For years it has been assumed that if we discovered an asteroid on collision course with Earth we would be able to alter its trajectory by exploding a nuclear weapon on its surface.

Such an idea has been behind many novels and two blockbuster movies this year. Deep Impact is currently on release and Armageddon will be shown later this year.

A scene from Deep Impact

But nudging an asteroid out of harm's way may not be easy, according to latest research published in the journal Nature.

Erik Asphaug, of the University of California, has used computer simulations to study the effects of powerful impacts on asteroids.

He found that the outcome of the impacts depends on the asteroid's internal structure.

Some could absorb a powerful explosion with little or no effect.

"It is a lot more difficult to nudge these asteroids around than we had thought," said Mr Asphaug.

Various other studies suggest that many of the hundreds of thousands of asteroids in our solar system are collections of debris left over from previous collisions.

They could be either a few large fragments held together by gravity or "rubble piles" consisting of many smaller pieces.

Only if the asteroid consisted of solid rock would it possible to destroy or move them with a nuclear detonation.

"Asteroids are not an imminent threat but in case we ever identify an asteroid or comet on a collision course, it would be best to know our enemy so that we can get it before it gets us," said Mr Asphaug.