This essay began as a minor footnote to a culture war post about the value of nuclear weapons, but subsequently exploded in size and I thought I might as well post it on its own.
A refresher on ICBMs
As the practical realities of intercontinental ballistic missiles (ICBMs) and their counter systems are the single most important element of modern geopolitics, surpassing every other element of national relevance by an order of magnititude at least, I can only presume every member of the motte has done extensive research on the subject already. But to go over the basics for those who might've forgotten:
ICBMs are large missiles that are launched high into space using a rocket engine, and then follow a ballistic trajectory back to earth. The impossibility of intercepting these missiles created and maintains MAD doctrine, and prevents a terribly costly arms race between ICBMs and counter-ICBMs. This impossibility might strike you as strange, considering an ICBM is - by its very nature - following a predictable ballistic arc which modern computers can extrapolate almost immediately. The problems with interception vary depending on when you're trying to stop an incoming ICBM, so let us examine each of the 4 parts of an ICBM's journey to valhalha.
The four parts of an ICBM's journey
Boost phase (part 1): This is the only phase in which the missile's rocket motor is activated, which naturally makes it light up on radar screens like a million candle flashlight. At the beginning of this phase the missile is stationary in the launch device as well, which compared to later phases makes it a rather easy target. The primary difficulty with boost phase interception is time and distance - the boost phase only lasts 60 to 300 seconds, and happens entirely within enemy territory. Which is to say, thousands of miles away from your anti-missile systems. Achieving boost phase interception is simply never going to be feasible unless you are actively at war with the relevant nation, and can park anti-missile systems basically on top of his silos. At which point you may as well have just destroyed the missiles with conventional air strikes, rather than waiting for them to fire.
The Boeing YAL-1 was an experimental plane designed to eliminate missiles in the boost phase, but was scrapped in 2010 for multiple reasons. Mostly because it was stupid and ridiculous. It would need to be flying over the missile launch site to have any hope of intercepting a missile, and people who are likely to fire ballistic missiles at us are also likely to shoot down big lumbering Air Force-aligned 747s loitering around in their air space before trying to fire ballistic missiles. Even then it only had a few shots before the chemicals in its laser needed to be replaced, which could only be accomplished by the plane landing at a friendly air base.
Post-boost phase (part 2): The post-boost phase is sometimes left out of ICBM stage descriptions, as it is a very brief window where not a great deal happens. But for the sake of completeness, it is this part of the missile's journey. The main rocket body falls away, and the PBV (post boost vehicle, also called the 'missile bus') releases its cloud of chaff, decoys, and warheads directed along their relevant trajectories (collectively called the 'threat cloud'). The PBV is an extremely tempting target for ABM (anti ballistic missile) defenses, as it's more exposed than the main rocket is during the boost phase and still contains all the panoply of war within itself. A single strike to a PBV that hasn't deployed its cargo would effectively neutralize the missile. Unfortunately the window of time to strike is minuscule, and hitting a PBV after its released its contents is worthless (like closing a barn door after the cows have already left). Further the post-boost phase still happens somewhat deep within enemy territory, and so it's unlikely friendly missile kill systems could arrive in time. Further-further the PBV is much harder to find on radar, as unlike the main rocket during the boost phase it isn't spewing rocket exhaust out of its back side.
As a result no ABM system I am aware of has been been theoretically floated for attempting to kill an ICBM in the post-boost phase.
Mid-course phase (part 3): The mid-course phase is the most hopeful place to intercept an ICBM's warhead. The warheads are up in space, completely exposed to any search radars you might have. The warheads will be in this phase for some 30 minutes, so there is plenty of time to launch interceptors. And the thrusting element has fallen away (e.g. the rocket), so you are trying to hit objects that are travelling in predictable ballistic free fall.
But it's not all roses. For intercontinental missiles, the warheads are 1600 km up - which makes them quite hard to see on radar and energetically intensive to reach. More importantly, the PBV has already distributed its cargo and so what was once one target is now dozens. MIRV'd missiles each contain many individual warheads (Trident missiles contain 8-14 warheads) that can all be directed to blow up cities, and additionally contain a great quantity of decoys. The invention of this "MIRV" technology is the heart of why missile interception has never been realistic. Every enemy missile must be met by dozens of your own, and therefore the defender will go bankrupt far before the attacker if it becomes an arms race.
