http://apod.nasa.gov/apod/ap130210.html
Asteroids in the Distance
This picture depicts asteroids outside of out planet in the asteroid belt. The summary states that asteroids striking the earth are a common occurrence So common, it happens daily! The asteroids that enter the atmosphere daily however are so small, that they burn up almost instantly when they enter the atmosphere. The larger the asteroid the rarer it is for it to enter our atmosphere. Should a large asteroid about 1 km in diameter strike the earth, it wouldn't be the impact that would cause global problems, but the dust clouds that would rise and affect the climate of the entire planet.