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It is quite common to find (pseudo?) scientific statements to the effect that blowing up asteroids which are heading our way is a very bad idea, and that the movie Armageddon got this point totally wrong. I would like to explore this a little bit more. I have particularly seen statements that breaking it up makes it significantly worse.

Shouldn't breaking up the asteroid into 2 drastically increase its surface area or cross-section when its going through the atmosphere? Wouldn't this then both increase the chance of burning up in the atmosphere itself, causing an air-burst (not too sure about this one) or at the very least slow down at a slightly higher rate? Wouldn't all of these reduce the damage done?

At the extreme, if the asteroid is broken down to the size of sand or nearly so, wouldn't the damage to earth be minimal?

Additionally I would figure this has a bit more finality to it rather than diverting the asteroid by rockets or gravity. This may not matter much.

This method would be lot more effective than the diversion methods if the asteroid were discovered too late as well wouldn't it?

I am thinking more carpet bomb than a single giant bomb such as in the movie.

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    $\begingroup$ The problem is obviously that several fragments can hit Earth and multiply the devastated area. The Shoemaker-Levy comet breaking up and hitting Jupiter in 1994 was a deterring example of this. Maybe that spectacular event has caused people to overestimate this problem. Earth isn't Jupiter. $\endgroup$ – LocalFluff Oct 16 '14 at 8:55
  • $\begingroup$ @LocalFluff right but as I said, wouldn't the cumulative damage be lower and more spread around? I can imagine a 100 Tunguskas(or a 1000 or how many ever) all over the earth would be better than 1 chicxulub $\endgroup$ – Karthik T Oct 16 '14 at 10:51
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    $\begingroup$ If the same mass (at least if smaller than Chicxulub) hits the ground on Earth, it would certainly be worse if it is distributed on a handful of sites, than all in one place. I tried to enter some parameters for the near Mars comet Siding Spring if it hit Earth, and just 200 km from ground zero tinyurl.com/msseu4s it seems to be no worse than a 7.4 earthquake, about the limit where windows break. Shoemaker-Levy was a nightmare scenario, I don't know how likely that is for Earth. My impression is that it depends on specific factors and that there is no general best way to deal with it. $\endgroup$ – LocalFluff Oct 16 '14 at 11:17
  • $\begingroup$ With any explosion, the radius is proportional to cubic root of the energy, a 1MT bomb destroys area of radius 10x the 1kT bomb - or area coverable by 100 1kT bombs (sum of yields 0.1MT). That's why MIRV and cluster bombs are such fearsome weapons. And that's why the asteroid broken into 1000 pieces will damage 10x the area it would otherwise. $\endgroup$ – SF. Nov 20 '15 at 11:07
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The general idea behind this is as follows. After a certain size of impact, there isn't really much that will increase the devastation. So if you take two rocks, each half the size of the first, and toss it, you will quite likely have 2 large impacts, instead of 1 large impact.

If you manage to make all of the pieces small enough, then there would be less overall devastation than a single large impact, because small bits will burn up. If every piece was, say, smaller than 10m in diameter, no pieces would survive the Earth's atmosphere. Of course, that much dust in the atmosphere would cause a "nuclear winter", but that's another issue entirely. Likely that would happen no matter what occurred, although it might be somewhat worse for the large amounts of dust.

So the question then becomes, at what size is a chunk of rock harmless, and at what size does it not significantly matter what size it is? Using this handy calculator, I've played around with that. I'm going to assume you either have 1 200m rock, or 8 100m rocks widely spread. The 100m rocks produce an explosion of 22.7 Mtons, and a 1.2 km crater. The 200m rock provides a 182 MTon blast, with a crater of 3 km in diameter. The total energy is slightly larger for the 100m rocks, and the crater area is slightly larger as well. In addition, with 8 hits instead of just 1 you are more likely to be in a populated area. A 400m rock would produce an explosion of about 1.45 GTon, with a crater of about 5 km, which is also less damage than 8 200m rocks would cause. Overall, more damage comes from small rocks than larger rocks of the equivalent size, unless you can make the rocks small enough to not hit the Earth at all.

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The main problem is that you will have no idea what the result of your attempt will be until you try it. Is the asteroid a rubble pile that will completely blow apart, or is it a solid object that will remain largely unaffected? Or some combination of the two with big solid pieces inside surrounded by small rubble? How big will the resulting pieces be? How many and which ones will remain on an Earth impact trajectory? Will you now have many remaining objects large enough on an impact trajectory that you have to take further measures to mitigate, multiplying the effort required with less time remaining?

Is this really an experiment you want to be doing when many lives are at stake and time is of the essence?

It is better to use a gravity tug approach, or maybe spray paint one pole side of the asteroid white to use the Yarkovsky effect to deflect it as a whole without disturbing its integrity.

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The idea in the movie was to split the asteroid into two. This is actually a sound idea - an explosion (unless it's absolutely massive compared to the recipient) won't change the center of mass of the object, but if you cut it in half AND give each half enough velocity so both halves miss the planet, you're good until the next one.

