ICBMs reach a velocity of about 5 km/sec., aren't reprogrammable in minutes to shoot at spsce rocks, and US ones at least carry weapons of less than 500 kt. This isn't going to do it. If we want to use nukes, which is questionable, the devices need to already be in high orbit to respond fast enough, have a radar guidance system, and be much more powerful. This is also a recipe to trigger WW 3.

I'm not suggesting it as a "minutes to go" safety measure. More of a "we have tons of rockets with nukes on them that can make it into space" just sitting around measure.

Making it to space isn't enough. They have to be able to escape completely and deliver a very large warhead to the asteroid.