LtGen James Abrahamson, USAF (Ret) and Ambassador Henry Cooper, who were directors on President Ronald Reagan’s Strategic Defense Initiative, had some thoughts on this in their recent Wall Street Journal Letter to the Editor.

Brilliant Pebbles, the space-based interceptor we advocated, promised a high probability of kill (over 90%) of all of a “limited” strike of up to 200 attacking re-entry vehicles—the number then controlled by a Russian submarine commander. It’s better than anything we have today. It became the SDI era’s first formally approved ballistic-missile defense system, with a validated cost estimate of $10 billion in 1988 dollars (now $20 billion) for concept definition and validation, development, deployment and 20 years operation of that constellation of 1,000 Brilliant Pebbles. This isn’t expensive….

Such a thing is particularly inexpensive compared to the cost of those attacking re-entry vehicles getting through to their military and urban targets.

The best defense is a good offense: we should be orbiting offensive weapons, also.  Keep in mind, too, that offensive weapons in orbit need not be nuclear in order to satisfactorily service targets: the energy released by impact after “falling” to earth from orbit compares very favorably with the energy released from nuclear warheads and with none of the radiation or fallout complications.