Comet composition. Space comets: danger or forced neighborhood The main body of a comet is called

COMETS (from the Greek κομήτης - hairy, shaggy), small in size and mass celestial bodies of the solar system, revolving around the Sun in highly elongated orbits and sharply increasing their brightness when approaching the Sun. Near the Sun, comets look like luminous balls in the sky, followed by a long tail (Fig. 1). Comets are icy celestial bodies (sometimes called cosmic icebergs) whose bright glow is created by the scattering of sunlight and other physical effects. The full name of comets includes the names of the discoverers (no more than three), the year of discovery, a capital letter of the Latin alphabet and a number indicating at what point in the year the comet was discovered, and a prefix indicating the type of comet (P - short-period comet, C - long-period comet , D - collapsed comet, etc.). Every year, about 10-20 comets can be observed in an amateur telescope.

Historically, the appearance of comets in the sky was considered a bad omen, portending misfortune and catastrophe. Disputes about the nature of comets (atmospheric or cosmic) continued for 2 thousand years and ended only in the 18th century (see Comet Astronomy). Significant progress in the study of comets was made in the 20th century thanks to flights to comets by spacecraft.

General information about comets. Comets, along with asteroids, meteoroids and meteor dust, belong to the small bodies of the solar system. The total number of comets in the solar system is extremely large, it is estimated at no less than 10 12 . comets are divided into two main classes: short-period and long-period comets with an orbital period of less than and more than 200 years, respectively. The total number of comets observed in historical time (including those in parabolic and hyperbolic orbits) is close to 1000. Of these, about 100 short-period comets are known to regularly approach the Sun. The orbits of these comets have been reliably calculated. Such comets are called "old", in contrast to the "new" long-period comets, which, as a rule, were observed in the inner regions of the solar system only once. Most short-period comets are included in the so-called families of giant planets, being in orbits close to them. The most numerous is the family of Jupiter, numbering hundreds of comets, among which more than 50 of the shortest period comets are known with a period of revolution around the Sun from 3 to 10 years. Less observed comets include the families of Saturn, Uranus and Neptune; to the latter, in particular, belongs the famous Halley comet.

The main reservoirs containing comet nuclei are located on the periphery of the solar system. This is the Kuiper belt, located near the plane of the ecliptic just beyond the orbit of Neptune, within 30-100 AU. e. from the Sun, and a spherical Oort-shaped cloud located approximately half the distance to the nearest stars (30-60 thousand AU). The Oort cloud periodically experiences gravitational perturbations from giant interstellar gas-dust clouds, the galactic disk and stars (during random encounters) and therefore does not have a clearly defined outer boundary. Comets can leave the Oort cloud, replenishing the interstellar medium, and return again. Thus, comets play the role of original probes of the regions of the Galaxy closest to the Solar System.

Due to similar perturbations, some bodies from the Oort cloud fall into the inner regions of the solar system, passing to highly elliptical orbits. These bodies, when approaching the Sun, are observed as long-period comets. Under the influence of gravitational perturbations from the planets (primarily Jupiter and other giant planets), they either replenish the known families of short-period comets that regularly return to the Sun, or move to parabolic and even hyperbolic orbits, leaving the solar system forever. The main source of short-period comets is the Kuiper belt. Due to Neptune's gravitational perturbations of Kuiper belt objects, a relatively small proportion of the icy bodies inhabiting the belt are constantly migrating into the inner regions of the solar system.

Comets orbiting. Comets move in orbits with a large eccentricity and inclination to the plane of the ecliptic. Movement occurs both in the forward (as with the planets) and in the opposite direction. Comets experience strong tidal disturbances when passing near planets, which leads to a significant change in their orbits (and, accordingly, difficulties in predicting comet motions and accurately determining ephemerides). As a result of these orbital changes, many comets fall on the Sun.

The results of calculations of the orbital elements of comets are published in special catalogs; for example, a catalog compiled in 1997 contains the orbits of 936 comets, over 80% of which have been observed only once. Depending on the position in the orbit, the brightness of comets varies by several orders of magnitude, reaching a maximum shortly after the passage of perihelion and a minimum at aphelion. The absolute stellar magnitude of comets in the first approximation is inversely proportional to R 4 , where R is the distance from the Sun. As a rule, short-period comets revolve around the Sun no more than a few hundred times. Therefore, their lifetime is limited and usually does not exceed 100 thousand years.

