Asteroids are small members of the solar system in heliocentric orbits concentrated between Jupiter and Mars. Since most of them have orbits that are roughly similar to those of the planets (low inclination and eccentricity), they have sometimes been called minor planets, although this term is no longer in common use. More important is the distinction between asteroids and comets, where the primary difference is one of composition. Most asteroids are rocky objects, composed of the same sorts of materials as the inner planets. Comets, in contrast, contain a substantial quantity of water ice and other frozen volatiles in addition to silicates and organic compounds. However, there are ambiguities in terminology. Comets that make frequent passes around the Sun may lose their volatiles and become indistinguishable from rocky asteroids. In addition, there are many volatile-rich objects being discovered in the outer solar system (beyond Neptune) that resist classification as either asteroids or comets. These are the Kuiper Belt Objects (KBOs), discussed in this topic in the entry for Comets.
The most comprehensive references for asteroids are the large multiauthor text topics published in the Space Science Series of the University of Arizona Press. See especially Asteroids II (1989) edited by R. Binzel and others, and Hazards Due to Comets and Asteroids (1994) edited by T. Gehrels. Soon to be published in the same series is Asteroids III (2002), edited by W. Bottke and others.

Discovery of Asteroids

Asteroids are too faint to be visible to the unaided eye, so their discovery belongs to the era of telescopic astronomy. On New Year’s Day in 1801, Giuseppe Piazzi at Palermo Observatory found the first asteroid, which he named Ceres for the Roman patron goddess of Sicily. This faint object (now known as the largest asteroid) orbited the Sun at a distance of 2.8 astronomical units (AU). It was at first hailed as the ”missing planet” in the large gap between the orbits of Mars and Jupiter. In the following few years, three more asteroids—Pallas, Juno, and Vesta—were found, also orbiting between Mars and Jupiter. These were even smaller than Ceres, although Vesta is slightly brighter due to its more reflective surface. Even combined, the masses of these four objects came nowhere near that of a real planet. Most of the asteroids are located in what is defined as the main asteroid belt, at distances from the Sun are between 2.2 and 3.3 AU, corresponding to orbital periods between 3.3 and 6.0 years.
The next asteroid was not discovered until 1845, but from then on, they were found regularly by visual observers who scanned the sky looking for them. By 1890, the total number had risen to 300. At that time, photographic patrols began, and the number of known objects rapidly increased and reached 1000 in 1923, 3000 in 1984, 5000 in 1990, and 20,000 in 2001. To be entered on the official list of asteroids, an object must be well enough observed to establish its orbit and permit its motion to be accurately calculated many years into the future. The responsibility for cataloging asteroids and approving new discoveries is assigned to the International Astronomical Union Minor Planet Center in Cambridge, Massachusetts (1). It is an interesting indication of growing interest that 198 years were required to find the first 10,000, but only two years were needed to find the second 10,000. Most recent discoveries are a by-product of the Space-guard Survey, a concerted search for near-Earth Asteroids (NEAs) that will be discussed in detail at the end of this article.
In addition to numerical designations (e.g., 4 Vesta, 1000 Piazzia), which are given in chronological order of determination of an adequate orbit, most asteroids have names, usually suggested by the discoverer. Initially, these were the names of Greek and Roman goddesses, such as Ceres and Vesta, and later expanded to include female names of any kind. When masculine names were applied, they were given the feminine Latin ending. More recently, the requirement of female gender has been dropped, and today asteroids are named for a bewildering variety of persons and places, famous or obscure. This article uses names rather than numbers to refer to specific asteroids.

Basic Asteroid Statistics

Our census of the larger asteroids is fairly complete by now, based primarily on ground-based surveys, complemented by infrared space observations, such as those from the Infrared Astronomy Satellite (IRAS) in 1983. It is likely that we have discovered all main-belt asteroids 25 km or more in diameter, and discovery should be more than 50% complete for diameters down to 10 km. Ceres, the largest, has a diameter just under 1000 km. The next largest asteroids are about half this size (see Table 1 for a listing of the dozen largest asteroids). The total mass of all of the asteroids amounts to only 1/2000 of the mass of Earth (less than 1/20 the mass of the Moon). Our knowledge is better for the closer asteroids in the inner part of the asteroid belt, and most of the larger undiscovered bodies are probably beyond 3 AU from the Sun.
There are many more small than large asteroids. An estimate of the relative numbers of objects of each size is interesting as a characterization of the asteroid population, and it is also closely related to the distribution of craters caused by the collision of asteroids with the planets and satellites in the inner solar system.
As a rule, many processes in nature, including those of fragmentation, result in approximately equal masses of material in each size range. Applying such a power law to the asteroids, we would find that there should be 1000 times more 10-km objects than 100-km ones, and a million more at 1km than 100 km. In other words, the number of objects of a given diameter is inversely proportional to the cube of their diameter. However, measurements of the asteroids indicate that the numbers do not rise this fast as size declines but increase more nearly as the inverse square of the diameter, resulting in a distribution where most of the mass is in the larger objects. This is why we are relatively certain of the total mass of the asteroids, even without having counted all of the small ones.
Table 1. The Largest Asteroids

