What would NASA do if...

the "washing machine-sized impacter" they just crashed into a comet millions of miles away from Earth, managed to change the orbit of the comet and sent it on a collision path towards us? The comet is supposed to be half the size of Manhatten Island in New York so it'll probably cause quite a bit of damage (read, total annihilation of mankind and the entire planet). Yeah, I know it's a remote chance...but what if? Would it be funny? Ironic? Moronic? You decide...