Asteroid Introduction
Solar system! Jupiter Mars Earth Asteroid Ida! Sun Comet

Historical survey of major asteroid discoveries
Early observations
Modern research
Development of classification systems

Orbits of asteroids
Distribution and Kirkwood gaps
Outer-belt asteroids
Trojan asteroids
Inner-belt asteroids
Near-Earth asteroids

The nature of asteroids
Rotation and shape
Size and albedo
Mass and density

The origin and evolution of the asteroids

Asteroid 4179 Toutatis

Asteroids also called minor planest, or planetoids. Asteroids are rocky metallic objects that orbit the Sun but are too small to be considered planets. It is because of their small size and large numbers relative to the nine major planets that asteroids are also called minor planets. The two designations are frequently used interchangeably, though dynamicists, astronomers who study individual objects with dynamically interesting orbits or groups of objects with similar orbital characteristics, generally use the term minor planet, whereas those who study the physical properties of such objects usually refer to them as asteroids.

Historical survey of major asteroid discoveries

Early observations

Distant stars

The first asteroid was discovered on Jan. 1, 1801, by Giuseppe Piazzi at Palermo, Italy. At first Piazzi thought that he had discovered a comet; however, after the orbital elements of the object had been computed it became clear that the object moved in a planetlike orbit between the orbits of Mars and Jupiter. Owing to illness, Piazzi was able to observe the object only until February 11, and, as no one else was aware of its existence, it was not reobserved before it moved into the daytime sky. The short arc of observations did not allow computation of an orbit of sufficient accuracy to predict where the object would reappear when it moved back into the night sky, and so it was lost. There matters might have stood were it not for the fact that this object was located at the heliocentric distance predicted by Bode's law of planetary distances proposed in 1766 by the German astronomer Johann D. Titius and popularized by his compatriot Johann E. Bode, who used the scheme to advance the notion of a "missing" planet between Mars and Jupiter. The discovery of Uranus in 1781 by the British astronomer William Herschel at a distance that closely fit the distance predicted by Bode's law was taken as strong evidence of its correctness. Some astronomers were so convinced that during an astronomical conference in 1796 they agreed to undertake a systematic search. Ironically, Piazzi was not a party to this attempt to locate the missing planet. Nonetheless, Bode and others, on the basis of the preliminary orbit, believed that Piazzi had found and then lost it. This led the German mathematician Carl Friedrich Gauss to develop in 1801 a method for computing the orbit of an asteroid from only a few observations, a technique that has not been significantly improved since. Using Gauss's predictions, the German astronomer Franz von Zach rediscovered Ceres on Jan. 1, 1802. Piazzi named this object Ceres after the ancient Roman grain goddess and patron goddess of Sicily, thereby initiating a tradition that continues to the present day: asteroids are named by their discoverers (in contrast to comets, which are named for their discoverers).

The discovery of three more faint (compared with Mars and Jupiter) objects in similar orbits over the next six years (Pallas, Juno, and Vesta, respectively) complicated this elegant solution to the missing-planet problem and gave rise to the surprisingly long-lived, though no longer generally accepted, idea that the asteroids were remnants of a planet that had exploded.

Following this flurry of activity, the search for the planet appears to have been abandoned until 1830, when Karl L. Hencke renewed it. In 1845 he discovered the fifth asteroid, which he named Astraea.

Modern research Asteroid 4 Vesta!
There were 88 known asteroids by 1866, when the next major discovery was made:

Daniel Kirkwood, an American astronomer, noted that there were gaps in the distribution of asteroid distances from the Sun (see below Distribution and Kirkwood gaps). The introduction of photography to the search for new asteroids by the German astronomer Max Wolf in 1891, by which time 322 asteroids had been identified, accelerated the discovery rate. By the end of the 19th century, 464 had been found. The asteroid designated 323 Brucia, detected by Wolf in 1891, was the first to be discovered by means of photography.

The first measurements of the sizes of asteroids were made in 1894 and 1895 by the American astronomer Edward E. Barnard, who used a filar micrometer (an instrument normally employed for visual measurement of the separations of double stars) to estimate the diameters of the first four asteroids. Barnard's results established that Ceres was the largest asteroid, with an estimated diameter of nearly 800 km. These values remained the best available until new techniques were introduced during the early 1970s (see below Size and albedo). The first four asteroids came to be known as "the big four," and, because all other asteroids were much fainter, they were believed to be considerably smaller as well.

