comet

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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 or another star. 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. In 1987 a comet orbiting the star Beta Pictoris was identified, and since then an increasing number of exocomets have been found.

The Orbits of Comets

Although the occurrence of many comets had been recorded, it was not until 1577 that the Danish astronomer Tycho BraheBrahe, Tycho
, 1546–1601, Danish astronomer. The most prominent astronomer of the late 16th cent., he paved the way for future discoveries by improving instruments and by his precision in fixing the positions of planets and stars.
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 suggested that they traveled in elongated rather than circular orbits. A century later Giovanni BorelliBorelli, Giovanni Alfonso
, 1608–79, Italian physiologist, physicist, astronomer, and mathematician; son of a Spanish infantryman. His wide interests led to original contributions in many fields, including anatomy, epidemiology, the study of fermentation, volcanology,
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 concluded that the orbits were parabolic and that comets passed through the solar system but once, never to return. In 1705, however, Edmond HalleyHalley, Edmond
, 1656–1742, English astronomer and mathematician. He is particularly noted as the first astronomer to predict the return of a comet and the first to point out the use of a transit of Venus in determining the parallax of the sun. In 1676 he went to St.
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 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 cometHalley's comet
or Comet Halley
, periodic comet named for Edmond Halley, who observed it in 1682 and identified it as the one observed in 1531 and 1607. Halley did not live to see its return in 1758, close to the time he predicted.
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). While some comets appear to have parabolic orbits (see parabolaparabola
, plane curve consisting of all points equidistant from a given fixed point (focus) and a given fixed line (directrix). It is the conic section cut by a plane parallel to one of the elements of the cone.
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), 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. WhippleWhipple, Fred Lawrence,
1906–2004, American astronomer, b. Red Oak, Iowa. After graduating from the Univ. of California, Berkeley (Ph.D. 1931), he accepted a position at Harvard, where he remained for the rest of his career.
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 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 windsolar wind,
stream of ionized hydrogen—protons and electrons—with an 8% component of helium ions and trace amounts of heavier ions that radiates outward from the sun at high speeds.
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 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. In 2014 Rosetta became the first space probe to orbit a comet's nucleus (that of Comet 67P); it also deployed a lander on the comet. 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 meteormeteor,
appearance of a small particle flying through space that interacts with the earth's upper atmosphere. While still outside the atmosphere, the particle is known as a meteoroid. Countless meteoroids of varying sizes are moving about the solar system at any time.
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); 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 astronomyX-ray astronomy,
study of celestial objects by means of the X rays they emit, in the wavelength range from 0.01 to 10 nanometers. X-ray astronomy dates to 1949 with the discovery that the sun emits X rays.
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) 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 in the Solar System

The Oort Cloud

The origin of the solar system's 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 proposed by J. H. OortOort, Jan Hendrik
, 1900–1992, Dutch astronomer. He confirmed (1927) Bertil Lindblad's theory of the Milky Way galaxy's rotation. In the 1950s he and his colleagues used radio astronomical means to map the spiral-arm structure of the galaxy.
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 and since modified, a spherical shell of more than 100 billion comets surrounds the solar system at a distance of 20,000 AU to 50,000 AU or greater (1 AU, or astronomical unit, being the mean distance from the earth to the sun); some astronomers have suggested an inner Oort cloud exists beginning at 2,000 to 5,000 AU and extending to 20,000 AU. While the comets move very slowly in this huge storage cloud, a passing star may change the orbits of bodies in the outer reaches of the Oort cloud 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; 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. KuiperKuiper, Gerard Peter or Gerrit Pieter
, 1905–73, American astronomer, b. the Netherlands. Kuiper is considered to be the father of modern planetary science for his wide ranging studies of the solar system.
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, 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 PlutoPluto,
in astronomy, a dwarf planet and the first Kuiper belt, or transneptunian, object (see comet) to be discovered (1930) by astronomers. Pluto has an elliptical orbit usually lying beyond that of Neptune.
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 not as a planet but rather as dwarf planetdwarf planet,
a nonluminous body of rock or gas that orbits the sun and has a rounded shape due to its gravity. Unlike a planet, a dwarf planet is not capable of clearing its orbit of smaller objects by collision, capture, or other means.
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 that is a member of the Kuiper belt. The discoveries of several Kuiper belt objects led to this view. ErisEris,
in astronomy, the largest known dwarf planet. Eris, whose highly eccentric elliptical orbit ranges from 38 AU to 97 AU and is inclined more than 44°, is the largest known object of the Kuiper belt (see comet), with a diameter (1,445 mi/2,317 km) slightly larger than
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, 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 VS2 (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 (1991), C. Sagan and A. Druyan, Comet (1997), D. H. Levy, Comets (1998), G. Kronk, Cometography (1999), and D. J. Eicher, Comets! (2013).

