Nuclear explosions in space?

DrewDad wrote:

Again, we're not talking about just the bomb matter. The mass of the bomb is irrelevant. It's the yield of the bomb that matters. We're using the asteroid's own material as reaction mass.

Okay. That's definitely the right way to go, but you'll probably need at least a few tenths of a percent of the meteor mass ejected in more or less one direction, so momenta don't cancel much. If you use a proximity burst and send ejecta every which way, you're going to need several times as much ejecta to obtain the same momentum. I still say landing and burying several bombs to get more ejecta and hopefully channel its direction is the only practical way to use bombs.

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In '61, the Soviets tested a couple of devices with measured yields in the 60-90 megaton range, and later claimed to have developed and manufactured warheads of considerably greater yield - 100+ megaton missile-deliverable warheads. Even allowin' for bluster, there no doubt are damned powerful fusion weapons. It has been calculated a 1000-Meter-Above-Surface 100 megaton blast would crater an area nearly 50 kilometers in diameter, with the center of that crater a few hundred meters deep. The crater's slope would be fairly mild. Nukes - particularly thermonukes - is nasty. The point to which I'm goin' here is that a quite appreciable fraction of the target object's material would be converted to light, heat, and gas/plasma - the matter would be "GONE" from a mechanical perspective.

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Brandon9000 wrote:

Okay. That's definitely the right way to go, but you'll probably need at least a few tenths of a percent of the meteor mass ejected in more or less one direction, so momenta don't cancel much. If you use a proximity burst and send ejecta every which way, you're going to need several times as much ejecta to obtain the same momentum. I still say landing and burying several bombs to get more ejecta and hopefully channel its direction is the only practical way to use bombs.

Brandon,
you are absolutely correct. Blasting all the material in one
direction is more effective than blasting it every which way.
An unfocussed blast creates a lot of momentum scattered sideways,
that cancels itself out when the vectors are added together.

But is it really "several times as much ejecta to obtain the same momentum"?
Let's calculate exactly how much!

If we assume an explosion on the surface of a meteor scatters material
uniformly in every direction, then it creates a expanding hemisphere
of debris flying outward from the meteor.

The volume of that hemisphere is 2/3 * pi * R^3.
The circle that forms the base of that hemisphere (flat on
the meteor's surface) has an area of pi*R^2.
The average height of the hemisphere is therefore volume/base, or 2/3*R.

This is the "vertical" component of the debris, that causes a
"downward" push on the meteor. It is the vertical vector of the momentum,
left over when all the "sideways" vectors cancel each other out.

So we have our answer. Instead of all the material going directly "upwards" R miles,
it gets dispersed out to the sides too, in a hemispherical pattern,
and on average only goes "upwards" 2/3*R instead.

It's not a multiple of 3 or 10 or "several times as much ejecta".

We have only one third less vertical momentum, so to make up
for the vertical momentum lost in a hemispherical blast,
we only need to project 50% more material in the explosion.

Or project the same material 50% faster.

Whatever form that material takes -- whether it's one chunk, several pieces,
dust, gas, light, or plasma -- it's still the same mass being exploded away at very high velocity.

Also, this hemispherical model does not take into account any reflections or
bouncing of material from the crater itself or the surface of the meteor.
That kind of reflecting and focussing of the blast would only make the
results even BETTER than 2/3*R.

Does this seem physically impossible or wrong?

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I know I keep hammerin' this, but I think somethin' both Brandon and Codeborg are missin' is that there's a big difference between "GONE" and "Goin' anywhere".

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CodeBorg wrote:

...Blasting all the material in one
direction is more effective than blasting it every which way.
An unfocussed blast creates a lot of momentum scattered sideways,
that cancels itself out when the vectors are added together.

But is it really "several times as much ejecta to obtain the same momentum"?
Let's calculate exactly how much!...Does this seem physically impossible or wrong?

I think your analysis is either correct or else contains enough truth to be acceptable. I can accept your result that the average momentum vector of a piece of ejecta has a vertical component 2/3 of what it would be were it directed perpendicular to the surface. Therefore, you would only need 3/2 the mass you would need in a focussed blast. I thought my comment was probably hyperbole. However, if you are using bombs, I don't think you can get enough ejecta without burying them. You will never alter the meteor's path appreciably without a mass of ejected meteor equalling at least a tenth or two of a percent of the meteor's mass. The idea that the blast from a nearby bomb can by itself much deflect a meteor of the size being discussed is certainly wrong.

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timberlandko wrote:

I know I keep hammerin' this, but I think somethin' both Brandon and Codeborg are missin' is that there's a big difference between "GONE" and "Goin' anywhere".

Could you please clarify this? We are discussing how much velocity change can be imparted to the meteor by bombs, which would seem to me to be the relevant consideration.

