I'm not sure if anyone has told you this yet or not, but that will NOT work, because ICBM's CANNOT escape the earth's orbital boundaries-- they are not meant to do so, after all. To use one, you would need to add a"superbooster", to get to the upper atmosphere, and THEN, maybe, you could use the original stages of the missile, to get out of the earth's orbital neighborhood.
Remember:: you must get to over 22,500 mph, to escape, the earth, and ,no matter what ,you would NOT want to nuke an asteroid inside of the earth's orbit. If you did, you would have Radioactive debris raining down , ALL OVER the planet.