comets

Comets are small Solar System bodies composed largely of volatile ices, such as water, carbon dioxide, and methane, mixed with dust and rocky material. When a comet approaches the Sun, solar heating causes these ices to sublimate, releasing gas and entrained dust that form a surrounding atmosphere called a coma. The solar wind and radiation pressure push material away, producing one or more tails that point away from the Sun. The nucleus, typically a few kilometers across, remains the solid core.

Most comets originate in the outer Solar System. Short-period comets, with orbital periods under about 200 years, come mainly from the Kuiper Belt just beyond Neptune. Long-period comets, with orbits spanning thousands to millions of years, come from the distant Oort Cloud, a roughly spherical reservoir surrounding the Solar System.

Observations of comets date from ancient times, and their appearances have influenced history and culture. The best-known periodic comet is Halley’s Comet, with an orbit of about 76 years. Notable modern comets include Hale-Bopp and, in 1994, the fragments of Comet Shoemaker-Levy 9 that collided with Jupiter, illustrating the dynamical interactions of comets with planets.

Comets are active at varying distances from the Sun, but activity increases as they near perihelion, when ices sublimate and release material. The coma can be tens of thousands of kilometers across, and the tails can extend millions of kilometers. Ion tails tend to be straight and point directly away from the Sun, while dust tails are broader and often curved by orbital motion.

Scientific significance: studying comets provides insights into the materials present in the early Solar System, including water ice and organic compounds. They are targets for spacecraft missions and are considered potential contributors to Earth's oceans and prebiotic chemistry.