COMET CLASSIFICATIONS

Hyperbolic and parabolic comets Many parabolic and hyperbolic comets may come from the Oort cloud, or have interstellar origin. The Oort Cloud is not gravitationally attracted enough to the Sun to form into a fairly thin disk, so comets originating from the Oort Cloud can come from roughly any orientation (inclination to the ecliptic), and many even have a retrograde orbit. A hyperbolic orbit means that the comet will only travel through the Solar System once, with the Sun acting as a gravitational slingshot, sending the comet out of the Solar System entirely unless its eccentricity is otherwise changed. Typically comets in the Oort Cloud are thought to have roughly circular orbits around the Sun, but their orbital velocity is so slow that they may easily be perturbed by passing stars and the galactic tide. The JPL Small-Body Database and the Minor Planet Center list comet orbits as having an assumed eccentricity of 1.0. (This is the eccentricity of a parabolic trajectory; hyperbolics will be those with eccentricity greater than 1.0.) The Kreutz sungrazers originate from the progenitor of the Great Comet of 1106. Although officially given an assumed eccentricity of 1.0, they have an orbital period of roughly 750 years and inclination of 144 degrees. Many of the Kreutz sungrazers do not survive perihelion, as their average perihelion distance is 0.0050 AU, and the radius of the Sun is 0.0046 AU; i.e. they pass 50,000 km above the surface of the Sun. The group of comets descended from the breakup of a comet in 326 AD. Many bright comets are members of the group, including Comet Ikeya–Seki, which broke in 3 pieces. Three other sungrazing groups, the Meyer, Marsden, and Kracht groups, have respectively a perihelion distance of 0.035, 0.044, and 0.049 AU, an inclination of 72, 13, and 26 degrees, and a period of at least a decade, 5.6, and 3–4 years. The Meyer sungrazing group has no discerned period. The Kracht and Marsden groups are periodic, both with periods of approximately 3 years. The Kracht group contains fewer members than the Meyer and Kreutz groups, probably as a result of their periodic nature, leading them to burn up more frequently. They are believed to be the parent bodies of the Southern Delta Aquariids meteor shower, occurring between July and August and usually having 15–20 meteors an hour. Interstellar objects generally have strongly hyperbolic orbits, for example the first known object of this class 1I/2017 U1 ʻOumuamua has an eccentricity of 1.192. Near-parabolic comets Near-parabolic comets have a very high eccentricity (generally 0.99 or higher) and a period of over 1,000 years that do not quite have a high enough velocity to escape the Solar System. Often, these comets, due to their extreme semimajor axes and eccentricity, will have small orbital interactions with planets and minor planets, most often ending up with the comets fluctuating significantly in their orbital path. These comets probably come from the Oort cloud, a cloud of comets orbiting the Sun from ~10,000 to roughly 50,000 AU. Long-period comets These comets have orbital periods between 200 and 1000 years. These comets come from the Kuiper belt and scattered disk, beyond the orbit of Pluto, with possible origins in the Oort cloud for many. Periodic comets Periodic comets (also known as short-period comets) are comets with orbital periods of less than 200 years or that have been observed during more than a single perihelion passage (e.g. 153P/Ikeya–Zhang). "Periodic comet" is also sometimes used to mean any comet with a periodic orbit, even if greater than 200 years. Periodic comets receive a permanent number prefix usually after the second perihelion passage, which is why there are a number of unnumbered periodic comets, such as P/2005 T5 (Broughton). Comets that are not observed after a number of perihelion passages, or presumed to be destroyed, are given the D designation, and likewise comets given a periodic number and subsequently lost are given D instead of P, such as 3D/Biela or 5D/Brorsen. In nearly all cases, comets are named after their discoverer(s), but in a few cases such as 2P/Encke and 27P/Crommelin they were named for a person who calculated their orbits. The long-term orbits of comets are difficult to calculate because of errors in the known trajectory that accumulate with perturbations from the planets. Several comets were lost because their orbits are also affected by non- gravitational effects such as the release of gas and other material that forms the comet's coma and tail. Unlike a long-period comet, the next perihelion passage of a numbered periodic comet can be predicted. Periodic comets sometimes bear the same name repeatedly (e.g. the nine Shoemaker–Levy comets or the twenty-four NEAT comets); the IAU system distinguishes between them either through the number prefix or by the full designation (e. g. 181P and 192P/Shoemaker–Levy are both "Comet Shoemaker–Levy"). In the literature, an informal numbering system is applied to periodic comets (skipping the non-periodic ones), thus 181P and 192P are known as Comet Shoemaker–Levy 6 and Comet Shoemaker–Levy 1, respectively. Non- periodic Shoemaker–Levy comets are interleaved in this sequence: C/1991 B1 between 2 and 3, C/1991 T2 between 5 and 6, C/1993 K1 and C/1994 E2 after Shoemaker–Levy 9. In comet nomenclature, the letter before the "/" is either "C" (a non-periodic comet), "P" (a periodic comet), "D" (a comet that has been lost or has disintegrated), "X" (a comet for which no reliable orbit could be calculated - usually historical comets), "I" for an interstellar object, or "A" for an object that was either mistakenly identified as a comet, but is actually a minor planet, or for an object on a hyperbolic orbit that does not show cometary activity. Some lists retain the "C" prefix for comets of periods larger than about 30 years until their return is confirmed Numbered comets These are periodic comets that are numbered by the Minor Planet Center after having been observed on at least two occasions. Their orbital periods vary from 3.2 to 366 years. Most of the numbered comets are members of the Jupiter-family. There are also Encke-type comets, Halley-type comets, Chiron-type comets, and a long-period comet (i.e. 153P). About a third of these bodies are also near-Earth comets. In addition, some are principally classified as minor planets and display characteristics of both an asteroid and a comet. Occasionally, comets will break up into multiple chunks, as volatiles coming off the comet and rotational forces may cause it to break into two or more pieces. An extreme example of this is 73P/Schwassmann–Wachmann, which broke into over 50 pieces during its 1995 perihelion. Halley-type comets These are comets with a period between 20 and 200 years, named after the first identified member, Halley's Comet. They orbit between the orbit of Jupiter and Pluto, and are thought to be long-period comets that slowly migrated inwards, or Jupiter family comets that had been slingshotted outwards by Jupiter's gravity Jupiter-family comets While Jupiter-family comets are officially defined by (2< TJupiter <3), they can also be loosely defined by any comet with a period of less than 20 years, a relatively low inclination, and an orbit coinciding loosely with that of Jupiter's. These comets are often patchily observed, as orbital interactions with the planet often cause comets' orbits to become perturbed, causing them to not be found at the expected position in the sky and subsequently lost.