Asteroid Fragmenting and Re-entry?

[Christopher]

Well, as with an orbital tether, the altitude lost from firing a projectile could be compensated for magnetically. A charged conductor moving in a planet's magnetic field can raise or lower itself with the right kind of current flow pushing or pulling against the field lines, though my memory is fuzzy on the specifics.

 

[Meredith]

In a discussion in the TNZ about keeping nuclear weapons in stock for use in asteroid defence, I raised the idea of using railguns (once the tech for them is nailed down) to pulverise asteroids into smaller chunks burn up in atmosphere, when someone stated this wasn't the case, since apparently it would be worse for us. It was almost a week after before I saw the thread, so I decided against necroposting, and decided maybe it was a question better placed to ask here.

Now, I would have been under the impression that if an asteroid is reduced to smaller elements, it would be the case that each section would burn up quicker than a single larger whole, since there's a larger surface area that can suffer friction from the atmosphere, thusly quicker to burn up during re-entry.

Am I wrong here?

You are correct.

Energy would be spread out in both space and time.

Same as blunderbuss.Spreads the energy out over a larger volume and distributes it over time.

>>>>>>>> . <<<<<<<<<< energy concentrated in a point generates hugh shock wave.

>>>>>>> O <<<<<<<<<<<, energy spread over bigger space and also over time...shock waves less intense.

Plus it won't make it far enough to impact the earth and throw crust material back into the sky. A LOT of the K-T boundary material is from Earth's own crust.

 

[T'Girl]

If you detect the asteroid soon enough, all you need to do is tweak its course a tiny bit.

The various solutions mention here are fine for the future. If a small or medium size asteroid was going to collide with Earth within a year, the ony plan would be to detonate a series of nukes next to it and hopefully alter it's path enough to miss us. Trying out different ideas on a asteriod that isn't going to hit us would be a good idea.

 

[SilentP]

^^As long as you don't accidentally knock it onto a path that will collide with Earth

 

[Crazy Eddie]

Contrary to the oft-repeated (and surprisingly thoughtless) line quoted by scientists who ought to know better, blasting an asteroid into smaller particles WOULD mitigate the consequences greatly. For three simple reasons

1) The breakup of the asteroid increases the overall surface area of the object that is exposed to atmospheric drag, allowing it to shed more of its kinetic energy in the upper atmosphere. Since the resulting fragments are moving more slowly, they will do less damage if they hit the ground.

2) The breakup of the asteroid increases the overall surface area that is exposed to entry heating, resulting in more of its mass being vaporized in the atmosphere and not reaching the ground at all. To illustrate this effect, try dropping an ice cube into a glass of warm water and see how long it takes to melt; then shatter a similar-sized ice cube with a hammer and do the same.

3) A heterogenous object--a comet, for example--will have varrying densities in different strata; as the whole object descends at speed its collision with the thickest layers of atmosphere would have the dynamics of a beer bottle hitting a brick wall; the resulting explosion would create an airburst that could cause massive destruction for dozens of miles. Shattering the asteroid/comet before it enters the atmosphere would eliminate this problem entirely.

 

[SilentP]

Contrary to the oft-repeated (and surprisingly thoughtless) line quoted by scientists who ought to know better, blasting an asteroid into smaller particles WOULD mitigate the consequences greatly. For three simple reasons

1) The breakup of the asteroid increases the overall surface area of the object that is exposed to atmospheric drag, allowing it to shed more of its kinetic energy in the upper atmosphere. Since the resulting fragments are moving more slowly, they will do less damage if they hit the ground.

2) The breakup of the asteroid increases the overall surface area that is exposed to entry heating, resulting in more of its mass being vaporized in the atmosphere and not reaching the ground at all. To illustrate this effect, try dropping an ice cube into a glass of warm water and see how long it takes to melt; then shatter a similar-sized ice cube with a hammer and do the same.

3) A heterogenous object--a comet, for example--will have varrying densities in different strata; as the whole object descends at speed its collision with the thickest layers of atmosphere would have the dynamics of a beer bottle hitting a brick wall; the resulting explosion would create an airburst that could cause massive destruction for dozens of miles. Shattering the asteroid/comet before it enters the atmosphere would eliminate this problem entirely.

The first two reasons were why I thought shattering an asteroid would be helpful. Thanks for wording it far more eloquently and clearly than I could.

I hadn't thought or heard of the 3rd point. Interesting

 

[strongmind]

Contrary to the oft-repeated (and surprisingly thoughtless) line quoted by scientists who ought to know better, blasting an asteroid into smaller particles WOULD mitigate the consequences greatly. For three simple reasons

1) The breakup of the asteroid increases the overall surface area of the object that is exposed to atmospheric drag, allowing it to shed more of its kinetic energy in the upper atmosphere. Since the resulting fragments are moving more slowly, they will do less damage if they hit the ground.

2) The breakup of the asteroid increases the overall surface area that is exposed to entry heating, resulting in more of its mass being vaporized in the atmosphere and not reaching the ground at all. To illustrate this effect, try dropping an ice cube into a glass of warm water and see how long it takes to melt; then shatter a similar-sized ice cube with a hammer and do the same.

3) A heterogenous object--a comet, for example--will have varrying densities in different strata; as the whole object descends at speed its collision with the thickest layers of atmosphere would have the dynamics of a beer bottle hitting a brick wall; the resulting explosion would create an airburst that could cause massive destruction for dozens of miles. Shattering the asteroid/comet before it enters the atmosphere would eliminate this problem entirely.