The active phase of the comet's existence ends when the stock of volatile substances in the nucleus is exhausted or the surface of the comet's nucleus is covered with a melted dusty ice crust, which occurs as a result of the comet's repeated encounters with the Sun. After the end of the active phase, the comet's nucleus becomes similar in physical properties to an asteroid, so there is no sharp boundary between asteroids and comets. Moreover, the opposite effect is also possible: an asteroid may begin to show signs of cometary activity when its surface crust cracks for one reason or another.

The irregularity of comets' orbits leads to an unpredictable probability of their collisions with planets, which further complicates the problem of asteroid-comet hazard. The collision of the Earth with a fragment of the nucleus of comets may have caused the Tunguska event of 1908 (see Tunguska meteorite). In 1994, more than 20 fragments of comets Shoemaker-Levy 9 (torn apart in the immediate vicinity of the planet by tidal forces) were observed to fall on Jupiter (Fig. 2), which led to catastrophic phenomena in Jupiter's atmosphere.

Structure and composition of comets. Comets are made up of a nucleus, an atmosphere (coma), and a tail. Irregular-shaped nuclei are small in size - from units to tens of kilometers and, accordingly, have a very small mass that does not have a noticeable gravitational effect on planets and other celestial bodies. Comet nuclei rotate about an axis almost perpendicular to the plane of their orbit, with a period from several units to several tens of hours. Comet nuclei are characterized by low reflectivity (albedo 0.03-0.04), so comets are not visible far from the Sun. The exception is comet Encke: the period of revolution of this comet is only 3.31 years, it is relatively close to the Sun and can be observed throughout the orbit.

The remaining elements of the cometary structure are formed when the comet approaches the Sun. Near the perihelion of the orbit, due to the sublimation of the substance of the nucleus and the removal of dust from its surface, a coma arises. The size of dust particles in a coma is mainly 10 -7 -10 -6 m, but larger particles are also present. A coma is a brightly glowing foggy shell with a diameter of over 100,000 km. Inside the coma, in the vicinity of the nucleus, the brightest clot is distinguished - the head of the comet, and outside the coma - the hydrogen corona (halo). A tail extending from the coma is tens of millions of kilometers long: a relatively faintly luminous band, which, as a rule, does not have clear outlines and is directed mainly in the direction opposite to the Sun. Intensive sublimation and removal of dust create a reactive force; this non-gravitational effect also affects the irregularity of cometary orbits.

Comet nuclei have a very low average density, usually not exceeding hundreds of kg/m 3 . This indicates the porous structure of the cores (Fig. 3), consisting mainly of water ice and some low-temperature condensates (carbon dioxide, ammonia, methane ice) with an admixture of silicates, graphite, metals, hydrocarbons, and other organic compounds. A significant proportion of the core is dust and larger stony fragments. The abundance of water ice in comets is explained by the fact that the water molecule is the most common in the solar system.

Measurements taken during the approach of spacecraft to the comet, in general, confirmed the hypothesis that the nucleus is a "dirty snowball". A similar model of the nucleus of comets was proposed in the middle of the 20th century by the American astronomer F. Whipple. Coma consists mainly of neutral molecules of water, hydrogen, carbon (C 2 , C 3), a number of radicals (OH, CN, CH, NH, etc.) and glows due to luminescence processes. It is partially ionized by short-wavelength solar radiation, which creates ions OH + , CO + , CH + , etc. When these ions interact with the solar wind plasma, the observed radiation in the UV and X-ray regions of the spectrum arises.

During the sublimation of ice, dust is simultaneously intensively removed into the atmosphere, due to which the tail of the comet is mainly created. According to the classification proposed back in the 2nd half of the 19th century by F. A. Bredikhin, there are three types of comet tails: I - straight and narrow, directed in the opposite direction from the Sun; II - wide, curved and somewhat deviated relative to the direction from the Sun; III - straight, short and strongly deviated from the direction from the Sun. In the 20th century, S. V. Orlov developed the physical basis of this classification in accordance with the mechanism of tail formation. A type I tail is created by plasma interacting with the solar wind, a type II tail by submicron dust particles exposed to light pressure, a type III tail by a combination of small and larger particles experiencing different accelerations under the action of gravitational forces and light pressure.

