Kinetic Energy Impactor

Objects like asteroids and comets may be deflected by hitting them with a high-speed spacecraft. Kinetic energy, or energy from motion, turns into an explosion of the object's material. As the material blows away from the object, it pushes like rocket exhaust to move it off course, preventing an impact with Earth. Kinetic energy equals the speed-of-impact-squared times mass.

Solar sails enable very high energy impacts with low mass spacecraft. A sail could spiral in towards the sun, then incline the orbit over until the spacecraft orbits the sun opposite the direction as the Earth (aka retrograde). When a retrograde sail hits an asteroid in a non-retrograde orbit, the speed of impact is about double the asteroid's speed around the sun. This dramatically increases the kinetic energy of impact: proportional to 4 times orbit-speed-squared. Dachwaldetal2006b:Dachwaldetal2007:Wie2008b