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aquitaine
2012-Apr-27, 05:29 AM
Here's a thought experiment: Ok, so lets say we have a big, kind of extinction level asteroid heading our way and we don't have time for a gravity tractor or something like that to push it out of the way. Would it be possible to destroy it in the upper atmosphere with nukes?

Jens
2012-Apr-27, 05:43 AM
One, I wonder if a nuclear weapon would be sufficient to destroy an object large enough to cause an extinction level event. You might have a rock hundreds of meters in diameter.

Secondly, there's a very good chance that you would crack the rock into pieces, and the amount of falling stuff would be the same, just distributed a little more.

djellison
2012-Apr-27, 05:50 AM
I doubt it would make a blind bit of difference. I doubt it would even be possible infact.

Here's a crude calculation for a 10km rock.
http://impact.ese.ic.ac.uk/cgi-bin/crater.cgi?dist=50&diam=10000&pdens=1500&pdens_select=0&vel=17&theta=45&tdens=2500&tdens_select=0

Specifically
"Energy before atmospheric entry: 1.13 x 1023 Joules = 2.71 x 10^7 MegaTons TNT"

27,100,000 MegaTons

If you roughly add up all of the USA's nuclear arsenal ( http://www.cdi.org/nuclear/database/nukestab.html ) it comes out at something like 300-500 Megatons.

A little under 0.002% of the energy of the inbound asteroid.

For all the good all the nuclear weapons in the world would do at that point...you might as well just shout at it.

Swift
2012-Apr-27, 12:46 PM
Lastly, by the time the asteroid is in the Earth's atmosphere it is minutes to seconds from impact, and, even if you managed to break it into pieces, all the pieces would still strike the Earth and cause just about as much damage as one big piece.

mutleyeng
2012-Apr-27, 01:27 PM
best thing you could do is start burrowing

glappkaeft
2012-Apr-27, 03:16 PM
Lastly, by the time the asteroid is in the Earth's atmosphere it is minutes to seconds from impact, and, even if you managed to break it into pieces, all the pieces would still strike the Earth and cause just about as much damage as one big piece.

Well technically a "big enough" explosion could make all the pieces miss the Earth but it would be a case where the cure is worse than the disease.

aquitaine
2012-Apr-27, 06:25 PM
I doubt it would make a blind bit of difference. I doubt it would even be possible infact.

Here's a crude calculation for a 10km rock.
http://impact.ese.ic.ac.uk/cgi-bin/crater.cgi?dist=50&diam=10000&pdens=1500&pdens_select=0&vel=17&theta=45&tdens=2500&tdens_select=0

Specifically
"Energy before atmospheric entry: 1.13 x 1023 Joules = 2.71 x 10^7 MegaTons TNT"

27,100,000 MegaTons

If you roughly add up all of the USA's nuclear arsenal ( http://www.cdi.org/nuclear/database/nukestab.html ) it comes out at something like 300-500 Megatons.

A little under 0.002% of the energy of the inbound asteroid.

For all the good all the nuclear weapons in the world would do at that point...you might as well just shout at it.

Ok, so if the entire arsenal was thrown at it, the best we could hope for is to break it up?

Lastly, by the time the asteroid is in the Earth's atmosphere it is minutes to seconds from impact, and, even if you managed to break it into pieces, all the pieces would still strike the Earth and cause just about as much damage as one big piece.

But would a more distributive impact lessen the amount of material being kicked up into the atmosphere, or not have it be thrown as high? The problem is asteroid impacts isnt the impact itself, its the nuclear winter from all that material being flung high into the atmosphere.

Secondly, there's a very good chance that you would crack the rock into pieces, and the amount of falling stuff would be the same, just distributed a little more.

So why not send them up in two waves, the first being just enough to crack it apart and the second aimed at getting as many pieces as possible?

djellison
2012-Apr-27, 09:43 PM
Ok, so if the entire arsenal was thrown at it, the best we could hope for is to break it up?

I must not have made myself clear. If you throw the entire arsenal at it - nothing will happen. In the roughly 10 seconds it takes to get from the top of the atmosphere to impact, it's creating a massive fireball that would probably vaporize any inbound weaponry.