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Energy can be lost to
- heat of vaporization
- denting/deforming the meteor
- light/heat absorbed by the meteor (not dispersed outwards)
- any chemical reactions that absorb heat

Can you think of any other forms that the bomb's energy might convert to ("GONE"),
that reduces the kinetic energy of the ejecta?

PS - Some neato bomb specs are available at
http://survival.anomalies.net/nukes.html
"HEAT: One third of the energy of a nuclear weapon is emitted in this form. It radiates in straight lines at the velocity of light, "

I presume that if tremendous heat is absorbed by the meteor, melting
and vaporizing a good deal of solid matter, that the kinetic energy of
the blast still remains the same. Roughly speaking, when a 150 Megaton
bomb makes a 2.1 Km crater, is it more from the
- one-third of the bomb's energy (heat) that vaporises it, or
- two-thirds of the energy (explosive) that pushes it away.

Maybe the crater is still produced by the bomb's kinetic energy,
regardless of how much matter is "GONE", changed state into vapor or plasma?

I'm still looking for the actual blast velocity - In the initial crater, can the
debris acheive anything like 1/10th light-speed? That'd be useful to know!
Wondering.

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CodeBorg wrote:

...I'm still looking for the actual blast velocity - In the initial crater, can the debris acheive anything like 1/10th light-speed? That'd be useful to know!
Wondering.

I'm kind of busy at work (staying late), but this I can address quickly. One tenth the speed of light is 67 million miles an hour. Surely, not even a nuclear explosion can accelerate dust, vapor, plasma, etc. to that kind of speed.

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Mebbe I'm not makin' my point clearly enough. While I doubt anything approachin' near-relativistic matter accelleration would be achieveable, my point is that a significant thermonuclear detonation will convert a significant portion of whatever is in proximity to it into somethin' other than matter. From a mechanical standpoint, the converted matter would be "GONE" - it would be literally vaporized. Now, while vapor - gas and/or plasma - has mass, in the overall scheme of things, particularly in the vacuum of space, that gas/plasma cloud is gonna dissipate into a very thin - on the order of a few attoms and the occasional molecule per cubic meter - hugely dispersed cloud - not an appreciable mass, no matter what its vector.

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Which are you talking about?
1) matter converted into energy (disappearing mass), or
2) solid matter converted into vapor (a thin gas/plasma cloud with the same mass as the solid)

From what I understand, #1 only occurs with nuclear reactions (fusion or fission), or by antimatter annihilations. The only mass lost when a nuclear
bomb goes off is when a small amount of uranium-235 splits (fission).
That's what accounts for the tremendous energy released.

No other atoms are destroyed.
All the surrounding matter has chemical reactions and physical reactions,
but not nuclear reactions such that matter becomes "GONE".

I could be wrong ... is there other info?

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CodeBorg wrote:

Which are you talking about?
1) matter converted into energy (disappearing mass), or
2) solid matter converted into vapor (a thin gas/plasma cloud with the same mass as the solid)

Both, more or less. Not that matter "disappears", per se, but as consequence of an appreciable thermonuclear blast an awful lot of it will be converted into into very, very diffuse matter - in very short order.

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1 gram of dense solid matter
is the same mass as
1 gram of very very diffuse gas.

They both work equally well as propellant ... with same momentum at high speed.

(sorry, editted my previous post)

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Some pretty equations and diagrams are at Fission and Fusion:

Quote:

1 kilogram of nuclear fuel could theoretically yield as much energy as 10 million tons of TNT!

So, a 100 Megaton bomb might convert only about 10 kilograms of mass into energy.
(disappearing mass, truly "GONE").

All the other kilograms of matter (mass) on the meteor and missile have to go SOMEwhere.

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I read what you typed, CodeBorg, and looked at the website - pretty animations indeed. However, I'm not talkin' about convertin' significant mass into energy, I'm talkin' about convertin' it into very much more difuse mass - a gas cloud. A large amount of the physical matter in near proximity to a substantial thermonuclear blast - like stuff within hundreds, if not a few thousand meters - of the blast center - will be melted to the point at which its matter state changes from solid to gas. Even a very, very brief exposure to the temperatures generated by a thermonuclear blast will do that to matter. It isn't "GONE" per se, I admit, but the density of the material, particularly if generated in the vacuum of space, will be infitessimally low per cubic meter of area, and that density will continue to decrease as the cloud expands at who-knows-what fantastic velocity - even after havin' slowed for hours, the cloud's expansion still will be quite rapid by terms which we understand. It would have insignificant mechanical effect, so from a mechanical perspective, it would be "GONE" - it would do nothin' noticeable - other perhaps than generate atmospheric auroral displays, when whatever was left of it got anywhere.

