comet [Gr.,=longhaired], a small celestial body consisting mostly of dust and gases that moves in an elongated elliptical or nearly parabolic orbit around the sun. Comets visible from the earth can be seen for periods ranging from a few days to several months. They were long regarded with awe and even terror and were often taken as omens of unfavorable events.

The Orbits of Comets

Although the occurrence of many comets had been recorded, it was not until 1577 that the Danish astronomer Tycho Brahe suggested that they traveled in elongated rather than circular orbits. A century later Giovanni Borelli concluded that the orbits were parabolic and that comets passed through the solar system but once, never to return. In 1705, however, Edmond Halley concluded that the comet observed in 1682 was the same one that had been described in 1531 and 1607, and he predicted that it would return again in late 1758 or early 1759. The comet was sighted on Christmas Day in 1758, and it returned again in 1835, 1910, and 1986 (see Halley's comet ). While some comets appear to have parabolic orbits (see parabola ), others return to the inner solar system in highly elongated orbits with periods ranging from a hundred to thousands of years. Still others return at shorter intervals of less than 10 years and reach aphelion (the orbital point farthest from the sun) near the planet Jupiter; these have been captured into their smaller orbits by Jupiter's gravitational attraction.

Structure of Comets

A comet far from the sun consists of a dense solid body or conglomerate of bodies a few miles in diameter called the nucleus. As it approaches the sun the nucleus becomes enveloped by a luminous "cloud" of dust and gases called the coma; this luminosity is caused by the molecules absorbing and reflecting the radiation of the sun. According to the icy-conglomerate theory proposed by F. L. Whipple in 1949, the nucleus consists of frozen water and gases with particles of heavier substances interspersed throughout, thus being in effect a large, dirty snowball, although more recent research has suggested that comets may contain a higher proportion of dust and rock than previously proposed. The Stardust probe—passed near Comet Wild 2 in 2004, collected particles from the coma, and returned the samples to earth in 2006—found evidence that many of the dust particles were formed at high temperatures not found in the Oort cloud and Kuiper belt (see below), where comets are believed to have formed. Data from the Deep Impact mission, which sent a projectile crashing into Comet Tempel 1 in 2005, suggests that suggests that the interior structure of comets may consists of layers of accreted material. As the comet approaches the sun, the solar wind drives particles and gases from the near the surface of the nucleus and coma to form a tail which can extend as much as 100 million mi (160 million km) in length. Thus the tail always streams out in the direction opposite the sun; i.e., it follows the head as the comet approaches the sun and precedes it as the comet passes perihelion (its closest point to the sun) and moves away.

Near the sun a comet can change drastically in size and shape; it may even split into two or more pieces, as Comet Biela did in 1846, and Comet West did in 1976. The comas of comets vary widely in size, some being the size of the earth or larger. However, the nucleus, which makes up virtually all a comet's mass, is small; in 1986 the Giotto and Vega spacecrafts observed Comet Halley's nucleus to be only about 6 mi (10 km) in diameter. Comets lose material and thus brightness with successive passages near the sun. Some of this material moves around the comet's orbit as a stream of meteoroids (see meteor ); when the earth passes through this path, a meteor shower is observed.

In 1992 the periodic comet Shoemaker Levy 9 made an extremely close passage of Jupiter. The tidal stresses induced by the giant planet's gravity shattered the comet's nucleus, estimated to have been 5–9 km (3–5 mi) in diameter, into more than 20 major fragments, the largest of which was about 4 km (2.5 mi) in diameter. Two years later, the returning fragmented comet crashed into Jupiter; observations from both terrestrial observatories and artificial satellites such as the Hubble Space Telescope yielded vast amounts of information about the structure of comets and about Jupiter's atmosphere.

In 1996 the Polar satellite discovered a constant rain of small comets impacting the earth. Unlike large comets, whose cores are estimated to be as much as 25 mi (40 km) in diameter, these are only up to 40 ft (12 m) wide. It is estimated that as many as 43,000 reach the earth each day and break up at altitudes of 600–15,000 mi (950–24,000 km). Also in 1996 the ROSAT satellite (see X-ray astronomy ) detected X-rays emanating from the Comet Hyakutake. This was completely unexpected, and can be explained by no known mechanism. Observation of more large comets passing through the solar system by orbiting X-ray observatories will be necessary to corroborate this finding.