Name Year of discovery Distance from Sun, AU Diameters, km Class
Ceres 1801 2.77 940 C
Pallas 1802 2.77 540 C
Vesta 1807 2.36 510 V
Hygeia 1849 3.14 410 C
Ineramnia 1910 3.06 310 C
Davida 1903 3.18 310 C
Cybele 1861 3.43 280 C
Europa 1868 3.10 280 C
Sylvia 1866 3.48 275 C
Juno 1804 2.67 265 S
Psyche 1852 2.92 265 M
Patientia 1899 3.07 260 C

From the observed distribution of sizes, we can estimate that there are more than 100,000 asteroids down to a diameter of 1 km. Although 100,000 sounds like a lot of objects, space in the asteroid belt is still empty. The belt asteroids occupy a very large volume, roughly doughnut shaped, about 100 million km thick and nearly 200 million km across. Typically the asteroids 1km or larger are separated from each other by millions of kilometers. They pose no danger to passing spacecraft. In fact, it was challenging to locate asteroids near enough to the trajectory of outward-bound spacecraft (such as Galileo and Cassini) to allow close asteroid flybys on the way to Jupiter.
Physical and Chemical Properties
As seen through a telescope without special image compensation (adaptive optics), an individual asteroid is an unresolved starlike point. The word asteroid means starlike. Before about 1970, almost nothing was known about the physical nature of asteroids, and research was confined to discovering and charting orbits and determining rotational rates from observations of periodic variations in brightness. In the past 30 years, however, new observing techniques used with large telescopes have revealed a great deal about the physical and chemical nature of the asteroids (2). These astronomical observations have been supplemented by key studies of meteorites and by close-up spacecraft observations of a few asteroids, including Gaspra, Ida, Mathilde, and Eros—the latter involved both orbital and landed investigations.
Asteroid sizes and shapes are determined directly from imaging with modern adaptive optics or from the Hubble Space Telescope (although the resolution, even with the largest telescopes, leaves much to be desired). High-precision radar imaging is also a powerful tool if the object comes sufficiently close to Earth. Size can also be measured by timing the passage of an asteroid in front of a star. Because we know exactly how fast the asteroid is moving against the stellar background, measuring how long the star is obscured yields a chord length for the asteroid that can be accurate to a few kilometers. If timings of the same event made from different locations on Earth are combined, the profile of the asteroid can be derived. Unfortunately, however, suitable events are rare, and only a dozen asteroids have been measured successfully by this method.
Most asteroid sizes have been estimated indirectly from their visible or infrared brightness. Given only the apparent visible-light brightness of the object, we can roughly estimate its size by assuming a reflectivity or albedo that is characteristic of average asteroids. Such diameters are typically uncertain by a factor of 2, implying an order-of-magnitude uncertainty in mass. Much more accurate are reflectivities determined by combining visible-band measurement of reflected light with infrared-band measurement of emitted heat radiation. Such diameters are good to 10% or better, and they require no arbitrary assumptions about reflectivity.
It is clear that the asteroids have a variety of surface compositions, as discussed further later. This variety leads to a wide range of surface reflectivity.
The majority of the asteroids are very dark, roughly the brightness of charcoal. Other types can have reflectance as high as white terrestrial rocks. To make sense of this diversity of material, one must add information on the spectral reflectance of the asteroids. It is particularly useful to compare the asteroids with extraterrestrial samples that reach Earth as meteorites. A few meteorites come from the Moon or Mars, but the great majority of them are fragments from asteroids. Unfortunately, the chaotic dynamic processes that deliver meteorites to our planet do not include traceable return addresses. One of the major challenges of meteoritics is to connect the samples we have to their parent bodies (or class of parent bodies) among the asteroids.
