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 Blowing up asteroid

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tealcstelescope



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PostSubject: Blowing up asteroid    Tue Aug 23, 2011 3:13 pm

I often hear on SGU that blowing up a large asteroid that is heading towards us would not work as a means of defence.

I don't understand why.

I understand it would result in the same mass hitting us at the same speed (minus whatever bits get blown off in other directions), but surely 20 tonnes of gravel would be different than a 20 tonne rock.

So, why would it not work as a viable technique if we only had short notice of an impending impact.
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astrolover

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PostSubject: Re: Blowing up asteroid    Tue Aug 23, 2011 3:14 pm

tealcstelescope wrote:
I often hear on SGU that blowing up a large asteroid that is heading towards us would not work as a means of defence.

I don't understand why.

I understand it would result in the same mass hitting us at the same speed (minus whatever bits get blown off in other directions), but surely 20 tonnes of gravel would be different than a 20 tonne rock.

So, why would it not work as a viable technique if we only had short notice of an impending impact.

There are at least a couple of major issues with a large asteroid:

(1) It might not all be gravel. Some of the pieces could be quite large, so instead of one large asteroid you might now have half a dozen somewhat smaller, but still large chunks.

(2) Even if you do end up with gravel, you are still dealing with a lot of gravel hitting the atmosphere. A lot of gravel hitting the atmosphere in a short time is going to produce a lot of heat, and could flash burn anything exposed. Then there's the nitric oxide that could be produced. A lot of that would be bad.

With both of those, breaking up a large asteroid could make things worse.
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TomK

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PostSubject: Re: Blowing up asteroid    Tue Aug 23, 2011 3:14 pm

There is also the simple calculation to be done - better to lose one large city or a hemisphere's worth? Above a certain size the actual size of the rock kind of stops mattering to the people underneath it. The destruction is total. So you either get that on one site or several.
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scopeman

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PostSubject: Re: Blowing up asteroid    Tue Aug 23, 2011 3:15 pm

My understanding is that a large part of the effects of an asteroid striking the Earth is the heat transfered into the atmosphere. As a first order approximation, that is only dependent upon the mass of the material and the speed it hits. Breaking up the asteroid into smaller pieces changes neither.

IIRC, in some ways smaller pieces are worse; for one, if they disintergrate in the atmosphere, all their energy gets put into the atmosphere. Large pieces striking the ground transfer some of their energy into the ground, where it does less significant heating.
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Clive_D



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PostSubject: Re: Blowing up asteroid    Tue Aug 23, 2011 3:16 pm

Also, if the asteroid is blown up with nuclear weapons, not only do you have a few thousand high velocity impacts, but a few thousand high velocity, radioactive impacts. This may be an exaggerated danger, but it will depend very much on the details of the warhead design.

As vaguely related aside, would there be any kind of electro-magnetic pulse from a few thousand tons of gravel whacking into the atmosphere?
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Astrotones



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PostSubject: Re: Blowing up asteroid    Tue Aug 23, 2011 3:17 pm

Much depends on " WHEN " the break up takes place. If you catch it earlier, the parts of the asteroid , after explosion, separate , each going in it's own direction, and consequently most of them won't strike earth at all.
It is quite possible that some parts , instead of hitting earth straight, will glance off the atmosphere, skipping off.
Much depends on the size of the asteroid in question. WE track the larger ones, and know them well.
So far, there are no direct strike NEO in our future. Close....but no .
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astro_alan

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PostSubject: Re: Blowing up asteroid    Tue Aug 23, 2011 3:17 pm

A 20 ton asteroid likely has a large dimension of less than three meters and will typically burn up in the atmosphere. To blow up a million ton asteroid = large dimension about 400 feet requires a large charge near the mass center. Asteroids come much larger, so we are needing gigatons placed miles below the surface. Worse; best estimates may be London will be totaled, but the actual path may graze Earth's atmosphere, killing no one. Blown up a million persons could die from a few large fragments. Alan.
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scopeman

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PostSubject: Re: Blowing up asteroid    Tue Aug 23, 2011 3:18 pm

Clive_D wrote:
Also, if the asteroid is blown up with nuclear weapons, not only do you have a few thousand high velocity impacts, but a few thousand high velocity, radioactive impacts. This may be an exaggerated danger, but it will depend very much on the details of the warhead design.


