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Micrometeoroid collected from the antarctic snow

A Micrometeoroid (also micrometeorite, micrometeor) is a tiny meteoroid; a small particle of rock in space, usually weighing less than a gram.

See also: Cosmic dust

Micrometeoroids are very small, typically metallic, pieces of rock broken off from larger chunks of rock and debris, often dating back to the formation of the solar system. Micrometeoroids are extremely common in space, particularly near the Earth. These tiny particles are a major contributor to space weathering processes. When they impact the surface of the Moon, or any airless body (Mercury, the asteroids, etc), the resulting melting and vaporization causes darkening and other optical changes in the regolith. In order to understand the micrometeoroid population better, a number of spacecraft (including Lunar Orbiter 1, Luna 3 and Mars 1) have carried micrometeoroid detectors.

While meteoroids tend to remain in stable orbits, micrometeoroids are more likely to fall to Earth, and can provide information on millimeter scale heating events in the solar nebula. Micrometeorites (as they are known upon arrival at the Earth's surface) can only be collected in areas where there is no terrestrial sedimentation, typically polar regions. Ice is collected and then melted and filtered so the micrometeorites can be extracted under a microscope.

See also: Space debris

Micrometeoroids pose a significant threat to space exploration. Their velocities relative to a spacecraft in orbit can be on the order of kilometers per second, and resistance to micrometeoroid impact is a significant design challenge for spacecraft and space suit designers (See Thermal Micrometeoroid Garment). While the tiny sizes of most micrometeoroids limits the damage incurred, the high velocity impacts will constantly degrade the outer casing of spacecraft in a manner analogous to sandblasting. Long term exposure can threaten the functionality of spacecraft systems.

Impacts by small objects with extremely high velocity are a current area of research in terminal ballistics. Accelerating objects up to such velocities is difficult; current techniques include linear motors and shaped charges. The risk is especially high for objects in space for long periods of time, such as satellites. They also pose major engineering challenges in theoretical low-cost lift systems such as rotovators, space elevators, and orbital airships.