FLINGING pebbles at an asteroid sounds like a fruitless task, but a new calculation shows that this could deflect an Earthbound rock.

It takes surprisingly little force to deflect an asteroid, provided it is done several years before the projected impact. Previous ideas have included landing an engine on the asteroid to push it away from a collision, and using mirrors or lasers to vaporise its surface and provide thrust to shift its course.

Alison Gibbings and Massimiliano Vasile, aerospace engineers at the University of Strathclyde, UK, have another solution. A 500-kilogram swarm of fingernail-sized spacecraft would, they calculate, deflect a fast-moving, 250-metre asteroid by nearly 35,000 kilometres – easily enough to avoid a collision, provided the swarm hits eight years, or about three orbits, before the expected Earth impact. A swarm could be launched from Earth in a single rocket. After release, pebbles could harness the thrust provided by reflected sunlight to steer themselves into a tight cloud directed at the asteroid.

Best of all, each pebble would be too small to crack the asteroid into still-dangerous pieces, the pair reported on 17 April at the Astrobiology Science Conference in Atlanta, Georgia.