Swarm of Mirrors

Illustration: M Vasile et al, University of Glasgow

Focusing sunlight onto an asteroid with space-based mirrors is the best way to deflect Earth-bound space rocks, a new study finds. The mirrors beat out nuclear blasts and "gravity tractors" in the study, which compared nine different deflection methods.

The spacecraft would be launched from Earth to hover near the asteroid and concentrate sunlight onto a point on the asteroid's surface. In this way, they would heat the asteroid's surface to more than 2100° C, enough to start vaporising it. As the gases spewed from the asteroid, they would create a small thrust in the opposite direction, altering the asteroid's orbit.

The scientists found that 10 of these spacecraft, each bearing a 20-metre-wide inflatable mirror, could deflect a 150-metre asteroid in about six months. With 100 spacecraft, it would take just a few days, once the spacecraft are in position.

To deflect a 20-kilometre asteroid, about the size of the one that wiped out the dinosaurs, it would take the combined work of 5000 mirror spacecraft focusing sunlight on the asteroid for three or more years. Launching and controlling 5000 spacecraft is a daunting prospect, but launching a few dozen spacecraft to deflect a smaller asteroid is within our capabilities.

Read more Are mirrors the best way to deflect asteroids
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