Speaking at the American Association for the Advancement of Science's annual meeting, Lu said the most viable tactic would be to use the gravitational pull of a spacecraft to alter the asteroid's trajectory.

"Some of the simpler methods that people have talked about have turned out to be, upon closer inspection, not as simple as you might think," Lu said.

"There is a random element to them. Things like hitting them with a bomb or flying a spacecraft into them -- you just don't know what the results of that are going to be. It could make things worse."

Lu said a small "tractor spacecraft" similar in size to those used in the Apollo missions would need to be deployed to deflect rogue asteroids.

"It would be positioned hovering in front or behind, with the intention to drag the asteroid off its trajectory with gravity," Lu said. "You can move an aircraft carrier with a tiny tug if you pull long enough."

US scientists are closely monitoring the progress of Apophis, which is scheduled to pass within about 32,000 kilometers (19,000 miles) of Earth in 2029.

Experts have said previously it is possible that Apophis, which could obliterate a country the size of England if it struck, may change its orbit when it swings by Earth in 2029, putting it on a collision course with the planet when it is due to pass by again in 2036.

How the international community should react to the threat of an asteroid will be the subject of a series of four workshops beginning later this year which will aim to draft a treaty to be tabled at the United Nations in 2009.

Russell Schweickart, the chairman of the Association of Space Explorers, whose members are astronauts, said there was a need for a set of internationally recognized procedures for dealing with asteroids.

"We know how to identify an asteroid, we know how to deflect it, the question is who will take on the responsibility for dealing with it. Who are the decision makers?" Schweickart said.

The problem lies in the fact that it is not possible to pinpoint early on which part of the globe an asteroid might strike when it is first identified as threatening the planet.

"When you look at the potential of where something like Apophis is going to hit, you end up drawing a line right across the planet. Eventually that line will shrink to a point," Schweickart said.

"But you will have to have made the decision to deflect the asteroid when it is still a line. If you wait until it's a point, it's too late. Just grab your martini, and go and watch it hit."

Schweickart said a working group of pre-eminent scientists, diplomats and experts in international law would seek to draft a treaty for consideration by the UN in two years' time.

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