B Destroying asteroids shown to be very difficult

jim mcnamara

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Charles El Mir et al, A new hybrid framework for simulating hypervelocity asteroid impacts and gravitational reaccumulation, Icarus (2018). DOI: 10.1016/j.icarus.2018.12.032

https://phys.org/news/2019-03-asteroids-stronger-harder-previously-thought.html

Modeling the destruction of asteroids in an effort to protect Earth from nasty collisions, has some new research. And a new model.

The takeaway is larger asteroids "reassemble themselves" from explosion fragments, under the asteroid's own gravity unless the explosion has far more energy than previous models indicated.

Assuming the model's results are meaningful -
In other words, we need a bigger bang. Which may mean that deflection of asteroids instead of breaking them into shards may be a more feasible approach. There is some discussion on this with Charles El Mir, the lead author, in the phys.org article.
 

mathman

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My impression is that deflection is the only feasible method ever being considered. Destroying a large asteroid would take an enormous amount of explosive, while a lot less energy would be needed to deflect..
 

jim mcnamara

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@mathman - your point is well taken and I think is what the phys.org article mentions as well.
 

LURCH

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I think it is safe to say that the object may indeed reassemble itself, but not entirely. A subsurface blast would eject a significant amount of material at speeds greater than escape velocity. So, unless the blast takes place at the exact center of gravity, the newly reassembled object will be on a different course. The ejected material would be on an even greater deviation from the original (collision) course.

Also discussed in this same Forum in the thread:
Ideas to protect the Earth from possible asteroid impacts

https://www.physicsforums.com/threads/ideas-to-protect-the-earth-from-possible-asteroid-impacts.959077/
 
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Same article were already presented here

Let me link the relevant wiki article
Wiki is hard to be considered a reliable source - but even there the 'blow it to pieces' approach seems to be ... missing, and the whole 'nuke it' section is actually about nuclear propelled deflection.

Can't help wondering that why scientific articles would aim against - movies? o_O
 

LURCH

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Thanks for the link, @Rive , that article makes a lot of the same points mentioned in the PF link in my Post#4. Nice to see that other people are thinking the same way.

The article makes some pretty flawed assumptions. For example;
For example, if there's an asteroid coming at earth, are we better off breaking it into small pieces, or nudging it to go a different direction?
These are presented as mutually exclusive scenarios, which they are not. To me the obvious question would be, “Why should we care if the object is shattered into a thousand pieces, so long as none of those pieces are headed for Earth?”
 
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To me the obvious question would be, “Why should we care if the object is shattered into a thousand pieces, so long as none of those pieces are headed for Earth?”
The 'none' is the problem. As last resort blowing one to pieces might be better than nothing since the atmosphere has a decent shielding effect for smaller pieces so the expected damage will be most likely reduced, but it is impossible to guarantee the all the (dangerous) major pieces will miss Earth - and there is not much intervention possibility left once it become a rubble cloud without shape and handle.
But as long as it is in one piece it can be 'nudged' as a whole.

So with the 'nuke it' option the hard part is actually to give it the biggest nudge in the most effective direction without blowing it to pieces (small debris does not count).

The model mentioned in the article might be helpful in calculating the best yield/depth and other parameters.
 

Grinkle

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So with the 'nuke it' option the hard part is actually to give it the biggest nudge in the most effective direction without blowing it to pieces (small debris does not count).
Does a nuclear explosion actually transfer very much momentum? Why would detonating a nuclear bomb adjacent to an object in space do anything except heat that object up? Why would it push the object appreciably? In an atmosphere a nuclear explosion heats up air and the resulting shock wave moves a whole lot of stuff. In a vacuum one only has the particles that are released in the nuclear explosion to do any momentum transfer - I imagine not much pushing.

In order to move the asteroid, one must use part of the asteroid mass itself for momentum transfer I think - how can it be possible to push the asteroid via explosion without ejecting part of the asteroid mass to push the rest in the other direction? Maybe you don't disagree and this is what you mean by nudge also.
 