The use of spectral data to characterize asteroids has yielded preliminary determinations of composition for approximately 1500 objects. These include a few asteroids that have metallic surfaces, presumably representing the surviving cores of objects that melted, differentiated chemically, and subsequently lost their stony crusts and mantles. Most, however, have rocky surfaces, that compare to the majority of meteorites, which are also rocky. Exact identifications are difficult, however, and usually we cannot specify the unique properties that identify an individual. With some notable exceptions, contemporary asteroid research, therefore, tends toward broad statistical studies rather than detailed investigation of particular objects. The exceptions are the handful of asteroids that have been visited by spacecraft or imaged at close range by radar, to be discussed further later.
Most of the well-observed asteroids fall into one of two classes based on their reflectivity (3). They are either very dark (reflecting only 3-5% of incident sunlight) or moderately bright (15-25% reflectivity). A similar distinction exists in their spectra. The dark asteroids are fairly neutral reflectors and do not have major absorption bands in the visible range to reveal their compositions, although some of them show spectral evidence of chemically bound water in the infrared. Most of the lighter asteroids are reddish and have the spectral signatures of common silicate minerals such as olivine and pyroxene. The dark gray asteroids have spectra similar to the carbonaceous meteorites, so they are called C-type asteroids. The lighter class is named the S-type, indicating silicate or stony composition. A third major group appears to be metallic (like the iron meteorites) and is called the M-type. There is also a variety of subclasses based on spectra and reflectivity, especially among the dark C-type objects. It is also increasingly clear that some process of ”space weathering” alters the optical properties of surface materials; it partially masks identification with specific meteorite types on Earth and blurs the distinctions that might otherwise be seen among asteroids of different subgroups.
Using the classification of the asteroids, we can look at the distribution in space of the broad C, S, and M types. At the inner edge of the belt, the S asteroids predominate. Moving outward, the fraction of C-type objects increases steadily, and in the main asteroid belt as a whole, the dark, carbonaceous objects make up 75% of the population, compared to 15% S and 10% of M and other types.
Beyond the main belt, all asteroids are very dark, but their colors are redder than the belt objects, and they do not look like any known carbonaceous meteorite (4). Because these objects are not represented in our meteorite collections, scientists hesitate to commit themselves concerning their composition. It is generally thought, however, that they are primitive objects and that a fragment from one of them would be classed as a carbonaceous meteorite, although of a kind different from those already encountered.
If the asteroids are still near the locations where they formed, we can use the distribution of asteroid types to map out the composition of the solar nebula, the original circumsolar assemblage of gas and dust from which the planetary system formed (5). Carbonaceous meteorites formed at lower temperatures than the other primitive stones, so we infer that the concentration of similarly composed C-type asteroids in the outer belt is consistent with their formation farther from the Sun, where the nebular temperatures were lower. It is also possible, however, that the asteroids formed elsewhere and were herded into their present positions by the gravity of Jupiter and the other planets. In that case, the C-type asteroids could have formed far beyond Jupiter and subsequently diffused inward to their present positions in the outer part of the asteroid belt. Similarly, the S-type asteroids near the inner edge of the belt could either have formed where we see them today, or they could have been gravitationally scattered to their present locations from still closer to the Sun. The solar nebula temperatures that we would deduce by applying these two alternative models are quite different. So far, however, we have not been able to settle on which model is preferred for the origin of the asteroids.


