Minor Bodies of the Solar System

So far we have considered only planets and planetary satellites. There is a great number of other bodies in the solar system, like dwarfplanets, asteroids, comets, meteoroids and interplanetary dust. However, there are no distinct borders between different types of objects. Some asteroids have similar features or origin as the comets, and some near-Earth asteroids are possibly cometary remnants where all volatile elements have disappeared. Thus our classification has been based more on the visual appearance and tradition than on real physical differences.

In 2006 the International Astronomical Union (IAU) in its General Assembly defined three distinct categories, namely planets, dwarf planets, and Small Solar System Bodies which include the rest of the Solar System bodies, like asteroids, Trans-Neptunian Objects, comets and meteoroids.

Dwarf planets. According to the IAU definition a dwarf planet is a celestial body that: (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.

The upper and lower limits to the size and mass of dwarf planets are not strictly specified. The lower limit, however, is determined by the hydrostatic equilibrium shape, but the size at which this happens may vary according to the composition and history of the object. It is estimated that up to 40-50 dwarf planets will be discovered in coming years.

Currently, there are three dwarf planets in the Solar System, namely Ceres, Pluto and Eris. Ceres was formerly counted as an asteroid, Pluto was a planet and Eris (2003 UB 313, known also by the nickname Xena) was the first Trans-Neptunian object which turned out to be larger than Pluto.

Pluto was discovered in 1930 at the Lowell Observatory, Arizona, after an extensive photographic search (Fig. 7.53). This search had already been initiated in the beginning of the century by Percival Lowell, on the basis of the perturbations observed in the orbits of Uranus and Neptune. Finally, Clyde Tombaugh discovered Pluto less than 6° off the predicted position. However, Pluto turned out to be far too small to cause any perturbations on Uranus or Neptune. Thus the discovery was purely accidental, and the perturbations observed were not real, but caused by minor errors of old observations.

Pluto has no visible disk as seen with terrestrial telescopes; instead, it resembles a point, like a star (Fig. 7.54). This fact gave an upper limit for the diameter of Pluto, which turned out to be about 3000 km. The exact mass was unknown until the discovery of the Plutonian moon, Charon, in 1978. The mass of Pluto is only 0.2% of the mass of the Earth. The orbital period of Charon is 6.39 days, and this is also the period of rotation of both bodies. Pluto and Charon rotate synchronously, each turning the same side towards the other body. The rotation axis of Pluto is close to the orbital plane: the tilt is 122°.

Mutual occultations of Pluto and Charon in 19851987 gave accurate diameters of each body: The diameter of Pluto is 2300 km and that of Charon, 1200 km. The density of Pluto turned out to be about

7.17 Minor Bodies of the Solar System

187

Fig. 7.53. A small portion of the pair of pictures where Pluto was discovered in 1930. The planet is marked with an arrow. (Lowell Observatory)

2100 kg m-3. Thus Pluto is not a huge iceball but about 2/3 of its mass is composed of rocks. The relatively small abundance of ices is possibly due to the low temperature during the planetary accretion when most of the free oxygen was combined with carbon forming carbon monoxide. The computed lower limit for water ice is about 30% which is fairly close to the value observed in Pluto.

Pluto has a thin methane atmosphere and there is possibly a thin haze over the surface. The surface pressure is 10-5-10-6 atm. Ithas been speculated that when

Pluto is far from perihelion, the whole atmosphere will become frozen and fall on the surface.

Pluto has three satellites. Two of them were discovered by the Hubble Space Telescope in 2005 (Fig. 7.54). They orbit Pluto counterclockwise twice the distance of Charon.

