Near Earth Objects, such as astroids. A 300 metre NEO would equal a 1,000 megaton explosion. Current estimates indicate that there could be up to 100,000 NEOs larger than 100-metres. Only 500 NEOs have been discovered so far. Click on the Planetary Societies link at the left to learn more.
Lets take a break from the short sighted view.
The worlds population quadrupled in the past 100 years. We in america are worried about not having enough power to feed the demand over the summer. How about seriously considering a scientific human presence on Mars, which in turn could lead to a manufacturing presence, in turn leading to a domestic colinization. No, too hard. Lets spend billions on a missile defense system that may or may not work.