The orbits of the belt asteroids are for the most part stable, their eccentricities are less than 0.3, and inclinations are below 20°. In the past, when presumably there were more asteroids, collisions may have been common, but by now the population has thinned to the point where each individual asteroid can expect to survive for billions of years between collisions. Still, with 100,000 objects 1 kilometer or more in size, a major collision somewhere in the belt is expected every few tens of thousands of years. Such collisions, as well as lesser cratering events, presumably yield some of the fragments that develop Earth-crossing orbits and eventually reach Earth as meteorites. In contrast, Earth-approaching NEAs have unstable orbits and typical dynamic lifetimes of only about 100 million years. Their numbers represent an equilibrium between inward scattering from the main belt and elimination either by colliding with the terrestrial planets or the Sun or by gravitational ejection from the solar system.
Given their history of collisions, there is no reason to expect that most asteroids are monoliths. Many may be rubble piles, consisting of loosely bound, low-density accumulations of debris that has reaccreted after a catastrophic disruption (6). In general, the energy required to disperse such debris completely is substantially greater than the energy needed to break up a target. One line of evidence for the existence of rubble piles comes from the highly elongated shapes of some small, rapidly spinning asteroids. These shapes are nearly what one might expect for an equipotential fluid and suggest such a reaccretion process. However, conclusive evidence of rubble piles awaited the first close-up spacecraft investigations, as recounted later.
The orbits of asteroids within the main belt are not evenly distributed. As shown in Fig. 1, some orbital periods are preferred, and others are nearly unpopulated. These unpopulated sections of the belt are resonance gaps, also known as the Kirkwood gaps for the nineteenth-century astronomer who discovered them. These gaps occur at orbital periods that correspond to resonances between these periods and the orbital period of Jupiter. Resonance takes place when the orbital period of one body is an exact fraction of the period of another. In this case, the underpopulated asteroid orbits correspond to periods that are one-half, one-third, one-quarter, etc., that of the 12-year orbital period of Jupiter. The Kirkwood gaps provide a clue to the origin of the asteroids or rather to the absence of a single large planet in the region between Mars and Jupiter. Presumably the dominant gravitational presence of Jupiter interrupted the accre-tionary process and dispersed the planetesimals in this part of the solar system. Most of the material ended up striking the inner planets or was ejected from the system, and only a small remnant remains in the asteroid belt today.
Histogram of orbital periods of the known asteroids. The deep minima are the Kirkwood gaps that correspond to periods that are in resonance with Jupiter (courtesy of Jet Propulsion Laboratory and NASA). Source: <>. This figure is available in full color at
Figure 1. Histogram of orbital periods of the known asteroids. The deep minima are the Kirkwood gaps that correspond to periods that are in resonance with Jupiter
Asteroidal orbits display other patterns in addition to the resonance gaps. An asteroidal family is defined as a group of objects that have similar orbits that suggest a common origin. These were first identified by Kiyotsuga Hirayama early in the twentieth century. About half of the known belt asteroids are members of families, nearly 10% belong to just three: the Koronos, Eos, and Themis families. Although not clustered together in space at present, the members of an asteroid family were all at the same place at some undetermined time in the past. Members of the same family tend to have similar reflectivities and spectra.
Apparently, the family members are fragments of broken asteroids, shattered in some ancient collision, and still follow similar orbital paths. According to some estimates, almost all asteroids smaller than about 200 km in diameter were probably disrupted in earlier times, when the population ofasteroids was larger. The families we see today may be remnants of the most recent of these inter-asteroidal collisions.

