What would we do if a big asteroid was coming toward earth?

The problem with your thinking on this one is... any asteroid large enough to cause that sort of widespread disaster, would be miles wide and thick. All the nukes in the world wouldn't be able to break something of that size up into just a bunch of pebbles that would disintegrate in our atmosphere, instead, you would have a trail of stadium sized chucks of rock, colliding with the Earth... and they would follow that same trajectory... we would be showered by this mive debris EVERY YEAR, just like we are when we p into comet debris in several meteor showers annually