A comet is a small Solar System body composed primarily of volatile ices, dust and rocky material that travels around the Sun on an orbit that can produce a visible atmosphere and tail when the object becomes sufficiently heated. The solid central body is called the nucleus. As a comet approaches the Sun, solar heating causes some surface and near surface ices to sublimate, releasing gas and entrained dust.
The escaping material forms a surrounding coma, and interactions with sunlight and the solar wind can produce tails extending away from the Sun. The word comet comes from the Greek word for long haired, reflecting the appearance of the coma and tail. Comets are scientifically important because many preserve relatively primitive materials from the early Solar System, especially volatile compounds that could not survive in the same form in the warm inner regions where the terrestrial planets formed. Their activity provides a natural demonstration of how solar radiation transforms small icy bodies.
A comet's nucleus is typically an irregular, dark body composed of a mixture of frozen volatile substances, dust, minerals and organic compounds. Water ice is a major constituent of many comets, while carbon dioxide, carbon monoxide and other volatile compounds can also be present. The exact composition varies between comets, and different regions of a single nucleus can have different physical properties. The nucleus is generally only a few kilometres across for many known comets, although sizes vary considerably.
Despite their icy composition, comet nuclei are often extremely dark because their surfaces contain carbon rich material and complex organic residues. When inactive and distant from the Sun, a comet can resemble an asteroid in appearance. The distinction becomes much clearer when solar heating activates volatile sublimation. Comet nuclei can contain layered or structurally complex material and can be highly porous, with low bulk densities compared with many solid rocks.
As a comet approaches the Sun, sublimation transforms the relatively inactive nucleus into an active cometary system. Sublimation occurs when solid material transitions directly into gas, allowing molecules to escape from the surface or subsurface. The released gas carries dust grains into space and creates the coma, a diffuse cloud that can become tens of thousands or even hundreds of thousands of kilometres across. The coma and nucleus together form the central active region of the comet.
The solar wind and solar radiation then interact with the escaping material. Ionised gas is influenced strongly by the solar magnetic field carried outward by the solar wind, producing an ion tail that generally points away from the Sun. Dust particles are affected by radiation pressure and their orbital motion, creating a dust tail that can appear curved. The visible tail therefore does not simply indicate the direction in which the comet is travelling.
Cometary orbits vary greatly. Short period comets generally complete an orbit in less than about two hundred years and often originate from the trans Neptunian region, particularly populations associated with the Kuiper Belt and scattered disc. Halley's Comet is the classic example of a short period comet, returning to the inner Solar System roughly every seventy five to seventy six years.
Long period comets can have orbital periods of thousands or millions of years and are thought to originate predominantly from the distant Oort Cloud. Some comets have highly elongated elliptical orbits that carry them from the outer Solar System into the inner planetary region. Gravitational interactions with planets, particularly Jupiter, can substantially alter cometary trajectories. A comet can be redirected into a shorter period orbit, ejected from the Solar System or placed on a trajectory that brings it closer to the Sun. Orbital evolution therefore strongly affects comet populations over astronomical timescales.
Comets have played an important role in the scientific study of the early Solar System. Because they formed in cold regions where volatile substances could remain frozen, they may preserve relatively primitive chemical material. Spacecraft missions have examined several comets directly, including missions to Halley's Comet, Comet Borrelly, Comet Wild 2, Comet Tempel 1 and Comet 67P Churyumov Gerasimenko.
The Rosetta mission provided especially detailed observations of Comet 67P, revealing cliffs, pits, jets, layered terrain and a highly irregular bilobed nucleus. Samples of cometary dust collected during the Stardust mission contained a mixture of primitive and high temperature processed materials, demonstrating that material in the early Solar System was transported over substantial distances. Comets therefore provide evidence that the early Solar System was dynamically and chemically interconnected rather than perfectly divided into isolated zones.
The role of comets in delivering water and organic compounds to planets has been extensively studied, particularly in relation to Earth's early history. Impacts by icy bodies could deliver water and carbon containing material, but the proportion of Earth's water attributable to comets remains an active scientific question.
Measurements of isotopic ratios, especially the deuterium to hydrogen ratio in cometary water, have shown that many measured comets do not match Earth's ocean water closely enough to account for all terrestrial water by themselves. Asteroids and other primitive bodies may also have contributed substantially. Comets nevertheless contain a rich inventory of organic molecules, and their impacts could have supplied chemically interesting material to the young Earth. This does not demonstrate that comets created life, because the origin of life involves many additional chemical and environmental conditions. It does show that small Solar System bodies can transport chemically important material between different planetary environments.
Cometary activity changes strongly with distance from the Sun. Different volatile substances become active at different temperatures and distances, so a comet may show weak or negligible activity far from the Sun and increasingly vigorous jets and outgassing as it approaches perihelion. Seasonal effects can occur if the nucleus has an irregular shape and a tilted rotation axis. Dust and gas loss gradually changes the mass and surface structure of the nucleus.
Repeated passages can deplete near surface volatiles and leave behind an insulating dust mantle, potentially reducing activity. In other cases, sudden fragmentation or outbursts can release large amounts of material. Some comets eventually break apart, collide with the Sun or a planet, or lose sufficient volatile material to become relatively inactive. The boundary between an extinct comet and an asteroid can therefore be physically complicated, especially for objects that retain an asteroid like orbit but contain a cometary history.
Comets are also responsible for several observable phenomena on Earth and in the night sky. When Earth passes through streams of dust left along a comet's orbit, the particles can enter Earth's atmosphere and produce meteor showers. The Perseid meteor shower is associated with debris from Comet Swift Tuttle, while the Leonids are associated with Comet Tempel Tuttle. These particles are usually tiny, and the resulting meteors are atmospheric flashes rather than intact pieces of the comet reaching the ground.
Comets can therefore affect Earth without directly striking it. Historically, bright comets were often interpreted as omens because their sudden appearance was unusual to observers. Modern astronomy instead explains their behaviour through orbital mechanics, sublimation, dust release, radiation pressure and solar wind interactions. Their dramatic appearance remains a visible connection between distant Solar System dynamics and phenomena observed from Earth.
