Beyond our planet, Earth, and the other terrestrial planets— scorched Mercury, enshrouded Venus, and frigid Mars—lie the rocky bodies of the asteroid belt. Any natural solar system object other than the Sun, a planet, a dwarf planet, or a moon is called a small body; these include aster- oids, meteoroids, and comets.
Beyond those remnants of the early solar system lie the planets of the outer solar system.
These planets, in order of their distance outward from the Sun, are Jupiter, Saturn, Uranus, and Neptune.
Many astrono- mers consider the asteroid belt a demarcation point between the inner solar system (consisting mainly of the terrestrial planets) and the outer solar system. Most of the rocky asteroids move around the Sun in ellip- tical orbits in the same direction of the Sun’s rotation. Such motion is termed prograde.
Looking down on the solar sys- tem from a vantage point above Earth’s North Pole, an observer would fi nd that prograde orbits are counterclock- wise. Orbits in a clockwise direction are called retrograde.
Between the orbits of Mars and Jupiter are a host of rocky small bodies, about 1,000 km (600 miles) or less in diameter, called asteroids that orbit in the nearly flatring called the asteroid belt. It is because of their small size and large numbers relative to the major planets that asteroids are also called minor planets.
The two designations have been used interchangeably, though the term asteroid is more widely recognized by the general public. Among scientists, those 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 .
MAJOR MILESTONES IN ASTEROID RESEARCh
The first asteroid was discovered on Jan. 1, 1801, by the astronomer 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.
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 suffi cient accuracy to pre- dict 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 astronomers searching for a “missing” planet between Mars and Jupiter during an astronomical confer- ence in 1796. (Unfortunately, Piazzi was not a party to this attempt to locate the missing planet.) In 1801, German mathe- matician Carl Friedrich Gauss developed a method for computing the orbit of an asteroid from only a few observations.
Using Gauss’s predictions, the German Hungarian astronomer Franz von Zach rediscovered Piazzi’s “lost” object on Jan. 1, 1802. Piazzi named this object Ceres after the ancient Roman grain goddess and patron goddess of Sicily, thereby ini- tiating a tradition that continues to the present day—asteroids are named by their discoverers.
The discovery of three more faint objects (at least when compared with Mars and Jupiter) in similar orbits over the next six years— Pallas , Juno , and Vesta —complicated this elegant solu- tion to the missing-planet problem and gave rise to the surprisingly long-lived though no longer 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 a fifth asteroid, which he named Astraea .
There were 88 known asteroids by 1866, when the next major discovery was made: Daniel Kirkwood , an American astronomer, noted that there were gaps (now known as Kirkwood gaps) in the distribution of asteroid distances from the Sun.
The introduction of photogra- phy to the search for new asteroids in 1891, by which time 322 asteroids had been identifi ed, accelerated the discov- ery rate. The asteroid designated (323) Brucia , detected in 1891, was the fi rst to be discovered by means of photography. By the end of the 19th century, 464 had been found; this grew to more than 100,000 by the end of the 20th century and to more than four times that number by 2009.
This explosive growth was a spin-off of a survey designed to find 90 percent of asteroids with diameters greater than 1 km (0.6 miles) that can cross Earth’s orbit and thus have the potential to collide with the planet.. Later Advances in Asteroid Studies During much of the 19th century, most discoveries concerning asteroids were based on studies of their orbits. The vast majority of knowledge about the physical characteristics of asteroids— for example, their size, shape, rotation period, compo- sition, mass, and density—was learned beginning in the 20th century, in particu- lar since the 1970s.
As a result of such studies, these objects went from being merely “minor” planets to becoming small worlds in their own right. In 1918 the Japanese astronomer Hirayama Kiyotsugu recognized cluster- ing in three of the orbital elements (semimajor axis, eccentricity, and inclina- tion) of various asteroids.
He speculated that objects sharing these elements had been formed by explosions of larger par- ent asteroids, and he called such groups of asteroids “families.” In the mid-20th century, calculations of the lifetimes of asteroids whose orbits passed close to those of the major planets showed that most such asteroids were destined either to collide with a planet or to be ejected from the solar system on timescales 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 these asteroids.
At first this source was thought 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.
