Earth at 'lower risk' of impact

Not just science fiction

By BBC News Online science editor Dr David Whitehouse

You can sleep a little easier tonight knowing the chances that the Earth will suffer a catastrophic collision with an asteroid over the next hundred years are not as high as previously thought.

The results of a sky survey have lengthened the odds of such an apocalyptic event occurring to about one in 5,000.

The survey found that our Solar System contains far fewer asteroids big enough to destroy civilisation than previously thought.

"It is clear that we should feel somewhat safer than we did before," said Zeljko Ivezic of Princeton University, US.

Rock and toll

The new estimate draws on observations of many more asteroids, particularly small faint ones, than were available in previous impact-risk estimates.

The faint objects have been detected by the Sloan Digital Sky Survey (SDSS) that is automatically mapping one-quarter of the sky. The survey is chiefly looking for objects beyond our Solar System, but it also records closer objects such as asteroids and comets.

The survey allowed astronomers to gauge the number and size of asteroids in the main asteroidal region - between the orbits of Mars and Jupiter - with improved accuracy.

"You do not know precisely the size of an object you are looking at unless you know what type it is," Ivezic said, noting that the SDSS provided information about the colour of objects which allowed the diameters of asteroids to be calculated.

Solar System history

Based on SDSS observations of 10,000 asteroids, the researchers estimate that the asteroid belt contains about 700,000 objects larger than one kilometre (0.6 miles) - the minimum size thought to pose a catastrophic risk to our planet.

Earlier estimates put the number of dangerous asteroids up to three times higher. Previous studies could only detect asteroids larger than 5 km (3 miles).

The new data allow a more accurate determination of the numbers and size of asteroids.

"The reason for this reduced number of smaller asteroids is an open question, which, if answered, may offer important clues about the history of the Solar System and the factors that shaped the asteroid belt," said Serge Tabachnik, of Princeton University.

"The SDSS study is a major advance in our understanding of the gross asteroid belt structure," said Robert Jedicke, of the University of Arizona, US.