“The geology P.A.G.E” made a nice post on fighting asteroids, but theres one claim there Ive seen many times, but never believed in. Its this:
So you shatter an asteroid the size of Texas and get what? Instead of 1 asteroid the size of Texas you get millions to billions of still very large asteroids hitting the Earth at roughly the same time… This result is possibly worse then doing nothing, if not just as bad.
Many people say this when talking about fighting asteroids. They say it with great assurance. They could be right, I just dont get why though, and that makes me sceptical until I see some hard proof. Im tired of hearing people say “well this just how it is, accept it”. Because for me it defies any experience I have with how things behave. Of cource, I have no experince of falling mega-asteroids, but I can down scale it to other things I do have experince of.
But first… Lets say you have a Texas-size asteroid hitting Earth Armageddon style – that would cause such massive destruction to the biosphere, atmospehre and litosphere so that very little or none of life on Earth would survive. The impact ejecta would cover the planet in dust and debris and the atmosphere would probably more or less disapear near the impact and become unlivable in the rest. We would se massive earthquakes and winds all over the planet. Total annihilation of basically everything.
The claim is that breaking the asteroid in to several pieces is at best just as bad, but probably worse (!)
And this is what I dont get. If you throw one kilogram of gravel at a window, the window might actually hold. If you through a stone of one kilogram at the same window, it most likely will break. Same speed, same force, same standard glass window. The force is spread out on several grains wich cannot cause the same force on the target.
This simple principle tells me that the same logic should apply to an impact event on Earth. One big is much more worse than several small. Now note that I say SEVERAL small. Im not talking about two pieces instead of one, Im talking about “millions of billions” of the same mass as one. Lets spray Earth with one million car-sized asteroids or one Texas-sized.
Of course there would be massive devestation still, but worse than from one single massive impact? Most likely several billion more tons of the asteroids would burn up in the atmosphere than it would from the single mass. And the force would be spread out on a larger area and for a longer duration. Instead of one second event you would get minutes of bombardment. The window might actually hold.
Or at least thats what I imagine. I wanna se real proof of why this isnt so.
s a n d b i a n 
This is the first I’ve heard of this, and I certainly don’t have the expertise to provide the solid argument you ask for either way. (I would also be interested in hearing it.) However, from the passage you quote, I think the point is that breaking the asteroid apart is not equivalent to “one kilogram of gravel”, but a more apt analogy might be that of a ten-kg rock being broken apart into 250g pieces. These are still large enough on their own that any one of them is likely to break the window. Only instead of one huge piece of rock breaking your window, you now have lots of smaller, but still more than big enough, rocks hitting the window in many places, and quite possibly also spreading out to break the upstairs window as well.
DrM: Ok. So the argument is that the individual pieces are “big enough to break the window on their own” – that kills my window-argument of course. But my comparison had a fault from the beginning. A planet is not a window of course…
Im trying to think of another possible analogy but its hard.
Im guessing the effect on the atmosphere would be roughly the same with a spread of smaller asteroids and a large one. Possibly worse with the spread as you said.
But the effect on the litosphere should be remarkably larger with the one big solid one – creating massive shockwave effects through the planets mantle and core – creating and releasing massive +9 earthquakes all over the world also – for years to come – and increasing the risk for massive volcanic eruptions as well that could alter the climate.
If you pound the planets surface with car-sized asteroids, millions of them, the effect on the litosphere would be superficial and local for each impact (it wont ignite the atmosphere or create tzunamis or earthquakes and so on). Asteroids of that size dont even create any big craters. You need sizes above 1 km to do some real havoc (“break the glass”).
But hit with “Texas” and the effect would reach and shake the core of the planet since something like that would physically make its way down to the mantle and litteraly bend and shake it and the core beneath.
If something like that would hit the Atlantic ocean, not only would it evaporate the ocean and create tzunamis that would cross the surface of the planet, it would easily break the litosphere below and eject it as well creating havoc on the atmosphere with gases and ash. If you look at know impact events in the geological record, and see what much smaller stones can do, the “Texas-sized” “planetoid” would be something almost in comparison to when the moon was created from earth ejecta.
I think that people underestimate the effects of that one massive strike when they compare the two things. I still would prefer to take my chances with the broken up asteroid.
If you look at the largets known impact crater on earth – http://en.wikipedia.org/wiki/Vredefort_crater – it is thought to have generated massive volcanic activity. And that was from a single asteroid some 5-10 km in size.
Just imagine what something roughly 1200 km in size would do then…
*Im guessing the effect on the atmosphere would be roughly the same with a spread of smaller asteroids and a large one.*
That’s not quote true. For the overall kinetic energy it doesn’t matter if it’s one big lump or a handful of gravel. But the surface of the smaller pieces is by orders of magnitudes higher. So is friction, and hence the energy transfer into the atmosphere.
So less energy hits the lithosphere because the atmosphere gets the lion’s share. This may mean that the panet is stripped of most fluids by such a massive “buckshot”.
I generally agree with you about the one-body impact, but I doubt if, in your words, the window would hold in the other szenario.
Lars: Aha. Thanks for your input. Much appreciated.
Interesting question. I tended to agree with DrM’s point, that breaking up an asteroid would cause a large number of too big and dangerous fragment. However, I decided to have a closer look at the problem with the help of some basic physics and the Internets. The result is clearly that in most cases many asteroids are worse.
See this blogpost for a more detailed discussion. (in Swedish. Try English with Google Translate.)
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