[Ichneumon]

Many people underestimate kinetic energy as an effective tool. If this comet were slammed with another comet of equal size going the opposite way at, say, 130,000 mph, there wouldn't be much left of either.

Yeah, but... If you have the technical ability to grab another comet and enough energy to redirect its trajectory to exactly where you want it to be so that it hits the first comet head-on, then you don't *need* the second comet, you can use the same tools to just redirect the first one away from an Earth-impact trajectory... ;-)

[RightWhale]

Funny thing about orbits. Unless they are parabolic or hyperbolic, which would be the case for an extra-solar comet, they go all the way around and come back again.