# Blowing up an asteroid/comet really potentially worse?

Often on television shows and in articles I see it mentioned that it'd always be bad to blow up an asteroid or comet because then the energy would just be spread out and cause even more damage.

According to some estimates I've seen around 100 tons (or more) of meteoroids hit the Earth each day. If all of this were combined together into a single asteroid, it could destroy an entire, large city.

Given that alone, it makes it seem to me that it be more logical to take the chance and blow up an asteroid and thereby trim its weight down, causing a lot more to be more easily burnt up on entry, so that when it did hit, it would cause less damage.

Is this logical at all? If my science/math/physics is incorrect, I want to understand why having more burn up by spreading out is worse compared to having it more concentrated and vastly more dangerous.

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If the 100 tons were ice and formed into a cube, it would be about 46.4 m on each side. A meteor this size would only affect a small area. When you get larger meteors, you would probably need more than one missile to fragment it into small enough chunks. – LDC3 May 30 '14 at 5:36
@LDC3 but what if it were rocky and approaching at a steep angle (> 45deg)? – Supuhstar Aug 21 '14 at 2:52
100 tons of ice = 109 cubic meters = 4.77 meter cube – MadBender Sep 30 '15 at 14:32

Well there are some things to consider. Initially if you could make sure that after you blow up an asteroid you will end up with numerous but small enough pieces so that they will either: one, burn up in the atmosphere or two, be headed away from Earth (and not hitting us five years later) then we are OK, and blowing up the asteroid with a missile would be a fair solution.

The problem here lies in the fact that we know little about the internal composition of asteroids in general, and presumably even less about a particular one, so it is very hard to predict exactly where the pieces of the asteroid generated by an impact are or aren't going to end up or, be headed towards or even its size.

Another scenario could be that if you effectively smashed the asteroid into small pieces that could then burn into the atmosphere, and if those pieces were coincidentally to end up being consumed by Earth's atmosphere, it would heat up provoking presumably an unpleasant day on Earth of course depending on the mass of the object.

But there is a much better solution than that Armageddon-Hollywood inspired one. It is call gravitational tethering. There is something we know, and we know very well about asteroids, and that would be their trajectory paths or orbits. Even when a new asteroid is discovered, its orbit can be computed pretty quick and with great accuracy (because we know the solar system's gravity very well). So if an asteroid is to impact Earth, it is likely that we will know with years, probably decades in advance. And so we can just send a space vehicle (called gravity tractor), with enough mass and time in advance, and place it just beside the asteroid, hence allowing us to tilt its orbit by just a tiny amount, due to the gravitational pull between the two objects. Now when you consider the effect of that tiny amount in the long run, it effectively deflects the path of the given asteroid from that of the Earth so that it won't hit us 20 or 30 years later.

And this is something we have control over, and something we can predict with great accuracy. It is the (safe) way to go.

If you are still not happy with my answer, you can listen to Neil de Grasse Tyson himself explaining it in this 5 min video.

Also check out this talk from the American Museum of Natural History on "Defending Earth from Asteroids" LINK

Further reference here.

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"So imagine you blow up a 10 km asteroid and you end up with multiple 600 m pieces of rock rushing towards Earth, then you just made the problem worse." Doesn't that presume though that none of these would burn up in the atmosphere more so than one solid piece? You may have 600 m smaller objects, but if it's 10km wide anyway, I don't see how you could make it worse. – simontemplar May 30 '14 at 4:35
Yes, in that sentence I am assuming that those pieces won't consume in the atmosphere and will produce multiple impacts on Earth surface. In any case, that was meant to work as an example of the complexity of that solution. And whether it is 10 or 30 km or 600 or 150 m pieces is irrelevant to the point I am trying to make. I just wanted to show you how blowing up an asteroid is potentially worse, which is what you asked; and provided you with what is most widely considered the right approach. – harogaston May 30 '14 at 5:03
I strongly suggest you check out this YouTube video – harogaston May 30 '14 at 5:12
I didn't ask the proper way to avoid an asteroid impact, I specifically asked whether or not spreading it out would actually lessen the impact because it would be in smaller pieces allowing more to burn up, if the answer is "no" on the basis that one isn't accounting for the fact the Earth has an atmosphere, then it's not what I'm asking about. – simontemplar May 30 '14 at 6:38
Asteroids is a child's play soon when almost all dangerously large ones will have been identified. Comets, however, don't give us 20 years, but maybe only 20 days. So you guys hanging out here say: astronomy.stackexchange.com/questions/2544/… A nuke on a converted ICBM might be a good thing to have prepared for that eventuallity. At least some of the mass would evaporate and some of the fragments miss Earth. And a second one could correct an unexpected outcome of the first. – LocalFluff May 30 '14 at 9:26

The asteroid that destroyed the dinosaurs was equivalent to about a billion hiroshima bombs or a million "Tsar Bombs" (biggest atom bombs). The counter force of energy involved to defuse that kinetic energy is a complex dynamic, scientifically designed and emulated involving: deflection, disintegration and reducing speed of impact.