The orbit of Pluto is different from planetary orbits. The eccentricity is 0.25 and the inclination is 17°. During its 250 year orbit, Pluto is closer to the Sun than Neptune for 20 years; one such period lasted from 1979 to 1999. There is no danger of Pluto and Neptune col-

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tographed Pluto and its three known moons in February 2006. The smaller moons were found in 2005, and were later named Nix and Hydra. Their diameter is estimated as 40-160 km. (M. Mutchler (STScI), A. Stern (SwRI), and the HST Pluto Companion Search Team, ESA, NASA)

Fig. 7.54. Even with large terrestrial telescopes Pluto is seen only as a point of light. The best views have been obtained from the Hubble Space Telescope showing some albedo differences on the surface. (Alan Stern/Southwest Research Institute, Marc Buie/Lowell Observatory, NASA and ESA). (right) The Hubble Space Telescope pho-

tographed Pluto and its three known moons in February 2006. The smaller moons were found in 2005, and were later named Nix and Hydra. Their diameter is estimated as 40-160 km. (M. Mutchler (STScI), A. Stern (SwRI), and the HST Pluto Companion Search Team, ESA, NASA)

liding, since Pluto is high above the ecliptic when at the distance of Neptune. Pluto's orbital period is in a 3:2 resonance with Neptune.

Since 1990's a number of distant Trans-Neptunian objects (TNOs) have been discovered. In the Kuiper belt, a vast collection of icy bodies beyond the orbit of Neptune, there are objects even larger than Pluto. One of them is Eris which is now classified as a dwarf planet. It was discovered in 2003, and it was for some time known by an unofficial name Xena. Eris is slightly larger than Pluto; the diameter is estimated to be about 2400 km. The semimajor axis of the orbit is 97 AU, orbital period 560 years and inclination 45°.

The third dwarf planet Ceres was the first asteroid discovered in 1801 by Giuseppe Piazzi. The diameter of Ceres is about 1000 km, thus exceeding the limit to be in the hydrostatic equilibrium. Contrary to Pluto and Eris, Ceres is a more close object. It orbits the Sun in the main asteroid belt between Mars and Jupiter. We will discuss asteroids and other Small Solar System Bodies below.

Asteroids. Asteroids form a large and scattered group of Sun-orbiting bodies. The oldest and best-known group form the main asteroid belt between Mars and Jupiter, with distances of 2.2-3.3 AU from the Sun (Fig. 7.56). The most distant asteroids are far beyond the orbit of Pluto, and there are a number of asteroids that come closer to the Sun than the Earth. Diameters of asteroids vary from hundreds of meters to hundreds of kilometers. The largest asteroid (1) Ceres is classified as a dwarf planet and the border between smallest asteroids and meteoroids is not specified. Structure and composition of asteroids range from comet-like icy and loose clumps of material to iron-nickel or stony hard and solid bodies.

An asteroid observer needs a telescope, since even the brightest asteroids are too faint to be seen with the naked eye. Asteroids are points of light like a star, even if seen through a large telescope; only their slow motion against the stellar background reveals that they are members of the solar system. The rotation of an asteroid gives rise to a regular light variation. The amplitude of light variation is in most cases well below 1 magnitude and typical rotation periods range from 4 to 15 hours.

At the end of year 2006 there were more than 140,000 numbered asteroids. The number of catalogued asteroids increases currently by thousands every month. It has been estimated that more than one million asteroids larger than 1 km exist in the Solar System.

The characteristics of the main belt asteroids are best known. Total mass of the main belt asteroids is less than 1/1000 of the mass of the Earth. The centre of the asteroid belt is at a distance of approximately 2.8 AU, as predicted by the Titius-Bode law (Sect. 7.19). According to a formerly popular theory, asteroids were thought to be debris from the explosion of a planet. This theory, like catastrophe theories in general, has been abandoned.