In 1918 the Japanese astronomer Kiyotsugu Hirayama recognized clustering in three of the orbital elements of various asteroids (semimajor axis, eccentricity, and inclination). He speculated that objects sharing these elements had been formed by explosions of larger parent asteroids and called such groups of asteroids "families."

The idea that Jupiter was responsible for interrupting the formation of a planet from the swarm of planetesimals accreting near a heliocentric distance of 2.8 AU was introduced in 1944 by O.J. Schmidt. In 1951 the Estonian astronomer Ernest J. Öpik calculated the lifetimes of asteroids with orbits that passed close to those of the major planets and showed that most such asteroids were destined to collide with a planet or be ejected from the solar system on time scales of a few hundred thousand to a few million years. Since the age of the solar system is approximately 4.6 billion years, this meant that the asteroids seen today in such orbits must have entered them recently and implied that there was a source for the asteroids. Öpik believed this source to be comets that had been captured by the planets and that had lost their volatile material through repeated passages inside the orbit of Mars.

The mass of Vesta was deduced by the German-born American astronomer Hans G. Hertz in 1966 from measurements of its perturbations on the orbit of 197 Arete. The first mineralogical determination of the surface composition of an asteroid was made in 1969 by Thomas McCord, John B. Adams, and Torrence V. Johnson of the United States, who used spectrophotometry to identify the mineral pyroxene in the surface material of 4 Vesta. In 1970 the first reliable albedos (reflectivities) and diameters of asteroids were determined by two groups of American astronomers-Joseph F. Veverka, Benjamin H. Zellner, and their colleagues, who used a technique based on polarization measurements, and David A. Allen and Dennis L. Matson, who employed infrared radiometry.

Development of classification systems Asteroid 1998 KY26
In 1975 Zellner, together with Clark R. Chapman and David D. Morrison, grouped the asteroids into three broad taxonomic classes, which they designated C, S, and M (see below Composition). They estimated that about 75 percent belonged to class C, 15 percent to class S, and 5 percent to class M.
The remaining 5 percent were unclassifiable in their system owing to either poor data or genuinely unusual properties. Furthermore, they noted that the S class dominated the population at the inner edge of the asteroid belt, whereas the C class was dominant in the middle and outer regions of the belt. In 1982 other American astronomers, Jonathan C. Gradie and Edward F. Tedesco, expanded this taxonomic system and recognized that the asteroid belt consisted of rings of differing taxonomic classes with the S, C, P, and D classes dominating the populations at heliocentric distances of approximately 2, 3, 4, and 5 AU, respectively (see below Composition). As more data became available from further observations, additional minor classes were recognized by the American astronomer David J. Tholen in 1984, by the Italian astronomer Antonietta Barucci and colleagues in 1987, and by Tedesco and colleagues in 1989.
Orbits of asteroids Asteroid belt Trojan asteroids in L5 trailing point Trojan asteroids in L4 leading point Jupiter Mars Earth Sun Main asteroid belt
Because of their large numbers, asteroids are assigned numbers as well as names. The numbers are assigned consecutively after accurate orbital elements have been determined. (For example, Ceres is officially known as 1 Ceres, Pallas as 2 Pallas, and so forth.) By mid-1991, more than 7,000 asteroids had been observed at two or more oppositions, and 5,000 of these were numbered. The discoverers have the right to choose a name for their discoveries as soon as they are numbered. Now the names selected are submitted to the International Astronomical Union for approval. Now in new millenium there are more than 20,000 numbered asteroids! Prior to the mid-20th century, asteroids were sometimes assigned numbers before accurate orbital elements had been determined, and so some numbered asteroids could not later be located.
These objects are referred to as "lost" asteroids. Until recently only asteroid 719 Albert remained lost.

The Minor Planet Center at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., U.S., maintains computer files for all measurements of asteroid positions. The Institute for Theoretical Astronomy in St. Petersburg, Russia, publishes each year the Ephemerides of Minor Planets, which contains the orbital elements of all numbered asteroids, together with their opposition dates and ephemerides.