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Timber,

If you take 10 kg and eject it from a rocket, it doesn't matter if you eject it as a solid or a plasma. All that matters is how fast you eject it.

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timberlandko wrote:

Even a very, very brief exposure to the temperatures generated by a thermonuclear blast will do that to matter. It isn't "GONE" per se, I admit, but the density of the material, particularly if generated in the vacuum of space, will be infitessimally low per cubic meter of area, and that density will continue to decrease as the cloud expands at who-knows-what fantastic velocity - even after havin' slowed for hours, the cloud's expansion still will be quite rapid by terms which we understand. It would have insignificant mechanical effect, so from a mechanical perspective, it would be "GONE" - it would do nothin' noticeable - other perhaps than generate atmospheric auroral displays, when whatever was left of it got anywhere.

If matter were blown off of the meteor, pretty much in one direction at high speed, it would serve to push the meteor in the other direction, whether it was gas, plasma, or even converted into energy (which won't happen). Momentum must be conserved and a bunch of matter of any type at all that goes off in one direction will send the meteor in the other. For that not to happen would violate momentum conservation. The only question we are discussing is getting enough matter going fast enough, and in close enough to a single direction.

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Hmmm... a gas or plasma might actually be BETTER and result in higher speeds.

Reasoning: If one third of the bomb's energy is released as heat, this might usually just radiate away into space at the speed of light. Only two-thirds of the bomb's energy is released as explosive force, tearing a crater into the solid meteor.

But if some of that heat energy was used to convert solids into gas or plasma, the nature of gas is that it's springy and expands by it's own pressure. The expansion of a hot gas or plasma ADDS to the kinetic energy of the explosion itself.

More of the bomb's energy would be used!
Some of the heat energy (that would otherwise be radiated away) is turned into kinetic energy of the gas, resulting in faster ejection of the mass.

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AAAARGGGGGHHHHH - I see I'm not makin' myself clear - my fault, no doubt. What I'm sayin' is that a substantial thermonuclear blast will convert a helluva lotta material unfortunate enough to be in near proximity of that thermonuclear blast into matter of insubstantial nature. Sure, the matter still will exist, and sure, the conservation of momentum will pertain, but a very substantial portion of the matter will be so diffuse as to have insignificant mechanical effect. It will be there, but that "THERE" will be a very, very much bigger, and constantly expandin', increasisin'ly ever-less-dense "THERE". The ultra-high-temperature fireball of a multi-megaton thermonuclear blast is immense, whether that blast occurs in a vacuum, under water, in contact with, or deeply buried within a mass. Anything within, and much that happens to be anywhere near, that immense, hotter-than-a-star fireball will be rendered mechanically insignificant.

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There's no such thing as "mechanically insignificant" in Conservation of Mass.
Once the initial explosion imparts momentum to everything, there is
no further mechanical interaction.

Take a crater full of solid rock or iron. Say it has a mass of 1000 tons.
Vaporize it into gas and plasma. It still has a mass of 1000 tons.
And it even has the same volume and density!

Then you eject that mass at 5000 mph out from the crater.
The SAME amount of momentum has still been imparted to both the
gas and the meteor that it pushes against.

It plain doesn't matter if the mass is solid, liquid, gas or plasma.
1000 tons is still 1000 tons.

There are no gears and levers involved for anything to be "mechanically significant".

And it especially doesn't matter what happens AFTER the matter leaves the crater!
Later, as it's moving through space and starting to become really dispersed, thin,
and vacuous, little wispy dust, . . . it has no involvement with the meteor anyways.
Even spread across the solar system, it still has a mass of 1000 tons!
Only the initial bump is where momentum is transferred.

The gas can make auroral displays (inside the Earth's magnetosphere) or a
pretty trail (like a comet tail) or become tiny interstellar dust. It
doesn't matter how thin it gets or insubstantial. After the initial bump,
it makes no difference what state it's in or what happens to it.

Am I still missing somethin'? Puzzled... maybe ... I dunno.

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I think what you are missin' is the effect of a thermonuclear blast. The first one-megaton yield thermonuclear blast occurred in 1952 at Bikini Atoll. The fireball was over a mile in diameter, and it is estimated some 80 million tons of earth, plus a similar-if-not-greater volume of water, was vaporized - not blown around, VAPORIZED. What recondensed into materially substantial - if still near-microscopic - matter fell as thousands of tons of "fallout" over tens of thousands of square miles through a period of several months. It took that long for much of it to make its way back to the surface of the planet from the upper reaches of the atmosphere, so insubstantial was it. A good bit of it still is in the upper atmosphere.

Edit to add - for reference, a cubic meter of crushed iron ore goes about 2500kg, natural sand and gravel weighs around 1650-1800kg/cubic meter.

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Nuclear explosions in space?

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