Study: Less Chance of Catastrophic Asteroid Strike

Nov. 8, 2001 -- Scientists say there's at least one thing people can worry less about these days: a giant asteroid obliterating Earth.

According to data gleaned from the Sloan Digital Sky Survey, the chance of a catastrophic collision with an asteroid over the next 100 years is about one in 5,000. That's a much sunnier prospect than previous estimates that concluded the chances were a third greater, or about one in 1,500.

"It is clear that we should feel somewhat safer," said Zeljko Ivezic,an astrophysicist at Princeton University and lead author of the study, which appeared in the November issue of the Astronomical Journal.

The Sloan Digital Sky Survey is a multi-institutional collaboration that maps a quarter of the sky and records images of closer objects that pass by its main telescope in New Mexico. Data from the survey indicate that about 700,000 asteroids, hefty enough to wipe out all civilization on Earth, are zooming around in dangerous proximity.

That's significantly fewer than the 2 million large asteroids that researchers previously concluded were winging around Earth's neighborhood.

More, Smaller Asteroids Seen

The estimate is based on actual observations of 10,000 asteroids, including more observations of smaller asteroids. Before, telescopes could only detect asteroids that were five kilometers (3.1 miles) or larger. Scientists used that data to approximate the number of smaller ones. The minimum asteroid size believed to pose a catastrophic threat to Earth is one-kilometer-wide (or six-tenths of a mile).

Asteroids cluster in two main bands in the solar system, one about 260 million miles from the sun and the other 300 million miles from the sun. Asteroids considered to be dangerously close to Earth have been nudged from these orbits and hover closer than 1.3 astronomical units, or about 120,800,000 miles to Earth.

By estimating the number of dangerously large asteroids in the two main belts, Ivezic and his team gauged how many similar-sized ones may be hovering near Earth.

Just One Catch

New work by a University of Arizona's Spacewatch program appears to affirm the Sloan team's findings. The Arizona Spacewatch project, led by Robert Jedicke, will soon publish similar results based on independent observations. This team based their findings on direct observations of near-Earth asteroids, rather than of asteroids in the more distant belts.

Despite this validation, there's still a catch.

The new estimates and older ones all rely on a single devastating event when it's believed a 10-kilometer (6.2-mile) asteroid crashed to Earth 65 million years ago and wiped out the dinosaurs.

Based on that event, researchers have calculated that such impacts happen about once every 100 million years. They then use this statistic to gauge the likelihood of smaller, more prevalent asteroids whamming into Earth.

As Ivezic points out, that single statistic may be a somewhat weak basis for further estimates, but it's all researchers have to go on.

"We can't know if the results are truly correct, but compared to past estimates, the figures have gone down steadily," he said. "It's still a firm assurance that our risk is very low."