Although most asteroids travel in fairly circular orbits, there are some notable exceptions. One of the most extreme of these is 3200 Phaethon, discovered by the Infrared Astronomical Satellite (IRAS) in 1983. (It was the first asteroid to be discovered by a spacecraft.) Phaethon approaches to within 0.14 AU of the Sun, well within Mercury's perihelion distance of 0.31 AU. Phaethon's aphelion (2.4 AU) is in the main asteroid belt. This object is the parent body of the Geminid meteor stream and, since the parent bodies of all other meteor streams identified to date are comets, it is considered by some to be a defunct comet. Another asteroid, 944 Hidalgo, is also thought by some to be a defunct comet because of its unusual orbit. This object, discovered in 1920 by Walter Baade at the Bergedorf Observatory near Hamburg, Germany, has a perihelion distance of 2.02 AU at the inner edge of the main asteroid belt and an aphelion distance of 9.68 AU just beyond the orbit of Saturn at 9.54 AU. Finally, there is the case of 2060 Chiron, discovered in 1977 by Charles Kowal at the Palomar Observatory near San Diego, Calif., U.S. This object was originally classified as an asteroid, but in 1989 the American astronomers Karen J. Meech and Michael J. Belton observed a dusty coma surrounding it, and in 1991 Schelte J. Bus, also of the United States, and his colleagues detected the presence of cyanogen radicals, a known constituent of the gas comas of comets. Chiron travels in an orbit that lies wholly outside of the asteroid belt, having a perihelion distance of 8.43 AU (between the orbits of Jupiter and Saturn) and an aphelion distance of 18.8 AU, which nearly reaches the orbit of Uranus at 19.2 AU. Because Chiron moves in a chaotic, planet-orbit-crossing orbit, astronomers believe that it will eventually collide with a planet or be permanently ejected from the solar system.

Distribution and Kirkwood gaps
About 95 percent of the known asteroids move in orbits between those of Mars and Jupiter. These orbits, however, are not uniformly distributed but exhibit "gaps" in the distribution of their semimajor axes. These so-called Kirkwood gaps are due to resonances with Jupiter's orbital period. An asteroid with a semimajor axis of 3.3 AU, for example, makes two circuits around the Sun in the time it takes Jupiter to make one and is thus said to be in a two-to-one (written 2:1) resonance orbit with Jupiter. Consequently, once every two orbits Jupiter and an asteroid in such an orbit would be in the same relative positions, and such an asteroid would experience a force in a fixed direction. Repeated applications of this force would eventually change the semimajor axes of asteroids in such orbits, thus creating a gap at that distance. Gaps occur at 4:1, 7:2, 3:1, 5:2, 7:3, and 2:1 resonances, while concentrations occur at the 3:2 (Hilda group), 4:3 (Thule), and 1:1 (Trojan group) resonances. The presence of secular resonances complicates the situation, particularly at the inner edges of the belt. An adequate explanation of why some resonances produce gaps and others produce concentrations has yet to be found.
Families Graph
Within the main asteroid belt are groups of asteroids that cluster in certain orbital elements (semimajor axis, eccentricity, and inclination). Such groups are called families and assigned the name of the lowest numbered asteroid in the family. Asteroid families are thought to be formed when an asteroid is disrupted in a catastrophic collision. Theoretical studies indicate that such catastrophic collisions between asteroids are common enough to account for the number of families observed. About 40 percent of all known asteroids belong to such families.

The three largest families (Eos, Koronis, and Themis) have been determined to be compositionally homogeneous. If the asteroids belonging to them are considered to be fragments of a single parent body, then their parent bodies must have had diameters of 200, 90, and 300 km, respectively. The smaller families have not been as well studied because their numbered members are fewer and smaller (and hence fainter). Nevertheless, it is known that some of the smaller families are compositionally inhomogeneous and that, at least in some cases, what are observed are pieces of a geochemically differentiated parent body. It is theorized that some of the Earth-crossing asteroids and meteorites reaching the terrestrial surface are fragments produced in collisions similar to those that produced the asteroid families.

