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.

The whole entire world currently produces 1.3 billion tons of steel annually, meaning it would take 833,315 years to just get together enough of the raw material to unleash the ultimate intergalatic fury.

And that's not even taking into account the other costs, which would include one wallet-busting superlaser and -- perhaps most importantly --  a thermal exhaust port made impervious to small craft fire.

In short, a Death Star is simply not in the cards. So if UFOs do land, our best bet is to see if we can hook the space invaders on reality TV and Taco Bell and then just sneak attack them when they pass out on the couch.