Yes, there will be some debris that will still hit you. Random chance as to wether you survive but that's better than the zero chance if Texas hits you at multi-km/s.

Where the movie failed was the timing - you're not going to catch up to an inbound rock, drill into it's core, insert a multi-megaton explosive and crack the thing nicely in two if you start inside the moon's orbit. If we do notice anything that big coming at us with 18 days left, we had a good run.

If you notice the impending doom years out you don't need to go to much trouble at all - a satellite-sized booster stage will change it's orbit enough to miss the planet. 12,000 km is a really small target, doesn't take much to miss it.

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    $\begingroup$ Actually, no. If you split the rock you must still supply just as much energy pushing the halves apart as you would if you just pushed the whole rock--but you also need to provide the splitting energy. Thus splitting always costs more energy than shoving. The only reason it could possibly be of value is you get more of the energy of the bomb that way. On the flip side you have to match orbits and land to place the bomb but with shoving it it doesn't matter how fast the bomb is moving relative to the target. $\endgroup$ – Loren Pechtel Nov 20 '15 at 0:36
  • $\begingroup$ (Continued) I can't imagine the savings in bomb weight being greater than the weight of the fuel needed for the velocity match. Thus the only case where splitting could possibly make sense is if for some reason you were limited by available boom rather than your launch weight. $\endgroup$ – Loren Pechtel Nov 20 '15 at 0:38
  • $\begingroup$ @LorenPechtel: To change the course of a single-piece asteroid would require a source both energy and momentum. Splitting an asteroid in half and propelling the two pieces on divergent paths so they miss the planet would require more energy, but need not require changing the combined momentum of the system. $\endgroup$ – supercat Feb 7 '18 at 22:55
  • $\begingroup$ @supercat While it doesn't change the combined momentum the center of mass won't impact anyway so that doesn't matter. (Now, normally the center of mass is inside the object but when you are looking at the center of mass of two disjoint pieces there's nothing there. If Marvin the Martian fires his annihilation cannon at the exact point where Earth is in it's orbit it's a clean miss.) $\endgroup$ – Loren Pechtel Feb 8 '18 at 3:05
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For meteorites large enough, blowing them apart before impact might not help "spread the damage", unless some of the chunks are diverted into missing the earth completely.

When a really large meteorite (like the dinosaur killer) hits, a lot of the kinetic energy of the incoming mass is transferred to high-velocity ejecta. Some of this will escape the earth; the rest will be in various elliptical orbits that must intersect the earth at some point in the future. When they do, they will deliver their kinetic energy to the atmosphere where they hit, a random point somewhere on earth.

But the amounts of energy are so high that this random spread of part of the impact energy uniformly all over the world will still raise the surface temperature to destructive levels...

Blowing the meteor into gravel or dust might just speed the process up a little...

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There have been lots of studies into breaking up an asteroid. This one and this one suggest that detonating a nuclear bomb inside an asteroid would be effective, breaking up the asteroid in many fragments and giving them enough delta-V that (if you do it early enough) many fragments will miss Earth altogether.

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While the existing answers all address the issue I would like to add a different perspective on what's going on here.

Blasts extend in three dimensions. However, on this scale humanity exists in only two dimensions. All the energy expended up or down does basically no damage. Thus damage scales at the 2/3 root of the energy.

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Another reason you don't want to blow up an asteriod in the manner of Armageddon is radiation (assuming nuclear explosives are used to blow up the in bound asteriod). I remember that when the movie came out, one of the criticisms I heard was the bomb irradiated gigatons of material that is about to impact Earth's atmosphere. The radiation dose would likely be enough to kill or sicken everyone on the planet.

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  • $\begingroup$ @MarkAdler: Really? I haven't seen the movie (heard nothing good about + I don't like Bruce Willis' movies). How big was the bomb they used to blow up the asteroid? I read at the Bad Astronomy web site that the asteroid was Texas-sized; I'm not a physicist, but I'd think it would take a bigger nucear weopon than anything we've detonated to blow that up. How much radiation would have come through to our atmosphere (and maybe the surface, depending on particle size)? $\endgroup$ – GreenMatt Oct 24 '14 at 18:49
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    $\begingroup$ You're right. I was going with your "megatons", but it according to this, it would have taken 50 billion megatons of yield to do the job. The paper also says that no yield was quoted in the movie. Though clearly they were thinking a much, much smaller bomb in the movie. $\endgroup$ – Mark Adler Oct 24 '14 at 19:01
  • $\begingroup$ @MarkAdler: Thanks. I hadn't really thought about the amount and just used megatons from habit. I changed megatons to gigatons in the answer. $\endgroup$ – GreenMatt Oct 24 '14 at 19:11
  • $\begingroup$ The radiation from the bomb wouldn't harm a fly. When you look at the thermal energy the debris from the Armageddon rock will dump into the atmosphere there won't be any flies left to be harmed. (Using low assumptions on the energy I still get a 7-figure temperature.) Besides, the fallout would be no worse than from such a bomb on Earth--an exceedingly minor threat. $\endgroup$ – Loren Pechtel Nov 20 '15 at 0:42

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