[Robert A Cook PE]

YO THE PHYSICS GROUP!

Assume you have a 100 Meg ton yield H-bomb (They're not common, since most nuclear bombs are much smaller, but a few were made by us and the Russians) ....

What size crater is created from a ground blast of that weapon?

It would appear that (at a minimum) a "totally blown up" asteroid/comet of 2x the crater size would be blown into so many small fragments that the remains would get heated and destroyed. (Granted, the residue is still present, but amll pieces are not a threat - the atmosphere slows them. As far as city-destroying solid blocks at least)

Radiation? Fallout?

Sure. Some. Spread out and dispersed across milions of square miles is a publicity, not practical, problem.

And trivial compared to the impact of a solid block 300-600 yards across on a country. City. Area.

[cogitator]

It would appear that (at a minimum) a "totally blown up" asteroid/comet of 2x the crater size would be blown into so many small fragments that the remains would get heated and destroyed.

However, you have the "Armageddon" movie problem; you have to insure that the object is totally blown up. If you just make a crater in the object; well, then you have an object of similar size with a large hole in it.