Basic Features

Many comets do not develop tails; moreover, comets are not surrounded by nebulosity during most of their lifetime. The only permanent feature of a comet is its nucleus, which is a small body that may be seen as a stellar image in large telescopes when tail and nebulosity do not exist, particularly when the comet is still far away from the Sun. Two characteristics differentiate the cometary nucleus from a very small asteroid—namely, its orbit and its chemical nature. A comet's orbit is more eccentric; therefore, its distance to the Sun varies considerably. Its material is more volatile. When far from the Sun, however, a comet remains in its pristine state for eons without losing any volatile components because of the deep cold of space. For this reason, astronomers believe that pristine cometary nuclei may represent the oldest and best-preserved material in the solar system.

During a close passage near the Sun, the nucleus of a comet loses water vapour and other more volatile compounds, as well as dust dragged away by the sublimating gases. It is then surrounded by a transient dusty "atmosphere" that is steadily lost to space. This feature is the coma, which gives a comet its nebulous appearance. The nucleus surrounded by the coma makes up the head of the comet. When it is even closer to the Sun, solar radiation usually blows the dust of the coma away from the head and produces a dust tail, which is often rather wide, featureless, and yellowish. The solar wind, on the other hand, drags ionized gas away in a slightly different direction and produces a plasma tail, which is usually narrow with nods and twists and has a bluish appearance.

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