Brownlee, Donald E., II Department of Astronomy, University of Washington, Seattle, Washington.
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A submillimeter extraterrestrial particle that has survived entry into the atmosphere without melting. Meteoroids are natural interplanetary objects that orbit the Sun, and they range in size from small dust grains to objects that are miles (kilometers) in diameter. Particles below 0.04 in. (1 mm) in diameter are considered micrometeoroids, and the micrometeoroids that enter the atmosphere without melting are called micrometeorites. Meteoroids of all sizes enter the atmosphere with velocities in excess of the Earth's escape velocity of 7.0 mi/s (11.2 km/s), and all but the smallest ones are heated sufficiently by air friction to produce at least partial melting. Micrometeorites survive entry without severe heating because they are small and they totally decelerate from cosmic velocity at high altitudes near 55 mi (90 km). In the thin air at such altitudes the power generated by frictional heating is low enough to be radiated away without a particle reaching its melting point, typically about 2400°F (1300°C) for common meteoritic samples. Larger objects penetrate deeper into the atmosphere before slowing down, and are melted and partially vaporized by friction with the comparatively dense air. Most of the mass of extraterrestrial matter that annually collides with the Earth is in the micrometeoroid size range, a total of about 104 tons (107 kg), but only a small fraction survives as micrometeorites. Usually only the particles smaller than 0.1 mm survive as true unmelted micrometeorites, although the survival of an individual micrometeorite depends on entry velocity, angle of entry, melting point, and density as well as size. The flux of micrometeorites falling onto the Earth's surface is approximately 1 per square meter per day (0.1 per square foot per day) for particles with diameters of at least 10 micrometers and approximately 1 per square meter per year (0.1 per square foot per year) for particles with diameters of at least 100 μm. See also: Meteorite
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