If the top kill fails, they'll attempt the junk shot/top kill, and if that fails, they'll have the top hat ready to put on the top of the BOP. There were talks of putting a second BOP on top of the first one and using that. Other than that I don't know. The ultimate solution is to drill relief wells (already underway) to pump the well full of drilling mud and seal it permanently. Of course, the drilling will take until August, so hopefully something works before that.