In a meteor impact, say the size of the one that killed the dinosaurs, does fussion ignite due to the powerful forces at work? If not on Earth, perhaps if it hit an object made of more unstable elements?

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To ignite fusion you need:
1) An extremely high temperature to give the atoms the kinetic energy they need to overcome the repulsive force generated by their electric charge. The actual temperature needed depends on what elements you want to fuse, expect something in the million Kelvin range.
2) An extremely high pressure (read density) for a considerable number of head-on collisions.
Even if the impact of a meteor would generate these conditions for a short time in a small area where it hits the earth's surface (I doubt the temperature goes above a few thousand Kelvin, but I haven't calculated this) temperature as well as pressure will decline immediately.
So the answer is a plain 'no'.

Another problem is that even if the temperature reached the required level, nickel and iron are at the bottom of the energy curve. They are endothermic (absorb energy instead of releasing it) for both fusion and fission.

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I see. So is that to say that there would be no radiation released in an impact like that?

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Well, the impact would vaporize, and eject into the atmosphere, any radioactive elements that happened to be at (and under) the site of the impact, or in the impactor itself. That might raise the background level by a fair amount, for a while.

In fact, we found the Chixulub impact because of the geological layer enriched in a particular isotope of iridium deposited post-impact. I don't think it was a particularly active isotope, and had nothing to do with the mass extinction, but it does illustrate the possibility of a spike in radioactivity after a "big smack".

Well, no nuclear energy. An object moving at a minimum of 11 kps still has a lot of kinetic energy that will be disipated on impact. The object that created Barringer crater in Arizona is thought to have been only a few tens of meters in diameter!

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There's a geology class this winter term about asteroid impacts. I may just take that...

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Unfortunately, we're down to about 17 ounces of the highly unstable stuff that does.

well, theres actually no radiation released in a fusion reaction/explosion either. The radiation from an H-Bomb is actually caused by the fision bombs used to create the heat needed for the fusion portion of the bomb to occur

I guess only when the asteroid would travel at a velocity that is some serious fraction of the speed of light... ;-)
When an asteroid hits Earth, I expect that radioactivity will be one of the least problems we have to face...

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I'm no expert, but I'm not sure that's is 100% true. Fusion does create neutrons and when those interact with other atoms they will probably create unstable isotopes. But a lot of the real dirty stuff does come from fision.

I recall some book, probably science fiction, that suggested a big enough hit could actually generate x-rays, just from the temperature of the event (black body radiation). I find that a little hard to believe, but I guess it is remotely possible.

But I echo what has been said about impacts, no fision - the blast, heat, firestorm, and global weather effects are going to be what kills you.

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It also depends on what you mean by "radiation." LIGHT, for example, is radiation. All of the energy in an atom bomb is released in the form of radiation.

Stellar fusion does give rise to a small amount of radioactive elements (elsewise, there'd be nothing around larger than Iron).

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Supposing that it impacted on the ocean. Could not the force of the impact and the energy release be enough to cause a small amount of the hydrogen in the water to fuse? Granted, it wouldn't be but for a tiny fraction of a second, but isn't it possible for SOME fusion to occur?

As a guess, even with a significant amount of hydrogen present at the impact point, temperature and pressure needed to sustain fusion will occur only if the impactor is at least as big as Moon. A "rogue moon" hitting Jupiter (or Sun for that matter) might ignite fusion reaction. Still, amount of energy such reaction would release will be tiny compared to the kintic energy of the impact.

I'm not so sure about that. I've heard that part of the "fun" in designing a device is making it strong enough that it holds together for an instant while the stuff is fissioning. The reaction ends when the stuff blows itself apart, not when there's no more U/Pu/Am/whatever. I'm thinking that some of the bomb's energy must be released as KE.

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Unfortunately, no. Most of the hydrogen is bonded to oxygen. That which isn't (1 in 10,000,000 molecules of water) is a free-floating proton without electrons (the electrons are left with the other half of the water molecule).
The oxygen-hydrogen bonds can be broken (in, say, extreme heat), but doing so will still give you the free-floating protons, and not real hydrogen.
Those protons can't fuse to form helium, because you must have the electrons there to form the neutrons.

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Two small errors in the last post (by Betenoire):

One, no electrons are needed to fuse hydrogen into helium. In the Sun for instance less than one percent of the fusions involve the input of an electron, most involve the output of a positron.

Two, surely if there's heat enough to cause fusion, there's also heat enough to ionize the atoms in the "water" (which under those conditions is no longer water). Take an ordinary pebblesized meteor entering the athmosphere. The energy of the impact is sufficient to melt and vaporize the iron in the meteor and leave behind a trail of ionized air that can be seen miles away (or is that just a grainsized meteor?).

Now looking at the calculations of the Bad Astronomer right here http://www.badastronomy.com/bad/movi...itageddon.html I estimate the average energy output to be around one megaton per square meter as a minimum. Enough to fuse a little hydrogen? I don't know, but it wouldn't surprise me.

True, a big enough impact might deliver enough energy to fuse hydrogen, but there's still the pressure issue. An impact, even on water, won't be forcing the hydrogen atoms closer together. Instead, as the water heats up, it'll expand, pushing them farther away.

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Which was, if you'll re-read my post, what I was suggesting ("The oxygen-hydrogen bonds can be broken (in, say, extreme heat), but doing so will still give you the free-floating protons, and not real hydrogen.").

And checking with the Wikipedia (source of all knowledge) I discover that I either do not recall correctly my high school chem and physics text books, or was mis-informed.
But, if a neutron is formed by the casting off of a positron (decreasing mass) from a proton, why is the neutron more massive? And how is it that the neutron decays by casting off an electron (decreasing mass) to become a proton (again, according to Wikipedia)? This would all seem a bit counter-intuitive and the neutron=proton + electron (rather than neutron=proton - positron (the result of which would seem to be a neutron mass less two positron/electron masses)) is what I recall from high school.

Edit: more questions after reading more of the Wikipedia

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