Earth is constantly bombarded by ‘micrometeorites’; tiny bits of asteroids and comets. Scientists have measured the typical accretion rate of these particles at between 20 and 60 million kilograms per year.
So, on average, more than 100,000kg fall to Earth each day.
The mass distribution of these particles peaks at about 1.5×10–5 grams, with a diameter of about 200 micrometres (1mm = 1,000 micrometres). So, on average, there’ll be about 6.7×1012 micrometeorites falling to Earth each day.
If these fall uniformly across Earth’s surface (which is not perfectly true), each 75m2 (807ft2) of Earth’s surface would receive one micrometeorite per day.
If we also assume these particles are falling vertically and the average human head has a diameter of 14.4cm (5.5in), then you have about a 1 in 4,600 chance of being hit by one if you stand in the same spot for 24 hours.
In other words, you’d have to stand in the same spot for almost 13 years to be sure of a micrometeorite being caught in your hair.
Of course, these are only average numbers and it’s possible that you have, at some point, collected cosmic dust in your hair while outside. But it doesn’t happen very often.
This article is an answer to the question (asked by Laura Wolf, County Durham) 'Is it true we collect space dust in our hair when we’re outside?'
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