According to various sources (1,2, and 3) lunar landings kick up a lot of dust at very high speeds. Is it possible that some dust was able to get fast enough to escape the moon, fall towards Earth, and survive reentry reasonably intact (not coming down in tiny dust particles)?
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2 Answers
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Short answer: No
Long answer:
The escape velocity of the moon is about 2.4km per second. The exhaust velocity of the of the LEM’s engines were about 3km per second. At first you might say “well that’s bigger, so it could work!” But don’t be so sure.
The exhaust is pointing into the ground, meaning that almost all of the dust that would be flying off is going into the wrong direction. In addition the exhaust would probably not have the capability to accelerate the dust to 100% of its speed, that’s just the best case. Not only does the dust have to escape the moon, it has to have the right amount of velocity to hit earth instead of flying out into nowhere or orbiting forever. Which is very unlikely. The dust would need to travel at a very specific angle to the moon. This might not even be possible for some of the moon landings because they might have been on the wrong area of the moon. I don’t know for sure though, as I haven’t done all the math to figure that out. And it also has to avoid any obstructions on the moon.
And even if the dust got all of that stuff right, It will still burn up in the atmosphere without a doubt.
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$\begingroup$ Re the last statement there ,some quick googling suggest that at low enough sizes the cross section/density means dust does survive re-entry agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/…, would be useful if there was a full version of that text somewhere. $\endgroup$ Jul 30, 2022 at 7:15
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3$\begingroup$ “without a doubt.” Doubt! Dust is less likely to burn up during entry, not more. By the square-cubed rule, area scales down with size slower than mass scales down. Mass generates heat; area dissipates heat, and catches air for deceleration. Dust particles then decelerate higher in the upper atmosphere, where aero-compression is less effective, and radiative cooling is more effective. $\endgroup$ Jul 30, 2022 at 13:12
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$\begingroup$ …of course, this is all secondary to the origin argument. Stochastics might not be an issue (we’re talking dust, who’s counting?) but vector (without randomization and thermalization) is. $\endgroup$ Jul 30, 2022 at 13:14
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$\begingroup$ One reason that the lightest atoms of an atmosphere (e.g. Earth's hydrogen and helium atoms) escape is that they gain escape velocity through multiple collisions. In any gas for example, while the average speed is roughly the speed of sound, there's always a tail out to much higher speeds. Particles get there through multiple, fortuitous collisions that occasionally keep kicking them in the same direction. They don't stay there for long though, unless the gas expands to the point where collisions no longer happen. Of course the exhaust-induced dust isn't a gas, but maybe, one particle, once... $\endgroup$– uhohMar 12 at 1:43
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“ Is it possible that some dust was able to …” “ survive reentry reasonably intact (not coming down in tiny dust particles)?”
Come now, you’re not supposing dust particles accreted in mid-flight to become something else (i. e., meteoroids)?
Okay, semantics aside. Non-dust (>30 micrometer) would strike other (mostly lesser) particles as part of the “ejecta” plume. Scattering directions would be, well, scattered. It’s only possible to speak on a population basis (like a gas of lunar material), so it’s folly to completely rule out literally zero particles having some threshold speed, in a specific (lunar-escape, eventually Earthbound) trajectory.
The issue is how many particles, which separates an academic (thought exercise) answer from a practical (can I get some of it) answer. The practical answer is no, people have tried catching dust for decades, and have given up trying to select lunar dust (in general) from the dust background. If you want an academic answer (does this phenomenon actually exist?), I’m sorry but I have to answer with another question: Why?
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$\begingroup$ You answer a related but different question. Then you say I have to answer our question with anotehr question. Is this not the definition of not an answer. @calnstrument $\endgroup$ Mar 12 at 1:14
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$\begingroup$ "Come now, you’re not supposing..." Answers should address the question as asked and be written with future readers in mind. Stack Exchange isn't a forum, it's not a conversation, answer posts should not address the OP directly more than absolutely necessary. Is it possible to adjust the wording so that it feels more like you are writing for future readers, rather than deriding your personal take on the question author's thinking? Thanks! $\endgroup$– uhohMar 12 at 1:37
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$\begingroup$ Because your question mixes together the theoretical (‘does it exist’) with an implication of effects/practicality (‘not negligible totals, or negligible size per particle’). So, which is it? If your real question is existence vs. non-existence, then of course mass came off a body, even if it’s ions. If your real question is survival onto Earth, yet not ions but something substantial, palpable, visible in optical microscope not e-microscope, then that’s a different bar to clear. Is it dust, or is it not dust? Or does your (English, nonscientific) definition of ‘dust’ differ from ours? $\endgroup$ Mar 14 at 16:52
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$\begingroup$ A pedagogical answer (‘yes, ions come off’) can be completely different from a practical answer, given arbitrary bars to clear (‘no, boulders neither escape nor reach Earth- Earth surface, or Earth-von-Karman?) Would that be a boulder on the Wentworth scale, or the Krumbein scale??? You don’t know the differing definitions of ‘dust’ we have… yet you asked us for ‘not dust’. $\endgroup$ Mar 14 at 16:53
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