Sunday afternoon, while waiting for the Super Bowl to start, some friends & I were talking astronomy over some beer. As you might imagine, the discussion staggered into VERY hypothetical territory.

We got onto the topic of asteroids and what they are made of and why some are one type and others are of other types. This begged the question could an asteroid be made of only one material, eg carbon? We decided that would be yes. That got us onto, if you could have a pure carbon asteroid, could you have one that was pure diamond? The conversation staggered through many materials until we got to plutonium.

Plutonium got the discussion on whether you could build a bomb from a pure PL asteroid. Then could you suround it with hydrogen and make a super H-bomb?

This got us onto could a human made H-bomb iniate a pure PL asteroid into a bomb rate Alpha? (self sustaining, very high rate fissioning) This is the basis for my question.

Does anybody know if you detonate a nuclear bomb on a pure PL asteroid, would the PL making up the asteroid participate in the nuclear explossion? If yes, would all or just part of the asteroid participate?
Again, if yes, what type of device would iniate the asteroid? Fission? Fussion?

I know this is a question posed by a bunch of dumb drunks but it has really piqued my curiosity. Any knowledgable feedback is appreciated.

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It's kind of a moot point since plutonium has such a short half-life that it doesn't exist naturally. I would think, though, that if you somehow had an asteroid made of pure, weapons-grade plutonium, that it would be an atomic bomb, i.e., it would fission immediately, and that would be it.

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Plutonium is Pu, not PL.

First problem is that plutonium is not an natural element. The second problem is that if you get enough of it together it will spontaneously start to energetically disassemble on you.

If you had a a large jumble of Pu, it would pop and fizz in all directions at different times due to random configuration. To get a "super" bomb you want nice even, controlled detonation, not a bunch of small poots that will fragment the asteroid stopping criticality.

I would think colliding two large chunks of neutron matter would be more enjoyable.

That's short half-life astronomically speaking, of course. To we humans, it's long enough.

Ahh something I know a little about for a change.

ToSeek is right in a sense, as soon as a cosmic ray or high volosity particle of dust or asteriod touched a Pure Plutonium body thats was unshielded it would detonate. (The shielding on a nuke warhead serves to protect both people from radiation decay, and the warhead itself against outside radiation or particle impacts).

If it was massive enough a Pure PL body could detonate under it's own weight. (I think the mass required would be about 1.4 of a metric tonne for this)

Actually any material except for Hydrogen can undergo Fission. If you had two wood balls and slammed them together at suffient velosity to start the cascade, even those would make an impressive Fission bomb.

The reason for Uranium and Plotonium is that they are so unstable, it's relatively easy to start the Fission Cascade by slamming pie shaped wedges of a sphere together with simple standard explosives.

Fussion reactions are much harder to achive (H-Hombs). It's triggered by a Fission bomb, that -must- have it's explosion contained for a short time, usually by use of a depleted uranium/lithium/lead or iron casing. The fission explosion is contained long enoung that the tempature reaches the point where the hydogen generated by the fission, can start to fuse together. Less material is needed fo the fission device itself, but the casing has to be huge by commparision or fusion tmeperatures and pressure is never achived.

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Seeing this post with your sig is amusingly disturbing

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Eagle Eye Observatory

Suppose the asteroid was made of francium?

Am I wrong , or is there so little of that partricular element that we don't really know what form it takes?

Or have they found more recently?

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It's kindof a moot point. Compared with our current tech, all astroids may as well be made of Unobtanium

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Eagle Eye Observatory

It would be gone in a matter of days.
Nobody ever "found" any francium -- like plutonium and other transuranian elements, it is too short-lived to be found in nature. It can and has been made in sufficient quantities to study. And yes, its form is well known -- it is an alkali metal with a very low melting point.

Fun tidbit -- if you dissolve enough francium salt in a glass of water, the water will boil spontaneously, thanks to the heat of francium decay.

