Wanted: Small Asteroid for Use As Slingshot to Slay a Goliath

It worked for David as he squared up to Goliath, and now scientists hope a slingshot will help save the planet. Scientists at the French space agency, CNES, have calculated how to capture an asteroid and manoeuvre it into a near-Earth orbit, from where it can be flung into the path of a larger asteroid that threatens to collide with Earth.

The plan joins a growing list of strategies put forward to protect our home planet should scientists discover an asteroid is on target to crash into us. Proposals have ranged from lobbing nuclear missiles into the asteroid's path to nudging it out of the way with sunlight and attaching rocketboosters to divert it.

Didier Massonnet and Benoit Meyssignac calculate that a 20-metre to 40-metre wide asteroid could be captured into an Earth orbit by first landing a small space module on it. The scientists believe it could be diverted by knocking material off its surface until it falls into a gentle orbit close to Earth.

Writing in the journal Acta Astronautica, the scientists show how the asteroid - termed "our David's stone" - could be kept in a holding position until it needed to be redirected to intercept a much larger "Goliath". Even if the incoming asteroid was 1,000 times heavier than the orbiting asteroid, the two would still collide at a speed of 23km a second, generating 4 million billion Joules of energy, the equivalent of 40 Hiroshima bombs.

"Such an asteroid capture would be one of the most remarkable achievements of mankind," the scientists write. "It would provide us with a David's stone against the potentially destructive giants which cruise above our heads."

Earlier this month an asteroid sped past Earth in one of the nearest misses recorded to date. Although asteroid XP14 missed by an estimated 260,000 miles - a similar distance to the moon - it is a close shave by astronomical standards. To ensure debris from an asteroid collision did not strike Earth the two asteroids would have to hit each around 500,000km away, the scientist say.

Monica Grady, a planetary scientist at the Open University, said: "It is certainly a very dynamic idea. The prospect of an asteroid impact is a very real problem, but we've got lots of satellites and so on orbiting the Earth already, and some of them are as big as a bus. Why not sacrifice one of the satellites that is monitoring the ice caps, or a GPS satellite, and fling that at an incoming asteroid?"