Asteroids up Close

Radar Studies. One of the most powerful tools for investigating asteroids is radar. There are two major planetary radar facilities, both of which were upgraded in the late 1990s. NASA operates the Goldstone (California) planetary radar facility as part of the Deep Space Net, and the 1000- foot Arecibo dish in Puerto Rico is operated by the National Astronomical and Ionospheric Center with NSF and NASA support. The two facilities are complementary—Arecibo has greater sensitivity, but Goldstone has greater sky coverage. Radar allows measuring range and velocity and permits us to define the rotational state precisely and to constrain the object’s internal density distribution. In addition, radar astronomers used measurements of echo power in time delay (range) and Doppler frequency (radial velocity) to construct geologically detailed three-dimensional models that sometimes rival the resolution of spacecraft imaging systems (7).
By 2001, radar had detected more than 120 asteroids, whose sizes are as small as 30 m. These include large objects in the main belt as well as more than 80 of the smaller NEAs. One of the early radar contributions was to search for direct evidence of metallic surfaces for a few asteroids from their high microwave reflectivity. Observations of M asteroids Psyche and Kleopatra provide the best evidence linking the M class to metallic composition. However, these two asteroids have provided numerous surprises. In spite of its apparently metallic surface, Psyche has a density of only about 2g/cm3, suggesting that its interior has extremely high porosity if composed of metal. Kleopatra is even stranger; it has a remarkable ”dog-bone” shape that suggests reaccretion of material after a catastrophic impact. There is also evidence of a low-density surface of uncon-solidated rubble on Kleopatra—again not what we would have expected by comparison with the lumps of iron-nickel in our meteorite collections (8).
The highest-resolution imaging has been achieved for asteroids that come very close to Earth. The largest of these is Toutatis, an elongated lumpy asteroid that provided early evidence that asteroids might not be monolithic (Fig. 2). At 5 km long, Toutatis is among the largest of the NEAs. Toutatis is also one of three asteroids found so far that are in slow, non-principal-axis spin states—perhaps evidence that they have received recent impacts (9). Among the interesting results of radar has been the discovery of three bifurcated objects (Castalia, Mi-thra, and Bacchus) that appear to be contact binaries. In several other cases, there is evidence of satellites orbiting asteroids. Satellites provide a way to calculate densities of the primary objects. Since the late 1990s, several asteroidal satellites (including the large C-type main-belt asteroids Eugenia and Antiope) have been discovered by using ground-based optical telescopes, and densities have also been measured for three of the asteroids visited by spacecraft. Most of the densities turn out to be surprisingly low (less than 2g/cm3), suggesting rather high interior porosity.
Figure 2. Shape model from radar images of Toutatis. Analysis of the delay-Doppler imaging sequence established that Toutatis is in a non-principal-axis spin state, and accurate determination of the asteroid’s rotation required inverting of the image sequence with a realistic physical model. These four views of the Toutatis computer model show shallow craters, linear ridges, and a deep topographic ”neck” whose geologic origin is not known. It may have been sculpted by impacts into a single, coherent body, or this asteroid might actually consist of two separate objects that came together in a gentle collision. Toutatis is about 5 km long (images courtesy of Steve Ostro, NASA Jet Propulsion Lab and Scott Hudson, Washington State University).
Shape model from radar images of Toutatis.
Spacecraft Flybys. Table 2 summarizes the four detailed spacecraft studies of asteroids. The main-belt asteroids Gaspra and Ida were flyby targets for Galileo on its way to Jupiter, and Mathilde was visited by the NEAR-Shoemaker spacecraft on its way to its primary target, Eros. The NEAR studies of Eros are discussed in the next subsection. Figure 3 illustrates these spacecraft targets on the same scale.
The Galileo flybys of two main-belt S-type asteroids revealed that both are highly irregular in shape, heavily cratered, and have only slight differences in color or reflectivity across their surfaces. Gaspra is undersaturated with craters, indicating a relatively young age (where age is the time since the last global-scale impact). In contrast, Ida is saturated with craters, and it appears to have a broken-up surface layer (a regolith) that is tens of meters thick (similar to that of the Moon). The discovery of a small satellite (Dactyl) in orbit around Ida permitted measuring its mass and density. The density is 2.6 g/cm3, similar to that of primitive rocks. Partly on this basis, it appears that these two S-type asteroids are probably coherent and are composed of materials similar to ordinary chond-rite primitive meteorites. However, the spectral mismatch between these objects and known chondrites in our meteorite collections continued to baffle investigators after these two flybys. In addition, the presence of large families of grooves or lineaments on both asteroids suggested that they had global-scale cracks resulting from past impacts.
Mathilde was the first main-belt C-type asteroid to be examined at close range. NEAR-Shoemaker found a unique shape for this asteroid, dominated by several apparent craters whose diameters are greater than the radius of the asteroid. Such a configuration is not possible for a “normal” rocky target because the formation of the most recent of these craters would have been expected to destroy preexisting giant craters or perhaps even to disrupt the target entirely.
Table 2. Spacecraft Encounters with Asteroids