The US program designed to intercept missiles during this phase is called the GMD (ground-based mid-course defense), which launches GBIs (ground based interceptors) into space to shoot down identified warheads. Each GBI costs $70 million dollars. So let's do some quick math. An Ohio-class submarine contains 24 SLBMs (submarine launched ballistic missiles), and each has 8-14 warheads. Let's go best case scenario here and say 8 warheads and no decoys. GBIs have a ~50% chance of successful interception, based off testimony in Congress which said 4 missiles fired at an object would give an interception probability in the 90s. Let's say each warhead must be intercepted at a 99% probability for it to be considered 'neutralized'. Therefore neutralizing all 192 warheads would require 1,344 GBIs, at a total cost of 94 billion dollars. For comparison, the entire Ohio sub itself only cost 2 billion dollars, giving a cost disparity of ~47 to 1 between defenders and attackers.
And this is the best case scenario, in more ways than one. The GBI's success record is based on it being used against rogue states with primitive decoys and stealth systems on their warheads. A more advanced nation could do a great many things to make the defender's job infinitely harder if not outright impossible.
As an example in the Gulf War an Iraqi ballistic missile was able to completely befuddle US missile defense systems with an advanced technique called "Being shoddily constructed and falling to pieces". The cloud of radar-reflective debris almost totally shielded the warhead and prevented US search radars from getting a lock. This was dumb luck on the Iraqi's part, but a more intelligent attacker could easily exploit the exact same concept to render their warheads much trickier. Or if not this technique, many more. Hitting cold, tiny objects 1600 km out travelling at many km/s is not an easy feat at the best of times, let alone if the other person is actively trying to confound you.
Terminal Phase (part 4): This is where the rubber meets the road, so to speak. The warheads come streaking back into the atmosphere at many kilometers per second, and smash into your stuff. The great difficulty here is the sheer speed the warheads are travelling at - there is simply not enough time to spot, launch and intercept before the warheads have already struck. The US program for interception in this stage is the Terminal High Altitude Area Defense (THAAD), which is the most successful of all the US interception systems by a quite high percentage. In other words - the THAAD is a CHAD. But for ICBMs specifically it is ineffective, due to the sheer mind-bending speed they achieve on re-entry (in excess of mach 20, compared to the THAAD's interceptor speed of mach 8). Fortunately short and medium range ballistic missiles aren't going quite so fast when they re-enter the earth's atmosphere, and so the THAAD is expected to be quite successful at intercepting those kinds of missiles. Hence the system's deployment against lower tier powers like North Korea who lack the technology to build true ICBMs and so are still vulnerable to the CHAD THAAD.
The Future
First, obligatory
Second, let's first acknowledge how far we've come. A 50% success rate for GMD, and a 90%+ success rate for THAAD, are frankly remarkable considering the great number of failures that have stained ABM technology for two generations. In some sense it's similar to AI, where there was a lot of over-hyping and over-promising with almost nothing tangible to show for all the money spent for decades. But then seemingly out of the blue, the technology jumped forward and now computers dumpster humans at Go. Or in the case of ABMs, gone form utterly worthless to potentially of value against 3rd rate powers. GMD + THAAD are already likely capable of deterring so-called 'nuclear blackmail' from rogue states, who lack the technological sophistication to employ advanced anti-ABM measures and lack the finances to counter US systems through saturation (aka missile spam).
But as to the question of ICBMs, that requires stepping into the realm of sci-fi because existing systems are not capable of stopping peer-level threats. The systems I'm about to discuss are variations on mid-course interception, as that is still the most realistic interception point along an ICBM's arc. So let's get retro and talk about space (space space space SPAAAACE).
The most near-term solution to the problem of MIRV ICBM interception is the MOKV, or Multi-Object Kill Vehicle. You can see a prototype MOKV being tested here. The basic idea is that unlike conventional missile interception, that relies on the defender firing one counter missile for every incoming warhead, you instead fire only 1 or 2 defending missiles each topped with many small MOKVs. The MOKVs, as the name implies, then engage many warheads in the ICBM's threat cloud at once. Effectively the defender is MIRV-ing his ABM missiles, to counter the MIRV-ing of the attacking missile. Historically this was considered impossible, as the kinetic kill competent of defensive missile systems lacked the ability to maneuver on their own - they go where the main rocket motor directed them, along an interception course that is only going to intercept a single enemy warhead.
But the complicated rocket motors inside each MOKV are hoped to give each one enough maneuverability so that an entire threat cloud could be engaged simultaneously once the kill vehicles have been boosted up to the 1600 km altitude required for interception. I suppose one way to view it would be the carrier vehicle is a 'mother ship' and the MOKVs are its 'space fighters'.
