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It's often said (or at least, I have read it somewhere on some occasion), that using nukes to combat a potential incoming meteoroid is not viable because it may split it into multiple incoming bogies.

Some questions though:

  1. If the nuke was detonated on one side of it, or several were detonated on one side, wouldn't that be enough to push it slightly off course and it would then sail right on by the earth. It would have to be done when it's still some distance away so that even a small nudge would have an effect.

  2. Even if it was split into a gazillion pieces - surely this would be better than one big ass piece as those gazillion pieces would just burn up in the atmosphere and some lucky gits will see the meteor shower of the century.

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    $\begingroup$ Did some railgun calcs a few years back. Turns out hitting the asteroid from the side in order to shift its trajectory is less efficient than hitting head on to slow it; momentum transfer and all. You only need to delay the thing for the time it takes Earth to move 1 diameter, about 12,756 km, along its orbit. That takes about 28 minutes. $\endgroup$ – Wayfaring Stranger Nov 19 '16 at 15:22
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    $\begingroup$ space.SE is more appropriate. 1) A nuke would not literally "push" the asteroid. It would heat one side of it and that would cause out gassing (geysers) of frozen volatiles that would propel it a bit like a temporary rocket engine. But if the asteroid rotates fast enough it would not necessarily redirect it. 2) Not if the result is several fragments each large enough to be disastrous. An asteroid can be anything from a fluffy snowball to a solid piece of iron. $\endgroup$ – LocalFluff Nov 19 '16 at 15:46
  • $\begingroup$ Just burning up in the atmosphere could easily involve enough heat production to bake large swathes of land. Direct heat xfer to the atmosphere is minimized if the thing comes in as a single piece. The heat derived from that single piece excavating a 30 km deep crater is a different matter. $\endgroup$ – Wayfaring Stranger Nov 19 '16 at 22:43
  • $\begingroup$ @WayfaringStranger I suspect you'd be hard pressed for a space rock to excavate a 30 km deep crater. Even 0.3 km deep may be pushing it, but at least it's a more plausible order of magnitude. $\endgroup$ – a CVn Nov 21 '16 at 15:16
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    $\begingroup$ Meteoroids are probably not what you're interested in. What distinguishes them from asteroids is that they are no greater than 1 meter in diameter. An object of this size would not be a large concern. You're probably interested in deflecting asteroids. $\endgroup$ – called2voyage Nov 22 '16 at 18:41
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If you have enough warning, there are more gentle and much more predictable approaches than using a nuke. If you don't have enough warning, then you're probably only going to get one shot at it, so you definitely don't want the unpredictability of a nuke imparting unknown velocities to unknown size pieces, only one of which would need to be large enough to be a significant threat to make this a risky venture. If you know that the object is not a rubble pile, e.g. a metallic asteroid, and you have very little time to get it off course, then a nuke might be a good option.

You certainly shouldn't throw nukes out of your toolbox, but their application needs to be considered very carefully. That statement probably applies to all potential uses of nukes.

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  • $\begingroup$ just noticed the nugget of wisdom at the end there. $\endgroup$ – uhoh Oct 31 '17 at 7:52
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As for the first part:

Nukes have one great advantage over any other scheme that has been proposed. Every other suggested system requires matching orbits with the target before you can attempt to move it. So long as your guidance system is good enough a nuke doesn't need this.

Your target is closing on the Earth at 10 km/sec, you have to fly out there, then cancel your outbound velocity and add 10 km/sec back towards Earth before you've done anything--and at that point even with the biggest booster your payload is awfully small. Your nuke, however, doesn't care if it's going 20 km/sec relative to the target. So long as the fuse explodes it at the right altitude it does it's job.

There's also the little detail that nukes pack an awful lot of energy per kg and the number of kg we can get to the target is limited.

As for the second part of this question:

The reason breaking it up is a very bad thing is the square-cube law. The energy expends in all three dimensions, thus it drops off at the third power of distance. We live on the surface, though, the area goes up only at the square of the distance.

Look at what happens if we split a rock into 8 equal parts. Energy = 1/8, distance = 1/2, area = 1/4. Each piece does 1/4 the damage of the whole rock but there are 8 pieces for a total of twice the damage. (More energy is expended on blowing things up, less on making a crater.)

Breaking the rock up is the worst thing you can do unless you can break it small enough the pieces expend their energy in the atmosphere.

