Bolts from beyond: "shooting stars" secrets

For two centuries, astronomers and geologists have recognized that the Earth is continually bombarded by small extraterrestrial objects called meteoroids. Each piece of this cosmic debris has its own orbit around the Sun. Because some of those orbits cross the Earth's, our planet and certain bits of the debris inevitably reach the same point at the same time and collide.

Every day, in fact, about a hundred tons of extraterrestrial material rain onto our planet, most in the form of grains of dust that float gently downward and land undetected. Some of that dust has been captured by collectors mounted on high-flying aircraft, but the great hope for obtaining significant amounts of it resides with the spacecraft Stardust, launched in 1999 and now on the other side of the Sun from Earth. Early in 2006 Stardust will return to Earth with samples of the interplanetary medium.

It is probably natural to think of meteorites--as the meteoroids that fall to Earth are called--as threatening, even dangerous, phenomena. The best-known meteorites, not surprisingly, are the ones that strike something important, perhaps one of us. Despite the impression left by Hollywood movies, however, people have been hit by meteorites only once or twice in recorded history, and those impacts led to only minor injuries. The only verified mammalian fatality from a meteorite impact in the past century was a dog unlucky enough to occupy the exact spot near Alexandria, Egypt, where a meteorite from Mars struck on a June day in 1911. Closer to home (and more typical), on October 9, 1992, a large meteorite that passed over the eastern United States in a mere forty seconds reached its ground zero in Peekskill, New York, where it demolished the rear end of an aged Chevrolet [see "nature.net," by Robert Anderson, page 63]

Truly large meteorites, such as the thirty-four-ton iron monster that the Arctic explorer Robert Edwin Peary brought from Cape York, Greenland, to New York's American Museum of Natural History in 1897, rank among the scarcest, and scariest, objects on Earth Fifty thousand years ago, a meteorite the size of a house and the weight of a destroyer struck near what is now the town of Winslow, Arizona, excavating a mile-wide hole known as the Barringer Meteorite Crater. Several much larger, though highly eroded, terrestrial impact craters have also been discovered, stark reminders that an object many miles in diameter strikes the Earth every 50 million to 100 million years.