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In certain circumstances, nothing could be done to avoid such a collision and ensuing destruction, and it would be best not to tell the public anything, said Geoffrey Sommer, of the Rand Corporation in Santa Monica, California.

"When a problem arises with high uncertainty, there is an opportunity to spin the problem to avoid global panic. If you can't do anything about a warning, then there is no point in issuing a warning at all," Dr Sommer told the association yesterday.

"If an extinction-type impact is inevitable, then ignorance for the populace is bliss. As a matter of common sense, if you can't intercept it and you can't move people out of the way in time, there's nothing you can do in terms of reducing the costs of the potential impact," he said.

"Overreaction not just by the public but by policy-makers scurrying around before the thing actually hits because we can't do anything about it anyway ... to a large extent you are better off not adding to your social costs," said Dr Sommer, who is also an adviser on terrorism.

The US National Aeronautics and Space Administration (Nasa) is conducting a 25-year survey of the sky to find asteroids wider than a kilometre which could have a devastating impact if they collided with Earth.

So far they have determined the orbits of about 60 per cent of these objects and none so far have a trajectory that threatens the world within the next couple of centuries, said David Morrison of Nasa's Ames laboratory in Moffat Field, California.

"There are, however, many things out there that we know nothing about," he said.

(independent.co.uk)