Let's say the little green man arrive and they aren't very friendly. We could just use some of that good old human ingenuity and build a Death Star from 'Star Wars' to zap their home planet, right?
Well, it doesn't seem likely. The folks at Centives.net net did some calculations, based on how other pieces of heavy weaponry are made, and determined 1.08x1015 tons of steel are needed to assemble a Death Star. And yes, that means 1 with fifteen zeros after it.