that can fully resolve all the physics of the impact," says Crawford. This type of simulation has already proven its fundamental accuracy. Two years ago Crawford and Boslough used the same computer code to simulate the Shoemaker-Levy impacts on Jupiter. Their calculations predicted that a visible plume would rise above the planet's rim, closely corresponding to what astronomers actually observed.

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. Skeptics point to the technological, political and economic difficulties in developing technology that could thwart an impact. Perhaps some persuasive images will help the cause.