comet

A minor member of the Solar System that travels around the Sun in an orbit that generally is much more eccentric than the orbits of planets. Typical comets have three parts: the nucleus, coma, and comet tail. The gas and dust tails of a comet appear only when the comet is near the Sun and always point away from the Sun. The nucleus is the permanent solid portion of a comet and is thought by the majority of astrophysicists to be a kilometer-sized dirty snowball; this model – the Whipple model – was first suggested by Fred L. Whipple in 1951. The Solar System contains only a few observable comets that have nuclei in the tens of kilometers size range. The numbers of actual (as opposed to observed) comets increase enormously as one goes to smaller and smaller sizes, but these are much less easy to see. The satellite missions to Halley's comet have revealed much about cometary composition and obtained the first image of a nucleus.

Cometary orbits fall into two classes. Short-period comets have periods of less than 200 years and orbits lying completely or almost completely inside the planetary system. They seem to have been captured into the inner Solar System by the gravitational attraction of the major planets. At any epoch about 50 of these are bright enough to be detected as they come to perihelion. They have a mean eccentricity of 0.56 and a mean inclination of 11°. Nearly all of them (Halley's comet is an exception) are moving in direct orbits, i.e. in the same direction as the planets. The second class have near-parabolic orbits with periods in excess of 200 years. These orbits are orientated at random, indicating that these comets do not come from any specific direction in space. Many of them have been seen only once (for example Kohoutek has a period in excess of 70 000 years).

The majority of comets observed from Earth have perihelions near the Earth. This is simply due to observational selection. The brightness of a comet is proportional to 1/rn δ2, where r is the distance between the comet and the Sun and δ is the comet–Earth distance. The power n is on average 4.2 but can vary widely around this value. On average a comet passes perihelion about 1000 times before decaying away. Cometary decay produces meteoroid streams.

Comets are named after their discoverers. In any year, say 1994, comets are designated 1994a, 1994b, etc., in order of discovery. Permanent designations 1994 I, 1994 II, etc., are given later, these being in order of date of perihelion passage.

There are two prevalent theories of cometary origin. In the first theory comets were produced as icy planetesimals in the Saturn–Neptune region, at the same time as the origin of the Solar System, and were then stored in the Oort cloud. In the second they are produced by gravitational accretion every time the Sun passes through an interstellar dust cloud. This latter theory predicts that the Solar System's cometary reservoir is periodically being topped up. The first theory predicts a cometary population that decreases with time.

Comet

 

a body of the solar system having the appearance of a nebulous object usually with a bright concentration—the nucleus—at the center and a tail.

General information A comet is observed when a small icy body, the nucleus of the comet, approaches to within 4–5 astronomical units (AU) of the sun, is heated by the sun’s rays, and begins to give off gas and dust. The gas and dust form a nebulous envelope (the comet’s atmosphere), sometimes called the coma, around the nucleus; together with the nucleus it constitutes the comet’s head. The comet’s atmosphere continuously dissipates into space and exists only as long as gas and dust continue to emanate from the nucleus. Under the influence of radiation pressure and owing to interaction with the solar wind, the gas and dust are carried away Árom the nucleus, forming the comet’s tail.

In most comets a bright “nucleus” (either starlike or diffuse) is observed in the middle of the head. This is the glow of the central, denser zone of gases around the comet’s true nucleus. The comet’s head and tail are not well defined; their apparent dimensions depend on the total intensity of gas and dust emissions from the nucleus and its proximity to the sun, on the one hand, and on the observational circumstances, primarily the background brightness of the sky, on the other.

Ancient Chinese annals contain significant information on sightings and motion of comets. But in Europe, in accordance with Aristotle’s ideas, it was thought until the 17th century that comets originated and traveled in the atmosphere, that they were terrestrial vapors that had risen upward and been ignited on approach to the “sphere of fire,” and that, moreover, their tails were flames driven by the wind. Tycho Brahe, studying the motion of the comet of 1577 among the stars, determined its parallax from observations in Denmark and Prague. It proved to be less than the lunar parallax, and thus the comet was more distant then the moon. This was proof that comets were celestial bodies, like the moon and the planets.