Asteroid Class Date Dimensions, km Density, g/cm3 Best resolution, m
Gaspra S 1991 18 x 11 x 9 — 50
Ida S 1993 60 x 25 x 19 2.6 25
Mathilde C 1997 66 x 48 x 46 1.3 160
Eros S 2000 31 x 13 x 13 2.67 0.1

Family portrait of spacecraft images of asteroids Gaspra, Ida, Mathilde, and Eros, shown to scale. Gaspra and Ida (both main-belt asteroids) were imaged by the Galileo spacecraft. Mathilda (main-belt) and Eros (near-Earth) were imaged by NEAR-Shoemaker (Images courtesy of NASA, the Caltech Jet Propulsion Laboratory, and the JHU Applied Physics Laboratory.) Source: < >.
Figure 3. Family portrait of spacecraft images of asteroids Gaspra, Ida, Mathilde, and Eros, shown to scale. Gaspra and Ida (both main-belt asteroids) were imaged by the Galileo spacecraft. Mathilda (main-belt) and Eros (near-Earth) were imaged by NEAR-Shoemaker
Only a “soft” target that has a less competent interior can absorb great shocks without internal disruption. This interpretation was reinforced by the measured density of 1.3 g/cm3, indicative of about 50% porosity. Thus, Mathilde became the first confirmed rubble-pile asteroid.
The NEAR-Shoemaker Mission to Eros. The most ambitious and successful spacecraft investigation of the asteroids was carried out by a small (Discovery-class) NASA spacecraft called the Near Earth Asteroid Rendezvous (NEAR) mission. It was further christened NEAR-Shoemaker in honor of Eugene Shoemaker, the father of asteroid geology. NEAR-Shoemaker missed its original rendezvous date with Eros in December 1998 due to a malfunction, but it recovered after one more trip around the Sun and finally arrived in February 2000. It achieved an initial high orbit, then gradually lowered its altitude during the next year, and studied Eros with a variety of instruments. The spacecraft obtained thousands of multispectral images and more than 10 million laser altimetry measurements, making Eros one of the best-mapped objects in the solar system (10,11).
After 1 year in orbit, NEAR-Shoemaker began a staged descent to the surface, taking pictures of ever-increasing resolution (Fig. 4). It landed on 12 February 2001 at an impact velocity of 1.6 m/s. Fortunately, the spacecraft was not damaged, even though it had not been designed for such a maneuver. Using its low-gain antenna, it continued radioing data from the surface for more than a week, providing the best measurements of elemental composition. The mission ended on 28 February when a command from Earth turned offthe spacecraft (12).
The quantitative measurement of radioactivity from K, Th, and U, as well as gamma-ray lines of Fe, O, Si, and Mg, demonstrated that Eros has a primitive composition equivalent to the low-iron group of ordinary chondrite meteorites. Eros is a normal class-S asteroid, so this in situ result finally settled questions that had remained open for decades concerning the nature (primitive or differentiated) of the S asteroids. The density of Eros (2.67 g/cm3) is also generally consistent with this meteorite identification, although it still implies a substantial bulk porosity of about 25%. Evidently asteroids, like many terrestrial sediments, are consistently less dense than the individual rocks of which they are composed.
Close-up of near-Earth asteroid Eros as seen from the NEAR-Shoemaker cameras at a range of just 7 km. Most of the scene (about 350 meters across) is covered by rocks of all sizes and shapes, but the floors of some craters are smooth, suggesting accumulation of fine mobile material. The smallest visible features are about 1 meter across. (Image courtesy of NASA and the JHU Applied Physics Laboratory.) Source: /http://>.
Figure 4. Close-up of near-Earth asteroid Eros as seen from the NEAR-Shoemaker cameras at a range of just 7 km. Most of the scene (about 350 meters across) is covered by rocks of all sizes and shapes, but the floors of some craters are smooth, suggesting accumulation of fine mobile material. The smallest visible features are about 1 meter across.
Long ridges seen in some of the images demonstrate that Eros is a consolidated and coherent body that has global-scale tectonics. As suspected for several other asteroids, Eros is a solid collisional fragment of a larger parent body (not a rubble pile), but it is also not a monolith because its interior has been heavily fractured. The surface is cratered, but there is a surprising deficiency of small craters, combined with an excess of boulders up to the 100-m size. There are actually more boulders than craters in the tens-of-meters sizes. Some measured slopes are greater than the angle of repose. Dark material has flowed down-slope, exposing underlying bright material. The effects of space weathering are evident in the different spectral reflectivity of exposures of differing age. Apparently Eros has a complex, mobile regolith, whose small-scale surface roughness is similar to that of lunar regolith (somewhat surprising because the gravity is so much less).
As noted in summer 2001 by MIT scientist Richard Binzel, “We’re getting to know asteroids as tangible objects, on the same scale and geologic sense that we know mountains on Earth.” And like terrestrial mountains, their interiors can be highly fragmented.

Trojan Asteroids

A particularly interesting group of dark, distant asteroids is orbitally associated with Jupiter. Although the gravitational attraction of this giant planet generally makes nearby asteroidal orbits unstable, exceptions exist for objects of the same orbital period as Jupiter, while leading or trailing it by 60°. These two stable regions are called the leading and trailing Lagrangian points, named for the mathematician who demonstrated their existence in 1772. While he was mathematically examining the possible motions of three mutually gravitating bodies, Lagrange found two regions where a small object could occupy a stable orbit within the gravitational fields of two larger objects. If the larger objects are Jupiter and the Sun, a small object in one of the Lagrangian points occupies one corner of an equilateral triangle, and the Sun and Jupiter are at the other two points.
The regions of stability around the two Lagrangian points are quite large: each contains several hundred known asteroids. The first of these Lagrangian asteroids was named Hektor when it was discovered in 1907. All of them are named for the heroes of the Iliad who fought in the Trojan War, and collectively they are known as the Trojan asteroids. Their spectra are distinctive, suggesting that they represent a group of special, primitive objects that have been trapped in this region of space since the birth of Jupiter. If we could detect the fainter members of these Trojan clouds, we might find that the Trojan asteroids are nearly as numerous as those in the main asteroid belt.

