Some who hang out here will note, via my usual channels, I was discussing the importance of warhead interception over the past few years. Our systems are capable, but the physics involved are deceptive. A 30-45 minute flight over the poles seems like a long time until you factor the region over which interception is possible given an interceptor's kinetic performance and the speed with which firing solutions must be generated and executed.
The engineers of the old Nike Zeus program specifically rejected concepts like THAAD not just because of the technical difficulty of guiding a telephone pole into a basketball at relative closures of mach 15+, but because of the simplicity with which a MIRV defeats the measure. They created the most advanced computing systems and signal processing systems to date - founded many of the computing concepts in the process - specifically to head off decoy systems and the like.
What was Nike Zeus? You launch a nuclear missile into the incoming missile and detonate to fizzle the nuclear fuel in the threat. Do this prior to MIRV separation, or close enough to it, and you can head off problems. All of this must be processed and decided within the order of seconds and a launch made while the incoming missile is below the horizon, itself going mach Jesus and accelerating.
I suppose one can hope the old rumors were true and there is a network of particle accelerators located throughout the U.S. - which would be the source of exotic lights and performances. I forget the specific name for it, but particle beams have the unusual property of being almost completely non-interactive with matter until they drop around 98% of their energy in an extremely short space. Making them a sort of "3d laser pointer" that can blast one specific spot to hell. Obviously would be useful for mucking with incoming nuclear warheads as you're basically looking at an old television tube on steroids (dramatic oversimplification of a steering yoke on a linear heavy ion accelerator with a Wakefield acceleration stage).
There were some interesting discussions about vast underground maglev rail networks which I believe would have been the cover for such a project. Some other tidbits I got about antimatter transport via underground networks from some contractor grapevine talk. 2+2=4!^2 if you squint just right.
Regardless, the system demonstrated the other day is an example of why primary target defense is a lost cause in the case of a focused, much less full strategic, exchange. We never committed to the Zeus system and while things like THAAD, SM-3, PAC-3, etc are capable of doing "something" - the exchange ratio will go quite poorly. Best case scenario is that we expend one missile for each of those submissions. Well - absolute "best case" is SM-3 block upgrades allow for situational engagement following the boost to orbit stage and compromise the deorbiter…. Maybe surviving warheads still detonate at altitude, maybe they just slam into the ground or water, somewhere.
But realistically, THAAD is engaging each of those 36 submunitions with very costly missiles. Assuming it can handle that many targets at once and score a 100% kill ratio, that was 36 high performance missiles to counter warheads from 1 launch vehicle - 1 missile.
That's very much not good.
Granted, the Zeus would not necessarily defeat non-nuclear strikes using these ballistic missiles. It would only reliably fizzle nuclear fuel and prevent proper detonation or the realizing of design yield.
SDI is an essential component of defense for the space age. If I go crazy and decide to tow a hunk of phenol resin and a giant warhead into a re-entry course for Zeon, or something, it's kind of a unifying concern of everyone on the planet to be able to make that plan generally fail. We need to get better at anti-rock rock throwing for our own mutual benefit.