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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Comet | 3/10 | https://en.wikipedia.org/wiki/Comet | reference | science, encyclopedia | 2026-05-05T13:31:54.670973+00:00 | kb-cron |
=== Bow shock === As a comet approaches the Sun, UV light ionizes gases in the coma. When the solar wind interacts with these ionized particles, mainly OH and CO group molecules, the solar wind plasma is decelerated in a large region around the comet: a cometary-mode bow shock forms. The cometary bow shock resembles the classical bow shock observed when a planetary magnetic field, like that around Earth, Jupiter, Saturn, or Neptune, interacts with the solar wind. However comets have no magnetic field. Like Mars and Venus, comets are said to have an induced magnetosphere.
=== Tails ===
A comet may have a dust tail, an ion tail, both or neither. The dust and ion tails both begin when sunlight vaporizes material from the comet. The dust tail contains particle about the size of human hairs. Radiation pressure from the Sun pushes these dust particles away, resulting in a gentle arc shape. Larger particles, the size of pebbles, a pulled towards the Sun, staying on the orbit of the comet. The ion tail contains molecule sized particles ionized by the Sun then captured by the magnetic field created by the interaction between these ions and the solar wind. The ion tail points directly away from the Sun following magnetic field lines. On occasions—such as when Earth passes through a comet's orbital plane, the antitail, pointing in the opposite direction to the ion and dust tails, may be seen.
=== Jets ===
Uneven heating can cause newly generated gases to break out of a weak spot on the surface of comet's nucleus, like a geyser. These streams of gas and dust can cause the nucleus to spin, and even split apart. In 2010 it was revealed that sublimation of dry ice (frozen carbon dioxide) can power jets of material flowing out of a comet nucleus. Infrared imaging of Hartley 2 shows such jets exiting and carrying with it dust grains into the coma.
== Orbital characteristics == Most comets are small Solar System bodies with elongated elliptical orbits that take them close to the Sun for a part of their orbit and then out into the further reaches of the Solar System for the remainder. Technically, these objects have a perihelion, their nearest point to the Sun of less that 4 AU, that is controlled by the gravity of Jupiter and not controlled by the gravity of another planet near the perihelion. Comets can be subdivided according to their aphelion distance, how far away from the Sun they can orbit. The four subdivisions, E for Encke, SP for short-period, I for intermediate, and L for long-period, have names derived from an earlier classification based on the length of their orbital periods. Single-apparition or non-periodic comets have parabolic or hyperbolic trajectories. The low mass and elliptical orbits cause comet orbits to be altered by gravitational forces from giant planets and these single-apparition comets orbit the Sun one time then permanently exit the Solar System.
=== Encke-type comet === At the shorter orbital period extreme, Encke's Comet has an orbit that does not reach the orbit of Jupiter, and is known as an Encke-type comet. They have aphelion less than 4 AU. Short-period comets with orbital periods less than 20 years and low inclinations (up to 30 degrees) to the ecliptic are called traditional Jupiter-family comets (JFCs). Those like Halley, with orbital periods of between 20 and 200 years and inclinations extending from zero to more than 90 degrees, are called Halley-type comets (HTCs). As of July 2025 there are 74 known Encke-type comets (six of which are classified as near-Earth objects (NEOs)), 109 HTCs (36 of which are NEOs), and 815 JFCs (153 of which are NEOs). Recently discovered main-belt comets form a distinct class, orbiting in more circular orbits within the asteroid belt.
=== Short period ===