In the rough-and-tumble world of asteroids, the 2,000 objects which are 1-kilometer wide may be of most concern to astronomers. But they represent only a small fraction of the asteroid population crossing Earth's path.
Estimates place the number of Earth-crossing asteroids at more than 150 million, ranging in size from 10 meters to nearly 40 kilometers, according to Arno Ledebuhr, a physicist at the Lawrence-Livermore National Laboratory in Livermore, Calif.
Astronomers place near-Earth asteroids into three groups, based on their orbits.
* Discovered in 1932, Apollo objects swing out beyond Earth's orbit. Their closest approach to the sun takes them well within Earth's path. Collisions with other objects knock off debris that falls to Earth as meteor showers.
* The Atens, discovered in 1976, form another class of Earth-crossers. Their orbits around the sun lie largely within that of Earth, crossing it when they near their farthest distance from the sun.
* The Amors orbit the sun wholly outside of Earth's orbit. But at their closest approach to the sun, they come close to Earth's path. They are thought to provide replacements for the Apollos and Atens destroyed by collisions or ejected by gravitational disturbances.
Ultimately, astronomers say these groups draw their material from the solar system's main asteroid belt between Mars and Jupiter. The objects either are knocked free by collisions or drawn out of the belt by the gravitational influence of Mars. In addition, some of the close-in asteroids may be the remnants of dead comets. One such asteroid, 3200 Phaethon, is responsible for the Geminid meteor shower each December.
So far, calculations based on close encounters and on the record of cratering on Earth suggest that about a quarter of the Earth-crossing asteroids will end their existence through collisions with a planet. The rest are expected to feel Jupiter's gravitational tug and be flung free of the solar system.