Of all the natural hazards facing us, an asteroid impact is potentially the deadliest, and also the most unpredictable. The latest news is not heartening. Wyn Evans has discovered an asteroid belt hovering just a few million miles beyond Earth's orbit.
Most asteroids circle the sun in a wide band between the orbits of Mars and Jupiter, but some nestle in zones of stability in the outer solar system. Evans, a physicist at Oxford, wanted to see if similar pockets of small asteroids could exist closer to home.
Using a mathematical simulation running on 20 desktop computers, he looked at the fates of imaginary objects plopped in orbits between the sun and Mars. Gravity from a passing planet usully swept the body from the solar system. But Evans found that asteroids could survive for billions of years in orbits some 8 million to 30 million miles outside the orbit of Earth.
Evans thinks there may be as many as 1,000 asteroids in this near-Earth asteroid belt; he has already identified three candidates. Although they generally stay in place, these objects will occasionally get knocked loose by the gravity of Earth and other planets. "There is some danger," Evans says, "because there are ways to eject objects out of the belt. But any Earth-crossing asteroid is potentially dangerous no matter where it came from."