And even if it COULD survive you're talking about 4 orders of magnitude difference between the energy of the impactor, and all the nuclear weapons you could throw at it.

I very much doubt any nuclear missile could hit it anyway....something at 15-20km/sec - that's more than double orbital velocity. You could hope to get in it's way if you could get there in time, but if you did, and if you detonated at the right time....it would be liking throwing confetti at a passing truck. Nothing would happen.

BigDon
2012-Apr-28, 12:04 AM
A bunch of smaller rocks that equal one Chixulub impactor, (The dino killer. Six miles across piece of space crap moving 21 miles per second, approaching from the south, if I recall correctly) is much worse for life on Earth for this reason:

The vast majority of all that energy from the Chixulub impactor was absorbed by the Earth itself. If you were to break that rock up into gravel without slowing it down significantly the *atmosphere* will then absorb all that energy. And THAT would have heated the whole atmosphere to incandensence!

There might have been some life left in the former sea bottoms, but nothing you wouldn't spray Lysol on.

publiusr
2012-Apr-29, 08:47 PM
Nukes work best for Orion pusher plate launches in atmo--but the asteroid has too much energy to overcome. People often overestimate nuclear power, and underestimate inertia.

glappkaeft
2012-Apr-30, 12:55 AM
The vast majority of all that energy from the Chixulub impactor was absorbed by the Earth itself.
IIRC a significant amount of the "overkill" is also radiated back into space.

HenrikOlsen
2012-Apr-30, 01:53 PM
Nukes work best for Orion pusher plate launches in atmo--but the asteroid has too much energy to overcome. People often overestimate nuclear power, and underestimate inertia.
Which is why I argue for penetrator nukes for steering (well before hitting atmosphere mind you, while really tiny nudges can still make it miss).

Get the nuke below the surface and the bits above the explosion will become reaction mass. it will probably require some experimentation/calculations to find the depth of optimal reflection, but it should make it quite a bit more efficient.

JustAFriend
2012-May-02, 03:30 AM
Unless it just grazes the atmosphere, an asteroid going 25,000mph would punch through
20 miles of atmosphere so fast everything would hit anyway even if you could hit it.

aquitaine
2012-May-03, 06:09 AM
I must not have made myself clear. If you throw the entire arsenal at it - nothing will happen. In the roughly 10 seconds it takes to get from the top of the atmosphere to impact, it's creating a massive fireball that would probably vaporize any inbound weaponry.

And even if it COULD survive you're talking about 4 orders of magnitude difference between the energy of the impactor, and all the nuclear weapons you could throw at it.

I very much doubt any nuclear missile could hit it anyway....something at 15-20km/sec - that's more than double orbital velocity. You could hope to get in it's way if you could get there in time, but if you did, and if you detonated at the right time....it would be liking throwing confetti at a passing truck. Nothing would happen.

You're talking about a large, unaerodynamic object already under great structural pressure from atmospheric entry. Wouldn't nuclear detonations in its path, just before it is reached, cause more pressure, which in addition to the hot temperatures destablize the object even further? Even the tempurature difference between the 10 million degree blast zone and the heat from normal re-entry (rapid heating and cooling) would add yet more stress to it.

And what I was talking about nuclear detonations at staggered altitudes in rapid succession, just as the pieces get to those points to vaporize at least some of them.

EDIT: I'll also point out that asteroids are generally not very stable to begin with. The shock from the blast can do a lot of structural damage to it.

A bunch of smaller rocks that equal one Chixulub impactor, (The dino killer. Six miles across piece of space crap moving 21 miles per second, approaching from the south, if I recall correctly) is much worse for life on Earth for this reason:

The vast majority of all that energy from the Chixulub impactor was absorbed by the Earth itself. If you were to break that rock up into gravel without slowing it down significantly the *atmosphere* will then absorb all that energy. And THAT would have heated the whole atmosphere to incandensence!

There might have been some life left in the former sea bottoms, but nothing you wouldn't spray Lysol on.

Yes, the vast majority of the Chixulub asteroid was absorbed by the Earth, in doing so it kicked up large quantities of dust and ash high into the atmosphere causing a nuclear winter. That is what killed the dinosaurs, not the actual impact.

