Thread by @StephenUCS

Here's the APS slide explaining how the US anti-missile system is supposed to work. This is hard work, & it is amazing the system can work even 50% of the time, which is about what it does in testing.

Now @LauraEGrego is discussing the challenges of countermeasures to the missile defense system. As she notes, this is not a new problem the APS found. It is well known. Here is the @UCSUSA video from more than two decades ago:

As @LauraEGrego notes, the estimated cost of this system, $90B, is a major investment, particularly when one recalls it is only intended to defense against a limited attack, and it doesn't work reliably.

The testing program over the last two decades has failed almost as often as it has succeeded, and those tests are not realistic. The conditions are made to facilitate success & countermeasures have been intentionally brighter - more visible - than the warheads. - @LauraEGrego

Now @LauraEGrego discusses the THAAD system - designed to intercept mid-range missiles, which may be adoptable for long-range defenses. The SMIII Block 2A _may_ be able to defend large parts of the country in theory, but still doesn't address the problem of countermeasures.

Fred Lamb now covers boost-phase defenses, intended to attack rising missiles in the first minutes of flight. In 2012, the National Academies recommended against investing in boost phase missile defenses, because the systems were not viable.

The new APS study looked at the issue again, & found slightly increased chances of success for air-based systems, like the one proposed by Dick Garwin & Ted Pistol: rlg.fas.org/refine.pdf It would launch interceptors off fighter jets.

This graphic summarizes the APS's concerns with boost-phase missile defense. In short, definitely not an easy solution: "very challenging."

A problem with boost phase is it destroys the missile, not the warhead, & it is possible the warhead could still keep flying & detonate, though it would be short of its target. Plus, for NKorea, intercepts for missiles against East Coast might have to happen over China. No bueno

Now on to space-based missile defense, which seem SO COOL but are SO CRAP! Here's the @APSphysics slide explaining the challenges:

Here is the @UCSUSA video on boost phase missile defenses, well worth watching:

Someone asked if missile defenses will work against hypersonic defenses. TL/DR answer: No.

Just to be clear, working 50% of the time in highly scripted tests designed to be "easy" for the system is an amazing technical achievement, but not one that should give anyone confidence the system in the real world, nor that the attempt won't have other adverse consequences.

In short, the US should abandon it's pursuit of long-range missile defenses, as they don't work, cost boatloads of $$, and won't make us more secure -- just the opposite in fact.