Due to this mechanism of formation, the position in space of type III tails is less clear, it does not coincide with the antisolar direction and is deflected backward relative to the orbital motion. Sometimes curved lines are observed in the structure of the tail - the so-called syndynam, or even a fan of sindynam, created by dust particles of different sizes.

The changes that occur with comets at different points of its orbit and during their lifetime are largely determined by non-stationary processes of heat and mass transfer in the porous core and the formation of an inhomogeneous surface structure from which sublimation occurs. Kinetic modeling of these processes made it possible to obtain an idea of ​​the state of the gas in the coma. Near the nuclei of active comets, the gas flow in the hemisphere facing the Sun is close to equilibrium, and the gas density rapidly decreases with distance from the surface of the nucleus. Due to the adiabatic expansion of the gas into the interplanetary vacuum, the temperature is a few kelvins at a distance of about 100 km from the core. In the vicinity of the axis of symmetry, a well-defined jet (jet) is formed, due to the intensive removal of gas and dust. (Several jets are visible in the image of the nucleus of Halley's comet, obtained during the flyby of the Giotto spacecraft near it.) Such uneven sublimation from the surface of the nucleus can be explained by thermal deformations that cause breaks and cracks in the surface crust of the comet.

As a result of the intense emission of dust from short-period comets, dust tori are formed along its orbit. These tori periodically cross the Earth in its orbit, which causes meteor showers.

The value of comets for cosmogony. The origin of comets is probably associated with the gravitational ejection of icy bodies from the formation of giant planets (see the article Cosmogony). Therefore, studies of comets contribute to solving the fundamental problem of the origin and evolution of the solar system. Comets are of great scientific interest, primarily from the point of view of cosmochemistry, since they contain the primary substance from which the solar system was formed. It is believed that comets and the most primitive class of asteroids (carbonaceous chondrites) retained in their composition particles of a protoplanetary cloud and a gas and dust accretion disk. As relics of the formation of planets (planetesimals), comets have undergone the least changes in the process of evolution. Therefore, information on the composition of comets makes it possible to impose fairly strict restrictions on the range of parameters used in the development of cosmogonic models.

At the same time, according to modern concepts, comets themselves could play an important role in the evolution of the Earth and other terrestrial planets as a source of volatile elements and their compounds (primarily water). As shown by the results of mathematical modeling, due to this source, the Earth could receive an amount of water comparable to the volume of its hydrosphere. Approximately the same amount of water could receive Venus and Mars, which speaks in favor of the hypothesis of the existence of ancient oceans on them, lost in the course of subsequent evolution. Comets are also considered as possible carriers of primary life forms. The problem of the origin of life on planets is associated, in particular, with the transport of matter inside and outside the solar system and migration-collision processes, in which comets play a key role.

Lit .: Orlov S.V. On the nature of comets. M., 1960; Dobrovolsky O. V. Comets. M., 1966; Physics and chemistry of comets. V.; N.Y., 1990; Yeomans D. Comets: a chronological history of observation; science, myth and folklore. N.Y., 1991; Comets in the post-Hailey era. Dordrecht, 1991. Vol. 1-2; Marov M. Ya. Physical properties and models of comets // Astronomical Bulletin. Exploration of the solar system. 1994. V. 28. No. 4-5; he is. Small bodies of the solar system and some problems of cosmogony // Uspekhi fizicheskikh nauk. 2005. V. 175. No. 6.

However, the orbits of comets are usually very elongated.

Part of their orbits lie very, very far from the Sun, and partly quite close to the Sun.

They are sometimes called "dirty snowballs" because they are small irregular formations of , and .

As the comet approaches the Sun, the ice begins to melt and boil, throwing out dust particles. These particles, together with form around the nucleus of a comet, which is called comet shell.

The shell is illuminated by the Sun. Sunlight repels, and it stretches into a long and brightly lit "tail".

Comets: shaggy wanderers of the universe

E. Halley predicted the date of the next appearance of this comet, and although he did not live to see this day, the prediction came true with brilliance.

Already in our time, in various historical chronicles, more than thirty references to the appearance of the "bearded star" have been found, which since the 18th century. began to wear Halley's Comet.

What are comets?

Halley established the most important fact - comets are members of the solar system and revolve around the sun.

However, we cannot observe them all the time, like other minor planets, because they have completely different orbits - so elongated that some of them come closer to the Sun than , and then move away to the Kuiper belt.

