IF WE COULD DIVERT IT TO HIT MARS, IT WOULD BE A START AT TERRAFORMING: Once in million years: Super-sized comet to pass Mars. It would donate a considerable store of volatiles, and the impact would liberate a lot of frozen atmosphere at the surface.

However, if I recall correctly — it’s been a while since I read Martyn Fogg’s magisterial treatment — it takes rather a lot of comets to create a Martian atmosphere, and if you try to shortcut by using bigger ones (like in excess of a mile in diameter for the head) the impact becomes so big that it blows a lot of the new volatiles right out into space. So, maybe not.