The Origin of Comets

The Oort Cloud

The origin of comets is still uncertain. They were once thought to have originated outside the solar system, but more recent theories suggest they were formed during the formation of the solar system and are permanent members of it. According to the storage-cloud hypothesis of J. H. Oort , a spherical shell of more than 100 billion comets surrounds the solar system at a distance of 75,000–150,000 astronomical units (1 astronomical unit, or AU, being the mean distance from the earth to the sun). While the comets move very slowly in this huge storage cloud, a passing star may change their orbits enough to force some of them into the inner part of the solar system. The mechanism for the Oort cloud's creation, however, is unclear, and it has been suggested that the Oort cloud may include a significant amount of material that originated outside the solar system and was gravitationally captured by the sun.

The Kuiper Belt

In 1951, G. P. Kuiper , noting that Oort's cloud of comets did not adequately account for the population of short-period comets (those making complete orbits around the sun in less than 200 years), proposed the existence of a disk-shaped region of minor planets outside the orbit of Neptune, now called the Kuiper belt, as a source for such comets. The Kuiper belt acts as a reservoir for these in the same way that the Oort cloud acts as a reservoir for the long-period comets. This theory was validated in 1992 with the discovery of the first of the more than 70,000 so-called transneptunian objects, bodies more than 60 mi (100 km) in diameter in an orbit 30–50 AU from the sun. Astronomers regard Pluto not as a planet but rather as a member of the Kuiper belt. The discoveries of several Kuiper belt objects have led to this view. Eris, an object discovered in 2003 (and originally nicknamed Xena), has an elongated orbit that extends to roughly three times the distance of Pluto's, has a diameter (1,500 mi/2,400 km) slightly larger than that of Pluto, and has a moon; Quaoar is more than half the size of Pluto; and Ixion and Varuna are almost half the size of Pluto. 2003 VS ₂ (roughly a fourth the size of Pluto) and a number of other Kuiper belt objects, called plutinos, have an orbital synchrony with Neptune like that of Pluto (Neptune completes three orbits around the sun in the same time that Pluto and the plutinos complete two orbits).

Bibliography

See D. Yeomans, Comets: A Chronological History of Observation, Science, Myth, and Folklore (1991); C. Sagan and A. Druyan, Comet (1997); D. H. Levy, Comets: Creators and Destroyers (1998); G. Kronk, Cometography: A Catalog of Comets (1999).

comet A small body, composed of ice and dust, in orbit around the Sun. The name derives from the Greek kometes, meaning ‘long-haired’. Comets are thought to exist in vast numbers in the Oort Cloud and Kuiper Belt, beyond the planets. From there they can be perturbed by the gravitational influence of passing stars into new orbits that bring them into the inner Solar System, where they become visible from Earth. When a comet is far from the Sun its nucleus is frozen solid and shines only by reflecting sunlight. As the nucleus nears the Sun it heats up and releases gas and dust, forming first a coma and, in some cases, a tail (see coma, cometary; nucleus, cometary; tail, cometary). The gas becomes ionized and emits light. Whereas the nucleus may be only 1 km or so across, the coma can extend for 10⁵ km or more from the nucleus and the tail for 10⁸ km}. Around the visible coma is an even larger cloud of hydrogen, detectable at ultraviolet wavelengths. Despite their size, a comet's coma and tail are of such low density that background stars can be seen through them. The mass of a typical comet is perhaps 10¹⁴ kg}.

 Each year over 100 comets are seen from Earth through telescopes; only a few ever become bright enough to be visible with the naked eye. Perhaps half are periodic comets, either new discoveries or known objects following predicted orbits. The most famous of these, and the brightest, is Halley's Comet. The remainder are new long-period comets appearing for the first time, with orbital periods of over 200 years. At the end of 2006 some 2500 comets were known, of which about 85% are long-period comets. During their passage through the inner Solar System comets can have their orbits altered by the gravitational influence of the planets, notably Jupiter. One spectacular example was Comet Shoemaker–Levy 9, which hit Jupiter in 1994.

 Some comets are discovered by amateur astronomers conducting deliberate searches, but most are found on images taken by professional astronomers; recently, over a hundred comets a year passing close to the Sun have been found on images taken by the Solar and Heliospheric Observatory (SOHO). Comets are named after their discoverers (now usually restricted to two names), or the spacecraft or survey which found them, and are also assigned a designation based on when they were discovered. According to a convention introduced in 1995, comets are identified by the year and a letter indicating the half-month in which they were discovered, plus the order of discovery in that half-month (e.g. C/1999 D3 would be the third comet discovered during the second half of 1999February). The names of periodic comets are preceded by P/ and a number indicating the order in which their periodicity was established (e.g. 1P/Halley, 2P/Encke). Comets that are defunct–either observed to have disintegrated or simply disappeared–are given the prefix D/ (e.g. 3D/Biela, D/1993 F2 Shoemaker–Levy). Comets for which there are insufficient observations to calculate an orbit are given the prefix X/.