The currently accepted theory assumes that asteroids were formed simultaneously with the major planets. The primeval asteroids were large chunks, most of them orbiting between the orbits of Mars and Jupiter. Due to mutual collisions and fragmentation, the present asteroids are debris of those primordial bodies which were never able to form a large planet. Some of the biggest asteroids may be those original bodies. The orbital elements of some asteroids are very similar. These are called the Hirayama families. They are probably remnants of a single, large body that was broken into a group of smaller asteroids. There are tens of identified Hi-rayama families, the largest ones including Hungarias, Floras, Eos, Themis, and Hildas (named after the main asteroid in the group).

The distribution of asteroids inside the asteroid belt is uneven (Fig. 7.56); they seem to avoid some areas known as the Kirkwood gaps. The most prominent void areas are at distances where the orbital period of an asteroid around the Sun (given by Kepler's third law) is in the ratio 1:3, 2:5, 3:7, or 1:2 to the orbital period of Jupiter. The motion of an asteroid orbiting in such a gap would be in resonance with Jupiter, and even small perturbations would tend to grow with time. The body would eventually be moved to another orbit. However, the resonance effects are not so simple: sometimes an orbit is "locked" to a resonance, e.g. the Trojans move along the same orbit as Jupiter (1:1 resonance), and the Hilda group is in the 2:3 resonance.

Many groups of asteroids orbit the Sun outside the main belt. These include the above-mentioned Trojans, which orbit 60° behind and ahead of Jupiter. The Trojans, which are close to the special points L 4 and L5 of the solution of the restricted three-body problem. At these Lagrangian points, a massless body can remain stationary with respect to the massive primaries (in this

Fig. 7.55. Left: Asteroid (951) Gaspra was photographed by the Galileo spacecraft in October 1991. The illuminated part of the asteroid is about 16 x 12 km. The smallest craters in this view are about 300 m across. Right: A mosaic of asteroid

case, Jupiter and the Sun). In fact, the asteroids are oscillating around the stationary points, but the mean orbits can be shown to be stable against perturbations.

Another large family is the Apollo-Amor asteroids. The perihelia of Apollo and Amor are inside the Earth's orbit and between the orbits of the Earth and Mars, respectively. These asteroids are all small, less than 30 km in diameter. The most famous is 433 Eros (Fig. 7.55), which was used in the early 20th century for determining the length of the astronomical unit. When closest to the Earth, Eros is at a distance of only 20 million km and the distance can be directly measured using the trigonometric parallax. Some of the Apollo-Amor asteroids could be remnants of short-period comets that have lost all their volatile elements.

There is a marginal probability that some Earth-crossing asteroids will collide with the Earth. It has been estimated that, on the average, a collision of a large asteroid causing a global catastrophe may take place once in one million years. Collisions of smaller bodies, causing damage similar to a nuclear bomb, may happen once per century. It has been estimated that there are 500-1000

(433) Eros was taken by the NEAR spacecraft from a distance of 200 km. The crater on top is about 5 km in diameter. The NEAR spacecraft orbited Eros for one year and finally landed on it in 2001. (JPL/NASA)

near-Earth asteroids larger than one kilometre in diameter but possibly tens of thousands smaller objects. Programs have been started to detect and catalogue all near-Earth asteroids and to predict the probabilities of hazardous collisions.

Distant asteroids form the third large group outside the main asteroid belt. The first asteroid belonging to this group (2060) Chiron, was discovered in 1977. Chiron's aphelion is close to the orbit of Uranus and the perihelion is slightly inside the orbit of Saturn. Distant asteroids are very faint and thus difficult to find.

Already in the 1950's Gerard Kuiper suggested that comet-like debris from the formation of the solar system can exist beyond the orbit of Neptune as an additional source of comets to the more distant Oort cloud. Later, computer simulations of the solar system's formation showed that a disk of debris should form at the outer edge of the solar system. The disk is now known as the Kuiper belt (Fig. 7.58).

