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I know that meteors that We see as shooting stars travel tens of thousands of kilometers per hour and are tiny as a grain of sand, but after hitting the atmosphere and burn I suppose that its ashes would touch the ground at some point. How long does it take? Can it be measured or observed?

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    $\begingroup$ Meteors that burn up do so at an altitude of the order of 100 km, depending on their incoming speed. Thus, their ashes will be scattered over thousands of square kilometers, making it impossible to find. $\endgroup$ – pela Nov 20 '17 at 15:08
  • $\begingroup$ It will also depend on the angle at which they enter the atmosphere. $\endgroup$ – Phiteros Nov 20 '17 at 19:25
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    $\begingroup$ I'm voting to close this question as off-topic because the assumption is wrong. No solid-state particles of measurable size need survive. $\endgroup$ – Carl Witthoft Nov 21 '17 at 15:02
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I think the best model we could use for this is nuclear test fallout, whose dispersion was monitored quite extensively. It's very, very dependent on weather patterns at altitude.

For example, the Castle Bravo test mushroom cloud reached at altitude of about 40km and hence hit the Jet Stream. In addition to this the wind speeds at lower altitudes were higher than ground speeds and this was not checked for, so the combination of factors spread the fallout downwind about 280 miles (about 430 km) in about 16 hrs. This was one reason for the irradiation of many people.

What these tests and other nuclear incidents show us is that material ejected into the high atmosphere can remain there a long time, even circling the globe like traces from Fukushima.

In theory trace particles from a meteor could remain in the upper atmosphere for days, months, even years. Most would be expected to fall within days, however.

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    $\begingroup$ That's a nice deductive answer. I'll add a related point on Tephra, where the very small particles (.01 mm and less) can stay in the air (by this article) sometimes for two or three years. geo.mtu.edu/volcanoes/hazards/primer/tephra.html $\endgroup$ – userLTK Nov 21 '17 at 9:27
  • $\begingroup$ The only problem with this analysis is that it makes assumptions about the resulting particle sizes as a meteor disperses. $\endgroup$ – Carl Witthoft Nov 21 '17 at 15:03
  • $\begingroup$ @CarlWitthoft If you're suggesting that after breaking up in the atmosphere nothing remains but gas (which seems to be your implication) I'd need some reasonable argument to justify that - it sounds like a more drastic assumption to me. $\endgroup$ – StephenG Nov 21 '17 at 16:43
  • $\begingroup$ Stephen, not gases, but perhaps particles of both mass and density small enough that even mid-to -upper atmospheric densities and wind strenghts are sufficient to make the "airborne half-life" on the order of years. $\endgroup$ – Carl Witthoft Nov 21 '17 at 16:45
  • $\begingroup$ @CarlWitthoft Hmmh. Have you a basis for estimating such a size and/or whether this isn't the case with material from fallout ? Just trying to tease out as solid a basis as possible for your suggestion. $\endgroup$ – StephenG Nov 21 '17 at 16:50

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