Here's something to worry about: A small asteroid -- a chunk of space debris perhaps a half-mile in diameter -- is on a collision course with Earth. A strike by a body that size could explode with the force of 25,000 hydrogen bombs, creating massive tidal waves and flinging enough dust into the atmosphere to block out sunlight for months. Civilization would collapse. Anyone who survived the initial impact would shortly die of starvation or exposure. Eventually all traces of human life would be extinguished from the planet.
Scientists say the chances of such an event occurring tomorrow are extremely small. But the Earth has been struck repeatedly in the past by objects whose orbits around the sun periodically cross its own. Evidence of such events are preserved on the Moon's cratered face; erosion on Earth has mostly erased the geological record, although relatively recent events, like the meteor impact that carved out the great Crater Lake in Oregon 60,000 years ago, are still visible. Early in this century a comet is thought to have exploded over Siberia, felling trees over a 100-mile radius. And scientists suspect the impact of an asteroid or comet may have been responsible for the extinction of the dinosaurs some 65 million years ago.