The first Trans-Neptunian asteroid (1992 QB1) was discovered in 1992, and in the beginning of year 2006

Fig. 7.56. (a) Most of the asteroids orbit the Sun in the asteroid belt between Mars and Jupiter. The figure shows the positions of about 96,000 catalogued asteroids on January 1, 2000, and the orbits and the positions of some major planets. The orbital elements of the asteroids are from the Lowell Observatory data base. (b) The total number of asteroids as a function of the distance from the Sun. Each bin corresponds to 0.1 AU. The empty areas, the Kirkwood gaps, are at those points, where the orbital period of an asteroid is in a simple ratio to the orbital period of Jupiter

Fig. 7.56. (a) Most of the asteroids orbit the Sun in the asteroid belt between Mars and Jupiter. The figure shows the positions of about 96,000 catalogued asteroids on January 1, 2000, and the orbits and the positions of some major planets. The orbital elements of the asteroids are from the Lowell Observatory data base. (b) The total number of asteroids as a function of the distance from the Sun. Each bin corresponds to 0.1 AU. The empty areas, the Kirkwood gaps, are at those points, where the orbital period of an asteroid is in a simple ratio to the orbital period of Jupiter there were about 1000 known members. The total number of Kuiper belt objects larger than 100 km in diameter is estimated to be over 70,000. Some of them may be even larger than Pluto. The Kuiper belt objects are remnants from the early accretion phases of the solar system. Several of the Trans-Neptunian objects are in or near a 3:2 orbital period resonance with Neptune, the same resonance as Pluto.

The exact sizes of asteroids were long unknown. Edward E. Barnard of the Lick Observatory determined visually the diameters of (1) Ceres, (2) Vesta, (3) Juno, and (4) Pallas in the 1890's (Fig. 7.57). Practically no other reliable results existed prior to the 1960's, when indirect methods applying photometry and spectroscopy were adopted. Moreover, several stellar occultations caused by asteroids have been observed since 1980's.

The first images of asteroids were obtained in the early 1990's. In 1991 the Galileo spacecraft passed asteroid (951) Gaspra, and in 1993 asteroid (243) Ida, on its long way to Jupiter (see Sect. 7.15). Finally, in 2001, the NEAR spacecraft landed on asteroid (433) Eros after orbiting it for one year.

Fig. 7.57. Sizes of some asteroids compared with the Moon. (Moon image, NASA)

Orbit of Binary Kuiper Belt Object >1998 WW31

Kuiper Belt and outer Solar System planetary orbits

The Oort Cloud (comprising many billions of comets)

Oort Cloud cutaway drawing adapted from Donald K. Yeomaris illustration (NASA, JPL)

The Oort Cloud (comprising many billions of comets)

Fig. 7.58. The Kuiper Belt is a disk-shaped cloud of distant icy bodies inside the halo of the Oort cloud. The short-period comets originate in the Kuiper belt, whereas a huge amount of icy bodies that form a source of long period comets resides in the Oort cloud (see Sect. 7.18). (JPL/NASA)

The images of asteroids (Fig. 7.55) show irregular, crater-filled bodies with regolith and pulverised rock on their surface. Some asteroids may once have been two separate objects that merged into one. In 1992 asteroid (4179) Toutatis passed the Earth only by 4 million kilometres. Radar images revealed a two-body system, where the components were touching each other. Double asteroids may be quite common, and there exist light curves of some asteroids which have been interpreted as results of twin bodies. Another example of a twin asteroid is 243 Ida that has a "moon", a smaller body gravitationally bound to it.

The composition of main belt asteroids is similar to that of iron, stone and iron-stone meteorites. Most asteroids can be divided into three groups, according to their photometric and polarimetric properties. 95% of the classified asteroids belong to the types C and S types. Metal-rich M type asteroids are rarer.

About 75 percent of asteroids belong to the type C type. The C asteroids are dark due to radiation darkening (geometric albedo p ~ 0.06 or less), and they contain a considerable amount of carbon (mnemonic C for carbon). They resemble stony meteorites. The material is undifferentiated and thus they belong to the most primordial bodies of the solar system. The reflectivity of silicate-rich S asteroids is higher and their spectra are close to those of stone-iron meteorites. Their spectra show signs of silicates, such as olivine, e. g. fos-terite Mg2SiO4 or fayalite Fe2SiO4. M type asteroids have more metals, mostly nickel and iron; they have undergone at least a partial differentiation.

