San Francisco

A new scale for assessing the risk of asteroid impacts was thrust into the limelight last week. The Palermo Technical Impact Hazard Scale is not intended for public consumption, but it sparked media interest when a two-kilometre-wide asteroid produced the scale's highest rating yet.

Since 1999, nearby asteroids have been rated on the Torino scale, in which zero represents no risk and 10 means that a globally devastating collision is almost certain. The scale was devised to communicate relative risks of asteroids to the public after media reports in 1998 that an asteroid might end civilization in 2028. Further calculations showed the asteroid to be harmless.

The Torino scale is not sensitive enough to help astronomers judge which of the dozens of nearby objects merit attention. “We wanted a way to look at a potential impact and to ask 'is it worth some big telescope time?',” says Steven Chesley, an astronomer at NASA's Jet Propulsion Laboratory in Pasadena, California, who led the development of the new scale.

For each asteroid, the Palermo scale rates the chance of its impact relative to the frequency with which objects of a similar size collide with the Earth. A rating of zero means that an impact is no more likely than a similar but as yet unidentified object striking the Earth before the asteroid being studied is scheduled to arrive. The value is expressed as a logarithm, so −2 implies that a collision is 1% as likely as the 'background' probability, and a rating of 2 means that an impact is 100 times more likely.

Initial measurements of the trajectory of the newly spotted asteroid — dubbed 2002 NT7 — indicated that it had roughly a 1-in-100,000 chance of striking the Earth in 2019. The asteroid is two kilometres across, and an object of this size is predicted to hit the Earth about every million years.

The figures gave 2002 NT7 a positive Palermo value — the first since the scale was adopted — and ensured widespread media coverage. “I was surprised to see a positive value within the first year of operation,” says Chesley.

Further measurements have now reduced 2002 NT7's Palermo value to below zero and ruled out the chances of a collision in 2019, although the possibility of it striking the Earth in 2053 remains open.

→ http://neo.jpl.nasa.gov/risk