After the discovery of the law of gravitation, methods for determining cometary orbits were worked out in the 18th and 19th centuries (E. Halley, H. Olbers). A new approach to comet research was proposed by F. Bessel (at the beginning of the 19th century) and developed by F. A. Bredikhin (in the second half of the 19th century), who began to study the physical nature of comets and the features of their internal structure. In particular, Bredikhin created a mechanical theory of comet shapes, which has played a large role in comet research. At the beginning of the 20th century an Austrian astronomer and the Soviet astronomer S. V. Orlov investigated the brightness of comets and established the law of the dependence of its variation on the comet’s distance from the sun. The modern era in comet research began in 1910 with the return of the bright Halley’s Comet, when photographic and spectroscopic methods of observation were widely used.

The unexpected apparitions of unusual celestial bodies, such as bright comets, have always made a strong impression on man. Therefore, it is not surprising that superstitious people saw the apparitions of comets as portents and linked them with local terrestrial events. Thus, for example, the appearance of a bright comet in 1811–12 was connected in Russia with Napoleon’s invasion, in Spain with the good grape harvest, and in Mexico with the discovery of silver ores.

The number of comets in the solar system is extremely large, probably reaching hundreds of billions. However, only a small number of comets are accessible to observation, namely, those passing within the orbit of Jupiter. Thus, in the period from 1850 to 1949 an average of about five comets were annually observed passing through perihelion, of which only one was visible to the naked eye. In the subsequent 20 years (1950–69), this number increased to nine per year owing to the intensification of comet searches. Table 1 lists the brighter comets of the 19th and 20th centuries and their greatest magnitude (where known).

Table 1. Bright comets
CometGreatest
 apparent magnitude
*Approximate
1811 I ...............+1
1823 ............... 
1843 I ...............–7
1858 VI ...............+ 0.2
1861 II ...............–4
1874 III ............... 
1880 I ............... 
1881 III ............... 
1882 II ...............–17
1901 II ...............–2
1910 1 ...............–5
1910 II Halley ...............–1
1927 IX ...............–6
1947 XII ...............–2
1948 XI ...............+1*
1957 III ...............4–2

By international agreement, comets are initially designated by the year of their discovery and a Latin letter in the order of receipt of information on their discovery. After reliable determination of their orbits, these preliminary designations are replaced by final ones containing the year, the ordinal number (Roman numeral) of perihelion passage, and the name(s) of the discoverer(s).

The brightness of a comet varies greatly. The brightest known comet was comet 1882 II, which approached to within a very small distance of the sun. Its brightness at perihelion reached a stellar magnitude of —17, that is, it gave off 60 times more light than the full moon. It was the brightest celestial object after the sun and was well visible during the day near the sun’s surface. However, most comets are visible only with telescopes.

A comet’s brightness increases rapidly with a change in the comet’s distance r from the sun; it also depends on the comet’s distance Δ from the earth. The stellar magnitude m of the comet’s head may be represented by the empirical relation m = m0 + 5 log Δ + 2.5 n log r. The Soviet astronomer B. Iu. Levin established another relation on the basis of physical considerations: Comet. In these formulas, m0 is the comet’s absolute brightness; n, A, and B are constants; and for most comets, n ≈ 4, that is, the brightness of the comet’s head varies approximately in inverse proportion to r4. Irregular fluctuations are sometimes superimposed upon the regular variation of a comet’s brightness with changes in r; these are possibly connected with solar activity. Many periodic comets are observed to have a secular decrease in brightness, which is explained by the exhaustion of the reserves of luminous material.

Orbits Up to 1971, about 1,000 systems of orbital elements had been computed for almost 600 comets. Results of these calculations are published in special catalogs. Thus, Porter’s catalog contains information on comet apparitions in the period from 239 B.C. to A.D. 1961. It mentions 829 apparitions of 566 individual comets, including 54 short-period comets (with periods p < 200 years) observed during two or more approaches to the sun, 40 short-period comets observed during only one approach, 117 long-period comets (with p > 200 years), 290 comets with parabolic orbits, and 65 comets with hyperbolic orbits, which, receding from the sun, will leave the solar system forever, traveling into interstellar space. Most of the orbits that are considered parabolic in actuality are apparently elongated ellipses. However, their eccentricities could not be determined owing to insufficient accuracy in observations. Hyperbolic orbits result from the perturbing influence of the large planets, primarily Jupiter, on the comets’ motions. Analysis of the motions of these comets in preceding years has led to the conclusion that each comet was approaching the solar system along an elliptical orbit until the moment when each one began experiencing an appreciable perturbing influence from the planets. The passage of a comet near a large planet results in sharp changes in its orbit. For example, the comet discovered in 1942 by the Finnish astronomer L. Oterma: until 1963 it had been traveling between the orbits of Mars and Jupiter, but after an approach to Jupiter it passed into an orbit between the orbits of Jupiter and Saturn.

