Micrometeorites: questions and answers
Short, clear answers to the questions people ask most about micrometeorites, from whether they are dangerous to whether you can sell one.
Micrometeorites are harmless, common and almost invisible. They land everywhere, including on you, but they are far too small and slow by the time they reach the ground to hurt anyone. Finding one takes patience, a magnet and a microscope.
Are micrometeorites dangerous?
Not on the ground. By the time they reach the surface, the air has slowed them to a gentle drift, and they are smaller than grains of sand. In space it is different: micrometeoroids travelling at many kilometres per second can pit spacecraft windows and damage equipment, which is why spacecraft carry dust shields.
Have micrometeorites ever landed on me?
Almost certainly, at some point. They fall everywhere, but at roughly one sizeable particle per square metre per year, the chance of one landing on you on a given day is small, and you would never notice it.
How big is a micrometeorite?
Less than about 2 mm by the usual definition, and most are between 0.05 and 0.5 mm, about the size of a grain of fine sand. See what micrometeorites are.
How many fall each year?
The best recent measurement suggests about 5,200 tonnes of micrometeorites reach the ground each year, with an uncertainty of roughly a quarter. More enters the top of the atmosphere, but much of it is vaporised. See how much falls.
Are they the same as shooting stars?
They are related. The bright meteors you see are made by grains about a millimetre to a centimetre across, most of which burn up completely. Micrometeoroids are smaller, make meteors too faint to see, and are more likely to survive to the ground.
Can I find one in my garden?
They land in gardens, but soil is full of other particles and constantly mixed, so finding one there is very hard. Roof sediment collected at the foot of a downpipe is much better, because the roof acts as a large, clean collecting surface. See search for them yourself.
Do I need to go on a roof?
No, and you should not. Rain brings roof sediment down the downpipes to ground level. Collect it there, with the owner's permission, wearing gloves.
Do all micrometeorites stick to a magnet?
Most cosmic spherules are attracted to a strong magnet because they contain magnetite or metal, though some glassy ones are only weakly magnetic. Using a magnet means you will miss some, but it removes most of the non-magnetic Earth material and makes the search manageable.
How can I be sure I have found one?
Only by measuring its chemistry, usually with a scanning electron microscope. Under an optical microscope, features such as a barred surface, a fine magnetite network and a metal bead are encouraging, but industrial spheres can look similar. See recognising a micrometeorite.
What are the round magnetic balls in my gutter?
Most are industrial: sparks from welding and grinding, fly ash from burning coal and oil, and dust from brakes, wheels and railways. A small number may be cosmic spherules.
Where do micrometeorites come from?
Mostly from dust shed by comets, particularly comets whose orbits are controlled by Jupiter, and from asteroid collisions. See where they come from.
How old are they?
The material in most micrometeorites formed about 4.6 billion years ago, with the Solar System. Some contain grains made in other stars even earlier. The time they have spent on Earth ranges from days, for fresh rooftop particles, to about 2.7 billion years for the oldest fossil ones.
Can micrometeorites come from Mars or the Moon?
In principle yes, because impacts throw material off both, but such particles would be a tiny share of the total and none has yet been firmly identified.
Do micrometeorites contain the building blocks of life?
Many contain carbon-rich organic matter, and some contain amino acids. They may have helped deliver such material to the early Earth. See water and organics.
Can I sell a micrometeorite?
There are no general laws against owning micrometeorites collected legally with permission, but they have little commercial value, and confirming them is difficult. Their value is scientific and personal. Material from Antarctica may not be collected privately at all.
Do micrometeorites fall on other planets?
Yes. They rain onto the Moon, where they strike at full speed and slowly churn the soil, and onto Mars, where the thin air lets many survive unmelted. Every planet and moon in the Solar System is sprinkled with space dust.
Is space dust making the Earth heavier?
Very slightly. Thousands of tonnes a year is tiny compared with the Earth's mass of about six million billion billion kilograms. The Earth also loses some gas to space, and the two effects are of a broadly similar order, so the overall change is negligible.
Is a micrometeorite the same as stardust?
Not quite. Stardust in the strict sense is grains made in other stars before the Sun formed, called presolar grains. Some micrometeorites contain a few of them, but most of their material formed in our own Solar System.
Why are scientists so interested in them?
Because they sample many more comets and asteroids than meteorites do, including fragile, carbon-rich bodies, and because they record the dust supply and even the atmosphere of the ancient Earth. See what they reveal.
Can children search for micrometeorites?
Yes, with an adult in charge. Children can wash, sieve and look through the microscope. They should wear gloves when handling sediment, wash their hands afterwards, and never taste samples, heat them, use chemicals or climb to collect them.