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Comment by JohnMakin | Hacker News Reader

So what happens when you intercept and blow up an ICBM in the atmosphere? Not good things, I think.

Nothing notably bad happens. Nuclear explosions are highly sensitive and engineered events. If you destroy the ignition mechanism, then it will become a dud.

FWIW, there has been a number of nuclear explosion tests in our atmosphere: https://en.wikipedia.org/wiki/Tsar_Bomba

If it's in the boost phase, it's over the enemy's country. If it's in reentry, it's only the RV (Reentry Vehicle). So some kg of Uranium or Plutonium gets scattered about, but that's it. Much better than vaporizing half a city.

Light might in theory reach an ICBM in boost, but anything you launch is going to need to go through its own acceleration phase to possibly hit the ICBM.

Further your ignoring submarines and early proximity detonation on the nukes. The most likely result of building such a system is an early nuclear EMP clearing orbit before the nukes get through anyway.

The NAS study seems to think kinetic interceptors can reach the booster in minutes but yes it requires a kick stage for the delta V.

That seems optimistic in any sort of realistic scenario. The everything works perfectly and there’s one ICBM and nobody modifies anything as we start launching all this stuff into orbit, then maybe.

How many satellites do you need to reach that kind of coverage? And how many interceptors per satellite to hit everything from one or more nuclear subs launching from the same location near a major city. How long does it take you to react without risking false positives making the system useless etc.

The problem just gets ugly fast.

That study said 1600 satellites are needed in a Delta Walker configuration (same as Starlink, but Starlink has more satellites already).

The number of interceptors per satellite was unclear but this USAF captain said "composite rocket motor and micro-missile system capable of fielding a rocket pod of 100 interceptors at a cost of approximately $60,000 per micro-missile" https://www.airuniversity.af.edu/Wild-Blue-Yonder/Article-Di...

The plutonium, uranium, and other high explosives fall back to earth. Pretty tame compared to what happens if you fail to intercept!

The first few ICBMs are expected to be detonated in the atmosphere to cause an EMP wave to kill the communication, including nearby satellites.

https://en.wikipedia.org/wiki/Nuclear_electromagnetic_pulse

Studies I've seen are not so clear that an EMP would kill many satellites. They would raise radiation levels 10x over normal for few months, but most Starlink satellites would be fine (as seen in the last solar CME event).

The US tested a nuke in space, dubbed Starfish Prime

It took out 3 satellites initially (there were very few at the time)

> In the months that followed, these man-made radiation belts eventually caused six or more satellites to fail

https://en.wikipedia.org/wiki/Starfish_Prime#After_effects

So it killed 5% of satellites initially and 15% in 6 months?

unintentionally, with 1 nuke from the 60s

Today, with modern nukes, if one was intentional, they could disrupt or destroy the space ecosystem

Pu-239 and U-235, the most common elements in nukes, are not very radioactive. Both alpha decay, meaning they're only really dangerous if inhaled as dust (or if they fission, obviously). Pu-239 has a half-life of around 24k years iirc, while U-235 is over 700 million years.

I mean... It's better than the ICBM warhead detonating.

Really any of the nuclear material will just disperse into the atmosphere relatively harmlessly, especially if it's somewhere over an ocean. Plutonium in seawater measures in the parts per billion anyway.

Depends on where it blows up, but if you hit it at its apogee, a lot of it will burn up in the atmophere.

>> if you hit it at its apogee, a lot of it will burn up in the atmophere.

Having several pounds of plutonium burn up in the atmosphere just means it will be spread over a very wide area which is much worse than a chunk of it falling somewhere.

Pu-239 is not particularly dangerous, however. It can cause cancer if inhaled in sufficient quantities — which doesn't need much, but is unlikely if it's scattered in the atmosphere.

U-235 is basically harmless.

The adverse effects are trivial compared to the other possible effects of an ICBM launch.

If you mean the subsequent detonation of an ICBM in the atmosphere, I think we are in uncharted waters there. The expert discussions of what effect the atmosphere-EMP would be is fascinating/horrifying to read.

An intercepted warhead would highly likely not explode.

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