Effects that cannot be explained by attraction by known bodies of the solar system (nongravitational effects) are also observed in the motion of a number of comets, primarily short-period ones. Thus, some comets experience secular acceleration, and others secular deceleration, which is apparently the result of the “rocket jet” effect of the streams of material being emitted from the nucleus.

It is customary to divide short-period comets into “families” according to their aphelion distances. Comets whose aphelia are situated near the orbit of Jupiter belong to the very numerous Jupiter family. Comets with aphelia near Saturn’s orbit belong to the Saturn family. Several long-period comets form an interesting group, the “sun-grazing” family. All of them have very small perihelion distances, 0.0055–0.0097 AU (that is, their perihelia are 0.5 to 1 solar radius from the sun’s surface), and approximately similar remaining orbital elements. It is highly probable that these comets are the products of the disintegration of one mother comet.

The orbital elements of several comets are given in Table 2.

Structure According to contemporary representations, a comet’s nucleus consists of water vapor with an admixture of “ices” of other gases (CO2, NH3) as well as stony material. Dust particles are partly released from the nucleus during the evaporation (sublimation) of the ices and are partly formed in its neighborhood as a result of the condensation of the molecules of nonvolatile and slightly volatile substances. The dust particles disperse sunlight, but the gas atoms and molecules absorb radiation at some long wavelength of the illuminating sunlight and then reradiate it. As a result of the emission of gas and dust particles from the sun-warmed nucleus, a reactive force arises, perhaps producing nongravitational effects in the comet’s motion. Intense emission occurs from the most warmed part of the nucleus’ surface, which, because of the nucleus’ rotation, is located not exactly facing the sun but slightly displaced in the direction of rotation. As a result, a component of the reactive force arises that either only accelerates the comet’s motion, if the nucleus’s rotation is in the same direction as the comet’s revolution around the sun, or retards it, if the rotation and revolution are in opposite directions.

The gas and dust released from the nucleus form the comet’s head. Molecules of water and other gases that are emitted from the nucleus by the action of solar radiation dissociate rapidly, producing observable, chemically active, free radicals. The radicals are broken up by the radiation from the sun, but much more slowly, as a consequence of which they succeed in propagating to a significant distance from the nucleus. The study of comet spectra indicates that comets contain the neutral molecules C3, C2, CN, CH, OH, NH, and NH2; the ionized molecules CO+, N+2, and CH+; and atoms of H, O, and Na. In rare cases, emission lines of Fe and other nonvolatile chemical elements have been observed in the spectra

Table 2. Orbital elements of several comets
CometTime of last perihelion, TPeriod of revolution p (years)Eccentricity eOrbital inclination iLongitude of ascending node ΩAngle from perihelion to node ωPerihelion distance q (AU)Aphelion distance Q(AU)Remarks
1970 I Encke1971 Jan. 9.923.3020.84715211°.9747334°2224185°.93830.3388974.09Shortest period
1957 IV Schwassmann-Wachmann I1957 May 12.8916.100.1314889°.4872321°.6094355°.82715.537747.21Small e, planetlike orbit
1910 II Halley1910 Apr. 20.1876.10.967297162°.215857°.8466111°.71900.58721235.31First comet whose orbit was determined
1965 VIII Ikeya-Seki (principal nucleus)1965 Oct. 21.188740.999915141°.8576346°296369°.04990.007785183“Sun-grazing”

of comets that have passed very close to the sun. Bright comets may have heads with diameters approaching millions of kilometers. The quantity of dust in the heads of comets varies: in some there is none and in others, the mass of dust can approach half the total mass of the head. The color and polarization of the light reflected by the dust particles in comet heads indicates that their size is approximately 0.25–5 jam.

