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. Smaller objects than this are classified as micrometeoroids or space dust. Most are fragments from comets or asteroids, while others are collision impact debris ejected from bodies such as the Moon or Mars.
When such an object enters the Earth's atmosphere at a speed typically in excess of 20 km/s, aerodynamic heating produces a streak of light, both from the glowing object and the trail of glowing particles that it leaves in its wake. At this point, the word meteor can be used to refer to the meteoroid, or to the streak of light caused by the meteoroid. Meteors are colloquially called "shooting stars" or "falling stars". A series of many meteors appearing seconds or minutes apart, and appearing to originate from the same fixed point in the sky, is called ameteor shower. Incoming objects larger than several meters (asteroids or comets) can explode in the air. If a meteoroid, comet or asteroid or a piece thereof withstands ablation from its atmospheric entry and impacts with the ground, then it is called a meteorite.
Around 15,000 tonnes of meteoroids, micrometeoroids and different forms of space dust enter Earth's atmosphere each year.
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