Subject: Minimum size of asteroid or comet needed to break up the Earth?

I know that a object of much smaller size could cause the 
destruction of most, if not all, life on Earth but what size 
object, relative to the Earth's size and mass, would cause the 
complete break up of the Earth?  How about the moon?  Could the 
break up of the moon cause the break up of earth?  I realize 
that chances are slim that this will ever happen but I was just 
wonder about it for the sake of knowledge.

Re: Minimum size of asteroid or comet needed to break up the Earth?