Look At All These Bombs!

I would expect the engines to need quite a long burn time to de-orbit an asteroid to collide with a planet. So, if you destroy the engines while it's still de-orbiting (and not on the final collision path) it would simply enter a slightly lower orbit but not destroy you.

Of course, based upon Neutrino's earlier comments here, you could put an asteroid in your own planet's orbit and when an enemy asteroid is on a collision course you smack your own into it and let the asteroid take the blow.

actually I just understood it. Nukes CAN be interplanetary, they can achieve escape velocity no problem as long as they're on low-orbit bodies. this makes a nuke launcher built on a meteor or moon, tons more valuable then a nuke launcher built on the earth even thought it's the same price!