Actually i think that a nuclear attack on the USA is not impossible for us to be targetted. In fact we are not that powerful if some terrorists were to randomly just attack us. In war it is different but terrorism is something even powerful countries can't widthstand. Personally I think if they were to launch a missle from half way across the world we could intercept it, but it is quite a possibility to take us from the inside and no I don't find it impossible and I fear such an attack happening on the United States, or any other country for that matter.