Outer-belt asteroids Asteroid 6489 Golevka!
The vast majority of asteroids have orbital periods between three years and six years-i.e., between one-fourth and one-half of Jupiter's orbital period. These asteroids are said to be main-belt asteroids.
Besides the few asteroids in highly unusual orbits, some of which were noted above, there are a number of groups that fall outside the main belt. Those that have orbital periods greater than one-half that of Jupiter are called outer-belt asteroids. There are four such groups: the Cybeles, Hildas, and Thule, named after the lowest numbered asteroid in each group, as well as the Trojan group, so called because all its members are named after characters from Homer's epic work about the Trojan War, the Iliad.
Trojan asteroids
Lagrangian points L1 L2 L3 L4 leading Trojan point L5 trailing Trojan Point Sun Jupiter
In 1772 the French mathematician and astronomer Joseph-Louis Lagrange predicted the existence and location of two groups of asteroids located near the (L4 and L5) equilateral triangular stability points of a three-body system formed by the Sun, Jupiter, and the asteroids. These are two of the five stable points in the circular, restricted three-body problem. (The other three stable points are located along a line passing through the Sun and Jupiter. Because of the presence of other planets, principally Saturn, the Sun-Jupiter-Trojan asteroid system is not a true three-body system, and so these other three points are not stable and no asteroids have been found near them. In fact, most of Jupiter's

Trojan asteroids do not move in the plane of its orbit but rather in orbits inclined by up to 40° and at longitudes that differ by as much as 70° from the longitudes of the true Lagrangian points.)

In 1906 Max Wolf discovered 588 Achilles near the Lagrangian point preceding Jupiter in its orbit. Within a year August Kopff had discovered two more: 617 Patroclus, located near the following Lagrangian point, and 624 Hector near the preceding Lagrangian point. It was later decided to name such asteroids after the participants in the Trojan War as given in the Iliad and, furthermore, to name those near the preceding point after Greek warriors and those near the following point after Trojan warriors. With the exception of the two previously named "spies" (Hector, the lone Trojan in the Greek camp, and Patroclus, the lone Greek in the Trojan camp), this tradition has been maintained.
The term Trojan has since been applied to any object occupying the equilateral Lagrangian points of other pairs of bodies. Searches have been made for Trojans of the Earth, Saturn, and Neptune, as well as of the Earth-Moon system, but so far none have been found. It was long considered doubtful whether truly stable orbits could exist near these Lagrangian points because of perturbations by the major planets. However, in 1990 an asteroid librating about the following Lagrangian point of Mars was discovered by the American astronomers David H. Levy and Henry E. Holt, thus reopening this question.

Although only about 65 Trojans have been numbered, photographic surveys have shown that there are about 2,300 such asteroids with diameters greater than 15 km. About 1,300 of these are located near the preceding Lagrangian point and 1,000 near the following Lagrangian point.

Inner-belt asteroids  
There is only one known group of inner-belt asteroids-namely, the Hungarias. The Hungaria asteroids, about 100 of which are known, have a mean semimajor axis of 1.91 AU; thus, their orbital periods are less than one-fourth that of Jupiter. Hungarias have nearly circular orbits (the mean eccentricity is 0.08) but large inclinations, the mean inclination being 22.7°. At least one dynamical family, the Hungaria family, exists within the Hungaria group. Because of the low eccentricities of their orbits, the mean perihelion distance of a Hungaria asteroid is 1.76 AU. Accordingly, a typical Hungaria cannot pass close to Mars, whose aphelion distance is 1.67 AU. A few Hungarias, however, have perihelion distances a few hundredths of 1 AU less than Mars's aphelion distance and so are shallow "Mars crossers" (see below) as well.
Near-Earth asteroids Asteroid 2063 Bacchus
Asteroids that can pass inside the orbit of Mars are said to be near-Earth asteroids. The near-Earth asteroids are subdivided into several classes. The most distant-those that can cross the orbit of Mars but that have perihelion distances (q) greater than 1.3 AU-are dubbed Mars crossers. This group is further subdivided into two groups: shallow Mars crossers (1.58 < q < 1.67 AU) and deep Mars crossers (1.3 < q < 1.58 AU).