To your second point, what I proposed would definately not turn the thing into gravel for all of it to burn up, although some of it probably would. But its a good bet that most of the pieces would survive entry and the lower altitude nuclear detonations, causing World War 2 like destruction over a certain area............but at least most of us would survive and could rebuild. If the whole thing slammed into the Earth, even that would be impossible.

Unless it just grazes the atmosphere, an asteroid going 25,000mph would punch through
20 miles of atmosphere so fast everything would hit anyway even if you could hit it.

While a warhead's reentry is much slower than an asteroids entry, we actually did have missiles designed to race up to the upper atmosphere and detonate a small nuke to destroy inbound Soviet nukes. And the Soviets in turn had a similair system, which is still online today. A big difference is after an ICBM launches there is a window of only ~20-30 minutes or so to determine where it's supposed to go and launch an intercept before the warheads impact. With an asteroid there would be a lot more lead in time, and that would help a lot regardless of the solution chosen.

Van Rijn
2012-May-03, 07:04 AM
You're talking about a large, unaerodynamic object already under great structural pressure from atmospheric entry.

That isn't relevant. It's far too massive and moving far too fast for that to be relevant. Mass and velocity are the only real issues here. In Lucifer's Hammer (which has some science issues and is a bit dated now, but describes the impact issue nicely) they use the example of a cubic mile of ice cream. And, a cubic mile of ice cream, hitting Earth like a comet or asteroid, would be an extinction level event or close to it. It would plow into the Earth, and it would only take it a few seconds to pass through the atmosphere first.

And what I was talking about nuclear detonations at staggered altitudes in rapid succession, just as the pieces get to those points to vaporize at least some of them.

As others have explained, there isn't nearly enough energy in the arsenal to do anything useful there (nukes MIGHT be useful if you can shift the direction of an asteroid in its orbit years before it would otherwise hit Earth - that's the scale of the mass involved). But if there was enough energy, it would essentially mean you were applying at least as much energy as the impact of the asteroid would produce, so it would mean *MORE* energy coming from this mess - it would make things worse.

glappkaeft
2012-May-03, 07:06 AM
To your second point, what I proposed would definately not turn the thing into gravel for all of it to burn up, although some of it probably would. But its a good bet that most of the pieces would survive entry and the lower altitude nuclear detonations, causing World War 2 like destruction over a certain area............but at least most of us would survive and could rebuild. If the whole thing slammed into the Earth, even that would be impossible.

Actually it's the other way around, many small spread out impacts are worse than one large. It works on most levels and is the idea behind cluster bombs and MIRV nuclear warheads. The only time when breaking up the asteroid works if it's so small that it burns up in the atmosphere AND also small/slow enough that the absorbed energy doesn't cause mayor damage. If the asteroid breaks in the atmosphere it will not spread out much (no matter the cause), the incoming velocity is just to high.

djellison
2012-May-03, 07:29 AM
YWouldn't nuclear detonations in its path, just before it is reached, cause more pressure, which in addition to the hot temperatures destablize the object even further?.

Noticeably?

No.

It's been pointed out, quantitatively and qualitatively why your idea isn't a good one.

Why are you still pushing it? Understanding your motives might help us.

aquitaine
2012-May-03, 03:20 PM
Noticeably?

No.

It's been pointed out, quantitatively and qualitatively why your idea isn't a good one.

Why are you still pushing it? Understanding your motives might help us.

Just making sure to cover all possibilities. I'm generally not in favor of simple, sweeping answers one way or the other, that's all.

Grashtel
2012-May-03, 11:37 PM
Just making sure to cover all possibilities. I'm generally not in favor of simple, sweeping answers one way or the other, that's all.
The problem is that sometimes the simple sweeping answer is the right one. For instance if an extinction level asteroid is detected late enough that its too close to Earth (and "too" close is much further away than the outer edge of the atmosphere) we're pretty much screwed with nothing that we can do other than either get to a deep long duration shelter or pick a favourite deity and start praying

JustAFriend
2012-May-04, 02:46 AM
And accept that we are not all powerful beings and our science is still pretty rudimentary...

Nukes aren't all that powerful when you realize our sun is making more energy every second than billions of our arsenals.