An extremely small meteoroid, typically the size of a grain of dust. Particles measuring less than 0.05 mm (0.002 inch) in diameter are classed as micrometeoroids. Despite their small size, their high velocities mean that micrometeoroids pose a significant threat to spacecraft and artificial satellites. ◇ A micrometeoroid that reaches the surface of the Earth, the Moon, or another celestial body is called a micrometeorite. The tiny size of micrometeoroids allows them to radiate away the heat generated by friction with the atmosphere, with the result that the proportion of micrometeoroids becoming micrometeorites is greater than the proportion of meteoroids becoming meteorites.