MARK GARLICK / SCIENCE PHOTO LIBRARY MARK GARLICK / SCIENCE PHOTO LIBRARY
Comet. Artwork of a comet passing behind the Earth and Moon. Comets are masses of ice, dust and rocky material. They tend to have long elliptical orbits around the Sun. When they near the Sun, the heat causes some of the ice to vaporise, forming a huge cloud (coma) around the comet body. Pressure from the solar wind causes a tail to form, which may be millions of kilometres long. Most comets have two tails. The ion tail (blue) comprises ionised gas carried away by the magnetic field of the solar wind. The dust tail (yellow) comprises dust blown away from the comet by radiation pressure from the Sun. Both tails always point away from the Sun.
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