There are comets that spend integers per revolution, and they appear in the earth's sky only once for humanity.

What are the celestial bodies that the ancient Greeks called the word "comet", meaning "shaggy" in translation?

The main mass of the comet is concentrated in a small dense core, which consists of ice, ammonia and methane, which are interspersed with small solid particles - dust grains and grains of sand.

While the comet is in the cold regions of the solar system far from the Sun, or even outside it, the nucleus looks like a small, surrounded by a light hazy shell - it is called a "coma".

As we approach our star, the core begins to heat up, the ice evaporates, and gases are ejected from the core, taking solid particles with them.

A comet has a tail, or rather, two tails - gas and dust, which, under the action of the sun, are extended in the direction opposite to the Sun.

Sometimes the gas and dust tails take on different forms - the particles of the substances of which they are composed react differently to solar radiation, and the length of the tails sometimes reaches 200 million km or more.

The tails of comets do not have sharp outlines and are almost transparent - stars are clearly visible through them. The gas and the smallest dust particles in them are extremely rarefied, and we can observe them only due to their own glow under the influence of ultraviolet radiation from the Sun.

As one of the astronomers noted, in fact, this is “visible nothing”.

Today, astronomers are aware of more than 400 comets with a short period of revolution, and 200 of them were observed twice and thrice.

Modern research on comets

In 1986, the Vega-1 and Vega-2 and Giotto spacecraft "visited" Halley's comet, transmitted images of its nucleus to the earth and analyzed the tail material. Scientists' assumptions about the composition of cometary nuclei were confirmed. The comet's nucleus is about 10 km in size and rotates around its own axis.

The main habitat of comets is on the farthest outskirts of the solar system - in the Oort cloud. There they spend most of their "life".

But sometimes, under the influence of other cosmic bodies, some of them change their orbits and begin to approach the Sun. That's when we see them in the night or evening sky.

However, the comet that decides to leave the Oort cloud is short - after all, with each passage near the Sun, it loses some of its substance. After 10-15 thousand years, comets completely evaporate.

The mass of an average comet is negligible - about a billion times less than the mass of the Earth, and the density of matter from their tails is almost equal to . Therefore, "bearded stars" do not affect the planets of the solar system. So, in May 1910, the Earth passed through the tail of Halley's comet without even feeling it.

But the collision of the nucleus of a large comet with our planet can cause extremely serious consequences for the Earth's magnetosphere. An example of such an event is the fall of debris from the comet Shoemaker-Levy, which astronomers around the world observed in July 1994.

In 2005, the American Deep Impact spacecraft went to the comet in order to ... ram it. He dropped a special one onto the comet, which collided with the comet's nucleus.

With more than 10 thousand tons of matter, the comet turned into gas and dust, and the instruments determined the composition of the substance that makes up its “head”.

A comet is a celestial body of small size, consisting of ice interspersed with dust and stone fragments. As it approaches the sun, the ice begins to evaporate, leaving a tail behind the comet, sometimes stretching for millions of kilometers. The tail of a comet is made up of dust and gas.

comet orbit

As a rule, the orbit of most comets is an ellipse. However, circular and hyperbolic trajectories along which ice bodies move in outer space are also quite rare.

Comets passing through the solar system

Many comets pass through the solar system. Let's focus on the most famous space wanderers.

Comet Arend-Roland was first discovered by astronomers in 1957.

Comet Halley passes near our planet every 75.5 years. Named after the British astronomer Edmund Halley. The first mention of this celestial body is found in Chinese ancient texts. Perhaps the most famous comet in the history of civilization.

Comet Donati was discovered in 1858 by the Italian astronomer Donati.

Comet Ikeya-Seki was noticed by Japanese amateur astronomers in 1965. Differed in brightness.

Comet Lexell was discovered in 1770 by the French astronomer Charles Messier.

Comet Morehouse was discovered by American scientists in 1908. It is noteworthy that photography was used for the first time in its study. Distinguished by the presence of three tails.

Comet Hale-Bopp was visible in 1997 to the naked eye.

Comet Hyakutake was observed by scientists in 1996 at a small distance from the Earth.

Comet Schwassmann-Wachmann was first noticed by German astronomers in 1927.

