I've been wondering about that myself. I thought they were just trying to 'change' it's current trajectory..., not blow it up? But I don't really know.
You mentioned debris from the thing possibly hitting earth.... well what if the debris hits other objects...., changing 'their' trajectory?
I understand what they are trying to learn. And I'm not usually a negative nelly when it comes to space exploration. But I've seen Armageddon half a dozen times, and something tells me that Harry Stamper is fictional, and Bruce Willis can't really save us (as he will probably be hiding under his couch).