A paper just posted on arXiv, “Painting Asteroids for Planetary Defense” calculates that, given sufficient warning of an impending Earth impact, simply coating the surface of an asteroid with a thin coat of reflective alkali metal deposited by an orbiting spacecraft would increase its albedo sufficiently for solar radiation pressure to divert it from hitting the Earth. Here is the abstract.
Asteroidal impact threats to the Earth will be predicted a century or more in advance. Changing an asteroid’s albedo changes the force of Solar radiation on it, and hence its orbit. Albedo may be changed by applying a thin (∼0.1μ) reflective coat of alkali metal, dispensed as vapor by an orbiting spacecraft. A complete coat reduces the effective Solar gravity, changing the orbital period. A Tunguska-class (50 m diameter) asteroid in a nominal orbit with perihelion 1 AU and aphelion 3 AU (a=2AU, e=0.5) may be displaced along its path by ∼1000km in 100 years, sufficient to avoid impact in a populated area, by application of one kg of lithium or sodium metal over its entire surface. Alternatively, coating one hemisphere of an asteroid in an elliptical orbit may produce a Solar radiation torque, analogous to but distinct from the Yarkovsky effect, displacing it by an Earth radius in ∼200 years. The time required scales as the square root of the asteroid’s diameter (the 1/6 power of its mass) because the displacement increases quadratically with time, making it possible to prevent the catastrophic impact of a km-sized asteroid with a minimal mass.