Comètes

Comètes are icy, dust-rich small bodies that orbit the Sun, typically in the outer Solar System. They originate from reservoirs such as the Kuiper Belt and the Oort Cloud and become visible when their ices sublimate as they approach the Sun.

Most comètes have three primary parts: a nucleus, a surrounding coma, and one or more tails. The nucleus, from a few hundred meters to tens of kilometers across, contains ice and rocky material. When warmed by the Sun, volatile ices sublimate, releasing gas and dust to form the coma, which can reach large sizes. Radiation from the Sun and the solar wind push dust away to form a dust tail; ionized gases form an ion tail that often points directly away from the Sun.

Orbits are typically highly elongated. Short-period comets complete orbits in under 200 years and are commonly linked to the Kuiper Belt; long-period comets originate in the distant Oort Cloud and may have periods measured in thousands or millions of years.

Comets have been observed since ancient times and have been visited or sampled by spacecraft, including Rosetta at 67P/Churyumov–Gerasimenko, Deep Impact at Tempel 1, and Stardust which returned dust from a cometary coma. Analysis of comets provides information about the early Solar System, including the distribution of water ice and organic compounds.

While not a major hazard, comets are studied for their scientific value and potential insights into volatile delivery to planets during formation.