Tunguska-like events once every 400 years
Satellite Study Establishes Frequency of Megaton-sized Asteroid Impacts
"In Hollywood films such as "Armageddon" and "Deep Impact" Earth is threatened by enormous asteroids. New research at The University of Western Ontario establishes a better baseline for the frequency of large impacts that may cause serious damage on the ground. Based on these new estimates the average chances the Earth will be hit by an asteroid impact capable of causing serious regional damage (roughly one megaton TNT equivalent energy) is close to once per century."
SteveS