The compositions and even sizes of the Trans-Neptunian objects are difficult to determine. They are dim, and due to their low temperature, the black-body radiation maximum is around 60 ¡m. This wavelength is almost impossible to observe on the Earth. Even the

Fig. 7.59. Top: Comet Mrkos in 1957. (Palomar Observatory). Lower left: The impactor of the Deep Impact spacecraft collided with the nucleus of comet Tempel 1 in July 2005. In this picture, the collision point is between the two sharp craters in the lower part of the body. The diameter of the nucleus is

Fig. 7.59. Top: Comet Mrkos in 1957. (Palomar Observatory). Lower left: The impactor of the Deep Impact spacecraft collided with the nucleus of comet Tempel 1 in July 2005. In this picture, the collision point is between the two sharp craters in the lower part of the body. The diameter of the nucleus is about 5 km. (Photo NASA) Lower right: A composite image of the nucleus of comet P/Halley taken by ESA Giotto spacecraft in 1986. The size of the nucleus is approximately 13 x 7 km. Dust jets are originating from two regions on the nucleus. (ESA/Max Planck Institut für Aeronomie)

estimations of the albedos, and therefore the diameter are very uncertain.

Colors of TNOs range from blue-grey to red and the distribution appears to be uniform. However, population of the low-inclination objects seem to be red and high-inclination objects blue. The unperturbed orbits of the low-inclination objects suggest that they represent a relic of the original population of the Kuiper belt.

Interpretations of the spectra are ambiguous and spectra may not describe the composition of the whole

Fig. 7.60. Orbits of short period comets projected to the plane of the ecliptic

object. The surface is altered by intense radiation, solar wind and micrometeorites and it can be quite different from the regolith and deeper layers underneath.

Small TNOs are probably mixtures of rock and ice with some organic surface material. The composition is similar to the comets. High density (2000-3000 kg m-3) of some large objects suggests a high non-ice content, similar to Pluto.

Comets. Comets are agglomerates of ice, snow, and dust; a typical diameter is of the order of 10 km or less. The nucleus contains icy chunks and frozen gases with embedded rock and dust. At its centre, there can be a small, rocky core.

A comet is invisible when far from the Sun; when it gets closer than about 2 AU, the heat of the Sun starts to melt the ice and snow. The outflowing gas and dust form an envelope, the coma around the nucleus. Radiation pressure and the solar wind push ionised gas and dust away from the Sun, resulting in the typical long-tailed shape of a comet (Fig. 7.59).

The tail is always pointing away from the Sun, a fact which was noticed in the 16th century. Usually, there are two tails, an ion tail (gas tail) and a dust tail. The partly ionised gas and very fine dust in the ion tail are driven by the solar wind. Some of the light is reflected solar light, but the brightness of the ion tail is mostly due to emission by the excited atoms. The dust tail is caused by the

Fig. 7.61. Comet Shoemaker-Levy 9 five months before its collision to Jupiter as seen by the Hubble Space Telescope. (JPL/NASA)

80 60 40 20

Fig. 7.62. A schematic diagram of the distribution of the semimajor axes of long-period comets. The abscissa is the inverse of the semimajor axis, 1/a [AU]-1. The Oort cloud is visible as a strong peak at the very small positive values of 1 /a. The orbits shown here are the "original orbits", i. e. computed backward in time to remove all known perturbations

Fig. 7.62. A schematic diagram of the distribution of the semimajor axes of long-period comets. The abscissa is the inverse of the semimajor axis, 1/a [AU]-1. The Oort cloud is visible as a strong peak at the very small positive values of 1 /a. The orbits shown here are the "original orbits", i. e. computed backward in time to remove all known perturbations radiation pressure. Because the velocities of the particles of the dust tail are lower than the velocities in the ion tail, the dust tail is often more curved than the ion tail.

