A new study portrays the paths of asteroids in the inner solar system as a vast Los Angeles-style traffic system crisscrossed with superhighways along which are hurtling huge, rocky projectiles. And in the middle of the highway network, on a possible collision path, is the planet Earth.

The study estimates that an armada of asteroids, 900 strong, all a kilometer in diameter or larger, present a potential hazard to life on Earth. Some pass within a few moon distances of Earth every year. "Sometime in the future, one of these objects could conceivably run into the Earth," warns astronomy researcher William Bottke at Cornell University. "One kilometer (about .6 of a mile) in size is thought to be a magic number, because it has been estimated that these asteroids are capable of wreaking global devastation if they hit the Earth."

Bottke is lead researcher on a U.S.-French team that has discovered the spatial and size distribution of a large group of asteroids called NEAs (for near-Earth asteroids), a vast system of orbiting rocks in inner space, ranging in size from mere specks to more than 64 kilometers (40 miles) in diameter. The astronomers believe the results of their observational and computer-based study will better quantify the likelihood of future catastrophic collisions with Earth. The survey also is expected to help observational astronomers in improving their search for hard-to-find asteroids that might pose a threat to the planet.

The team's report, "Understanding the Distribution of Near-Earth Asteroids," appears in the latest edition (June 23) of the journal Science. The authors, besides Cornell's Bottke, are astronomers with the Spacewatch group at the University of Arizona's Lunar and Planetary Laboratory and at the Observatoire de la Côte d'Azur in Nice, France.

Calculating which, if any, of the 900 asteroids identified in the study could hit the Earth is tricky, says Bottke. "The problem is that fewer than half of these Earth-threatening asteroids have been discovered so far. Of those we have found, we can accurately predict whether they will strike the Earth over the next hundred years or so, but we can't project out several thousands of years. So it's possible some of these asteroids eventually will move onto an Earth-collision trajectory. It's a dangerous place out there."

The new predictions for the distribution of NEAs in the inner solar system, say the astronomers, imply that 40 percent of the kilometer-or-larger asteroids near Earth already have been discovered. The remaining 60 percent, however, might be more difficult to find, says Bottke. "Most of these asteroids are too far from Earth to be easily detected or are located in regions of the sky that are challenging for astronomers to survey."

The study's authors refer to their survey as a "NEA treasure map" indicating in which orbits most NEAs spend their time. The researchers say the new estimate of the number of large asteroids is about half of that predicted by similar types of analyses reported in the past decade and is slightly larger than an estimate published recently in the journal Nature.

For many decades there has been good evidence that most of the small chunks of rocky or iron material that slam into the Earth's atmosphere daily are chips off old blocks of asteroids. Most of the asteroids in the solar system revolve around the sun on independent orbits, corralled between Mars and Jupiter in a formation known as the main belt. Occasionally, two of these asteroids -- some of them hundreds of miles in diameter -- slam into each other at great speed, causing chunks of all sizes to be blasted off the surfaces.

Most of this material continues to orbit the sun in the main belt. But sometimes the newly formed asteroids migrate to unstable regions of the asteroid belt known as resonances, areas where the tiny gravitational kicks produced by nearby planets such as Mars, Jupiter or Saturn can significantly change asteroid orbits. In some cases, these changes are enough to swing asteroids into a possible future collision path with the Earth.

To find the location of these potentially threatening and hard-to-find projectiles, the researchers used the results of the Spacewatch group's 10-year search for asteroids in the solar system during which it has discovered about 100 NEAs. The problem is that this tally is only a small fraction of the predicted number of NEAs. Using a statistical technique to compensate for the big gaps, Spacewatch astronomers were able to calculate the total number of NEAs but not their approximate location. To obtain the orbits of the undetected NEAs, Spacewatch astronomers combined their NEA population estimates with theoretical models, produced by the Cornell and Nice researchers, which show how asteroids in the main belt are transported to the near-Earth environment.

Other authors of the study were Robert Jedicke of the University of Arizona and Alessandro Morbidelli, Jean-Marc Petit and Brett Gladman of the Observatoire de la Côte d'Azur. The study was funded by NASA and the European Space Agency.