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Sorry, no. For all elements lighter than iron, fission is an endothermic reaction -- it takes in energy. Slamming two wood (C, N, O, H) balls together at relativistic velocity may break some nuclei into smaller fragments (i.e. fission), but that will actually reduce the kinetically-induced explosion. For elements heavier than iron such fission will release energy, but will not cause a cascade. For the latter to happen, a nucleus must easily break up from absorbing a neutron -- something only very heavy nuclei do.

No, it is because when the wedges of a sphere are brought together, there is suddenly less surface through which (continuously generated) neutrons can escape. They are what sets off a fisison cascade, not the kinetic energy of wedges slamming together. The reason these wedges must slam with explosive force instead of mechanically brought together is to hold the sphere together long enough for most of it to fission. If you push the wedges together by hand (for example), cascade will still occur, but will shatter the sphere to pieces and stop the reaction after only a fraction of a percent of the uranium manages to fission. That is called "fission fizzle".

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There's a good nano-second by nano-second account, not sure if it is entirely accurate, but in a chapter of clancey's Sum of all Fears (the book) he outlines a "fusion fizzle."

Still quite an explosion...

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Forget diamond asteroids, how about some diamond planets

http://www.space.com/scienceastronom...ts_050208.html

Moreover, the critical mass of Pu-239 is about 10 kg. A few other isotopes are somewhat higher, but they're all under 100 kg, I think, so it would be hard to call that an asteroid. More just a lump of metal.

As i said it's something i know a -little- about Know how to make them, don't know all the nuclear physics behind them.

So Fussion has a threshold upto Iron...and Fission down to Iron.

That's actually starting to expain some things about why Stars behave the way they do.

Are there any other nuclear like reactions besides Fission, Fusion and Matter/Anti Matter?

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There is no problem that cannot be solved by a suitable application of high explosives - US Army Demolitions School

I just saw Hayley's comet, she waved, Said "why you always running in place? Even the man in the moon disappeared, Somewhere in the stratosphere" - Shinedown

http://worldsofothersuns.home.comcast.net/

Having dealt with the design and control of processes for U, Pu, Np, etc., I'd be really interested in the causes for the natural origin of a Pu asteroid. :-k

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"Naturally occurring" has a specific set of cirmcumstances attached to it. Pu is not naturally occurring on earth, today. But that does not mean it wasn't present 4.6 billion years ago when the earth was formed, or formed in the Oklo "natural reactor" 2 billion years ago, or would not be formed somewhere else in the universe in, say, a supernova.

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True. That was a sloppy and hurried statement. I think unlikely, and not readily available element for asteroidal purposes would be closer.


Given the half-life of Plutonium is 24,000 years (or 240,000 for a effective disappearance of the element) there simply is not enough time for it to form an "asteroid". Even if there was, the infalling chunks would be constantly acting as a "gun-assembly" nuclear weapon blasting what little material that had accreted back into dust.

Russ. What did you guys decide as far as an asteroid made of diamond? I'd have to say no on this, as a "naturally forming" asteroid will have never been under the pressures necessary to turn carbon into diamond.

Granted, if the diamond rock had formed on some planet, which then somehow got creamed by another planet or some such thing, then maybe yeah, but it doesn't sound like that's what you guys meant.

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First, I'd like to thank all of you who replyed. Your information and witty comments are fun to read. Sorry I was so long in getting back to my own conversation, life intervened.

To dgavin: You mention that any PL mass over 1.4 metric tons will spontaniously fission. It was my understanding that PL was not inherently radioactive and therefore had to be "iniated" to fission. This is why you can hold a block of it in your bare hand. This was the reason it is desirable for A-bombs in ICBM's. It could just sit there for years and still be ready to rock if the time came.

In bombs a thing called a zipper shoots protons at the critical mass to initiate the fission process just before the mass collision. There must be a sheet of paladium to absorb neutorons to keep them from poisoning the reaction. Without the paladium, less than a quarter of the PL will be able to fission.

To Grey: I thought "critical mass" was the mass below which a self sustaining, high alpha reaction could not be maintained. It is the minimum amount of fissal mass you must have for the desired reaction to occur. Am I wrong about that?

To All: Thanks again. I look forward to further discussion.