The first two reasons were why I thought shattering an asteroid would be helpful. Thanks for wording it far more eloquently and clearly than I could.

I hadn't thought or heard of the 3rd point. Interesting

That is what I said.

 

[T'Girl]

The pieces would also impact over a wider area. "Blowing up" an asteroid is said to be the difference of a rifle bullet vs a shotgun blast. If you had to choose between being hit by a single bullet and a single pellet, the choice is obvious.

 

[Jadzia]

If we look at the blast damage estimates of nuclear weapons, wikipedia says:


At 1 Kilotonne :
Urban areas completely levelled, 0.2 km
Destruction of most civilian buildings, at 0.6 km
Moderate damage to civilian buildings, at 1.7 km

At 1 Megatonne (1000 times the energy) :
Urban areas completely levelled, 2.4 km
Destruction of most civilian buildings, at 6.2 km
Moderate damage to civilian buildings, at 17 km

If this is correct, then you can see that equal damage is only 10 more distant with the 1MT bomb, even though it is a massive 1000 times more energetic.

We can infer that the radius of destruction is proportional to Impact Energy ^ (1/3).

Comparing this with an asteroid:

A moderate sized asteroid might collide with the energy of 5MT. That would cause "destruction of most civilian buildings" upto 10km from it's point in impact.

If broken up into 1000 pieces, you would have 1000 small asteroids, each colliding with the energy of 5KT. Each one would cause "destruction of most civilian buildings" upto 1km from it's point in impact. A much smaller area, but there are ONE THOUSAND of them.

See pictures below for drawn-to-scale effect.
Red = Completely levelled
Orange = Destruction of most civilian buildings
Yellow = Moderate damage to civilian buildings

 

[T'Girl]

A moderate sized asteroid...
small asteroids ...

Scary. For your calculations, what sizes of asteroids would be "moderate" and "small" please?

 

[SilentP]

Also, do your calculations account for the additional amount of material that is incinerated in each of the asteroid fragments due to additional surface area that is heated due to the fragmetation of equal amount of asteroid? (if that horribly mangled sentence made sense...)

 

[Jadzia]

It's the high speed that gives it the enormous energy. If the asteroid comes in at 30000 mph, then to produce the effects in the above pictures:

Picture 1: A moderate sized asteroid 50-100 meters across, striking the earth at 30000 mph, releases in the order of 10^16 Joules, or 5 megatonnes TNT equivalent.

Picture 2: Blasted into 1000 smaller chunks, and assuming no additional burning up in the atmosphere, each would be 5-10 meters across at impact, striking the earth at 30000 mph, releases in the order of 10^13 Joules, or 5 kilotonnes TNT equivalent EACH.

Talking about burning up on entry, ones this small (5-10 meters) might do that, so they may not strike the ground with this much energy. But this burning effect isn't something that scales up, since air drag and friction works on surface area, not on mass or volume.

On an asteroid 10 times larger, drag and friction are going to be around 100 times greater, but that's trying to slow a rock that's 1000 times more massive. So the slowing/reducing effects are 1/10 of the effectiveness.

This effects of the air becomes less significant as things get more massive. On small asteroids, say upto 10 meters across it is significant. On something 100 meters across, air friction will have much less effect. On something 1000 meters across, air will do nothing.

 

[SilentP]

But this burning effect isn't something that scales up, since air drag and friction works on surface area, not on mass or volume.

Is your point in regards to this sentence stating that the fragments have to be really small to be completely burned up by reentry, so that fragmenting is a futile gesture?

Anyone know the maximum size a fragment can be to get vapourised by the atmosphere before hitting the surface? Speed would also come into this, since drag and air friction would increase with speed.

 

[Jadzia]

But this burning effect isn't something that scales up, since air drag and friction works on surface area, not on mass or volume.

Is your point in regards to this sentence stating that the fragments have to be really small to be completely burned up by reentry, so that fragmenting is a futile gesture?

I'm saying that it's possible that fragmenting makes things worse, not better.

Generally, an asteroid broken into N^3 equal fragments will collectively do upto N times more ground area destruction than the one large asteroid on its own.

However, the smaller those fragments are, the more they will be reduced in potency by the air, so it will be collectively less than N times. It would have to be less than 1 to give a better outcome. And the air will do a lot to protect us from small asteroids.

I don't have the time to model the ablation right now as I have other things to do. You can crudely do it with Stokes law, but it's more correctly done with a hypersonic model.

 

[SilentP]

But this burning effect isn't something that scales up, since air drag and friction works on surface area, not on mass or volume.

Is your point in regards to this sentence stating that the fragments have to be really small to be completely burned up by reentry, so that fragmenting is a futile gesture?

I'm saying that it's possible that fragmenting makes things worse, not better.

Generally, an asteroid broken into N^3 equal fragments will collectively do upto N times more ground area destruction that the one large asteroid on its own.

Which is where the shotgun analogy comes from, I'm guessing.

However, the smaller those fragments are, the more they will be reduced in potency by the air, so it will be collectively less than N times. The air will do a lot to protect us from small asteroids.

I don't have the time to model the ablation right now as I have other things to do. You can crudely do it with Stokes law, but it's more correctly done with a hypersonic model.

No worries, I'm just checking to see that the model you briefly demonstrated with us is the start and not the whole picture. Thanks for clarifying