The next most distant group of near-Earth asteroids are the Amors (1.017 < q < 1.3 AU). Amor asteroids have perihelion distances greater than the Earth's present aphelion distance (Q) of 1.017 AU and therefore do not at present cross the planet's orbit. Because of strong gravitational perturbations produced by their close approaches to the Earth, however, the orbital elements of all the Earth-approaching asteroids, except the shallow Mars crossers, change appreciably on a time scale of a few years or tens of years. For this reason about half the known Amors, including 1221 Amor, are part-time Earth crossers. Only asteroids that cross the orbits of planets, such as the Earth-approaching asteroids and objects like 944 Hidalgo and 2060 Chiron, suffer significant changes in their orbital elements on time scales shorter than many millions of years. Hence, the outer-belt asteroid groups (Cybeles, Hildas, Thule, and the Trojans) do not interchange members.

There are two groups of near-Earth asteroids that deeply cross the Earth's orbit on an almost continuous basis. The first of these to be discovered were the Apollo asteroids, 1862 Apollo being detected by the German astronomer Karl Wilhelm Reinmuth in 1932 but lost shortly thereafter and not rediscovered until 1978. Apollo asteroids have semimajor axes (a) that are greater than or equal to 1 AU and perihelion distances that are less than or equal to 1.017 AU; thus, they cross the Earth's orbit when near the perihelia of their orbits. For the other group of Earth-crossing asteroids-named Atens after 2062 Aten, which was discovered in 1976 by Eleanor F. Helin of the United States a<1.0 AU and Q > 0.983 AU, the present perihelion distance of the Earth. These asteroids cross the Earth's orbit when near the aphelia of their orbits.

It is estimated that there are roughly 100 Atens, 700 Apollos, and 1,000 Amors that have diameters larger than about one kilometre. Because these asteroids travel in orbits that cross the Earth's orbit, close approaches to the Earth occur, as well as occasional collisions. For example, in January 1991, an Apollo asteroid with an estimated diameter of 10 metres passed by the Earth within less than half the distance to the Moon.

The nature of asteroids

Asteroid 243 Ida!
Rotation and shape
Asteroid rotational periods and shapes are determined primarily by monitoring their changing brightness on time scales of hours to days.

Short-period fluctuations in brightness caused by the rotation of an irregularly shaped or spotted body (a spotted body being a spherical object with albedo differences) give rise to a light curve (a graph of brightness versus time) that repeats at regular intervals corresponding to an asteroid's rotation period. The range of brightness variation is more difficult to interpret but is closely related to an asteroid's shape or spottedness.

Rotational periods have been determined for more than 400 asteroids. They range from 2.3 hours to 48 days, but the majority (more than 80 percent) lie between 4 hours and 20 hours. Periods longer than a few days may actually be due to precession caused by an unseen satellite. The mean rotational period is roughly 10 hours for the entire sample. The largest asteroids (those with diameters greater than about 175 km), however, have a mean rotational period close to 7 hours, whereas this value is about 10 hours for smaller asteroids. The largest asteroids may have preserved their primordial rotation rates, but the smaller ones have almost certainly had theirs modified by subsequent collisions. The difference in rotation rates between the larger and the smaller asteroids is believed to stem from the fact that large asteroids retain all of their collisional debris from minor collisions, whereas smaller asteroids retain more of the debris ejected in the direction opposite to that of their spins, causing a loss of angular momentum and thus a reduction in speed of rotation.

Major collisions can completely disrupt smaller asteroids. The debris from such collisions makes still smaller asteroids, which can have virtually any shape or spin rate. Thus, the fact that no rotational periods shorter than about 2.5 hours have been observed implies that the material of which asteroids are made is not strong enough to withstand the centripetal forces that such rapid spins would produce.

For mathematical reasons it is impossible to distinguish between the rotation of a spotted sphere and an irregular shape of uniform reflectivity on the basis of observed brightness changes alone. Nevertheless, the fact that opposite hemispheres of most asteroids appear to have albedos differing by no more than a few percent suggests that their brightness variations are due mainly to changes in their projected, illuminated, visible, cross-sectional areas. Hence, in the absence of evidence to the contrary, it is generally accepted that variations in reflectivity contribute little to the observed rotational light-curve amplitude. Asteroid 4 Vesta is a notable exception to this generalization because it is known that the difference in reflectivity between its opposite hemispheres is sufficient to account for much of its light-curve amplitude.

