28 July 1999
by Kate Melville
First, the good news - there are fewer potentially devastating "doomsday" asteroids than scientists previously believed we flying towards our planet. But the bad news? Sometime in the coming century, one big one could hit the Earth fair and square. There may be 500 to 1,000 big asteroids and other near-Earth objects (NEOs) that pose a threat to civilization, David Rabinowitz of the Jet Propulsion Laboratory said at a news conference at Cornell University. That is less than the old estimate of 1,000 to 2,000 potentially deadly NEOs.
But the problem is that only 15 percent to 20 percent of the potentially nastiest asteroids have been identified. At least six teams of scientists are working to find 90 percent of the biggest near-Earth objects within a decade.
A "doomsday" asteroid is defined as one with a diameter greater than .6 mile (1 km), which could cause global climatic catastrophe if it collided with Earth. Debris from such a collision would be predicted to cause worldwide clouding and cooling, with possibly disastrous effects on crops and animals.
Such asteroids come along perhaps once every 100,000 years or more, according to the newly approved Torino scale that assesses how asteroid and comet collisions would affect life on Earth.
A "doomsday" rock would rate a Torino 10, the highest rating. But the vast majority of asteroids rate a zero, "events having no likely consequences," according to the Torino scale.
Richard Binzel of the Massachusetts Institute of Technology, who devised the Torino scale, said the chance of a 10-type impact in the next century ranged from one in 1,000 to one in 10,000. There is a one-in-three chance of a Torino 8 strike, which could cause localized destruction, in the 21st century, Binzel said.
Andrea Milani, an asteroid impact expert at the University of Pisa in Italy and others who are working to predict such asteroid strikes ruled out several likely collision candidates in the last three years, but prediction can be tricky.
An asteroid that appears on a probable collision course with Earth one day can be ruled out as a threat the next because of gaps in knowledge about the paths the asteroids follow, said Paul Chodas of the Jet Propulsion Laboratory.
And once ruled out as threats, some asteroids can return to the threat lists, Chodas said in an interview. Some asteroids can be eliminated as threats for centuries, while others need to be studied year by year.
It is not enough to discover the asteroids; most must be tracked to determine which way they will go, Chodas said.
In three recent cases, astronomers estimated -- and then revised their estimates to eliminate the threat -- that asteroids might strike Earth several decades in the future. Chodas said it was entirely possible that a killer asteroid could emerge with far less advance warning.