Paris, March 3 (AFP): A space rock capable of sub-continent scale devastation has about a one in 1,000 risk of colliding with the Earth early next century, the highest of any known asteroid, watchers said.

The rock, 2004 VD17, is about 500 metres long and has a mass of nearly a billion tonnes, which ? if it were to collide ? would deliver 10,000 megatonnes of energy, equivalent to all the world’s nuclear weapons.

Spotted on November 27, 2004, VD17 was swiftly identified as a rock that potentially crossed Earth’s orbit, with a 1 in 3,000 risk of collision on May 4, 2102.

Further observations and calculations have prompted the risk on that day to be upgraded to “a bit less than 1 in 1,000,” Nasa Near-Earth Object expert David Morrison said yesterday in an email circular.

“The risk of an impact within the next century (is) higher than that of any other known asteroid,” he said, stressing, however, that the likelihood of a hit was small.

“Fortunately, it is nearly a century before the close pass from VD 17.”

This should provide ample time to refine the orbit and, most probably, determine that the asteroid will miss the earth.” VD17, which was previously categorised as a grade green (“merits careful watching”) on the Torino scale of neo hazards, has been upgraded to grade yellow, “meriting attention”.

On the other hand, there are no near-term opportunities for additional observations, so VD17 will probably remain at Torino scale 2 for quite some time.

The Torino Scale is a method for categorising the impact hazard associated with near-Earth objects such as asteroids and comets. It is intended as a tool for astronomers and the public to assess the seriousness of collision predictions

There are two more grades beyond this, orange (“close encounter”) and red (“collision is certain”), involving objects capable of inflicting regional or global devastation.