`Red ten' means a global catastrophe

A DANGER scale has been devised to assess the potential devastation that could result from a collision between an asteroid and Earth.

Scientists said that the 10-point gradient for assessing the risks posed by any new objects detected in space could be likened to the Richter scale for earthquake hazards.

The International Astronomical Union - the official body representing astronomers around the world - has formally adopted the scale for classifying near-Earth objects according to their potential danger.

Richard Binzel, professor of planetary sciences at the Massachusetts Institute of Technology, in the United States, devised the scale because of public concern about doomsday scenarios, depicted in films such as Deep Impact. "The purpose is to place newly discovered objects into context. If we discover an object that was a certain impactor and of large enough size we do have a responsibility for making that information known," he said.

The scale starts at nought, where the object is too small to worry about; six is a close encounter with significant threat;and ten is capable of causing global catastrophe). Each number has a designated colour, with white given for events with "no likely consequences", orange for "threatening events" and red for "certain collisions".

A "red ten" is a certain impact with an asteroid capable of causing extensive, global climatic catastrophe, similar to the collision 65 million years ago which is thought to have wiped out the dinosaurs.

Since scientists began searching for asteroids with orbits similar to that of Earth, they have discovered an ever-increasing number with the potential to collide with the planet.

Professor Binzel said nobody would lose any sleep over asteroids classified at the lower end of the scale but those given a higher designation would need to be monitored carefully. "Scientists haven't done a good job of communicating to the public the relative danger of collisions with an asteroid. [They] should have some means of clearly communicating about it so as to clearly inform but not confuse or unnecessarily alarm the public," he said.

Less than 20 per cent of the estimated population of asteroids capable of causing global destruction have been detected to date, and scientists believe it could take another 10 years to identify a further 70 per cent. It is calculated that one object of this size hits Earth every 100,000 years, with smaller asteroids - capable of substantial local destruction - colliding at a rate of up to one every 50 years.

"What I hope the scale will accomplish is to put in perspective whether an object merits concern. This is a case of a high-consequence but low- probability event. It's difficult for human nature to figure out what level of anxiety we should assign to an approaching asteroid," Professor Binzel said.