Keep in mind, I'm thinking a B83's "physics package" - we're talking eighty (80) times the power of the bomb that was used in Hiroshima.
At point-blank range on a 1000 meter-wide asteroid - part of it would be vaporized.
And it would probably affect the trajectory a bit, too.
If we were faced with a near-certain impact, and not enough lead time to implement nudging orbital corrections (but enough lead time to do something other than confirming that our salvation plan is in order), more drastic/risky/uncertain outcome plans would certainly be considered and perhaps tried.
Did you ever read "The Cold Equations" or see the fairly recent Sci-Fi channel movie? Not necessarily the greatest story (it was written in the 1950s, after all), but if an asteroid is discovered on collision course with Earth, all of us will learn about the implacability of "the cold equations" soon enough.
I hope we never have to face that situation. But I think that it's worthwhile to keep thinking about what might be done if we do.