But Yeomans and others said all the estimates overlap because there are wide margins of error in estimating the number of large, potentially Earth-whacking asteroids and how often they really hit the planet.
"What's important to us is not the long-term average frequency [of impacts] but whether in fact anything is going to hit us in our lifetimes or our grandchildren's," said NASA planetary scientist David Morrison, who chairs an international working group on near-Earth objects (NEOs). "The important thing is not the statistics, but whether or not there is an asteroid out there with our name on it."
The simulations tracked the paths of thousands of cyberspace asteroids with varying orbits for 100 million years. The simulations were compared with Spacewatch's observations to estimate there are 900 near-Earth asteroids a kilometer wide or larger. The margin of error indicates the number could be between 790 and 1,010 such asteroids.
"It means that people's chances of dying in an asteroid impact have not changed significantly from the previous estimates," said astronomer Ted Bowell, of Lowell Observatory in Flagstaff, Arizona. "The odds are low."
The study indicated that when asteroids collide in the Asteroid Belt between Mars and Jupiter, fragments are not immediately kicked into orbits that threaten Earth. Instead, the rocks drift into unstable "resonance" zones where gravity from Jupiter, Saturn, Mars and other planets slowly nudges them, over millions of years, into orbits that cross Earth's path.
"We can almost map a network of superhighways that are taking asteroids out of the main Asteroid Belt and delivering them into orbits where they can hit the Earth," Bottke said. "Just like on a real superhighway, there are places where asteroids, just like cars, tend to congregate."
But don't ask to see the map.
"It's all a computer model and it's all in numbers," said David Brand, a spokesman for Cornell in Ithaca, New York. "I asked them [the researchers] to draw something up. But they said it's impossible -- too complicated."
The study dealt with kilometer-wide and larger asteroids because they can kick up enough debris to chill Earth's climate -- triggering crop failure and starvation -- and set off huge sea waves that could inundate low-lying coastal areas.
Yeomans said it would take about a 6-mile-wide (10-kilometer-wide) asteroid to cause mass extinction like the impact believed to have wiped out the dinosaurs and most other creatures 65 million years ago.
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