Asteroid light-curve amplitudes range from zero to a factor of 6.5 in the case of the Apollo asteroid 1620 Geographos. A light-curve amplitude of zero is caused by viewing an asteroid along one of its rotational poles, while the 6.5 to 1 variation in brightness is believed to result from either of two possibilities: Geographos is a cigar-shaped object that is viewed along a line perpendicular to its rotational axis, or it is a pair of objects nearly in contact that orbit each other around their centre of mass.

The mean rotational amplitude for asteroids is about a factor of 1.3. These data, together with the assumptions mentioned above, allow astronomers to estimate asteroid shapes, which occur in a wide range. Some asteroids, such as 1 Ceres, 2 Pallas, and 4 Vesta, are nearly spherical, whereas others, like 15 Eunomia, 107 Camilla, and 511 Davida, are quite elongated. Still others, as, for example, 624 Hektor, 1580 Betulia, and 4769 Castalia (which appears in radar observations by the American astronomer Steven J. Ostro to resolve as two spheres in contact), apparently have bizarre shapes.

Size and albedo

The most widely used technique for determining the sizes of asteroids is that of thermal radiometry.
Mir spacestation

This technique makes use of the fact that the infrared radiation (heat) emitted by an asteroid must balance the solar radiation it absorbs. By using a so-called thermal model to balance the intensity of infrared radiation with the intensity at visual wavelengths, investigators are able to arrive at the diameter of the asteroid. Several other techniques-polarimetry, speckle interferometry, and radar-also are used, but they are limited to brighter, larger, or closer asteroids.

The only technique that measures the diameter directly (i.e., without having to "model" the actual observations) is that of stellar occultation. In this method, investigators measure the length of time that a star disappears owing to the passage of an asteroid between the Earth and the star. Then, using the known distance and the rate of motion of the asteroid, they are able to determine the latter's diameter uniquely. The results of this method have made it possible to reliably calibrate the indirect techniques, thermal radiometry in particular. As a consequence, asteroid diameters obtained by means of such techniques are now thought to have uncertainties of less than 10 percent. Because passages of asteroids in front of stars are rare and best applied to fairly spherical asteroids and because only one cross section is measured, the majority of asteroid sizes have been obtained using indirect techniques.

Asteroid 1 Ceres, with a diameter of about 930 km, is the largest, followed by 2 Pallas at 535 km and 4 Vesta at 520 km. The fourth largest asteroid, 10 Hygiea, has a diameter of about 410 km. There are two asteroids with diameters between 300 and 400 km and about 24 with diameters between 200 and 300 km. In total, there are about 30 asteroids with diameters greater than 200 km. The smallest known asteroids are members of the Earth-approaching groups, since these asteroids can approach the Earth to within a few hundredths of 1 AU. The smallest routinely observed Earth-approaching asteroids measure less than 0.5 km across. It has been estimated that there are 250 asteroids larger than 100 km in diameter and perhaps 1,000,000 with diameters greater than 1 km.
A parameter closely related to size is albedo, or reflectivity. This property also provides compositional information. Albedo is the ratio between the amount of light actually reflected and that which would be reflected by a uniformly scattering disk of the same size. Snow has an albedo of approximately 1 and coal an albedo of about 0.05.

An asteroid's apparent brightness depends on both its albedo and diameter as well as on its distance. For example, if 1 Ceres and 4 Vesta could both be observed at the same distance, Vesta would be the brighter of the two by about 15 percent. Vesta's diameter, however, is only about 55 percent that of Ceres. It appears brighter because its albedo is around 0.35, compared with 0.09 for Ceres.

Asteroid visual geometric albedos range from just under 0.02 to over 0.5 and may be divided into four albedo groups: low (0.02-0.07), intermediate (0.08-0.12), moderate (0.13-0.28), and high (greater than 0.28). About 78 percent of the known asteroids are low-albedo objects, and most of them are located in the outer half of the main asteroid belt and among the outer-belt populations. More than 95 percent of outer-belt asteroids belong to this group. Roughly 18 percent of all known asteroids belong to the moderate-albedo group, the vast majority of which are found in the inner half of the main belt. The high-albedo asteroids make up the remaining 4 percent of the asteroid population. For the most part, they occupy the same regions of the main asteroid belt as the moderate-albedo objects.

