Formed by the impact of an asteroid or a small comet that smashed into Earth about 65 million years ago, the Chicxulub Crater in Mexico's Yucatán Peninsula may measure nearly 200 miles across. An impact of this magnitude would have created huge ocean waves and a global dust cloud that blocked sunlight for years.

approach it closely. Hundreds more are likely to exist. Finding them is no easy task because they are faint and move quickly across the sky. Nevertheless, astronomers hope to spot most of the truly dangerous ones--those measuring a half mile across or more--by early in the twenty-first century.

If you think that's frivolous, you need look no further than Jupiter itself. In 1993 the planetary scientists Eugene and Carolyn Shoemaker and the astronomer David Levy discovered a comet that looked strangely flattened. They and their colleagues soon realized that the comet, named Shoemaker-Levy 9, was trapped by Jupiter's gravitational field. The planet's intense tidal forces had torn the comet into 21 pieces, each about a mile wide, and stretched them into a 100,000-mile-long chain that looked like a string of pearls (page 58). Orbital calculations revealed that these pieces would plow directly into Jupiter on their next pass by the planet. Astronomers eagerly set up their telescopes for the first recorded collision of objects in the solar system.

The results were breathtaking. By July 1994 the chain was over a million miles long. The fragments took a week to slam into the planet, one after the other. Several of them carved dark, long-lived scars in the atmosphere while blasting plumes of gas thousands of miles into space. The seventh fragment exploded with an energy equivalent to 6 million megatons of TNT--nearly 1,000 times the power of all the nuclear weapons on Earth--and created a fireball 2,000 miles high. If any of the 12 largest pieces had struck Earth, the human race probably would have been obliterated.

A blast with the force of a 10-megaton bomb flattened trees in the Tunguska region of Siberia in 1908. Scientists think the destroyer came from space, probably a small comet that detonated in midair.

In one of the ironies of nature, Jupiter's disruptive presence in our solar system is also a blessing. Without Jupiter's tendency to sweep aside intruders such as incoming comets, Earth almost certainly would be more heavily bombarded. However, an asteroid did sneak through 65 million years ago, gouging a deep hole near Mexico's Yucatán Peninsula and probably dooming the dinosaurs. A mile-wide crater in the Arizona desert bears witness to an impact 50,000 years ago by a rock as wide as a football field. And just the other day, in cosmic terms, a vast stretch of forest near the Tunguska River in Siberia was flattened in 1908 when some object--probably a small comet--detonated in the air with the force of a 10-megaton bomb. These events serve as reminders that motion in our seemingly tranquil universe can unleash great violence with little warning.