Has the world made sufficient advances in missile defense tech to deter those fancy ICBMs with multiple independent warheads? If Putin woke up tomorrow bored of the Ukraine and decided to hunt moose in Canada with that arsenal of his, could any of the reentry vehicles be intercepted or is it adios, planet?
If we were given a month’s notice including the exact time and trajectory of ONE warhead, we might be able to intercept it.
Mostly what Lumpy said. Around the time missile defense was being developed in the 1970s and 1980s, both the USA and the USSR developed “penetration aids” that they added to their high end missiles. Not only do some of the higher end missiles have as many as 10 MIRV warheads, they also have a variety of pen aids such as inflatable reflective balloons that are identical to the warhead on radar.
One nasty countermeasure I read about is that some soviet reentry vehicles were built to “fail deadly” - damage to the reentry vehicle would cause the warhead to detonate. This could apparently happen so fast that impact with an interceptor would not be finished when the warhead would be detonating. The detonation would occur at high altitude, but the flash would blind the defense system from seeing or intercepting the other reentry vehicles.
This is why you rarely hear about missile defense and why a relatively small amount of money is being spent on it. Stopping one North Korean ICBM, if they ever got one working, is at least possible, but any major power with a nuclear arsenal will be able to shoot right through a practical defense system built with existing technology.
While I saw conjecture about ways that could defeat the early “Star Wars” proposals, I never saw that anybody actually developed let alone fielded systems like you mentioned. Have a cite?
In the same article “During the late 1970s, the Soviet Union fielded a large number of heavy (defined by throw weight), increasingly accurate MIRV ICBMs like the SS-18. These missiles carried as many as 10 warheads along with up to 40 penetration aids”
That’s 50 separate targets on radar. It is true that upon reentry the pen aid balloons will be quickly blown away - but waiting for reentry is really cutting it close. You’ve only got a matter of seconds before the warhead reaches airburst height at that point.
The current missile defense systems send a kill vehicle for each target. That means against an SS-18, you would need to fire 50 interceptors at a minimum.
The entire point of a MIRV is that a single booster can carry multiple reentry vehicles which are independently directed to separate targets. If the booster or post-boost vehicle isn’t intercepted prior to deploying its payloads, it now requires interceptors for each RV, and unless there is a very high probability of intercept the defender has to dedicate multiple interceptors per RV. For instance, if you have a probability of intercept P[SUB]i=1[/SUB] = 90%, P[SUB]i=2[/SUB] = 99%, but if P[SUB]i=1[/SUB] = 80%, P[SUB]i=2[/SUB] = 96% and for P[SUB]i=1[/SUB] = 50%, P[SUB]i=2[/SUB] = 75%. Given that a single RV which successfully makes the target zone will result potentially billions of dollars of damage or deaths of hundreds of thousands of deaths (or more), doing any less just invites an opposing force to fly enough boosters to overwhelm the system.
Say what?: Total to date – $164.7B. That is more than the entire man-on-moon effort (US$109B in 2010 dollars).
Stranger
One Halloween party, when I worked in that field, a coworker came dressed in a garbage bag that was covered with condoms (and I think a few ‘toys’). When asked what he was, he said ‘I’m a pen aid.’ 
Fighting Muslim terrorists, the USA has spent several thousand billion, depending upon your exact figures. Even if the terrorists obtained a nuke, it is highly unlikely they would obtain enough weapons and successfully deploy them to destroy more than a single city or 2.
Ballistic missile defense, if working versions were practical, could save hundreds of millions of lives. If it worked, it would be more valuable than every other endeavor by the Department of Defense combined. I’d say spending $164B on it is chump change.
However, since you need to fire several interceptors per reentry vehicle or contact that cannot be distinguished from an RV, and interceptors cost somewhere around the same approximate cost as an ICBM each, it’s clear to see why this can’t work. If there’s 50 radar contacts from an SS-18, and you fire 3 interceptors per contact, that’s 150 interceptors expended. This means that the USSR/Russia needed to purchase one additional SS-18 for every 150 interceptors NATO purchased. This is why it is not a feasible technology at the present time - the adversaries can spend 1/150 the funds and be able to guarantee they get through.
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Might this nuke detonation not also affect some other MIRVs too?
Yes, yes, Islam is the religion of peace. This does not change the basic undeniable fact that the several thousand billion spent were all fighting groups of people who almost all happened to be Muslim and who commonly used religiously driven attacks such as the suicide bombing* There are terrorists of other religions out there, but these wars were not fought against them.
*A guerilla attack that happens to result in the attacker needing to self sacrifice to finish the mission is one thing. However, suicide bombers do not even attempt to escape. That’s the difference between a religiously motivated attack and one that is a conventional guerilla tactic.
Here’s a documentary on the subject: https://www.youtube.com/watch?v=we4lY-GEzMk