Def. a relatively small (sand- to boulder-sized) fragment of debris in a solar system is called a meteoroid.
"As of 2011 the International Astronomical Union officially defines a meteoroid as a solid object moving in interplanetary space, of a size considerably smaller than an asteroid and considerably larger than an atom".
The visible path of a meteoroid that enters the Earth's atmosphere (or another body's) atmosphere is called a meteor, or colloquially a shooting star or falling star. If a meteoroid reaches the ground and survives impact, then it is called a meteorite.
Beech and Steel, writing in Quarterly Journal of the Royal Astronomical Society, proposed a new definition where a meteoroid is between 100 µm and 10 m across. Following the discovery and naming of asteroids below 10 m in size (e.g., 2008 TC3), Rubin and Grossman refined the Beech and Steel definition of meteoroid to objects between 10 µm and 1 m in diameter. The near-Earth object (NEO) definition includes larger objects, up to 50 m in diameter, in this category. Very small meteoroids are known as micrometeoroids (see also interplanetary dust).
The composition of meteoroids can be determined as they pass through Earth's atmosphere from their trajectories and the light spectra of the resulting meteor. Their effects on radio signals also give information, especially useful for daytime meteors which are otherwise very difficult to observe.
The light spectra, combined with trajectory and light curve measurements, have yielded various compositions and densities, ranging from fragile snowball-like objects with density about a quarter that of ice, to nickel-iron rich dense rocks.
In meteoroid ablation spheres from deep-sea sediments, "[t]he silicate spheres are the most dominant group."
From these trajectory measurements, meteoroids have been found to have many different orbits, some clustering in streams (see Meteor showers) often associated with a parent comet, others apparently sporadic. Debris from meteoroid streams may eventually be scattered into other orbits. ... Meteoroids travel around the Sun in a variety of orbits and at various velocities. The fastest ones move at about 26 miles per second (42 kilometers per second) through space in the vicinity of Earth's orbit. The Earth travels at about 18 miles per second (29 kilometers per second). Thus, when meteoroids meet the Earth's atmosphere head-on (which would only occur if the meteors were in a retrograde orbit), the combined speed may reach about 44 miles per second (71 kilometers per second). Meteoroids moving through the earth's orbital space average about 20 km/s.
- Peter M. Millman (1961). "A report on meteor terminology". JRASC 55: 265–267.
- Glossary International Meteor Organization. Imo.net. 2008-11-18. Retrieved 2011-09-16.
- Martin Beech, Duncan Steel (September 1995). "On the Definition of the Term Meteoroid". Quarterly Journal of the Royal Astronomical Society 36 (3): 281–284. )
- Rubin, A.E.; Grossman, J.N. (January 2010). "Meteorite and meteoroid: New comprehensive definitions". Meteoritics & Planetary Science 45 (1): 114–122. doi:10.1111/j.1945-5100.2009.01009.x. )
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- M.B. Blanchard, D.E. Brownlee, T.E. Bunch, P.W. Hodge, F.T. Kyte (January 1980). "Meteoroid ablation spheres from deep-sea sediments". Earth and Planetary Science Letters 46 (2): 178-90. doi:10.1016/0012-821X(80)90004-7. http://www.sciencedirect.com/science/article/pii/0012821X80900047. Retrieved 2012-01-02.
- Report on Orbital Debris. NASA. NASA Technical Reports Server. Retrieved 1 September 2012.