Fred Whipple introduced in 1950's a "dirty snowball" theory to describe the cometary structure. According to this model, cometary nuclei are composed of ice mixed with gravel and dust. The observations have revealed that the classical dirty snowball model is not quite accurate; at least the surface is more dirt than snow, also containing organic compounds. Several chemical compounds have been observed, including water ice, which probably makes up 75-80% of the volatile material. Other common compounds are carbon monoxide (CO), carbon dioxide (CO2), methane (CH4), ammonia (NH3), and formaldehyde (H2CO).

The most famous (and also best known) periodic comet is Halley's comet. Its orbital period is about 76 years; it was last in perihelion in 1986. During the last apparition, the comet was also observed by spacecraft, revealing the solid cometary body itself for the first time. Halley is a 13 x 7 km, peanut-shaped chunk whose surface is covered by an extremely black layer of a possibly tar-like organic or other similar material. Violent outbursts of gas and dust make an exact prediction of its brightness impossible, as often noticed when cometary magnitudes have been predicted. Near the perihelion, several tons of gas and dust burst out every second.

Cometary material is very loose. Ablation of gas and dust, large temperature variations and tidal forces sometimes cause the whole comet to break apart. Comet Shoemaker-Levy 9 which impacted into Jupiter in 1994 was torn apart two years earlier when it passed Jupiter at a distance of 21,000 km (Fig. 7.63). The impact of Shoemaker-Levy 9 showed that there can be density variation (and perhaps variation in composition, too) inside the original cometary body.

Comets are rather ephemeral things, surviving only a few thousand revolutions around the Sun or less. The short-period comets are all newcomers and can survive only a short time here, in the central part of the solar system.

Since comets in the central solar system are rapidly destroyed, there has to be some source of new short-period comets. In 1950 Jan Oort discovered a strong peak for aphelia of long period comets at a distance of about 50,000 AU, and that there is no preferential direction from which comets come (Fig. 7.62). He proposed that there is a vast cloud of comets at the outer reaches of the solar system, now know as the Oort cloud (Fig. 7.60). The total mass of the Oort cloud is estimated to be tens of Earth masses, containing more than 1012 comets.

A year later GerardKuiper showed that there is a separate population of comets. Many of the short period comets, with periods less than 200 years, have the orbital inclination less than 40°, and they orbit the Sun in the same direction as the Earth. The orbital inclination of long period comets are not peaked around the plane of the ecliptic but they are more random. Kuiper argued that the short period comets originate from a separate population of comets that resides in a disk-like cloud beyond the orbit of Neptune. The area is now known as the Kuiper belt (Fig. 7.60).

Occasionally perturbations from passing stars send some of the comets in the Oort cloud into orbits, which bring them into the central parts of the solar system, where they are seen as long-period comets. Around a dozen "new" comets are discovered each year. Most of these are visible only with a telescope, and only a couple of times per decade one can see a bright naked-eye comet.

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7.17 Minor Bodies of the Solar System

195

Fig. 7.63. Meteors are easy to capture on film: one just leaves shutter open for an hour or so. Stars make curved trails on the a camera loaded with a sensitive film on a tripod with the film. (L. Häkkinen)

Some of the long period comets are put into short period orbits by the perturbations of Jupiter and Saturn, whereas some others can be ejected from the solar system. However, there are no comets that have been proven to come from interstellar space, and the relative abundances of several isotopes in the cometary matter are the same as in other bodies of our solar system.

The origin of the Oort cloud and Kuiper belt is different. The Oort cloud objects were formed near the giant planets and have been ejected to the outer edge of the solar system by gravitational perturbations soon after the formation of the solar system. Small objects beyond the orbit of Neptune had no such interactions and they remained near the accretion disk.

