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A meteoroid is a small rocky or metallic body travelling through space. Meteoroids are significantly smaller than asteroids, and range in size from small grains to 1 meter-wide objects. Objects smaller than this are classified as micrometeoroids or space dust.〔 (【引用サイトリンク】publisher=Science Dictionary )〕〔〔 〕〔 (【引用サイトリンク】publisher=The free dictionary )〕 Most are fragments from comets or asteroids, whereas others are collision impact debris ejected from bodies such as the Moon or Mars.〔 (【引用サイトリンク】title=Meteoroid )〕〔 (【引用サイトリンク】title=Meteors & Meteorites )〕〔 〕〔 〕 When a meteoroid, comet or asteroid enters the Earth's atmosphere at a speed typically in excess of , aerodynamic heating of that object produces a streak of light, both from the glowing object and the trail of glowing particles that it leaves in its wake. This phenomenon is called a meteor or "shooting star". A series of many meteors appearing seconds or minutes apart, and appearing to originate from the same fixed point in the sky, is called a meteor shower. If that object withstands ablation from its passage through the atmosphere as a meteor and impacts with the ground, it is then called a meteorite. Around 15,000 tonnes of meteoroids, micrometeoroids and different forms of space dust enter Earth's atmosphere each year.〔 〕 == Meteoroids == In 1961, the International Astronomical Union defined a meteoroid as "a solid object moving in interplanetary space, of a size considerably smaller than an asteroid and considerably larger than an atom".〔〔 In 1995, Beech and Steel, writing in ''Quarterly Journal of the Royal Astronomical Society'', proposed a new definition where a meteoroid would be between 100 µm and 10 meters across.〔 Following the discovery of asteroids below 10 m in size, Rubin and Grossman refined the Beech and Steel definition of meteoroid to objects between 10 µm and 1 m in diameter.〔 According to Rubin and Grossman the minimum size of an asteroid is given by what can be discovered from Earth-bound telescopes, so the distinction between meteoroid and asteroid is fuzzy. The smallest asteroid ever discovered (based on absolute magnitude) is with an absolute magnitude of 33.2,〔 and an estimated size of 1-meter.〔 Objects smaller than meteoroids are classified as micrometeoroids and cosmic dust. The Minor Planet Center does not use the term "meteoroid". 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Meteoroid」の詳細全文を読む スポンサード リンク
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