Mass and density  

Asteroid masses are low and have little effect on the orbits of the major planets. In the past, the masses of several have been measured by noting their effect on the orbits of other asteroids that they approach closely at regular intervals. In 1989 the American scientists E. Myles Standish and Ronald W. Hellings used radio-ranging measurements that were transmitted from the surface of Mars between June 1976 and August 1980 by the two Viking landers to determine distances to Mars with an accuracy of about 10 metres. Because the largest asteroids (Ceres, Pallas, and Vesta) cause perturbations in Mars's orbit in excess of 50 metres on time scales of 10 years or less, they were able to use the measured departures of Mars from its predicted orbit to estimate the masses (given below) of these three asteroids. Since the diameters of these three largest asteroids also have been determined and since their shapes are either spherical or ellipsoidal, their volumes are known as well. Knowledge of the mass and volume of an asteroid allows its density to be computed.

The mass of the largest asteroid, Ceres, is 1.0*10^24 grams (0.0002 the mass of the Earth). The masses of the second and third largest asteroids, Pallas and Vesta, are only 0.28 and 0.30 times the mass of Ceres. The mass of the entire asteroid belt is roughly three times that of Ceres. Most of the mass in the asteroid belt is concentrated in the larger asteroids. The 10th largest asteroid has only 1/60 the mass of the largest, and about 90 percent of the total mass is contained in asteroids with diameters exceeding 100 km. Ninety percent of the total mass of the asteroids is located in the main belt, 9 percent is in the outer belt and Jupiter Trojan asteroids, and the remainder is distributed among the inner belt and planet-crossing asteroid populations.

The densities of Ceres, Pallas, and Vesta are 2.3, 3.4, and 4.0 grams per cubic centimetre, respectively. These compare with 5.4, 5.2, and 5.5 g/cm3 for Mercury, Venus, and the Earth, respectively; 3.9 g/cm3 for Mars; and 3.3 g/cm3 for the Moon. The density of Ceres is similar to that of a class of meteorites known as carbonaceous chondrites, which contain a larger fraction of volatile material than do ordinary terrestrial rocks and hence have a somewhat lower density. The density of Vesta is similar to those of the rocky planets. Insofar as Ceres, Pallas, and Vesta are typical of asteroids in general, it can be concluded that asteroids are rocky bodies.

The combination of spectral reflectance measurements (measures of the amount of reflected sunlight at various wavelengths between about 0.3 and 2.6 micrometres [mm]) and albedos is used to classify asteroids into various taxonomic groups. If sufficient spectral resolution is available, these measurements also can be used to infer the composition of the surface reflecting the light.

This can be done by comparing the asteroid data with that obtained in the laboratory using meteorites or terrestrial rocks or minerals.

By the end of the 1980s, spectral reflectance measurements at visual wavelengths between 0.3 and 1.1 mm were available for about 6,000 asteroids, while albedos were determined for roughly 2,000. Both types of data were available for approximately 400 asteroids. The Table summarizes the 15 taxonomic classes into which the asteroids can be divided on the basis of such data.
Asteroids of the B, C, F, and G classes have low albedos and spectral reflectances similar to those of carbonaceous chondritic meteorites and assemblages produced by hydrothermal alteration and/or metamorphism of carbonaceous precursor materials. Some C-class asteroids are known to have hydrated minerals on their surfaces, whereas Ceres (a G-class) probably has water present as a layer of permafrost. K- and S-class asteroids have moderate albedos and spectral reflectances similar to the stony-iron meteorites, and they are known to contain significant amounts of silicates and metals, including the minerals olivine and pyroxene on their surfaces. M-class asteroids are moderate-albedo objects, may have significant amounts of nickel-iron metal in their surface material, and exhibit spectral reflectances similar to the nickel-iron meteorites. P-, D-, and T-class asteroids have low albedos and no known meteorite or naturally occurring mineralogical counterparts, but they may contain a large fraction of carbon polymers or organic-rich silicates or both in their surface material. R-class asteroids are very rare; only one (the high-albedo asteroid 349 Dembowska) has been identified with certainty. The surface material of Dembowska has been identified as being most consistent with a pyroxene and olivine-rich composition analogous to the pyroxene-olivine achondrite meteorites. The E-class asteroids have the highest albedos and spectral reflectances that match those of the enstatite achondrite meteorites. V-class asteroids have reflectance properties closely matching those of one particular type of basaltic achondritic meteorite, the eucrite. The match is so good that some believe that the eucrites exhibited in museums are chips from the surface of a V-class asteroid that were knocked off during a major cratering event. The V class is confined to the large asteroid Vesta and a few very small Earth-approaching asteroids.