It is now known that most such objects come from regions in the main asteroid belt near Kirkwood gaps and other orbital resonances.
Geography of the Asteroid Belt Geography in its most literal sense is a description of the features on the sur- face of Earth or another planet. Three coordinates—latitude, longitude, and altitude—suffice for locating all such features. Similarly, the location of any object in the solar system can be speci- fied by three parameters—heliocentric ecliptic longitude, heliocentric ecliptic latitude, and heliocentric distance.
Such positions, however, are valid for only an instant of time since all objects in the solar system are continuously in motion. Thus, a better descriptor of the “loca- tion” of a solar system object is the path, called the orbit, that it follows around the Sun or, in the case of a planetary satellite (moon), the path around its parent planet.
All asteroids orbit the Sun in ellipti- cal orbits and move in the same direction as the major planets. Some elliptical orbits are very nearly circles, while others are highly elongated (eccentric).
An orbit is completely described by six geometric parameters called its elements. Orbital elements, and hence the shape and orien- tation of the orbit, also vary with time because each object is gravitationally act- ing on, and being acted upon by, all other bodies in the solar system.
In most cases, these gravitational effects can be accounted for so that accurate predictions of past and future locations can be made and a mean orbit can be defined. These mean orbits can then be used to describe the geography of the asteroid belt. Names and Orbits of Asteroids Because of their widespread occurrence, asteroids are assigned numbers as well as names.
The numbers are assigned con- secutively after accurate orbital elements have been determined. Ceres is officially known as (1) Ceres, Pallas as (2) Pallas, and so forth. Of the more than 450,000 asteroids discovered by 2009, about 47 percent were numbered. Asteroid discov- erers have the right to choose names for their discoveries as soon as they are num- bered. The names selected are submitted to the International Astronomical Union (IAU) for approval. Prior to the mid-20th century, aster- oids were sometimes assigned numbers
before accurate orbital elements had been determined, and so some numbered aster- oids could not later be located. These objects were referred to as “lost” asteroids. The final lost numbered asteroid, (719) Albert, was recovered in 2000 after a lapse of 89 years. Many newly discovered aster- oids still become “lost” because of an insufficiently long span of observations, but no new asteroids are assigned num- bers until their orbits are reliably known.
The Minor Planet Center at the Harvard-Smithsonian Center for Astro- physics in Cambridge, Mass., maintains computer files for all measurements of asteroid positions. As of 2009, there were more than 60 million such positions in its database.
Distribution and Kirkwood Gaps The great majority of the known asteroids move in orbits between those of Mars and Jupiter. Most of these orbits, in turn, have semimajor axes, or mean distances from the Sun, between 2.06 and 3.28 AU, a region called the main belt. The mean distances are not uniformly distributed but exhibit population depletions, or “gaps.”
These so-called Kirkwood gaps are due to mean-motion resonances with Jupiter’s orbital period. An asteroid with a mean distance from the Sun of 2.50 AU, for example, makes three circuits around the Sun in the time it takes Jupiter, which has a mean distance of 5.20 AU, to make one circuit. The asteroid is thus said to be
in a three-to-one (written 3:1) resonance orbit with Jupiter. Consequently, once every three orbits, Jupiter and an aster- oid in such an orbit would be in the same relative positions, and the asteroid would experience a gravitational force in a fixed direction.
Repeated applications of this force would eventually change the mean distance of this asteroid—and others in similar orbits—thus creating a gap at 2.50 AU. Major gaps occur at distances from the Sun that correspond to resonances with Jupiter of 4:1, 3:1, 5:2, 7:3, and 2:1, with the respective mean distances being 2.06, 2.50, 2.82, 2.96, and 3.28. The major gap at the 4:1 resonance defines the nearest extent of the main belt; the gap at the 2:1 resonance, the farthest extent. Some mean-motion resonances, rather than dispersing asteroids, are observed to collect them. Outside the limits of the main belt, asteroids cluster near resonances of 5:1 (at 1.78 AU, called the Hungaria group), 7:4 (at 3.58 AU, the Cybele group), 3:2 (at 3.97 AU, the Hilda group), 4:3 (at 4.29 AU, the lone asteroid (279) Thule), and 1:1 (at 5.20 AU, the Trojan groups). The presence of other resonances, called secular resonances, complicates the situation, particularly at the sunward edge of the belt. Secular res- onances, in which two orbits interact through the motions of their ascending nodes, perihelia, or both, operate over timescales of millions of years to change the eccentricity and inclination of aster- oids. Combinations of mean-motion and secular resonances can either result in long-term stabilization of asteroid orbits at certain mean-motion resonances, as is evidenced by the Hungaria, Cybele, Hilda, and Trojan asteroid groups, or cause the orbits to evolve away from the resonances, as is evidenced by the Kirkwood gaps. Near-Earth Asteroids Asteroids that can come close to Earth are called near-Earth asteroids (NEAs), although only some NEAs actually cross Earth’s orbit. NEAs are divided into sev- eral classes.