Near-Earth Asteroids

Asteroid populations that can impact Earth are of special interest to us. They are generally referred to as Near-Earth Asteroids (NEAs) or Earth-crossing asteroids (ECAs). Because of their unstable, planet-approaching orbits, the NEAs have impacted the surfaces of the planets in the inner solar system (including Earth) and have influenced both geologic and biological evolution. There is reason to expect further impacts in the future, so the NEAs are a topic that has profound political and societal overtones. The impact hazard represents the intersection of asteroid science and public welfare and governmental policy (13).
It is highly improbable that a large (diameter > 1 km) NEA will hit the Earth within our lifetimes, but such an event is entirely possible. In the absence of specific information, such a catastrophe is equally likely at any time, including next year. Recognition that Earth (and Moon) are impacted by asteroids and comets is less than a century old, and it was not even securely proven that the prominent Meteor Crater (Arizona) was of impact origin until the work of Eugene Shoemaker in 1960. The fortunate fact that the atmosphere protects us from impacting bodies smaller than a few tens of meters in diameter (except for the rare iron meteorites) has the perhaps unfortunate consequence that we have almost no direct experience with cosmic impacts.
Tunguska and Meteor Crater. On the timescale of a human lifetime, the 1908 Tunguska impact in Siberia is the most notable. It was estimated (primarily from barographic and seismic records) that it had an explosive energy of ~ 15 megaton (TNT equivalent) when it disintegrated about 8 km above the ground. The impactor had the force of a large contemporary nuclear weapon. The explosion affected an unusually remote part of the world, and the first expedition to study Tunguska was delayed by two decades. At the time, before the existence of an Earth-crossing asteroid population was recognized, it was naturally suggested that the culprit was a small comet. Other fringe-science explanations included the impact of a mini black hole and the crash of a UFO spacecraft. Not until the 1990s did numerical modeling of the entry physics clearly indicate that a comet (low-density, friable material) of this kinetic energy would disintegrate at very high altitudes and could not penetrate into the troposphere (14). Now we recognize that the event in Tunguska was simply the most recent example of an ongoing bombardment of Earth by NEAs.
A better known site of asteroidal impact is Meteor Crater (also called Bar-ringer Crater) in northern Arizona. In this case, an iron asteroid about 40-50 meters in diameter struck about 50,000 years ago and formed a crater slightly more than 1 km in diameter. The energy of this impact was approximately the same as that of Tunguska (about 15 megaton), but because of the greater strength and density of the projectile, the explosion occurred at or below the surface, and a crater was formed.
Impacts and Extinctions. NEAs entered the scientific and popular mainstream in the 1980s when they were identified as the possible agents of biological mass extinctions. Alvarez and others (15) proposed that the dinosaur-killing KT mass extinction was due to an impact by a comet or asteroid, inferred from the chemical signature of extraterrestrial material in the boundary layer at the end of the Cretaceous. This bold hypothesis received general acceptance after the 200km- diameter Chicxulub crater in Mexico (still among the largest craters identified on Earth) was discovered and it was dated exactly to the age of the KT extinction.
The most revolutionary insight of Alvarez and his colleagues was not that impacts take place on Earth (which was obvious), but that even small impacts (on a geological or astronomical scale) can severely damage the fragile terrestrial ecosystem. From the size of the Chicxulub crater, the energy of the KT impact is estimated at about 100 million megaton, and a consistent value of the size of the impactor (10-15 km in diameter) is derived from the observed extraterrestrial component in the boundary layer. Immediate effects ofthe impact included blast and the generation of a tsunami (because the impact occurred in a shallow sea). However, the primary agents of global stress appear to have been a short-lived firestorm from atmospheric heating of ejecta followed by a persistent (months to years) blackout due to particulates suspended in the stratosphere (16). Large land animals (such as the dinosaurs) were incinerated within a few minutes of the impact, and the marine ecosystem collapsed a few weeks later as a result of the global blackout. Fortunately, impacts of this size are exceedingly rare; they occur at average intervals of the order of a hundred million years. Today, there is no NEA comparable to the KT impactor that can hit Earth. However, we have no such assurance of immunity from smaller impacts.
Impacts from asteroids and comets have influenced the biological history of our planet in a variety of ways. It is widely thought that carbonaceous asteroids have been the dominant source of Earth’s water and other volatiles, including many organic compounds required for originating life. At the same time, the impact environment of early Earth must have challenged the development of life and may have led to short episodes in which the oceans boiled away and the planet was sterilized. The phenomenon has been called the ”impact frustration of life” (17). After the end of the heavy bombardment of Earth about 3.8 billion years ago, impact catastrophes of this dimension were not possible. However, the Earth must have experienced dozens (or more) of impacts of the size of the KT event that punctuated biological evolution with occasional episodes of dramatic environmental stress. Impacts have been suspected in several other mass extinctions besides the KT, but in no other case is the evidence truly compelling. However, we know that these impacts have happened, and it is entirely plausible that they played a major role in biological evolution.