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  • $\begingroup$ This assumes the damage of those 1/8 fragments is due to their area. This is of course not the case. If the damage done by a 10 km asteroid was limited to a 10 km area, we wouldn't care that much. The problem is that the 10 km asteroid packs a lot of energy and can devastate a far larger area. $\endgroup$ – MSalters Nov 22 '16 at 10:01
  • $\begingroup$ @MSalters I made no such assumption. Of course the rock damages an area far beyond it's own size. Note, also, that I was specifically talking about energy dropping off with distance. $\endgroup$ – Loren Pechtel Nov 22 '16 at 10:43
  • $\begingroup$ The perfect example why breaking an asteroid up is a bad idea is the MIRV missile, a dozen small nukes deal way more damage than 1 big nuke. $\endgroup$ – Blake Walsh Nov 22 '16 at 11:39
  • $\begingroup$ @BlakeWalsh: Critical difference: a MIRV is independently targeted; it can hit 12 dispersed targets . Asteroids are unguided. $\endgroup$ – MSalters Nov 22 '16 at 12:17
  • $\begingroup$ @MSalters okay a more accurate comparison would be a MRV missile. $\endgroup$ – Blake Walsh Nov 22 '16 at 12:34
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Judging by the amount of rock vaporized in underground nuke tests, and the size of an average asteroid, todays nuclear weapons are easily capable of vaporizing all but the most massive of asteroids.

The largest nuclear detonation by the US was Castle Bravo at 15Mt. This created a crater 2km in diameter and 75m deep. Thats 3.1x10^8 cubic meters. But thats a cone shape, so a lot of energy radiated in upward directions. Lets say very approximately half, giving 6x10^8 m^3 of rock vaporization and/or dispersal. That volume is equivalent to a sphere of radius 530m, assuming we send Bruce Willis to bore a hole to the center of the asteroid. According to wikipedia, there are about 31 million asteroids of less than that size, and about 1m greater. So given that the Russian Tsar Bomba was several times larger again, I think its fair to say that with the most powerful of todays nukes detonated at the core we are talking about enough energy to totally vaporize the vast majority of asteroids. Any chunks that might remain would be blown outwards at high speed by the extremes of pressure in the center of the vaporized and rapidly expanding plasma cloud, and so be thrown far enough from the original impact trajectory that they'd miss earth (assuming you hit it some weeks or months away).

i didnt manage to find much more than speculation regarding the effects of nuclear detonations in open space, beside an asteroid. Lacking any atmosphere for blast waves, the energy is released as radiation, which would likely vaporize the surface of the asteroid, propelling it sideways and off course.

I also found this, which seems interesting, although i didnt watch all of it.

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    $\begingroup$ An obvious difference would be that an underground nuclear explosion still operates within an atmosphere, whereas an asteroid-targetted nuclear weapon would be operating in a vacuum. This answer would be better if you add some relevant citations and examples to it. $\endgroup$ – a CVn Nov 22 '16 at 15:36
  • $\begingroup$ @michael kjrling yup, added $\endgroup$ – Innovine Nov 22 '16 at 18:19
  • $\begingroup$ Of course, if the nuke is detonated below the surface, there'll very quickly be an extremely high pressure atmosphere of plasma and superheated gasses from the vaporizing rock. I can't imagine a bit of vacuum around the bomb would matter all that much $\endgroup$ – Innovine Jan 14 at 20:50
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Nukes are a horrible idea to use for asteroids. First nuclear detonations do not work the same way in space as they do in atmosphere. In space a nuclear device creates a large deadly cloud of radiation but not much more. It loses all of its blast component and with no atmosphere to heat, most of its thermal component. So basically if you use a Nuke on an asteroid all that will result is you will have a very large highly irradiated asteroid on the same course. Maybe you might be able to use them if you coul drill a hole and fill it with a reactive gas like oxygen or hydrogen so you could get some of the blast and thermal component back, but this would take weeks possibly months to accomplish and the end result isn't guaranteed and would only work on asteroids that weren't very dense. Against a nickle/iron asteroid it would have very reduced to no effect at all.

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    $\begingroup$ Since the amount of radiation released at close range is intense enough to vaporize solids, I'm not sure your reasoning is accurate. Shock-subliming the outer layer off an asteroid makes a fine reaction engine, if a bit inefficient. $\endgroup$ – Nathan Tuggy Oct 31 '17 at 7:03

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