Ask HN: What happens if a nuclear missile is air-destroyed before detonation?

Keep in mind that in order for a nuclear explosion to occur, a very precisely timed series of explosions must occur within the device to trigger fission (and then potentially fusion, depending on the type of device). If the inbound missile is kinetically damaged or destroyed before it gets the chance to trigger, then the conventional explosives will likely detonate within the missile in a way that does not reach criticality, resulting in a conventional explosion with a small amount of fallout from the nuclear core that wasn't triggered being dispersed.

https://www.youtube.com/watch?v=zVhQOhxb1Mc has a good explanation of how nuclear weapons work.

It may be possible that a missel is disabled and when it hits the ground, produces a much lower yield explosion than designed for, in particular a plutonium device, if I recall the video and extrapolate with sufficient accuracy

It is basically impossible for a plutonium bomb to generate anything resembling what you think of as a nuclear explosion unless it is deliberately triggered and experiences ideal ignition conditions.

If it were to crash into the ground, be blown up by an intercepting missile, or otherwise malfunction, you'll just get a normal-bomb-sized regular conventional explosion and a bunch of toxic and only mildly radioactive plutonium strewn all over the place.

Just adding about the so-called "dirty bombs" (https://en.wikipedia.org/wiki/Dirty_bomb). It's not dirty because it's lethal but rather because the construction of these bombs are rather crude, which allows nearly everyone with some knowledge of bomb building to construct them.

Scenario I: The too-badly-damaged-to-properly-detonate nuclear warhead is now a piece (or maybe lots of pieces) of dangerous radioactive junk. Bad. But many orders of magnitude less bad than a successful nuclear detonation would have been.

Scenario II: Your defenses knocked out the warhead's delivery system, but did not damage the actual (small, tough) nuclear warhead or its control systems badly enough to disable those. My bet is that the standard design spec. is "when fully armed and near-ish to the target(s), if a critical failure occurs in the delivery system, then detonate immediately". Hopefully your missile defenses intercepted it far, far away from anything that you care much about.

Are we talking about boost phase, midcourse or terminal phase defense?

In all of these cases, a successful intercept will happen tens if not thousands of kilometers from the target.

A boost phase interception would happen above the launch area. A midcourse intercept would essentially happen in space and a terminal intercept would happen at the edge of the atmosphere.

I'd recommend reading about Aegis, THAAD and GMD. Also see https://en.wikipedia.org/wiki/Ballistic_missile_flight_phase...

Of course there are also stealth bombers and cruise missiles. In the case of stealth bombers, you either shoot them before the bombs are dropped or you don't (Barring some extremely conveniently located point defenses, like C-RAM)

The essence of MAD is that a strike is never "the" nuclear missile. It will consist of many missiles, more warheads, and yet more decoys.

https://youtu.be/2yfXgu37iyI?t=136