Meteoroids. Solid bodies smaller than asteroids are called meteoroids. The boundary between asteroids and meteoroids, however, is diffuse; it is a matter of taste whether a ten metre body is called an asteroid or a me-teoroid. We could say that it is an asteroid if it has been observed so often that its orbital elements are known.

When a meteoroid hits the atmosphere, an optical phenomenon, called a meteor ("shooting star") is seen (Fig. 7.63). The smallest bodies causing meteors have a mass of about 1 gram; the (micro)meteoroids smaller than this do not result in optical phenomena. However, even these can be observed with radar which is able to detect the column of ionised air. Micrometeoroids can also be studied with particle detectors installed in satellites and space crafts. Bright meteors are called bolides.

The number of meteoroids increases rapidly as their size diminishes. It has been estimated that at least 105 kg of meteoritic material falls on the Earth each day. Most

of this material is micrometeoroids and causes no visible phenomena.

Due to perspective, all meteors coming from the same direction seem to radiate from the same point. Such meteor streams (meteor showers) are, e.g. the Perseids in August and the Geminides in December; the names are given according to the constellation in which the radiation point seems to be. On the average, one can see a few sporadic meteors per hour. During a strong meteor shower one can see even tens of meteors per minute, although a normal rate is some tens per hour.

Most of the meteoroids are small and burn to ashes at a height of 100 km. However, larger bodies may come through and fall to the Earth. These are called meteorites. The relative speed of a typical meteoroid varies in the range 10-70 km/s. The speed of the largest bodies does not diminish in the atmosphere; thus, they hit the Earth at their cosmic speeds, resulting in large impact craters. Smaller bodies slow down and drop like stones but impacts of large bodies (diameter meters or more) may cause large-scale disaster.

Iron meteorites or irons, composed of almost pure nickel-iron, comprise about one quarter of all meteorites. Actually the irons are in a minority among meteoroids, but they survive their violent voyage through the atmosphere more easily than weaker bodies. Three-quarters are stony meteorites, or stone-iron meteorites.

Meteoroids themselves can be divided into three groups of roughly equal size. One-third is ordinary stones, chondrites. The second class contains weaker carbonaceous chondrites and the third class includes cometary material, loose bodies of ice and snow which are unable to survive down to the Earth.

Many meteor streams are in the same orbit as a known comet, so at least some meteoroids are of cometary origin. Near a perihelion passage, every second several tons of gravel is left on the orbit of a comet. There are several examples of meteorites that have their origin in the Moon or Mars. Debris of large impacts may have been ejected into space and finally ended up on the Earth. Some meteoroids are debris of asteroids.

Interplanetary Dust. Two faint light phenomena, namely zodiacal light and gegenschein (counterglow) make it possible to observe interplanetary dust, small dust particles reflecting the light of the Sun (Fig. 7.64).

Fig. 7.64. A projection of the entire infrared sky created from Earth as zodiacal light is an S-shaped glow across the image. observations of the COBE satellite. The bright horizontal band (G. Greaney and NASA) is the Milky Way. The dust of the solar system, visible on the

This weak glow can be seen above the rising or setting Sun (zodiacal light) or exactly opposite the Sun (gegenschein). The interplanetary dust is concentrated near the plane of the ecliptic. The typical sizes of the particles are in the range of 10-100 ^m.

Solar Wind. Elementary particles hitting the Earth originate both in the Sun and outside the solar system. Charged particles, mainly protons, electrons and alpha particles (helium nuclei) flow continuously out of the Sun. At the distance of the Earth, the speed of this solar wind is 300-500 km/s. The particles interact with the solar magnetic field. The strength of the solar magnetic field at the Earth's distance is about 1/1000 of that of the Earth. Particles coming from outside the solar system are called cosmic rays (Sect. 15.8).

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