Earth is a tiny speck. The odds of getting hit by space debris is infinitesimal. Yet why do we have mass extinctions on Earth from asteroids, and with such regularly, in our world's history?
Marc Davis and Piet Hut have a theory, that a red dwarf, or a brown dwarf, which is to dim to see from Earth, is in a distant orbit around our sun. Every 27 million years it approaches close enough to throw off the orbits of asteroids and comets, resulting in the Earth getting a pummeling.
A very layman's explanation of this theory is found on a website run by professor Muller.
Earth Catastrophe Cycle. Our poles have been on the move for the past 150 years or so, and are accelerating, as is the other related phenomena.