In accordance with the classification developed in the second half of the 19th century by F. A. Bredikhin, there are three types of comet tails:Type I tails are straight and directed away from the sun, Type II tails are curved and arc back with respect to the comet’s orbital motion, and Type III tails are almost straight but slant noticeably backward. In certain mutual arrangements of the earth, the comet, and the sun, the backward-slanting tails of Types II and III are seen from the earth as being directed toward the sun (anomalous tails). The physical interpretation of the classification of comet tails proposed by Bredikhin was significantly worked out in succeeding years and by the 1970’s had obtained the following form. Type I tails are plasma and consist of the ionized molecules CO+, N+2, and CH+, which are carried with large accelerations away from the sun by the action of the solar wind. Type II tails are formed by various-size dust particles that are continuously being emitted from the nucleus. Type III tails appear when an entire cloud of dust is emitted from the nucleus at the same time. Under the influence of radiation pressure, the different-size dust particules receive different accelerations and the cloud is stretched into a band forming the comet’s tail, the “synchrone.” In rare cases, a straight sodium tail is observed, directed along the plasma tail (Type I). Under the action of radiation pressure, the neutral molecules present in the comet’s head acquire approximately the same acceleration as the dust particles and therefore move in the direction of a Type II tail. However, their lifetime before photodissociation (or ionization) by solar radiation is only several hours. Therefore, they do not succeed in advancing very far into a Type II tail. Sometimes they are observed in small quantities in only the initial section of the tail.

Particles of the same size that are continuously emitted from the nucleus and that move under the influence of the same acceleration are distributed in space along curved lines—“syndynes.” Type II tails are a fan of syndynes corresponding to dust particles of different sizes. The apparent shape of a Type II tail is therefore determined by the distribution of dust particles according to size. Thus a visible tail of Type II is the belt of maximum brightness within the fan.

The greatest length is attained, as a rule, by Type I tails, which can extend to hundreds of millions of kilometers. However, their density does not apparently exceed 102–103 ions/cm3.

Laboratory experiments on comet modeling greatly contribute to a better understanding of the nature of comets. It has been possible, in particular, to reproduce the sublimation of dusty cometary ices with the ejection of meteoric particles from the nucleus as well as the formation of ionized structures resembling Type I tails. Artificial clouds (artificial comets) have been created from the vapors of alkali metals with the help of geophysical rockets and space probes at altitudes ranging from several hundred to tens of thousands of kilometers. This has paved the way for the modeling of comets in deep space. Under consideration is the possibility of sending a space probe to a periodic comet during its return to the sun in order to directly study the composition, magnetic fields, and other physical properties of comets.

Origin and evolution Theory, observation, and experiment indicate that during its return to the sun a comet loses a significant portion of its material, so that during its lifetime it cannot complete more than several hundred or thousand revolutions around the sun; this time is exceedingly small from a cosmogonical viewpoint. Since, nevertheless, comets are also observed in the present epoch, there must exist some source for replenishing their number. According to one hypothesis, developed by the Soviet astronomer S. K. Vsekhsviatskii, comets are the result of powerful volcanic eruptions on the large planets and their satellites. According to another hypothesis, proposed by the Dutch astronomer J. Oort, comets that are now observable arrive in the sun’s vicinity from a gigantic comet cloud (formed at the same time that the giant planets were formed) surrounding the solar system and extending to distances of up to 150,000 AU. Under the influence of perturbations from the attraction of stars, some comets of this cloud may pass to orbits with small perihelion distances and thus become observable.

REFERENCES

Bredikhin, F. A. O khvostakh komet. Moscow-Leningrad, 1934.
Orlov, S. V. O prirode komet. Moscow, 1958.
Vsekhsviatskii, S. K. Fizicheskie kharakteristiki komet. Moscow, 1958.
Dobrovol’skii, O. V. Komety. Moscow, 1966.
Fesenkov, V. G. “Solnechnoe kometnoe oblako i mezhzvezdnoe prostranstvo.” Zemlia i Vselennaia, 1965, no. 4.
Richter, N. B. Statistik und Physik der Kometen. Leipzig, 1954. (English translation, The Nature of Comets. London, 1963.)
The Moon, Meteorites, and Comets. Edited by B. M. Middlehurst and G. P. Kuiper. Chicago-London. 1963. Chapters 15–20. Nature et origine des comètes. Liège, 1966.

O. V. DOBROVOL’SKII

comet

[′käm·ət]
(astronomy)
A nebulous celestial body having a fuzzy head surrounding a bright nucleus, one of two major types of bodies moving in closed orbits about the sun; in comparison with the planets, the comets are characterized by their more eccentric orbits and greater range of inclination to the ecliptic.

comet

a celestial body that travels around the sun, usually in a highly elliptical orbit: thought to consist of a solid frozen nucleus part of which vaporizes on approaching the sun to form a gaseous luminous coma and a long luminous tail