Asteroids belonging to the class most distant from Earth—those asteroids that can cross the orbit of Mars but that have perihelion distances greater than 1.3 AU—are dubbed Mars crossers. This class is further subdivided into two: shallow Mars crossers (perihelion dis- tances no less than 1.58 AU but less than 1.67 AU) and deep Mars crossers (perihe- lion distances greater than 1.3 AU but less than 1.58 AU). The next most distant class of NEAs is the Amors. Members of this group have perihelion distances that are greater than 1.017 AU, which is Earth’s aphelion dis- tance, but no greater than 1.3 AU.
Amor asteroids therefore do not at present cross Earth’s orbit. Because of strong gravita- tional perturbations produced by their close approaches to Earth, however, the orbital elements of all Earth-approaching asteroids except the shallow Mars cross- ers change appreciably on timescales as short as years or decades. For this reason, about half the known Amors, including (1221) Amor, the namesake of the group,
are part-time Earth crossers. Only aster- oids that cross the orbits of planets—i.e., Earth-approaching asteroids and idiosyn- cratic objects such as (944) Hidalgo and Chiron—suffer significant changes in their orbital elements on timescales shorter than many millions of years. There are two classes of NEAs that deeply cross Earth’s orbit on an almost continuous basis. The first of these to be discovered were the Apollo asteroids, named for (1862) Apollo, which was dis- covered in 1932 but was lost shortly thereafter and not rediscovered until 1978.
The mean distances of Apollo asteroids from the Sun are greater than or equal to 1 AU, and their perihelion distances are less than or equal to Earth’s aphelion dis- tance of 1.017 AU; thus, they cross Earth’s orbit when near the closest points to the Sun in their own orbits. The other class of Earth-crossing asteroids is named Atens for (2062) Aten, which was discovered in 1976. The Aten asteroids have mean dis- tances from the Sun that are less than 1 AU and aphelion distances that are greater than or equal to 0.983 AU, the perihelion distance of Earth; they cross Earth’s orbit when near the farthest points from the Sun of their orbits.
The class of NEAs that was the last to be recognized is composed of asteroids with orbits entirely inside that of Earth. Known as Atira asteroids after (163693) Atira, they have mean distances from the Sun that are less than 1 AU and aphelion distances less than 0.983 AU; they do not cross Earth’s orbit.
The orbital characteristics of NEAs mean that some of these objects make close approaches to Earth and occasion- ally collide with it. In January 1991, for example, an Apollo asteroid (or, as an alternative description, a large meteor- oid) with an estimated diameter of 10 metres (33 feet) passed by Earth within less than half the distance to the Moon. Such passages are not especially unusual. On Oct. 6, 2008, the asteroid 2008 TC3, which had a size of about 5 metres (16 feet), was discovered and crashed in the Nubian desert of the Sudan the next day. The collision of a sufficiently large NEA with Earth is generally recognized to pose a great potential danger to human beings and possibly to all life on the planet. Because of the small sizes of NEAs and the short time they spend close enough to Earth to be seen, it is unusual for such close passages to be observed. An example of a NEA for which the lead time for observation is large is (99942) Apophis. This Aten asteroid, which has a diameter of about 300 m (984 feet), is predicted to pass within 32,000 km (19,884 miles) of Earth—i.e., closer than
communications satellites in geosta- tionary orbits—on April 13, 2029. During that passage, the probability of the aster- oid hitting Earth is thought to be near zero. In 2006, however, it had been esti- mated that Apophis would have about 1 chance in 50,000 of colliding with Earth during the following close approach, on April 13, 2036.