 Comets are believed to be icy planetesimals left over from the formation of the outer planets. The total population of the Oort Cloud and Kuiper Belt may be 10¹² objects, with a combined mass greater than the Earth. The main component of cometary ice is frozen water, plus some methane (CH₄), carbon monoxide (CO), and carbon dioxide (CO₂). Several other carbon-containing molecules have also been detected, including formaldehyde (H₂CO), hydrogen cyanide (HCN), and methyl cyanide (CH₃CN). These same molecules are also found in interstellar nebulae, similar to the nebula from which the Solar System formed. Small (less than 1 mm) dust particles released from comets around perihelion contribute to the inner Solar System's zodiacal dust cloud. Larger dust particles, of millimetre and centimetre size, from periodic comets give rise to meteor streams.

Comet Capture

Comets are the most volatile -rich minor bodies in the solar system. It has been suggested that impacts with comets and asteroids provided Earth with much of its water. Although most comets are less accessible than near-Earth asteroids , their high water content makes them an economically attractive resource for space mining. The possibility that some near-Earth asteroids are extinct or dormant cometary nuclei means that this water-rich resource may be more accessible than was once thought.

Recent spacecraftand ground-based studies of comets have confirmed and refined Whipple's "dirty snowball" model for cometary nuclei. Cometary material is composed principally of water ice and other ices (including CO, CO₂, CH₄, C₂ H₆, and CH₃ OH) mixed with cosmic dust grains. The passages of most Oort cloud comets through the inner solar system are not predictable. In addition, the highly elongated and inclined trajectories of these comets make them difficult targets with which to match orbits. In contrast, Jupiter-family comets tend to have predictable, well-determined orbits with short periods and low inclinations. Therefore, a future mining mission would most likely target a Jupiter-family comet.

The capture of an active comet as a source of water and other volatile elements is a difficult proposition. In the vicinity of Earth the jet-like gas that flows from a comet's nucleus would have a stronger influence on its trajectory than any force humans could apply to the comet. This behavior would make transporting an active comet into a suitable near-Earth orbit, and maintaining it there, very unlikely. The Earth-impact hazard posed by a sizable comet^* or comet fragment in an unstable near-Earth orbit would be unacceptable. For example, even if the trajectory of a cometary fragment could be manipulated to produce capture into a high-Earth orbit, bringing the material down to low-Earth orbit (e.g., to the space station) would be difficult. The Moon's gravitational pull would make the trajectory extremely difficult to predict and control.

Capture into a lunar orbit would also be problematical. Lunar orbits tend to be unstable because of gravitational influences from Earth and the Sun. Another difficulty that must be resolved is the current uncertainty about the consistency of cometary nuclei. Not only is the bulk density of cometary nuclei unknown (estimates range from 0.3 g/cm³ to greater than 1 g/cm³; liquid water has a density of 1 g/cm ³), we do not know the cohesiveness of this material. Such uncertainties make it impossible to predict the mechanical properties of cometary material and the way a comet nucleus would react to a "nudge" to change its trajectory. A comet nucleus may or may not behave as a rigid object does; it might instead break up into fragments when a force is applied to change its orbit.

A more attractive approach to harvesting cometary material would be to send a robotic spacecraft to mine the comet. Returning fine-grained material and/or liquid water to Earth orbit would greatly lower the risks. A cargo spacecraft would be easier to control than a comet fragment, and even if an uncontrolled atmospheric entry occurred, the water and/or fine-grained material would vaporize or rain down harmlessly onto Earth's surface.

see also Asteroid Mining (volume 4); Comets (volume 2); Kuiper Belt (volume 2); Living on Other Worlds (volume 4); Oort Cloud (volume 2); Natural Resources (volume 4); Resource Utilization (volume 4); Terraforming (volume 4).

Humberto Campins

Bibliography

Whipple, Fred Lawrence. "A Comet Model. I: The Acceleration of Comet Encke."Astrophysical Journal 111 (1950):375-394.

^*A "sizable" comet in this context means greater than about 100 meters, depending on the density of the material.

Comets

Throughout human history comets have been regarded as auguries of disasters such as famine, plague, or war. The most recent outbreak of widespread concern that a comet might portend disaster occurred in 1973 when the comet Kohoutek was announced. For the first time in more than a generation, there arose the possibility that a bright comet, plainly visible with the naked eye, would be seen by the majority of people. A variety of speculations on the spiritual and prophetic implications of the comet were made, but the comet did not prove to be as spectacular as hoped, and none of the predicted changes signaled by its appearance occurred. No such speculation seems to have occurred at the time of the return of Halley's Comet in 1986.

