This configuration was accepted as a tested and available alternative to missile interceptors that were proposed during the Bush administration but which are still under development.

It takes 90 to 200 minutes (or one to two orbits) for the missile interceptor to get close enough to its target.

'Basic calculations for a missile interceptor,' she said in a matter-of-fact voice.

The missile interceptors could reach an altitude of 90 miles, the document suggests.

The adviser also said officials could more quickly expand the defense to include sea-based missile interceptors.

Critical elements of the program, such as the booster rocket for the missile interceptor, have yet to be tested.

A system of missile interceptors launched from ships would cost between $50 billion and $64 billion through 2015, the report said.

The one where he said Pentagon leaders "don't rule out" putting nuclear tips on our ballistic missile interceptors?

The plan would shift the program's focus from space-based missile interceptors to ground-based defenses.

That system called for basing the first 100 missile interceptors in Alaska, along with the radar needed to guide them to their targets.