The first measurements of the sizes of individual asteroids were made in the last years of the 19th century. A filar microm- eter, an instrument normally used in conjunction with a telescope for visual measurement of the separations of dou- ble stars, was employed to estimate the diameters of the first four known aster- oids. The results established that Ceres was the largest asteroid, having a diame- ter estimated to be nearly 800 km (497 miles). These values had remained the best available until new techniques for finding albedos (reflectivities) and diam- eters, based on infrared radiometry and polarization measurements, were intro- duced beginning about 1970. The first four asteroids came to be known as the “big four,” and, because all other asteroids were much fainter, they all were believed to be considerably smaller as well. The first asteroid to have its mass determined was Vesta—in 1966 from measurements of its perturbation of the orbit of asteroid (197) Arete. The first mineralogical determination of the sur- face composition of an asteroid was made in 1969 when spectral reflectance mea- surements identified the mineral pyroxene in the surface material of Vesta. Size and Albedo About 30 asteroids are larger than 200 km (124 miles). The largest, Ceres, has a diameter of about 940 km (584 miles). It is followed by Pallas at 530 km (329 miles), Vesta at 520 km (323 miles), and (10) Hygiea at 410 km (255 miles). Three aster- oids are between 300 and 400 km (186 and 249 miles) in diameter, and about 23 between 200 and 300 km (124 and 186 miles). It has been estimated that 250 asteroids are larger than 100 km (62 miles) in diameter and perhaps a million are larger than 1 km (0.6 miles). The small- est known asteroids are members of the near-Earth group, some of which approach Earth to within a few hundredths of 1 AU. The smallest routinely observed Earth- approaching asteroids measure about 100 metres (328 feet) across. The most widely used technique for determining the sizes of asteroids (and other small bodies in the solar system) is that of thermal radiometry. This technique exploits the fact that the infrared radiation (heat) emitted by an asteroid must bal- ance the solar radiation it absorbs. By using a so-called thermal model to balance the measured intensity of infrared radia- tion with that of radiation at visual wavelengths, investigators are able to derive the diameter of the asteroid. Other
remote-sensing techniques—for example, polarimetry, radar, and adaptive optics (techniques for minimizing the distorting effects of Earth’s atmosphere)—also are used, but they are limited to brighter, larger, or closer asteroids. The only techniques that measure the diameter directly (i.e., without having to model the actual observations) are those of stellar occultation and direct imaging using either advanced instruments on Earth (e.g., large telescopes equipped with adaptive optics or orbiting observatories such as the Hubble Space Telescope) or passing spacecraft. In the method of stel- lar occultation, investigators measure the length of time that a star disappears from view owing to the passage of an asteroid between the star and Earth. Then, using the known distance and the rate of motion of the asteroid, they are able to determine the latter’s diameter as projected onto the plane of the sky. The necessary techniques for imag- ing asteroids directly were perfected during the last years of the 20th century. They (and radar) can be used to observe an asteroid over a complete rotation cycle and so measure the three-dimen- sional shape. These results have made it possible to calibrate the indirect tech- niques, thermal radiometry in particular, such that diameter measurements made with thermal radiometry on asteroids larger than about 20 km (12 miles) are thought to be uncertain by less than 10 percent; for smaller asteroids the uncer- tainty is about 30 percent.
