Micrometeorite

(astronomy) A very small meteorite or meteoritic particle with a diameter generally less than a millimeter.

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 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. 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). 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. See also Meteorite.

Micrometeorites are of particular interest because they are samples of comets and asteroids, small primitive bodies that have survived without major change since the earliest history of the solar system. Some of these particles are generated by collisions in the asteroid belt, while others are released from comets when these bodies approach the Sun and ice volatilization releases dust grains and propels them into space. Once released from a parent comet or asteroid, particles survive only for a few thousand to a hundred thousand years, depending on size, before they are either destroyed or collide with a planet. Particles are destroyed either when they collide with other particles or when they spiral into the Sun because of the Poynting-Robertson drag, an effect of sunlight that causes the orbits of small particles to decay. During exposure in space, the small particles accumulate large amounts of helium implanted by the solar wind, and they also are riddled with radiation damage tracks produced by solar cosmic rays, high-energy particles accelerated from solar flares. See also Asteroid; Comet; Cosmic rays; Solar wind; Sun.

The collection and laboratory analysis of micrometeorites provide an important source of information on the nature of materials in comets and asteroids. Most micrometeorites are collected in the stratosphere with aircraft such as the U2, which is capable of flying at an altitude of 12 mi (20 km) where terrestrial particles as large as 10 μm are rare. Micrometeorites are collected from the stratosphere by direct impact onto sticky plates that are extended from aircraft wings into the ambient airstream. After a cumulative exposure of many hours, the plates are returned to a clean room where the microscopic particles are picked off with needles and placed onto mounts where they can be studied by electron microscopes, mass spectrometers, and other instruments. The collection of micrometeorites in the stratosphere is usually limited to the size range from 2 to 100 μm in diameter. Most particles larger than this limit melt to form cosmic spherules during atmospheric entry and are not true micrometeorites. See also Cosmic spherules.