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It would be nice to have the linked wiki article as bottom line for any discussion about 'nuke an asteroid'.
 

Grinkle

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@Rive Yes, sorry, should have read that first.
 

jbriggs444

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The model mentioned in the article might be helpful in calculating the best yield/depth and other parameters.
how can it be possible to push the asteroid via explosion without ejecting part of the asteroid mass to push the rest in the other direction? Maybe you don't disagree and this is what you mean by nudge also.
It seems clear that Rive is contemplating a sub-surface detonation which does eject part of the asteroid mass to obtain effective thrust.

I'd be thinking in terms of a well-aimed armor piercing warhead with a delay fuse timed to achieve a burst at the desired depth. The guys who design missiles are pretty good at that stuff.
 

PAllen

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Actually, it seems to me not critical that any asteroid mass be ejected. Given an isotropic explosion at some point on the asteroid surface, nearly half the momentum ejected by the explosion goes to deflecting the asteroid. The tendency of the asteroid to coalesce if initially blown apart is only to the good for the purposes of deflection. With current technology, I don’t see any better deflection approach than a properly placed nuke.
 

Tom.G

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Given an isotropic explosion at some point on the asteroid surface, nearly half the momentum ejected by the explosion goes to deflecting the asteroid.
Hmm... seems to me that the mass is in the plasma of the fireball, which would be the gross mass of the bomb and casing. Doesn't seem like that would be very effective in moving a megaton rock.

Here is a slightly informative link. https://www.labroots.com/trending/space/12897/here-s-what-d-happen-detonated-nuclear-bomb-space

Try a Google search for nuclear bomb explosion, or for nuclear bomb explosion in space
 
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it seems to me not critical that any asteroid mass be ejected.
When available energy is to be converted to impulse, it is about the classic E= 1/2mv2 equation. With digging out the impulse it is p=2E/v, which is actually inversely proportional to the speed for any given energy.
There is energy loss and many other considerations, but at the end this is the basis of the deflection.

With abundant energy and low mass, you get a flashy explosion but just low impulse at the end: to get more impulse you need low speed and just the more mass the better.
Of course the speed still should be above the escape velocity of the asteroid in question to get effect.

So the key point of the 'nuke it' approach is to determine the optimal yield and depth of detonation for the material constitution of a specific asteroid to avoid breaking it up yet providing the most mass barely escaping it: this would provide the maximal thrust. (The direction of the thrust to get maximal effect is a different matter.)

That's why I dare to say that the linked model seems to be spot on, yet at the form it was presented it is just a capital misfire.
 
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PAllen

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When available energy is to be converted to impulse, it is about the classic E= 1/2mv2 equation. With digging out the impulse it is p=2E/v, which is actually inversely proportional to the speed for any given energy.
There is energy loss and many other considerations, but at the end this is the basis of the deflection.

With abundant energy and low mass, you get a flashy explosion but just low impulse at the end: to get more impulse you need low speed and just the more mass the better.
Of course the speed still should be above the escape velocity of the asteroid in question to get effect.

So the key point of the 'nuke it' approach is to determine the optimal yield and depth of detonation for the material constitution of a specific asteroid to avoid breaking it up yet providing the most mass barely escaping it: this would provide the maximal thrust. (The direction of the thrust to get maximal effect is a different matter.)

That's why I dare to say that the linked model seems to be spot on, yet at the form it was presented it is just a capital misfire.
That's of course true, but impulse is still proportional to energy given that plasma velocity will not vary much as bomb yield increases. While it is not as efficient as optimal burial, a large enough energy produces a large impulse. It is also technologically easier.
 

LURCH

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It also appears to me that the researchers were pretty responsible in their wording, saying that their new model shows some “possible” differences from previous models, and the old conclusions are “called into question,” while the writers of the article make it sound like the old models have now been proved wrong. Always a good idea in these situations to remember that all we have now are two or more models that do not agree. We have no observations to support one over the other. The new model includes factors that the old model did not, and that is a good thing, but there is still a lot of room for factors that no one has even guessed at yet.
 

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