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Obviously it's not going to be all that big as the intercept would occur only a few minutes before impact and the rubble is still going to hit. However, if the pieces are small enough they go boom in the upper atmosphere and the distance between the boom and people on the ground provides a lot of protection. Split it into a bunch of pieces and it's like splitting one big firecracker into a string of smaller ones--the shock waves for the most part don't add together.

This is of no benefit unless the fragments are small enough they can't get through the atmosphere, shooting at too big a rock makes things worse.

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    $\begingroup$ It's debatable whether an ICBM could hit an asteroid. Many of them are very old and simplistic (launch computers run floppy disks etc.) and they are designed to hit stationary targets from suborbital trajectories. Reprogramming an ICBM to hit a small asteroid moving at many relative km/s would be very difficult. Even if doable, I doubt any ICBMs have the fuel to launch on an Earth-escape trajectory (likely required to intercept asteroids) $\endgroup$ – Dragongeek Jan 4 at 4:24
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    $\begingroup$ @Dragongeek No ICBM could do a deep space intercept. I'm saying to launch it basically straight up into the path of the incoming rock--that gives an intercept several thousand miles out. It's close enough that Earth-based radars could guide it. $\endgroup$ – Loren Pechtel Jan 4 at 4:50
  • $\begingroup$ Several thousand miles out is relative. $\endgroup$ – Magic Octopus Urn Jan 4 at 6:01
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    $\begingroup$ +1 because the premise is completely fine (see comments 1, 2). $\endgroup$ – uhoh Jan 4 at 6:45
  • $\begingroup$ that answer was since deleted so I'll repost parts of the comments here: (see the ICBM-related comments below Is 678 km the new altitude record for a rocket shot “straight up” (vertical launch)? and answers, and also the Python and plots in, and the numbers in answer(s) to How to decide if the rocket most recently tested by North Korean is classified as long-range or medium-range launch?) $\endgroup$ – uhoh Jan 5 at 3:24
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Leaving aside the practicalities of modifying an ICBM for intercept this is tricky to estimate since the ideal situation involves reducing the object to chunks under 20 meters or so evenly, and nuclear weapons are not exactly precision sieving tools. A first order approximation would be to look at the crater sizes of various surface and subsurface nuclear tests. This suggests that breaking a sub 100 meter object into vapor and very small chunks would be possible, but going larger the details of exactly how the detonation happens starts to matter a lot, since ideally you need not only to hit but get a subsurface detonation.

Castle Bravo produced a 2km crater, suggesting that careful placement of a very high vield device could fragment a km sized object, but not probably not as a missile vs asteroid intercept in LEO, and probably at least one chunk would be larger than the 20 meter target size.

This in turn suggests that you cannot do much against extinction level objects noting that as diameter increases volume to be broken up goes up by the cube.

Note that the crater sizes for these smaller asteroids are only kms across, unfortunate for the town under it but not earth shattering. The big concern is the dust produced, and since even after a space detonation the fragments will still hit earth a LEO burst will if anything produce more dust, and also make it radioactive. Airbursting nuclear weapons is normally relatively clean but this detonation will effectively be a ground burst, where that irradiated ground is then spread across a large area for pretty much maximum level of fallout for a given bomb size.

Depending on just how the irradiation occurs it is possible the debris landing around the original target will salt the earth enough to be worse than just a cratering event.

So for a small size range up to 1-2 km across if done just right an ICBM extensively modified for mid course guidance could certainly save a town from becoming a crater or at least reduce the amount of direct damage, but it will also make things problematic everywhere else and therefore shooting becomes a political decision rather than purerly an engineering one.

This complicates a nation seriously researching or testing such a capability.

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  • $\begingroup$ Of course it will be radioactive but we've done atmospheric testing without catastrophe. I'm thinking of a city-killer rock. A bit of radioactivity falling on a city is far preferable to it being turned into a crater. I also doubt it could be underground, even a burrowing warhead couldn't survive that impact. $\endgroup$ – Loren Pechtel Jan 4 at 14:29
  • $\begingroup$ @loren Pechtel, Agree that actually doing a subsurface burst is hard, but if you cannot then the sized rock you can kill goes way down - vacuum is a rubbish medium for propagating bursts efficiently. Hazard wise note that this is not a 'cleanish' airburst, there will be a lot of fallout, will make an edit. $\endgroup$ – GremlinWranger Jan 4 at 22:37
  • $\begingroup$ I wouldn't say it's problematic everywhere else--a ground burst released nowhere near as much radioactivity as Chernobyl. A destroyed city won't release a lot of radioactivity but it's going to release a huge amount of toxins. And unless you're shooting at a lump of cobalt it shouldn't be any worse than a normal ground burst. $\endgroup$ – Loren Pechtel Jan 5 at 0:29
  • $\begingroup$ @GremlinWranger if it is a problem that vacum is a poor shock propagator, why not hit it in the upper parts of the atmospher to use air as a shock propagator. I realize that this makes the timeing more demanding. $\endgroup$ – lijat Jan 6 at 22:54
  • $\begingroup$ @lijat, the aim is to spread the mass of the asteroid over the largest possible volume of atmosphere, to maximise drag. So a sub 100km shot will be trading slightly increased blast effect for needing to add more energy to achieve the same spread. That comment was more about the problems of looking at fireball photos of surface or low altidue shots and extrapolating to space. Detonation will produce lots of high energy particles, which will get absorbed by the bulk of the object but not do much to break it up. $\endgroup$ – GremlinWranger Jan 7 at 8:15

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