The missile "defense" system is a joke. The supposed "tests" they have run have all bin rigged: the incoming missiles have had tracking devices for the interceptors to home in on.
Frontline has done a report which details all of the shenanigans the Star Wars lobby went through in order to make this pretend threat real. This is nothing other than the military-industrial-congressional complex at its finest.
You can watch the program here:
http://www.pbs.org/wgbh/pages/frontl.../missile/view/