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Weird you should bring it up. I just got an email yesterday from space.com and it linked to one of its articles: diamond planets

Something I'm hazy about and would like to ask, now that its a subject. What exactly happens at critical mass? By that I mean, what process is occuring in the material itself?

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Though we have no proof, we thought that in the nova stage (note, not supernova) of a carbon core star, there should be enough temp and press to create diamond. This diamod would be shattered into small chuncks (20-40miles avg diameter) and blown into the interstellar space. It would eventually be part of the planitary disk around a new forming star. Most of the diamond debris would end up in the planets and star but some would end up in the stellar system as meteroids and asteroids.

Sir A.C. Clarke speculated about this in his sequal to 2001, 2061. He had a mountain sized diamond collide with Europa. Two astronauts who crash land on Europa pick up a 10lb. piece as a souvineer of their adventure. :roll:

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It's just one of those damn things of which there are many few. -- Dan Blocker

First, it is Pu, not PL. Second, it most certainly IS radioactive, with half-life of 24,000 years. Isotopes with such long half-life ARE safe to hold in your hand because they emit relatively few particles per second, but they do emit some. Also, while it's true that with such long half-life a plutonium bomb core will not noticeably decay over the bomb's lifetime, that fact had little to do with plutonium's desirability -- half-life of U-235 is several billion years.

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All of the really heavy nuclei occasionally undergo spontaneous fission. Since the heavy nuclei have a larger share of neutrons than the lighter ones, this usually means you get two smaller pieces plus a few spare neutrons released. A couple of those neutrons may collide with nearby atoms, and for the materials that can undergo chain reactions, that extra neutron is exactly what it takes to trigger fission of that nucleus as well. If you only have a little bit of the stuff, many of the neutrons escape to the outside and never interact with another atom. However, as you accumulate more material, the likelihood that a neutron will trigger another fission reaction before it makes it out increases. At the critical mass, enough of the released neutrons from any given splitting nucleus interact with other nuclei and trigger fission in turn so that the reaction is sustaining. In a very short period, all of the nuclei split, and you get a big kaboom. As has been mentioned, since all of this is liberating energy, one trick to getting an actual explosion is to keep the material together long enough for most of the nuclei to undergo fission.

You're half correct. Below the critical mass, the reaction won't sustain itself. But above the critical mass, the chain reaction is spontaneous, and doesn't need any kind of trigger. The simplest bombs just take several pieces of material each of which is smaller than the critical mass, but with a total mass higher than that, and slams them together. It is true, though, that some use more sophisticated techniques like the one you describe to try to get more of the material to undergo fission before the bomb blows itself apart. That's just an efficiency issue, though.

The thing that I was really pointing out was that the critical mass is way lower than dgavin was suggesting. On the order of a few tens of kilograms at the very most, and as dense as these materials are, that's not very big.

You also have to remember that "critical mass" is a misnomer. It really should be "critical mass-to-surface ratio". The bigger is a piece of fissionable material, the more neutrons it generates per second. The greater is its surface area, the more neutron can escape. If it generates more neutrons than can escape, you get chain reaction. A sphere has lowest surface-to-volume ratio, so when physics textbooks define "critical mass" they usually assume it is a sphere. But you can have much more plutonium (or uranium) without a chain reaction if it is, say, in the form of a thin sheet.

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Or, a lot of pieces in different cities.

I always thought a "chain reaction" is what you got from wearing too many low-karat disco medallions.

OK, OK! I'll stop using MY abriviationand start using the official periodic table of the elements symbol. :roll: picky, picky, picky :roll:

Well, I guess I misspoke myself. I know PU (there are ya happy?) is radioactive, that's why it feals warm to the touch, but thought that it was not enough so to be able to self iniate a chain reaction (high alpha). Your comment in a later post about the better term being "critical mass to surface area" clears it up some for me.

So my question now is: If you have a large piece >1.4 metric tonnes, that is highly irregular in shape (high surface area to mass ratio) would it be stable or still go supercritical by itself? Would that depend on it's shape? Thanks in advance for your answer.

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You guys crack me up.

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It's just one of those damn things of which there are many few. -- Dan Blocker