It's a hugely exaggerated danger, remember more than 2000 nuclear tests were done including many hundreds of atmospheric tests, with pretty much no significant non-local effect.
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TomK

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PostSubject: Re: Blowing up asteroid    Tue Aug 23, 2011 3:19 pm

scopeman wrote:
My understanding is that a large part of the effects of an asteroid striking the Earth is the heat transfered into the atmosphere. As a first order approximation, that is only dependent upon the mass of the material and the speed it hits. Breaking up the asteroid into smaller pieces changes neither.

IIRC, in some ways smaller pieces are worse; for one, if they disintergrate in the atmosphere, all their energy gets put into the atmosphere. Large pieces striking the ground transfer some of their energy into the ground, where it does less significant heating.

Yes, if you're going to have a large impact, the best you can manage is to have it hit somewhere on land. It would be horrible, but less horrible than having it hit the ocean, and if you spread it out enough to deposit all its energy in the atmosphere, it would be like millions of nuclear air bursts, all at once.
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Clive_D



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PostSubject: Re: Blowing up asteroid    Tue Aug 23, 2011 3:20 pm

scopeman wrote:
Clive_D wrote:
Also, if the asteroid is blown up with nuclear weapons, not only do you have a few thousand high velocity impacts, but a few thousand high velocity, radioactive impacts. This may be an exaggerated danger, but it will depend very much on the details of the warhead design.


It's a hugely exaggerated danger, remember more than 2000 nuclear tests were done including many hundreds of atmospheric tests, with pretty much no significant non-local effect.

Maybe it is exaggerated, but the comparison does not hold up for me.

The tests were done in isolated areas and timing was well spread apart.

Therefore; it was boom-dissipate-boom-dissapate-boom...

The asteroid would be more like carpet bombing.
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Clive_D



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PostSubject: Re: Blowing up asteroid    Tue Aug 23, 2011 3:21 pm

astro_alan wrote:
A 20 ton asteroid likely has a large dimension of less than three meters and will typically burn up in the atmosphere. To blow up a million ton asteroid = large dimension about 400 feet requires a large charge near the mass center. Asteroids come much larger, so we are needing gigatons placed miles below the surface. Worse; best estimates may be London will be totaled, but the actual path may graze Earth's atmosphere, killing no one. Blown up a million persons could die from a few large fragments. Alan.

I'm not so sure a meteor that size would be likely to burn up.
I used a very simple calculator, which assumes among other
things that the meteoroid does not explode or break apart as
it enters.

Inputs to the program:

Initial mass: 20,000 kg
Initial speed: 40 km/s
Entry angle: 45 degrees
Entry height: 120 km
Density: 2.2 g/cm^3

The program results:

Original diameter: 2.589 m
Final diameter at impact: 1.512 m
Final mass at impact: 3,981 kg
Speed at impact:1.967 km/s
Time in atmosphere: 6.31 s
Impact energy: 7,700 MJ
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Stuper Nova

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PostSubject: Re: Blowing up asteroid    Tue Aug 23, 2011 3:21 pm

I can't think of anything except a very minor increase in the background radiation level. Nuclear bomb tests is responsible for "a small fraction" of the manmade/artificial part (20%) of the annual average dose of 3.6 mS.
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Stuper Nova

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PostSubject: Re: Blowing up asteroid    Tue Aug 23, 2011 3:22 pm

Clive_D wrote:
scopeman wrote:
Clive_D wrote:
Also, if the asteroid is blown up with nuclear weapons, not only do you have a few thousand high velocity impacts, but a few thousand high velocity, radioactive impacts. This may be an exaggerated danger, but it will depend very much on the details of the warhead design.


It's a hugely exaggerated danger, remember more than 2000 nuclear tests were done including many hundreds of atmospheric tests, with pretty much no significant non-local effect.

Maybe it is exaggerated, but the comparison does not hold up for me.

The tests were done in isolated areas and timing was well spread apart.

Therefore; it was boom-dissipate-boom-dissapate-boom...

The asteroid would be more like carpet bombing.

