Comet

A comet is a small Solar System body that originates from the outer parts of the solar system and becomes visibly active as it approaches the Sun. When close enough to the Sun, the heat causes volatile ices to sublimate and release gas and dust, forming a surrounding atmosphere called the coma. The solar wind and radiation pressure then shape the ejected material into one or more tails that typically point away from the Sun; the ion tail consists of gas and is often bluish, while the dust tail is broader and can appear yellowish. Tails can extend millions of kilometers into space.

Nucleus: The solid core of a comet, typically 1 to 10 kilometers in diameter, is made of ices such as water, carbon dioxide, carbon monoxide, methane, and ammonia, mixed with dust and rocky material. The nucleus itself is usually dark and irregular in shape.

Origin and orbits: Most comets originate in the outer Solar System. Short-period comets, with orbital periods less than about 200 years, largely come from the Kuiper Belt, whereas long-period comets originate from the distant Oort Cloud. Some comets, including Halley-type and Jupiter-family comets, are dynamically influenced by the planets and have relatively comparatively sunward orbits.

Observation and study: Comets have been observed since antiquity and are of interest as repositories of primitive material from the early Solar System. Space missions such as Rosetta to comet 67P/Churyumov-Gerasimenko and Deep Impact have studied nuclei, outgassing, and coma composition, contributing to understanding of solar-system formation and evolution.