The UN is fighting a global asteroid threat the only way the UN does best: it's drafting a pointless treaty.
More study. More talk. More committees no doubt.
Scientists will push the "gravity tractor" solution to an asteroid threat. The only problem with trying to push an asteroid off a collision path with earth instead of blowing it to smithereens is that you need to do it years in advance.
Not to mention that the spacecraft that would have to be built and launched for such a mission would require expertise not even developed yet. Seeing all this we should all hope that an asteroid isn't discovered anytime to soon on a collision path with earth.
All that we could really do is pray.
I'm convinced that the only real preemptive defense against an asteroid type threat is to be a more than one world species. Space colonization ensures that not all our marbles are in one basket. It also increases our knowledge of space exploration which could aid evacuations in a time of emergency, not to mention give us the expertise aforementioned that is crucial for the "gravity tractor" scenario.
Because really if an Apophis type asteroid were ever on a collision course with earth, the chances of us getting a spacecraft built and launched early enough to "nudge" it off a collision course with earth is very unlikely.