In a meeting yesterday with government officials, scientists set out the risks to the Earth from rogue asteroids, and described possible courses of defensive action.
Though this is one aspect of strike policy on which the political manifestos are silent, there is growing consensus among astronomers that rocket-launched nuclear weapons may be the best - and perhaps only - way to avoid a global cataclysm.
There are an estimated 100,000 objects larger than a kilometre across which could come closer than Mars and so pose a threat to the Earth - though scientists admit that the figure could be much larger.
"You either want to pulverise it, so that you have much smaller objects which are burnt up in the atmosphere, or divert its path," said David Hughes, reader in astronomy at Sheffield University. "Any old explosive will do. And when you're launching rockets, you want the most efficient explosive load you can pack in."
Duncan Steel, an astronomer at the University of Adelaide, in Australia, who runs a project called Spaceguard, which logs any dangerous asteroids, said: "They are a significant hazard. But we shouldn't be building rockets yet. What we should do is plot the orbits of those which could threaten us and predict when they would strike. Then you have enough warning to do something about it."
A spokeswoman for the Department of Trade and Industry said that a nuclear strike against an asteroid "would need international co-operation before such action could be considered. It's being treated as a hypothetical."
The Earth is certainly at risk from a future asteroid impact. The planet has been hit repeatedly by celestial bodies of various sizes, sometimes with drastic results. About 65 million years ago, an 160-kilometre-wide asteroid travelling at 20,000mph struck near to Mexico's Yucatan Peninsula - causing the death of the dinosaurs.
In June 1908, an asteroid less than 100 metres wide crashed into the atmosphere above Tunguska, Siberia, with the force of a 15-megaton bomb and flattened trees across hundreds of square kilometres.
And sometime in the next million years, scientists have calculated, the 22-kilometre asteroid Eros, presently in the vicinity of Mars, will fall towards the Earth.
In June, an internal civil service report advised the Ministry of Defence to employ three people to sift through the current data on the threat of a cosmic impact.
But Mr Hughes thought the risks of a devastating collision were low.
"Life has survived. We are here. These asteroids have been hitting the Earth continually since its origin, the rate of asteroidal impact has been going down since its origin, and life has survived," he said.Reuse content