We get buzzed by asteroids every day. Most of them go completely un-noticed.
The total number of people on Earth keeping their eyes peeled for Earth orbit crossing objects could staff a couple of McDonalds restaurants.
http://en.wikipedia.org/wiki/Spaceguard
Small asteroids and comet debris enter our atmosphere all day long. Most burn up from friction before they reach the ground.
It's just a matter of time until one big enough to cause some real damage makes it to the ground.
Meteor Crater in Arizona was caused by a rock about 55 yards wide by the time it hit. When it hit 50,000 or so years ago, pretty much all planet and animal life within 200 miles of the impact was destroyed.
They estimate that one was traveling just a bit under 30,000 miles per hour (8.33 miles per second) when it hit. Our atmosphere is about 30 miles thick, so at 30,000 mph it would take 3.6 seconds to go 30 miles.
At those speeds air is dense enough it can't get out of the way and it gets compressed. When a gas is compressed it heats up. So, an area around the impact point gets cauterized by super-heated air just before something that big and that fast hits.
The dinosaurs (and about half of all other species) were wiped out 65 million years ago by a big impact. That one left a crater 120 miles across and 30 miles deep. Meteor Crater is only 3/4 of a mile across.
http://en.wikipedia.org/wiki/Chicxulub_crater
http://en.wikipedia.org/wiki/Comet_Shoemaker-Levy_9