Much of the radiation from the blast would concevibly stay in space unless a armagedonesque drill-and-blow operation was attemted. Even if it didn't, it's still just one bomb, radiation just isn't *that* dangerous. There where 162 bomb tests in 1962 with no serious consequences.
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scopeman

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PostSubject: Re: Blowing up asteroid    Tue Aug 23, 2011 3:23 pm

Stuper Nova wrote:

Much of the radiation from the blast would concevibly stay in space unless a armagedonesque drill-and-blow operation was attemted. Even if it didn't, it's still just one bomb, radiation just isn't *that* dangerous. There where 162 bomb tests in 1962 with no serious consequences.


My point is that it's not "just one bomb". A shattered meteor with several large destructive pieces is several bombs.
And; maybe the radiation won't be an issue, but the shockwave can be.

Let's say a large shattered asteroid produces dozens (or even hundreds if it's large enough) destructive pieces. What you may get is dozens of Tunguskas over a period of minutes spread across a large area of the Earth.

Quote :
There where 162 bomb tests in 1962 with no serious consequence

I repeat... Spread over minutes instead of hours, days or months, and potentially in populated areas.
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astrolover

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PostSubject: Re: Blowing up asteroid    Tue Aug 23, 2011 3:24 pm

There is another way of explaining it. An asteroid impact is comparable to an explosion of a nuclear weapon.

A nuclear weapon is described by its yield (Y). However, a more important parameter is called effective yield, which describes surface area destroyed in a blast: Y_eff = Y^2/3. Effective yield increases with 2/3 power of device yield, because explosion energy is dissipated in THREE dimensions, while the target surface flat. Or, imagine that you explode a warhead on the ground. In the first approximation, it will excavate a spherical crater with the same dimensions as the fireball. Since fireball volume is proportional to Y, its diameter is proportional to Y^1/3. So the destroyed area is proportional to the square of its diameter, i.e. Y^2/3. You do not care how deep the crater is, only what is its horizontal extent.

So, a 10MT warhead has an effective yield of 10^(2/3)=4.6MT. But, two warheads 5MT each are equivalent to a single 2*5^(2/3)=5.8MT warhead in terms of surface damage. That's 26% more damage just by splitting the charge! Effective yield of 10 1MT warheads is 10*1^(2/3)=10MT. Now, it gets really interesting. 100 devices 0.1MT each give you an effective yield of 21.5MT, out of combined device yield of 10MT! (Coincidentally, this is why largest nuclear weapons currently in service are 1.2MT, despite designs as big as 50MT being successfully tested).

So, assume you have a 10MT equivalent asteroid. If it hits in one piece, its effective yield (in terms of surface damage) will be 4.6MT. If you split it in 100 pieces, you will increase its effective yield to 21.5MT, i.e. 4.7 times. So you have 4.7 times more surface damage! Also, that damage will be spread out. A 10MT asteroid hitting (say) London would basically obliterate the city, and rest of the UK would be undamaged. However, 100 0.1MT asteroids hitting random spots in the UK and Europe would cause widespread damage to energy and transportation networks, hampering rescue and rebuild efforts.
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TomK

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PostSubject: Re: Blowing up asteroid    Tue Aug 23, 2011 3:25 pm

The composition of the asteroid. It could be composed of compounds that will destroy the ozone layer or change the composition of the atmosphere. It does not matter whether big pieces or little pieces hit, it will still destroy the atmosphere. Wink
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astro_alan

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PostSubject: Re: Blowing up asteroid    Tue Aug 23, 2011 3:26 pm

So for a ten million ton asteroid (typicaly not very close to a sphere) several gigatons are needed near the mass center, and there may still be several pieces over 100,000 tons, each of which can destroy a large city.
One hundred each 100 megaton, evenly spaced over the surface area at a depth of 10 meters will produce lots of gravel, but may leave 50% of the interior intact to drop in the ocean instead of Siberia, or the Sierra desert. A multi gigaton blast 10 meters below the surface, will produce lots of gravel which is ejection mass, so it can change a likely hit to a likely miss, if it explodes when the asteroid is facing the optimum direction. Typical asteroids rotate about 3 times per day, but some rotate much faster. Tiny errors are likely in the optimum direction as well as the timing = changing a likely hit to a certain hit is not improbable. More optimistic is all the big pieces miss Earth and not enough gravel hits the outer atmosphere to produce third degree burns on exposed skin or start fires with rare exceptions.
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