By Dan Fielder, Updated Mar 24, 2022
Often called a "dirty snowball," a comet is a blend of ice, gas, and dust that never coalesced into a planet or asteroid during the early formation of our solar system. With highly elliptical orbits, comets swing close to the Sun and then venture far beyond the outermost planets.
The nucleus, or core, is the solid heart of the comet. It is mostly ice and dust cloaked by a dark organic coating. While frozen water dominates, other volatile species—such as carbon dioxide, ammonia, carbon monoxide, and methane—are commonly present. Most cometary nuclei are under 16 km in diameter. As a comet approaches the Sun, solar heating vaporizes the ices, releasing gases that escape the nucleus.
The coma is a vast, spherical envelope of gas and dust that surrounds the nucleus. Together, the nucleus and coma form the comet’s visible head. The coma can span up to a million kilometres, consisting of material that has sublimated directly from the nucleus, skipping the liquid phase entirely.
According to Solarviews.com, as the comet absorbs ultraviolet light, chemical reactions release hydrogen, which escapes the comet’s gravity and forms a large hydrogen envelope. This cloud, invisible from Earth because our atmosphere absorbs its light, can extend for millions of kilometres and has been detected by spacecraft.
The dust tail is shaped by solar radiation pressure and the solar wind, pushing dust particles away from the coma. It typically points directly away from the Sun, bending slightly due to the comet’s motion. Although the acceleration is slow, the dust tail can stretch up to 10 million kilometres, fading as the comet moves farther from the Sun.
Solar wind particles ionize some cometary gases, creating an ion tail. Though less massive than the dust tail, ionized particles accelerate rapidly, producing a nearly straight tail that extends opposite the Sun. Ion tails can reach lengths exceeding 100 million kilometres.