"Young" comets have a bluish tint. This is due to the presence of a large amount of ice. As the comet rotates around the sun, the ice melts and the comet takes on a yellowish tint.

Most comets originate from the Kuiper Belt, a collection of frozen bodies near Neptune.

If the tail of a comet is blue and turned away from the Sun, this is evidence that it consists of gases. If the tail is yellowish and turned towards the Sun, then there is a lot of dust and other impurities in it that are attracted to the luminary.

Study of comets

Scientists obtain information about comets visually through powerful telescopes. However, in the near future (in 2014), the launch of the ESA Rosetta spacecraft is planned to study one of the comets. It is assumed that the device will be near the comet for a long time, accompanying the space wanderer on her way around the Sun.

Note that earlier NASA launched the Deep Impact spacecraft to collide with one of the solar system comets. Currently, the device is in good condition and is used by NASA to study icy space bodies.

People watching a star falling in the sky may have a question, what is a comet? This word in Greek means "long-haired". During the approach to the Sun, the asteroid begins to heat up and takes on an effective form: dust and gas begin to fly away from the surface of the comet, forming a beautiful, bright tail.

The appearance of comets

The appearance of comets is almost impossible to predict. Scientists and amateurs have been paying attention to them since ancient times. Large celestial bodies rarely fly by the Earth, and such a sight fascinates and frightens. In history there is information about such bright bodies that sparkle through the clouds, eclipsing even the moon with their glow. It was with the advent of the first such body (in 1577) that the study of the motion of comets began. The first scientists were able to discover dozens of very different asteroids: their approach to the orbit of Jupiter begins with the glow of the tail, and the closer the body is to our planet, the brighter it burns.

It is known that comets are such bodies that move along certain trajectories. Usually it has an elongated shape, and is characterized by its position relative to the Sun.

The comet's orbit may be the most unusual. From time to time some of them return to the Sun. Scientists say that such comets are periodic: they fly near the planets after a certain period of time.

Comets

Since ancient times, people have called any luminous body a star, and those behind which trailed tails have been called comets. Later, astronomers discovered that comets are huge solid bodies, representing large ice fragments mixed with dust and stones. They come from distant space and can either fly past or revolve around the Sun, periodically appearing in our sky. Such comets are known to move in elliptical orbits of various sizes: some return once every twenty years, and some appear once every hundreds of years.

periodic comets

Scientists know a lot of information about periodic type comets. Orbits and return times are calculated for them. The appearance of such bodies is not unexpected. Among them are short-term and long-term.

Short-period comets are those that can be seen in the sky several times in a lifetime. Others may not appear in the sky for centuries. One of the most famous short-period comets is Halley's Comet. It appears near the Earth once every 76 years. The length of the tail of this giant reaches several million kilometers. It flies so far from us that it seems like a stripe in the sky. Her last visit was recorded in 1986.

comet fall

Scientists know many cases of asteroids falling on planets, and not only on Earth. In 1992, the giant Shoemaker-Levy came very close to Jupiter and was torn apart by its gravity into numerous pieces. The fragments stretched into a chain, and then moved away from the planet's orbit. Two years later, the chain of asteroids returned to Jupiter and fell on it.

According to some scientists, if an asteroid flies in the center of the solar system, then it will live for many thousands of years until it evaporates, flying once again near the Sun.

Comet, asteroid, meteorite

Scientists have identified the difference in the value of asteroids, comets, meteorites. Ordinary people call by these names any bodies seen in the sky and having tails, but this is not correct. From a scientific point of view, asteroids are huge boulders floating in space in certain orbits.

Comets are similar to asteroids, but they have more ice and other elements. When approaching close to the Sun, comets develop a tail.

Meteorites are small rocks and other space debris smaller than a kilogram in size. They are usually seen in the atmosphere as shooting stars.

Famous comets

Comet Hale-Bopp was the brightest comet of the twentieth century. It was discovered in 1995, and two years later it became visible in the sky with the naked eye. It could be observed in the sky for more than a year. It is much longer than the radiance of other bodies.

Comet ISON was discovered in 2012. According to forecasts, it was supposed to become the brightest, but, approaching the Sun, it could not meet the expectations of astronomers. However, it was dubbed the "comet of the century" in the media.

