A comet is a small astronomical body, typically a few kilometers across, that orbits the Sun. Comets contain icy chunks and frozen gases with bits of embedded rock and dust, and possibly a rocky core, aptly described as a "dirty snowball." Comets are leftovers from the formation of the Solar System and are believed to exist in vast numbers in the Oort Cloud and, to a lesser extent, in the Kuiper Belt. From these regions they can be perturbed by the gravitational influence of passing stars or interstellar clouds and thrown into new, highly elliptical orbits (see ellipse) that bring them into the inner Solar System. (Encyclopedia of Science, by David Darling, https://www.daviddarling.info/encyclopedia/C/comet.html )
Halley's Comet or Comet Halley, officially designated 1P/Halley, is a short-period comet visible from Earth every 75–79 years. Halley is the only known short-period comet that is regularly visible to the naked eye from Earth, and thus the only naked-eye comet that can appear twice in a human lifetime.] Halley last appeared in the inner parts of the Solar System in 1986 and will next appear in mid-2061. Halley's periodic returns to the inner Solar System have been observed and recorded by astronomers around the world since at least 240 BC. But it was not until 1705 that the English astronomer Edmond Halley understood that these appearances were reappearances of the same comet. As a result of this discovery, the comet is named after Halley. (Wikipedia, The Free Encyclopedia, https://en.wikipedia.org/wiki/Halley%27s_Comet )
A long-period comet is a comet with a period of more than 200 years and as much as several million years. Long-period comets, together with Halley-type comets, are now believed to come from the Oort Cloud, an enormous reservoir of frozen cometary nuclei orbiting the Sun at a distance of tens of thousands of astronomical units. The gravitational influences of passing stars, giant molecular clouds, and the central regions of the Galaxy, are thought to be instrumental in occasionally perturbing the orbits of some of the Oort objects and causing them to plunge toward the inner solar system on highly elliptical paths characteristic of long-period comets. (Encyclopedia of Science, by David Darling, https://www.daviddarling.info/encyclopedia/L/longperiod.html )
A short-period comet is a comet with a period of less than 200 years. Short-period comets are now subdivided into Jupiter-type comets, such as comets Encke and Tempel 2, which have periods of less than 20 years; and intermediate-period or Halley-type comets, with periods between 20 and 200 years. Jupiter-type comets are believed to originate in the Kuiper belt, which surrounds the Sun at distances between about 30 astronomical units (the distance of Neptune) and 50 astronomical units (7.5 billion km, 4.7 billion miles). The gravitational influence of the outer planets, Neptune and Uranus, is thought occasionally to perturb some of the Kuiper belt objects causing them to take up the orbits characteristic of comets in the Jupiter family. Halley-type comets, together with long-period comets, however, appear to come from the Oort cloud. (Encyclopedia of Science, by David Darling, https://www.daviddarling.info/encyclopedia/S/shortperiod.html )
The solid, centrally located part of a comet. The nucleus is a mass of dust and frozen gases. When heated by the Sun, the gases sublimate and produce an atmosphere surrounding the nucleus known as the coma, which is later swept into an elongated tail. Reliable measurements of cometary nuclei indicate sizes from a few km to 10 or 20 km. The nucleus of Comet Hale-Bopp is one of the largest (perhaps 40 km). The composition of the nucleus is determined by measuring the composition of the coma (except for 67P/Churyumov-Gerasimenko). The dominant volatile is water, followed by carbon dioxide (CO₂), carbon monoxide (CO), formaldehyde (H₂CO), methanol (CH₃OH), methane (CH₄) at a few percent level (with respect to water) and many other molecules at a lower level. (An Etymological Dictionary of Astronomy and Astrophysics, by M. Heydari-Malayeri, https://dictionary.obspm.fr/index.php?showAll=1&formSearchTextfield=comet+nucleus )
A comet tail—and coma—are features visible in comets when they are illuminated by the Sun and may become visible from Earth when a comet passes through the inner Solar System. As a comet approaches the inner Solar System, solar radiation causes the volatile materials within the comet to vaporize and stream out of the nucleus, carrying dust away with them. Separate tails are formed of dust and gases, becoming visible through different phenomena; the dust reflects sunlight directly and the gases glow from ionisation. Most comets are too faint to be visible without the aid of a telescope, but a few each decade become bright enough to be visible to the naked eye. (Wikipedia, The Free Encyclopedia, https://en.wikipedia.org/wiki/Comet_tail )
The solid, centrally located part of a comet. The nucleus is a mass of dust and frozen gases. When heated by the Sun, the gases sublimate and produce an atmosphere surrounding the nucleus known as the coma, which is later swept into an elongated tail. Reliable measurements of cometary nuclei indicate sizes from a few km to 10 or 20 km. The nucleus of Comet Hale-Bopp is one of the largest (perhaps 40 km). The composition of the nucleus is determined by measuring the composition of the coma (except for 67P/Churyumov-Gerasimenko). The dominant volatile is water, followed by carbon dioxide, carbon monoxide, formaldehyde, methanol, methane at a few percent level (with respect to water) and many other molecules at a lower level.
The most famous comet orbiting the Sun once about every 75 years. The last time it appeared was in 1986, and it is predicted to return in 2061. Its earliest recorded sighting is traced back to 240 BC in China. In 1705 Edmond Halley used Newton's new theory of gravitation to determine the orbits of comets from their recorded positions in the sky as a function of time. He found that the bright comets of 1531, 1607, and 1682 had almost the same orbits. He concluded that these appearances must belong to a single recurring comet, and predicted its return for 1758. Halley's comet is the first known periodic comet, hence its designation 1P/Halley.
The most famous comet orbiting the Sun once about every 75 years. The last time it appeared was in 1986, and it is predicted to return in 2061. Its earliest recorded sighting is traced back to 240 BC in China. In 1705 Edmond Halley used Newton's new theory of gravitation to determine the orbits of comets from their recorded positions in the sky as a function of time. He found that the bright comets of 1531, 1607, and 1682 had almost the same orbits. He concluded that these appearances must belong to a single recurring comet, and predicted its return for 1758. Halley's comet is the first known periodic comet, hence its designation 1P/Halley.
The solid, centrally located part of a comet. The nucleus is a mass of dust and frozen gases. When heated by the Sun, the gases sublimate and produce an atmosphere surrounding the nucleus known as the coma, which is later swept into an elongated tail. Reliable measurements of cometary nuclei indicate sizes from a few km to 10 or 20 km. The nucleus of Comet Hale-Bopp is one of the largest (perhaps 40 km). The composition of the nucleus is determined by measuring the composition of the coma (except for 67P/Churyumov-Gerasimenko). The dominant volatile is water, followed by carbon dioxide, carbon monoxide, formaldehyde, methanol, methane at a few percent level (with respect to water) and many other molecules at a lower level.