Some near earth objects travel with speeds of dozens of kilometers per second. Would a head-on collision between two of them create suitable conditions for nuclear fusion?
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$\begingroup$ I'm only a quarter joking when I say I'm surprised someone hasn't commented with "Fast, you say? Fast relative to what, old chap?". $\endgroup$– White PrimeSep 7, 2022 at 17:37
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4 Answers
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A different approach, ignoring any details of the collision. Note extreme simplifications will occur, do not try this at home...
The easiest hydrogen isotope fusion process is D-T, with a cross section that peaks at around 100keV. At 10keV it is more than 3 orders of magnitude lower. Lets ignore the isotope issue, and say we need H (protium) at at least 10keV to produce some fusion in the collision. So, what is the velocity of a 10keV H atom? Since that is non-relativistic, all we need is $E = {1 \over 2} m v^{2}$ and make sure we get all the units accounted for properly. Cranking through that all we get that a 10keV H atom has a velocity of a bit under 1.4 million meters per second, of some 1400 km/sec.
Dozens of kilometers per second doesn't cut it.
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Not even close.
This would have to be a rough "Fermi" estimate,
Lets suppose the asteroids are 50km in diameter and moving at a relative velocity of 50km/s. And so lets say the collision takes about 1 second.
Such asteroids have a mass of about 4e17 kg and so a momentum of 2e22 Ns The impulse to cause these asteroids to stop in about a second is 2e22 Ns, and so the forces involve are on the same order of magnitude, but these forces are spread over the whole asteorid's cross section. A rough estimate for the pressures in the collision would be about 1e13 N/m²
The temperature in the collision would be intense. There is evidence of impact temperatures on Earth being over 2000C. Let us suppose that the temperature is even higher, 10000C, instantaneously
This is nowhere near the temperature and pressure for fusion. The centre of the sun has a pressure of 2.5e16 N/m² and a temperature of 15 million C The energy release by an asteroid impact is substantial, but this is spread out over the whole asteroid, and so the temperature is rather less impressive.
These values, especially the value for the temperature, are so much more than the pressure and temperature in an asteroid collision that even though the calculation has been approximate, it is clearly far shorter than that needed to start fusion.
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“If two fast asteroids collided, would…”
We have meteorites from two fast asteroids colliding. Mesosiderites, and likely CB-group, too.
Got that? We have meteorites… which means the meteorites survived. They didn’t react via fusion, they didn’t plasma-fy, they didn’t even vaporize… even liquifying is a bit semantic.
We have hard evidence in our drawers, cases, and gloveboxes that two fast asteroids don’t react via nuclear processes. They barely reacted via chemical processes.
We have meteorites from asteroids colliding.
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3$\begingroup$ This just proves that some asteroid can survive the collision, doesn't it? I mean, we could be seeing only the ones that collided slowly enough not to be vaporized ;) $\endgroup$– PrallaxSep 7, 2022 at 17:39
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$\begingroup$ Don’t know when to quit, do you. Jon Custer already showed you are off by ~2 orders of magnitude, yet you still fish for a loophole. ~50,000 catalogued meteorites, of which meteoriticists can study nanogram samples, and you still tell them how to do their own jobs. 50,000 meteorites tell us the early Solar System was a billiard-game break, over and over again, for about 500 million years… and you still tell us how to do our own jobs. $\endgroup$ Sep 10, 2022 at 13:31
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$\begingroup$ Sometimes it is difficult to judge the tone of a written sentence. I will presume good intent and interpret you comment as a humourous remark! $\endgroup$– PrallaxSep 10, 2022 at 22:00
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Fusion is hard; it's really hard (insert paraphrase about a long way to the chemist's down the road). OTOH, if these two rocks contained a lot of fissile material, you might be able to cause a fission-based explosion.
A self-sustaining chain reaction is relatively easy (tho' in reality I can't imagine random asteroids of huge age still being highly fissile), but physically containing the reaction long enough to get more than the initial radiation flash is more difficult. A high-speed collision would emulate the original Little Boy mechanism.
answered Sep 7, 2022 at 13:21