A comet is a small celestial body composed primarily of volatile ices mixed with dust and rock. Early descriptions called them "dirty snowballs" because they contain frozen gases and particulate material in a compact nucleus. Modern observations show a range of sizes, shapes and surface compositions. A concise description of their makeup is that comets are balls of ice and dust: composition overview containing water ice, frozen carbon compounds and silicate grains (ice and volatiles) embedded in a dark matrix, traveling through outer space.
Structure and visible features
The dense, solid core of a comet is called the nucleus. When a comet approaches the Sun, solar heating causes ices to sublimate and release gas and dust, forming a fuzzy atmosphere called the coma and often one or more tails. Tails are not fixed behind the nucleus: an ion (gas) tail points away from the Sun because it is shaped by the solar wind, while a dust tail is pushed by sunlight and follows a curved path. A comet's nucleus typically has a very low reflectivity; for example, observations of Halley's Comet showed a surface that reflected only a few percent of incident light. These differences distinguish comets from rocky asteroids (asteroids), which lack volatile-driven comae and bright tails.
Origins and orbits
Comets originate in two principal reservoirs at the fringes of the solar system. Short-period comets, with orbits measured in decades to centuries, are commonly associated with the Kuiper belt and scattered disc beyond Neptune (Kuiper belt). Long-period comets, which may take thousands or millions of years to return, are thought to arise from the distant, roughly spherical Oort cloud. Many comets have orbital inclinations that carry them far from the plane of the planets; their orbital inclinations are often high and not confined to the ecliptic plane (orbital inclinations).
Behavior, lifetimes and fragmentation
As comets repeatedly pass near the Sun they lose material and can become inactive or disintegrate. Some break apart under thermal stress, tidal forces or internal gas pressure. Historical examples include Comet Biela, which fragmented in the 19th century, and Comet Shoemaker–Levy 9, whose pieces impacted Jupiter in 1994 (Shoemaker–Levy 9). Groups of comets observed to follow similar orbits are often interpreted by astronomers as fragments from a single parent body (astronomers' studies).
Observation, study and examples
Comets have been important both culturally and scientifically. They were once regarded as omens but are now valuable probes of primordial material from the early solar system. Telescopic studies and spacecraft encounters have transformed our understanding: close flybys and missions such as Giotto and Rosetta provided in‑situ data and images of nuclei. Scientific summaries and mission pages describe these efforts (mission archives). The dark, low-reflectance surfaces of many nuclei relate to their albedo and the distribution of organic-rich material.
Why comets matter
Comets supply unique information about the composition and physical conditions of the early solar nebula because they have preserved volatile compounds that are otherwise rare in planetary materials. Studying cometary chemistry and dynamics informs models of planetary formation and delivery of water and organics to the inner solar system. Observational techniques range from ground-based telescopes to space missions, and amateur observers still play a role when bright comets visit the inner solar system.
