asteroid_risk_revised_011108 Scientists said Wednesday that the risk of Earth being hit by a civilization-destroying asteroid had gone down, but other researchers said the claims are flawed and the risk has not changed at all.
The claims involved a new Princeton University study, which used data from the Sloan Digital Sky Survey to show that the main asteroid belt, between Mars and Jupiter, contains about 700,000 space rocks larger than 1 kilometer (0.62 miles). Previous estimates had put the count at around two million.
The researchers involved in the study, led by Zeljko Ivezic of Princeton University, said their work reduced the odds that an asteroid larger than 1 kilometer would hit Earth sometime in the next 100 years. They say the odds had been reduced from 1-in-1,500 over the next century to 1-in-5,000.
Asteroids that large are widely considered capable of global destruction.
"It is clear that we should feel somewhat safer than we did before we had the Sloan survey data," Ivezic said in a press statement.
But Donald Yeomans, an asteroid expert at NASA's Jet Propulsion Laboratory, said the 1-in-5,000 figure has been accepted for years and is based on a population of Near Earth Asteroids (NEAs) -- those that are thought to be near enough to our planet that gravity could lure them in sometime soon.
The Sloan survey did not study Near Earth Asteroids, but instead looked at objects primarily in the main asteroid belt. Asteroids that far away represent no threat to Earth over the next century and well beyond, Yeomans said. They are orbiting the Sun in relatively comfortable fashion.
Another respected asteroid researcher called the Princeton claims embarrassing.
Scientists at NASA and
elsewhere work together to find and track NEAs, especially those 1 kilometer and larger. Between 700 and 1,200 are thought to exist. Roughly 500 have been found. All are on courses that pose no threat to Earth.Asteroids larger than 1 kilometer are suspected of hitting Earth every 100,000 to 300,000 years, according to widely accepted estimates based partly on a handful of terrestrial craters. But Earth tends to bury or erode the evidence. So the estimate is based also on craters on the Moon, which do not erode quickly but which provide a glimpse into what likely happens on Earth.
Smaller asteroids hit Earth more frequently and can wipe out a city. Objects the size of a bus or smaller tend to burn up as they zoom through Earth's atmosphere, and therefore they pose little or no threat.
Millions and millions of smaller rocks are thought to inhabit the asteroid belt.
Researchers did not question the validity of the Princeton study's findings regarding the reduced number of asteroids in the main belt.
"The Sloan study is a major advance in our understanding of the gross asteroid belt structure," said Robert Jedicke, an asteroid expert at the University of Arizona.
The survey involved several institutions mapping a quarter of the sky, so the new estimate represents an extrapolation to the entire sky. The survey data also allowed the astronomers to better gauge the size of asteroids by studying their composition. An asteroid's size is estimated in part by the amount of light it reflects, which is tied to the surface composition.
The results were published in the November issue of the Astronomical Journal.
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