> An article by Phil Plait (The Bad Astronomer) says that each square meter of surface is hit by 1 - 2 micrometeorites per year.<p>A much cooler way to put this is that if you spend 1 hour per day on average not under a roof, and if we ignore some complications, your expected micrometeorite-strikes-per-lifetime is about 0.5:<p><pre><code> (1/24 of the time outside) * (1.5 per meter squared per year) * (80 years) * (pi * 8 inches * 6 inches) = 0.486 [1]
</code></pre>
where the last term here is an approximate cross-sectional area of the human head as viewed from above. (Micrometeorites reach terminal velocity at a very high altitude, so they should be falling almost vertically as they strike the ground, minimizing your cross-sectional area unless you happen to be lying down.)<p>Someone doing manual labor or other primarily-outdoor work has an expected-micrometeorites-per-lifetime much greater than 1, and therefore is overwhelmingly likely to have space dust in their hair at least once in their life. If you spend 8 hours outside per day (so your workday + some miscellaneous non-work time that covers days you're not working), your expected lifetime impacts is ~4, so your chance of >= 1 impact is 1 - Pois(0; 4) = 1 - e^-4 = about 98%.<p>A better calculation would account for the fact that Earth's orbital motion makes the relative motion of impacts non-uniform in the same way the front of a moving car is hit by more raindrops than the rear. This means most strikes occur around dawn, because that's the time when you're on the "front" of the Earth as it moves in its orbit.<p>Early-morning workers, like agricultural, sanitation, or custodial workers, might be out for the peak of impacts nearly every day, while the average reader here who drifts into the office hours after dawn (if at all) and spends most of their outdoor time in the evening might almost entirely miss their opportunity to have a bit of space fall on their head. (I think this is rather poetic: if you want to touch the stars, go pick up trash!)<p>-----<p>[1] <a href="https://www.wolframalpha.com/input?i=%281+hour%2F24+hour%29+*+%286+in*8+in%29+*+%281.5+%2F+%28m%5E2+*+year%29%29+*+80+year" rel="nofollow">https://www.wolframalpha.com/input?i=%281+hour%2F24+hour%29+...</a>