Explosions in the sky: Should we worry about asteroids?

It is often said that our atmosphere protects us from most asteroids. Very few are big enough to reach the ground without burning up. Crater-forming impacts are rare because there aren't many space rocks sufficiently massive to pass through the atmosphere. The record of impacts on Earth consists mainly of holes in the ground in various stages of erosion. But what about the ones that ldquoburn uprdquo? Do they really disappear harmlessly, without a trace-or have we been missing the geological evidence?