The most dangerous thing for the earth is a concentrated short blast on the earth's crust which causes nuclear winters and runaway volcanic activity and sending mineral clouds into the sky as opposed to the ice that arrived through the atmosphere, and it's difficult to say beyond which point it's better to scorch the earth surface than to shock it.

It's a very technical study. The energy is either released as heat or a shock wave or both, so if you wanted to reduce damage to the earth you would have to measure weather it would withstand a massive shock wave better than massive heat.

Ultimately the current SENTRY asteroid monitoring system and future asteroid safeguards will give enough time so that it's more a choice of deflecting the asteroid entirely than breaking it in such a way that it would all be still on course with the planet.

A grain of dust is enough to deflect a billiard ball by a large span, given enough distance, so the most efficient use of energy is one of deflection.

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This is from Phil Plait's "Bad Astronomy" page in which he dissects some of the scenes from the movie "Deep Impact." In the interest of science and as long as I give him proper credit, I'm sure he doesn't mind my quoting him. Here's the link if you want to read the whole thing yourself: http://www.badastronomy.com/bad/movies/di2.html

Bad: Minutes before final impact, the astronauts blow up the second comet, and we are treated to a spectacular light show. Good: Aaaaarrgg! This was the Biggest Baddest Astronomy in the movie. Blowing up a comet does no good at all, and might even make matters worse. Just because the pieces are smaller doesn't mean you have changed anything. If every piece still impacts the Earth (by that I mean actually is stopped by the Earth or its atmosphere) you are still dumping all the kinetic energy of The Comet into the Earth's atmosphere! That's a HUGE amount of energy, dumped in practically all at once. It would still create a massive explosion, dwarfing all of our nuclear bombs combined. Even if you could somehow soften the blow, all that heat would wreak havoc with our weather. Some people actually think it might be better to simply let a big one hit rather than blow it up, because the Earth itself can absorb the energy of impact better than the atmosphere can. This is still argued, though. I'd prefer not to try any experiments!

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I suggest that you edit in a link to badastronomy.com/bad/movies/di2.html, and use the quote feature > to show that this is a direct quote. – andy256 May 13 '15 at 23:56
But wouldn't dissipating the total combined kinetic energy over a larger area make it less catastrophic? – Scottie May 14 '15 at 16:05
No. You're looking at huge amounts of kinetic energy. A stony asteroid about the size of the dinosaur killer would have about 1.1^23 Joules of kinetic energy. That works out to about 21,638 Joules per Kg of atmosphere, and that's assuming an even distribution of kinetic energy. Down in the troposphere, where most of the weather occurs and we live, I'd think the energy density would be even higher. It's really better to let the asteroid expend most of its energy locally instead of globally. – BillOer May 14 '15 at 19:46
Surface to volume ratio: en.wikipedia.org/wiki/Surface-area-to-volume_ratio A single large piece won't have time to couple effectively with the atmosphere so as to convert its kinetic energy into heat. That transfer will happen mostly at the point of impact. A gazillion small particles will vaporize in the atmosphere, resulting in an enormous heat blast. It's telling that the military prefers nuclear airbursts rather than ground detonation for non-hardened targets. You get a wider radius of destruction that way. – Wayfaring Stranger Oct 1 '15 at 13:57

The OP is obviously correct. If you knew an asteroid was headed for Earth and had the choice of either doing nothing (and letting it strike) or cutting it in half and letting both halves strike, you would split it. Nuking it into a million pieces would spread out the impact, which would make the collision less dangerous. Would you rather get shot in the chest with a bullet or 100 BBs?

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It really depends where it hits. A large asteroid or comet hitting in the middle of the ocean would create large waves, but probobly not enormous. If it hits Antarctica for example, it makes a hole in the ice, again, not a huge deal. Now if you split it and half of the object that was going to hit Antarctica now half of it hits Australia and the other half, south America - you haven't helped yourself very much. A fun bit of research would be to find one and point it at an enemy nation by nudging it slightly. But they don't hit the earth often enough for that to be very practical. – userLTK May 13 '15 at 1:23