In the past century comets have also figured in speculations about the history of the earth. In Ragnarok: the Age of Fire and Gravel (1883), Ignatius Donnelly assembled legends and religious beliefs tending to show that the earth was affected by a collision with a comet that created the Pleistocene Ice Age. In the 1950s, Immanuel Velikovsky connected the theme of a comet disaster with biblical prophecy in his book Worlds in Collision.

Sources:

Donnelly, Ignatius. Ragnarok: The Age of Fire and Gravel. New York: Harper's, 1883. Reprinted as The Destruction of Atlantis: Ragnarok. Blauvelt, N.Y.: Rudolf Steiner Publications, 1971.

Melton, J. Gordon. "Comet Kouhotek: Fizzle of the Century." Fate 27, no. 5 (May 1974): 58-64.

Velikovsky, Immanuel. Worlds in Collision. Garden City, N.Y.: Doubleday, 1950.

comet Small, icy Solar System body in orbit around the Sun. The solid nucleus of a comet is small, that of Halley's comet measures just 16 × 8km (10 × 5mi), and comprises rock and dust particles embedded in ice. As the comet approaches the Sun, evaporation begins, and jets of gas and dust form the luminous coma. Later, radiation pressure from the Sun and solar wind may send dust and gas streaming away as a tail, as much as 150 million km in length. There are three main types of comet: short-period comets often have their aphelia at approximately the distance of Jupiter's orbit. Long-period comets have aphelia near or beyond Neptune's orbit. The third type are either called non-periodic, or hyperbolic. Their periods may be as much as several million yaers. See also meteor

comet family A group of comets which possess similar orbital characteristics, either because of a common origin or as a result of planetary perturbations. Members of some families, for example the Jupiter comet family, do not have a common origin, and may have been captured from a wide range of original orbits. It was once believed that the gravitational influences of Saturn, Uranus, and Neptune also produced comet families, but the apparent association of comets with these planets is now thought to result from resonances with Jupiter's orbital period. Other families, for example the Kreutz sungrazers, clearly have a common origin from the break‐up of a precursor body.

comet A small body composed of meteoric dust and frozen ices (H₂O, CO₂, CO, HCHO) in a highly elliptical or parabolic orbit around the Sun. The average perihelion distance is less than 1 AU, and the average aphelion distance is about 10⁴AU. Comets are derived from the Oort cloud and have average lifetimes of about 100 passages. Comet nucleii are irregular in shape, a few kilometres in diameter and have a low density (100–400 kg/m³). Due to solar radiation, they emit gas and dust, forming the characteristic tail, when within a few astronomical units of the Sun; the dust composition appears to resemble that of primitive carbonaceous chondrites.

COMET-PLANET COLLISIONS

Many comets are in Earth-crossing orbits and collisions do occur between comets and planets. A spectacular example of such a collision was the impact of comet Shoemaker-Levy 9 with Jupiter in July 1994. It is now well established that an impact with an asteroid or comet created a large crater at the edge of the Yucatan Peninsula 65 million years ago. Known as the Chicxulub impact, this event almost certainly caused the extinction of the dinosaurs. Efforts are underway to study the population of potential hazards from both comets and asteroids in sufficient detail to predict and prevent future large impacts.

com·et / ˈkämit/ • n. a celestial object consisting of a nucleus of ice and dust and, when near the sun, a “tail” of gas and dust particles pointing away from the sun. DERIVATIVES: com·et·ar·y / ˈkämiˌterē/ adj.

comet XIII (XII in L. form). — (O)F. comète — L. comēta — Gr. komḗtēs long-haired, sb. comet (for astḕr komḗtēs ‘long-haired star’), f. komân wear the hair long, f. kómē hair of the head, tail of a comet.

comets see asteroids and comets

comet •dammit, Hammett, Mamet •emmet, semmit •helmet, pelmet •remit • limit • kismet • climate •comet, grommet, vomit •Goldschmidt •plummet, summit •Hindemith •hermit, Kermit, permit •gannet, granite, Janet, planet •magnet • Hamnett • pomegranate •Barnet, garnet •Bennett, genet, jennet, rennet, senate, sennet, sennit, tenet •innit, linnet, minute, sinnet •cygnet, signet •cabinet • definite • Plantagenet •bonnet, sonnet •cornet, hornet •unit •punnet, whodunnit (US whodunit) •bayonet • dragonet • falconet •baronet • coronet •alternate, burnet •sandpit • carpet • armpit • decrepit •cesspit • bear pit • fleapit •pipit, sippet, skippet, snippet, tippet, Tippett, whippet •limpet • incipit • limepit •moppet, poppet •cockpit • cuckoo-spit • pulpit • puppet •crumpet, strumpet, trumpet •parapet • turnspit

Comet (or COMET) (ˈkɒmɪt) Committee for Middle East Trade
• computer-operated management evaluation technique