Winner
Diverse in Unity
Ouch, 50 m accuracy isn't very much if you would be trying to hit an 100 m rock, especially if you want to achieve more than just change its rotational period.
Those billions poured into SDI reseach could come handy here, if the US would try something like that. Maybe use an upscaled version of this cute little guy here:
http://www.youtube.com/watch?v=LC97wdQOmfI&feature=related
Nice
Although we'd still probably need something quite heavy, let's say over 10 tonnes. That of course makes the whole mission harder, since no heavy-lift launch vehicles are available. It would probably require some pretty complicated orbital trajectory changes to hit it at the right time at the right point on its orbit around the Sun (and at a right angle). That means more propellant, which means more mass in LEO. On the other hand, if there is enough time, I don't see anything that could make it impossible.
If there isn't enough time, let's nuke it
I am sure the US DoD would be happy to test their bunker-buster nukes on an actual rock, albeit one that's in space BTW, there is a little segment where this asteroid is being discussed: LINK.
