Asteroid to pass 'close by'

It's a close encounter of the most nerve-wracking kind. Tomorrow the passing of the nearest asteroid to Earth in decades will be closely watched by scientists who say the half-mile wide space rock is also one of the largest to zip by.

The Monday afternoon fly-by will be just that - astronomers have been following the path of XP14 since it was discovered in 2004 but say it will pass by 432,821km from Earth. That's about 1.1 times farther away than the moon.

Asteroids are believed to have collided with the Earth hundreds of times over centuries and may have been the cause of dinosaur extinction. More than three dozen tiny asteroids have flown near Earth in the past few years but nothing like the size of tomorrow's visitor.

Alan Gilmore of Canterbury University's Mount John Observatory said sky watchers are interested because "in terms of Earth approaching asteroids, it will be a substantial object". Don Yeomans of NASA's Near Earth Object Programme said it was unusual for something of this size to come near our planet. But he reiterated: "It's not Earth-threatening."

To see the asteroid will require a "decent size" telescope, as it will look like a streaking dot against a background of stationary stars, according to experts. The Northern Hemisphere will get the best views.

Jenny McCormik, the astronomy educator at Auckland's Stardome Observatory, said her overseas counterparts will be waiting to "ping the asteroid with a radar" in an attempt to gather data on XP14.

The Goldstone Observatory in the Mojave Desert will be headquarters for many of the asteroid watchers. Scientists will use a giant radar beacon to bounce signals off the asteroid as it flies by. The return signal may help determine the rock's shape and its future course through space.

Already, predictions are that XP14 will have 10 more close encounters with Earth this century.

Asteroids are usually found in the inner solar system among planets and usually move in elliptical orbits between Jupiter and Mars. Experts say a 2km-long asteroid would explode with more power than the world's nuclear weapons arsenal and would then darken the sun, causing mass starvation and threatening civilisation. Recent impacts have included a 10m-long asteroid which detonated in mid-air over the Mediterranean Sea.

Professor Jack Baggaley of Canterbury University said warnings are rarely issued for asteroids because of previous false alarms. "It's a bit of a political problem because eventually one will hit the Earth."