The most famous is Halley's comet. She played an important role in the history of astronomy, including helping to derive the law of gravity. The first scientist to describe the heavenly bodies was Gallileo. His information was processed more than once, changes were made, new facts were added. Once Halley drew attention to a very unusual pattern of the appearance of three celestial bodies with an interval of 76 years and moving almost on the same trajectory. He concluded that these were not three different bodies, but one. Later, Newton used his calculations to build a theory of gravity, which was called the theory of universal gravitation. Halley's comet was last seen in the sky in 1986, and its next appearance will be in 2061.

In 2006, Robert McNaught discovered the celestial body of the same name. According to assumptions, it should not have glowed brightly, however, as it approached the Sun, the comet began to quickly gain brightness. A year later, it began to glow brighter than Venus. Flying near the Earth, the celestial body made a real spectacle for earthlings: its tail curved in the sky.

Word "comet" is of Greek origin. It can be translated as "caudate" , "hairy" , "shaggy" .


This definition accurately characterizes the celestial body, since the "tail" of gas and dust is a characteristic feature of most comets.

A comet is a celestial body that, relative to other bodies in outer space, has a relatively small mass, usually of an irregular shape, composed of frozen gases and non-volatile components.

Comets move in space in certain orbits. The comet's orbit around the Sun is an extremely elongated ellipse. Depending on how far from the star the comet is, its appearance changes.

Away from the Sun, the comet looks like a blurry cloud. When approaching it, under the influence of solar thermal energy, the comet begins to evaporate gas. The gas "blows off" particles of solid matter that make up the comet, and they take the form of a cloud around the nucleus, forming a coma. It happens that the coma swells to a huge size.


Due to evaporation and the action of the solar wind, the comet "grows" a tail of dust and gas, thanks to which it received its name.

Characteristics of comets

Conventionally, a comet can be divided into three parts - the core, coma, tail. Everything in comets is absolutely cold, and their glow is only the reflection of sunlight by dust and the glow of ultraviolet-ionized gas.

Core

The core is the heaviest part of this celestial body. It contains the bulk of the comet's mass. It is rather difficult to study the composition of the comet nucleus precisely, since at a distance accessible to the telescope, it is constantly surrounded by a gaseous mantle. In this regard, the theory of the American astronomer Whipple was adopted as the basis for the theory of the composition of the comet nucleus.

According to his theory, the nucleus of a comet is a mixture of frozen gases mixed with various dusts. Therefore, when a comet approaches the Sun and heats up, the gases begin to "melt", forming a tail. However, there are other assumptions about the composition of the core.

One of them claims that the comet has a loose structure of dust with very large pores - a kind of cosmic "sponge". The “sponge” is incredibly fragile: if you take even a very large piece of a comet, you can easily tear it apart with your hands.

Tail

The tail of a comet is its most expressive part. It is formed near a comet as it approaches the Sun. The tail is a luminous strip that stretches from the nucleus in the opposite direction from the Sun, "blown away" by the solar wind.

It consists of gases and dust that evaporate from the nucleus of a comet under the influence of the same solar wind. The tail glows brightly - thanks to it, we have the opportunity to observe the flight of these celestial bodies.

Differences of comets from each other

Comets differ from each other in mass and size. Some of them are heavier, others are lighter, but still these celestial bodies are very small compared to the rest of the bodies in the universe. In addition, the observer (if he is very lucky) can see that different comets have different glows and shapes. It depends on what gases evaporate from the surface of their nuclei.

Comet tails can also vary in length and shape. For some, it stretches across the entire visible sky: in 1680, the inhabitants of the Earth could observe the Big Comet with a tail of 240 million kilometers. Some comets have a straight and narrow tail, others are slightly curved and wide, deviating to the side; still others are short and pronouncedly curved.

Differences between comets and asteroids

Asteroids, like comets, are small celestial bodies. However, asteroids are larger than comets: according to the international classification, they include bodies whose diameter exceeds 30 m. Until 2006, the asteroid was even called a minor planet. Indirectly, this was served by the fact that asteroids have satellites.

Asteroids and comets have a number of other differences from each other.

First, an asteroid and a comet differ in their composition. An asteroid is made up mostly of metals and rocks, while a comet, as we already know, is made up of frozen gases and dust.


This implies the second difference - the asteroid does not have a tail, since there is nothing to evaporate from its surface. Unlike comets, asteroids move in a circular orbit and tend to unite into belts.

And lastly, there are several million known asteroids, while there are only 3,572 comets.