The technological hurdles to MOKV are high, and their development program was cancelled for being technologically infeasible as late as the 2000s. But in 2017 US aerospace firms announced plans to resume development of the MOKV concept, which indicates either we've silently achieved some sort of advance in the technology or the money is finally good enough to get serious engineering interest from major corporations.
Either way it is a technology to watch, as it offers the tantalizingly realistic possibility of upending MAD - at least as far as ICBM and SLBM basd systems are concerned. Russia's ridiculous doomsday torpedo would naturally not be countered this way, and so some version of mutually assured destruction would remain in play at least between these two powers - even if MOKVs did turn out to be all that and a bag of potato chips.
A somewhat similar concept to MOKVs is Brilliant Pebbles, or kinetic kill micro-satellites. The idea was an umbrella of tens of thousands of tiny missiles launched high into orbit over many years that would destroy incoming MIRV'd warheads at a fraction of the cost of the warheads themselves if war were declared. The problem with this approach, and why it was scrapped in the '90s (massive unresolved technological difficulties not withstanding) is that if failed all three of the Nitze criteira. Which are, in order:
1) The technology actually has to work (Brillaint Pebbles didn't)
2) The technology has to be able to survive counter-attack (Brilliant Pebbles couldn't)
3) The new technology must win the cost exchange (Under realistic conditions, Brilliant Pebbles wouldn't)
The key flaw, aside from early '90s technology not being advanced enough, was that the Soviets could just screw with the pebbles themselves. Either through blinding them with ground based lasers (which take a tiny amount of energy to fry the pebble's optics), or engaging them with cheap ASAT (anti-satellite) missiles, or firing a cordon of anti-pebble defensive missiles with each ICBM launch (which would punch a hole in the 'pebble network' at a fraction of the cost of the network itself). Hence why MOKVs are each only deployed upon detection of enemy missile launch, rather than hanging out in the sky 24/7 which you'd think would make more sense.
Next let's talk about laser-shaped elephant in the room: Lasers! As Raytheon's own website points out, the technological hurdles to lasers are still massive. First, you'd need some way to generate enough electrical energy to fry an ICBM coated with lots of shielding and greebles that would all have to be burned way before the missiles / warheads could be destroyed. Next you need some way to focus the high energy laser beam into an ultra-precise pinpoint effective out to thousands of kilometers. And finally you need to miniaturize all this technology and mount it into a plane or missile, because lasers have a line-of-sight problem that limits their range to the horizon if they're mounted on the ground. Finally, as the YAL-1 demonstrates, you need the ability to fire many shots per 'reload'.
The potential upsides of lasers are enormous, enabling a single plane or missile the capability of shooting down dozens of incoming warheads for only a few dollars in electricity with interception times literally as fast as our universe allows anything to ever happen. Basically free and instant as far as the military industrial complex is concerned. But the technological hurdles are just as enormous, and will likely relegate this technology to sci-fi for most (if not all) of my life time.
But don't worry laser fans, sixth generation fighter jets might incorporate laser technology to shoot down incoming anti-air or tactical ballistic missiles in only 10 to 20 years. It's not ICBM-interception level laser technology, but you'll at least somewhat get to enjoy the high tech futuristic laser battles Reagen promised us all those years ago.
Finally railguns. As all fans of the greater good know, railguns propel a tiny mass at extreme speed - which is exactly what you'd want for a missile interception system. It's cheap, fast, and unlike lasers can follow parabolic arcs which means it can shoot over a horizon. But this technology is still in its infancy, even compared to lasers, which is why last month's Naval report to congress on its moonshot projects dedicated almost all of its time talking about solid state laser technology rather than railguns. As a result there's nothing much to really say on this front - the technology is just too primitive in 2019 to really speak authoritatively.
Conclusion
None of these technologies address the detection and tracking problems, which are still an unresolved question. Even if MOKVs work or laser-grids worked or railguns were perfected, you need to still spot and shoot down the warheads with radar and there are a great many things that could be done to confound that process. So-called penetration aids (hehe) are costly in terms of payload weight, and so are avoided when possible. But it's not hard to envision a future where ICBMs are just as unstoppable as they are now, but merely dedicate say 1/2 of the payload mass to penetration aids while employing a resultantly smaller but still highly deadly physics package. The Teller-Ulam bomb configuration is, for better or worse, very easily scaled to suit the needs of the mission.
As a result, at least for the foreseeable future, mutually assured destruction through ICBMs is here to stay. For which I find myself rather relieved. The invention of truly effective ABM systems would be profoundly destabilizing, and provoke an arms race - if not outright war. The strange thing is we are much safer living under an umbrella of nuclear death than we are without it. But that's getting into CW territory, so I'll cut myself off here.