Earth is really such a small, small target in the vast emptiness that's the Solar system. I've heard estimates that stipulate Earth always has a few trojan asteroids visiting Earth for a year or more before the Moon throws them out; however they're too small to do any damage or even track. Example: http://neo.jpl.nasa.gov/j002e3/j002e3c.gif

Never-the-less I still believe that learning how to wrangle an asteroid in deep space should be our top priority; even more so than getting to Mars. It is on the cusp of our abilities and there is so much beneficial technology learning how to do it even ignoring the tangible benefits of mining them. Most importantly some day eventually an extinction level asteroid will be headed our way. When that time comes I want us to be in a position where companies are in a bidding war to see who gets to snag it, and not have the whole planet terrified that we might not be able to stop it.

.//edit Not to mention, anybody can learn how to blast a tin can to Duna and get it back to Kerban. But rendezvousing with an asteroid in deep space? Christ that's difficult!