"The best way to divert an asteroid from a potentially catastrophic collision course with Earth is to deploy a "swarm" of mirror-bearing satellites to focus a beam of sunlight onto its surface, scientists say."

"They concluded the easiest and safest approach was "solar sublimation" – concentrating sunlight so that rock on the surface material turns to gas, creating a rocket-like thrust capable of nudging the asteroid onto a new orbit."

http://www.telegraph.co.uk/earth/main.jhtml?view=DETAILS&grid=&xml=/earth/2007/10/04/sciaster104.xml