micrometeoroid, a small grain, generally less than a few hundred micrometres in size and composed of silicate minerals and glassy nodules but sometimes including sulfides, metals, other minerals, and carbonaceous material, in orbit around the Sun. The existence of micrometeoroids was first deduced from observations of zodiacal light, a glowing band visible in the night sky that comprises sunlight scattered by the dust. Spacecraft have detected these particles as far out in space as past the heliopause, which indicates that the entire solar system is immersed in a disk of dust, centred on the ecliptic plane.
Every object in the solar system can produce dust by outgassing, cratering, volcanism, or other processes. Most micrometeoroids are believed to come from the surface erosion and collisions of asteroids and from comets, which give off gas and dust when they travel near the Sun.
The orbits of micrometeoroids are easily altered by interaction with the light and charged particles (solar wind) that emanate from the Sun. The smallest particles, less than 0.5 micrometre (μm; 0.00002 inch) in size, are blown out of the solar system. Drag effects from sunlight and the solar wind cause larger particles to spiral toward the Sun, some on paths that intercept planets or their moons.
Because of their high speed (in the tens of kilometres per second), micrometeoroids as small as a few hundred micrometres in size pose a significant collision hazard to spacecraft and their payloads. An impact can, for example, puncture a vital component or create a transient cloud of ions that can short-circuit an electrical system. Consequently, protection against micrometeoroid impacts has become a necessary element of space hardware design. Components of the Earth-orbiting International Space Station use a “dust bumper,” or Whipple shield (named for its inventor, the American astronomer Fred Whipple), to guard against damage from micrometeoroids and orbiting debris. Spacesuits intended for extravehicular activity also incorporate micrometeoroid protection in their outer layers.
Analyses of micrometeorites found in Greenland ice indicate that about 6 tons of interplanetary dust, between the size of 50 μm and 700 μm (0.002–0.028 inch), strike Earth every day. (When found in Earth’s atmosphere or on its surface, micrometeoroids are often referred to as micrometeorites or cosmic dust particles.) Particles from space larger than a few hundred micrometres—i.e., meteoroids—are heated so severely during deceleration in the atmosphere that they vaporize, producing a glowing meteor trail. Smaller particles experience less-severe heating and survive, eventually settling to Earth’s surface.
Using high-altitude research aircraft, the U.S. National Aeronautics and Space Administration has collected cosmic dust particles directly from Earth’s stratosphere, where the concentration of terrestrial dust is low. Particles larger than 50 μm are relatively uncommon there, however, which makes their collection by aircraft impractical. These larger particles have been collected in sediment that has been filtered from large volumes of melted polar ice. Micrometeorites have even been collected from urban rooftops. Spacecraft missions have been developed to retrieve dust particles directly from space. The U.S. Stardust spacecraft, launched in 1999, flew past Comet Wild 2 in early 2004, collecting particles from its coma for return to Earth. In 2003 Japan’s space agency launched its Hayabusa spacecraft, which also returned small amounts of surface material, comprising fragments and dust, from the near-Earth asteroid Itokawa for laboratory analysis.
Some micrometeorites gathered from the stratosphere are the least-altered samples of early solar system dust that have been studied in the laboratory. They provide clues to the temperature, pressure, and chemical composition of the nebular cloud from which the solar system condensed 4.6 billion years ago. (See solar system: Origin of the solar system.) The continuous accretion of micrometeorites on early Earth may have contributed organic compounds that were important for the development of life. A few micrometeorites are thought to contain preserved interstellar grains—samples of matter from outside the solar system. (See interstellar medium.) Spacecraft sample-return missions to comets and asteroids have provided scientists on Earth the opportunity to study even better-preserved material from the birth of the solar system.