Last week's Jovian pyrotechnics are both a replay of what has already happened here on Earth -- and a preview of what's to come. That's the growing conclusion of astronomers the world over who are realizing that much of the solar system, including Earth, has been engaged in a gigantic game of planetary pinball for millions of years. Bombardments are business as usual in the solar system, and someday, scientists predict, Earth will once again be visited by a starry messenger large enough to pack a wallop. It will blast a crater, spawn tidal waves, cloud the atmosphere with sun-blocking dust and, if the past is any guide, wipe out much of life on Earth. A recent scientific study in the prestigious journal Nature contends that statistically speaking, the chances of dying from a cosmic collision are comparable with those of perishing in an airplane crash, tornado or flood.
The legacy of this continuing celestial slamdance is written all over Earth's face. Even though the normal churning of the planet's crust sweeps much of the impact damage under the rug, scientists have uncovered evidence of more than 140 craters left by incoming space debris, such as the 1-mile-wide Meteor Crater in Arizona. Several years ago, researchers in the Yucatan Peninsula discovered the faint remnants of a huge crater that is widely believed to have been formed in a cosmic collision some 65 million years ago that caused the extinction of 70 percent of the species living on Earth, including the dinosaurs. In 1908, a comet or asteroid hit a remote area of Siberia, flattening trees for miles and sending out seismic waves that were picked up in the United States.
Astro ammo. The main ammunition in these cosmic bombardments is comets and asteroids, bits of rock and ice that are thought to be leftovers from the early days of the solar system. Most asteroids lie in a belt between Jupiter and Mars; comets are thought to reside in clouds at the far reaches of the solar system. These balls of dirt and ice are occasionally knocked out of their placid positions by gravitational perturbations and wander into the solar system's interior. Sometimes the paths of these wanderers intersect with that of Earth. In 1993, a truck- size asteroid passed within 90,000 miles -- less than half the distance to the moon and a close call by astronomical standards.
The Earth experiences a devastating collision about once every 100 million years. Blasts like the one that hit Siberia, which wouldn't cause long-term climatic problems but could wipe out a city, occur every few centuries. Every day, Earth is visited by some 20 tons of rock, mostly tiny particles no bigger than grains of sand.
Though the prospect of a planetary pummeling might send chills down the spine, scientists are quick to point out that ancient collisions have much to do with the shape of modern life. After all, the evolution of primates -- including humans -- might not have been possible without the sudden demise of the dinosaurs. Indeed, comet encounters may have delivered both the planet's water and the original organic molecules -- the once alien stuff that made life on Earth possible.