Among the larger asteroids (those with diameters greater than 100 km), the C-class asteroids are the most common (accounting for about 65 percent by number) followed, in decreasing order, by the S (15 percent), D (8 percent), P (4 percent), and M (4 percent) classes. The remaining classes constitute less than 4 percent of the population by number. In fact, there are no A-, E-, or Q-class asteroids in this size range, only one member of the R and V classes, and between two and five members of each of the B, F, G, K, and T classes.

The distribution of the taxonomic classes throughout the asteroid belt is highly structured, as can be seen from the figure. Some believe this variation with distance from the Sun means that the asteroids formed at or near their present locations and that a detailed comparison of the chemical composition of the asteroids in each region will provide constraints on models for the conditions that may have existed within the contracting solar nebula at the time the asteroids were formed.

The origin and evolution of the asteroids  
Available evidence indicates that the asteroids are the remnants of a "stillborn" planet. It is thought that at the time the planets were forming from the low-velocity collisions among asteroid-size planetesimals, one of them grew at a high rate and to a size larger than the others. In the final stages of its formation this planet, Jupiter, gravitationally scattered large planetesimals, some of which may have been as massive as the Earth is today. These planetesimals were eventually either captured by Jupiter or another of the trans-Jovian planets or ejected from the solar system. While they were passing through the inner solar system, however, such large planetesimals strongly perturbed the orbits of the planetesimals in the region of the asteroid belt, raising their mutual velocities to the average five kilometres per second they exhibit today. These high-velocity collisions ended the accretionary collisions by transforming them into catastrophic disruptions. Only objects larger than about 500 km in diameter could have survived collisions with objects of comparable size at collisional velocities of five kilometres per second. Since that time, the asteroids have been collisionally evolving so that, with the exception of the very largest, most present-day asteroids are either remnants or fragments of past collisions.
While breaking down larger asteroids into smaller ones, collisions expose deeper layers of asteroidal material. If asteroids were compositionally homogeneous, this would have no noticeable result. Some of them, however, have become differentiated since their formation. This means that some asteroids, originally formed from so-called primitive material (i.e., material of nonvolatile solar composition), were heated, perhaps by short-lived radionuclides or solar magnetic induction, to the point where their interiors melted and geochemical processes occurred. In certain cases, temperatures became high enough for iron to form. Being denser than other materials, the iron then sank to the centre, forming an iron core and forcing basaltic lavas onto the surface. At least one asteroid with a basaltic surface, Vesta, survives to this day. Other differentiated asteroids were disrupted by collisions that stripped away their crusts and mantles and exposed their iron cores. Still others may have had only their crusts partially stripped away, which exposed surfaces such as those visible today on the A-, E-, and R-class asteroids.
Collisions were responsible for the formation of the Hirayama families and at least some of the planet-crossing asteroids. A number of the latter enter the Earth's atmosphere, giving rise to sporadic meteors. Larger pieces survive passage through the atmosphere, and some of these end up in museums and laboratories as meteorites. The very largest produce craters such as Meteor Crater in Arizona in the southwestern United States, and one may even have been responsible for the extinction of the dinosaurs some 65 million years ago. This extinction may have been triggered by an explosion resulting from the impact of an asteroid measuring roughly 10 km in diameter. (Some investigators believe that a cometary body rather than an asteroid may have caused such an explosion.) Fortunately, collisions of this sort are rare. According to current estimates, a few asteroids of 1-km diameter collide with the Earth every 1 million years.

Last updated: May 7, 2001.