The Asteroid Impact Hazard

The average frequency of impacts by NEAs as a function of kinetic energy is illustrated in Fig. 5, adapted from a graph published in 1983 by Shoemaker (18). Comparison of this size-frequency distribution with the expected environmental damage caused by impacts of different energy leads to the conclusion (19) that the greatest risk is from large impacts, those that create a global ecological catastrophe. The threshold for global catastrophe is in the vicinity of 1 million megatons of energy, corresponding to an NEA whose diameter is about 2 km. Below this threshold, impacts create regional or local disasters, but the population (and social stability) of the planet are not threatened.
Although impacts below this million-megaton threshold are much more frequent, the total hazard from the sum of all such smaller impacts is less. Unlike more familiar natural hazards, the impact risk is primarily from extremely rare events—literally unprecedented in human history. The impact hazard represents the extreme case of a calamity of low probability but high consequences, including the possible end of civilization as we know it. It is logical to concentrate first on mitigating the risk from global catastrophes. Later, it may be desirable to extend mitigation efforts to smaller impacts that are much more likely to happen within our lifetimes, although they do not threaten society as a whole.
Plot of frequency of impacts on Earth vs. impact energy for near-Earth asteroids (NEAs). The power law is a long-term average derived primarily from lunar cratering and the current number and distribution of known NEAs. Shown plotted at their estimated energies are the Hiroshima nuclear bomb, the Tunguska impact of a small asteroid in Siberia (1908), and the KT impact that led to the extinction of the dinosaurs (65 million years ago).
Figure 5. Plot of frequency of impacts on Earth vs. impact energy for near-Earth asteroids (NEAs). The power law is a long-term average derived primarily from lunar cratering and the current number and distribution of known NEAs. Shown plotted at their estimated energies are the Hiroshima nuclear bomb, the Tunguska impact of a small asteroid in Siberia (1908), and the KT impact that led to the extinction of the dinosaurs (65 million years ago).
The preceding discussion treats impacts as if they are random statistical events, but they are in fact clearly deterministic. There either is or is not an NEA on a trajectory to collide with the Earth within, say, the next century. Any discussion of mitigation must recognize that these events can be predicted and even eliminated by deflecting a threatening NEA. The key requirement is adequate warning time. This is the philosophy behind the international ”Spaceguard Survey” being carried out by ground-based optical telescopes equipped with state-of-the-art wide-field detectors and automated search capability (20). The NEAs are found as they repeatedly fly past Earth at typical distances of tens of millions of kilometers. If one of them should be on course for a future collision, it should be discovered decades (or more) in advance. The initial goal of Spaceguard is to discover and catalog at least 90% of all NEAs larger than 1km in diameter within 10 years (by 2008). The focus is on NEAs of this size because 1 km is near the lower bound for an impact that can cause a global catastrophe. However, the observers continue to discover more NEAs below 1 km than above it, and over time the survey will extend completeness to smaller sizes.
The threat of impacts and the requirement to survey the NEAs have been recognized by the governments of the United States, the United Kingdom, Japan, and the Council of Europe, as well as by many ad hoc technical panels. The current Spaceguard Survey is being carried out by half a dozen observing teams primarily supported by NASA and the USAF. More than half the discoveries are being made by the Lincoln Lab/MIT group called LINEAR. As of the end of 2001, more than 500 of the estimated 1000 NEAs larger than 1 km have been found, and their orbits have been calculated. We can say with assurance that none of the discovered NEAs poses any threat on the timescale of a human lifetime, but of course we still cannot speak for the objects not yet discovered (21). How far the survey will be extended, and what plans will be developed for possible planetary protection against impacts, are questions for society as a whole, not just the small number of scientists who are currently studying NEAs.


Asteroid. A small (diameter less than 1000 km) rocky or metallic solar system object in heliocentric orbit, generally of moderate or low orbital eccentricity and inclination. The majority are located in the asteroid belt between Mars and Jupiter. Sometimes called minor planet.
Asteroid Belt. The region in the solar system where most asteroids are found, between Mars and Jupiter. Specifically, objects in the main asteroid belt have orbital periods between 2.2 and 3.3 years.
Astronomical unit (AU). The mean distance of Earth from the Sun, approximately 150 million km.
Megaton (MT). Unit of energy equivalent to one million tons of TNT, 4.3 x 1015 joule.
Meteorite. Any extraterrestrial material that survives passage through the atmosphere and falls to Earth’s surface. Most meteorites are fragments of as-teroidal parent bodies.
Near-Earth Asteroid (NEA). An asteroid whose an orbit brings it close to Earth (perihelion distance less than 1.3 AU) or, especially, an asteroid in an Earth-crossing orbit. Sometimes subdivided into Amor, Apollo, and Aten subgroups.
Regolith. The fragmented, dusty, porous, upper layer of material on a planetary surface; essentially the equivalent of Earth’s soil for an object like the Moon that has little or no atmosphere or water.

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