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A comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process called outgassing.
This produces a visible atmosphere or coma, and sometimes also a tail. These phenomena are due to the effects of solar radiation and the solar wind acting upon the nucleus of the comet.
Comet nuclei range from a few hundred meters to tens of kilometers across and are composed of loose collections of ice, dust, and small rocky particles.
The coma may be up to 15 times Earth's diameter, while the tail may stretch beyond one astronomical unit.