Classification of Asteroids
In the mid-1970s astronomers, using information gathered from studies of
colour, spectral reflectance, and albedo, recognized that asteroids could be grouped into three broad taxonomic classes, designated C, S, and M. At that time 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 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. Within a decade this taxonomic sys- tem was expanded, and it was recognized that the asteroid belt comprised overlap- ping rings of differing taxonomic classes, with classes designated S, C, P, and D dominating the populations at distances from the Sun of about 2, 3, 4, and 5 AU, respectively. As more data became avail- able from further observations, additional minor classes were recognized. Rotation and Shape The rotation periods and shapes of asteroids are determined primarily by monitoring their changing brightness on timescales of minutes to days. Short- period fluctuations in brightness caused by the rotation of an irregularly shaped asteroid or a spherical spotted asteroid (i.e., one with albedo differences) produce a light curve—a graph of brightness ver- sus time—that repeats at regular intervals corresponding to an asteroid’s rotation
period. The range of brightness variation is closely related to an asteroid’s shape or spottedness but is more difficult to interpret. In the early years of the 21st century, rotation periods were known for more than 2,300 asteroids. They range from 42.7 seconds to 50 days, but more than 70 percent lie between 4 and 24 hours. In some cases, periods longer than a few days may actually be due to precession (a smooth slow circling of the rotation axis) caused by an unseen satellite of the asteroid. Periods on the order of minutes are observed only for very small objects (those with diameters less than about 150 metres [492 feet]). The largest asteroids (those with diameters greater than about 200 km [1214 miles]) have a mean rota- tion period close to 8 hours; the value increases to 13 hours for asteroids with diameters of about 100 km (62 miles) and then decreases to about 6 hours for those with diameters of about 10 km (6 miles). The largest asteroids may have pre- served the rotation rates they had when they were formed, but the smaller ones almost certainly have had theirs modi- fied by subsequent collisions and, in the case of the very smallest, perhaps also by radiation effects. The difference in rota- tion periods between 200-km-class (124 mile) and 100-km-class (62 mile) aster- oids is believed to stem from the fact that large asteroids retain all of the collision 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 dis- rupt smaller asteroids. The debris from such collisions makes still smaller aster- oids, which can have virtually any shape or spin rate. Thus, the fact that no rotation periods shorter than about 2 hours have been observed for asteroids greater than about 150 metres (492 feet) in diameter implies that their material strengths are not high enough to withstand the centrip- etal forces that such rapid spins produce. It is impossible to distinguish mathe- matically 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 sides of most aster- oids appear to differ no more than a few percent in albedo suggests that their brightness variations are due mainly to changes in the projection of their illumi- nated portions as seen from Earth. Hence, in the absence of evidence to the contrary, astronomers generally accept that varia- tions in reflectivity contribute little to the observed amplitude, or range in bright- ness variation, of an asteroid’s rotational light curve. Vesta is a notable exception to this generalization because the differ- ence in reflectivity between its opposite hemispheres is known to be sufficient to account for much of its modest light- curve amplitude.
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, Jupiter, grew at a high rate and to a size larger than the others. In the final stages of its formation, Jupiter gravitationally scattered large planetesimals, some of which may have been as massive as Earth is today. These planetesimals were eventually either cap- tured by Jupiter or another of the giant planets or ejected from the solar system. While they were passing through the inner solar system, however, such large plane- tesimals strongly perturbed the orbits of the planetesimals in the region of the asteroid belt, raising their mutual veloci- ties to the average 5 km (3 miles) per second they exhibit today. The increased velocities ended the accretionary colli- sions in this region by transforming them into catastrophic disruptions. Only objects larger than about 500 km (310 miles) in diameter could have survived collisions with objects of comparable size at collision velocities of 5 km (3 miles) per second.
Geographos is an asteroid that passes inside Earth’s orbit. Geographos was discovered in 1951 by American astronomers Albert Wilson and Rudolf Minkowski at the Palomar Observatory. In 1994 radar observations found that Geographos has dimensions of 5.11 × 1.85 km (3.18 × 1.15 miles) and is thus the most elongated object in the solar system. That same year the U.S. spacecraft Clementine was scheduled to fl y by Geographos after leaving lunar orbit, but a com- puter malfunction canceled that portion of the mission. Hermes The binary asteroid Hermes has an eccentric orbit that brings it near Earth. It was discovered in October 1937 by German astronomer Karl Wilhelm Reinmuth when it approached within about 742,000 km (461,000 miles) of Earth; announcement of this near pas- sage occasioned some fear that it might collide with Earth. Hermes was subse- quently lost and was not observed again until 2003. Radar observations of Hermes showed that it was actually two asteroids that orbit each other every 14 hours.
The asteroid Icarus has a very eccentric orbit with an eccentricity of 0.82 and also approaches quite near to the Sun (within 30 million km [19 million miles]). It was discovered in 1949 by Walter Baade of the Hale Observatories (now Palomar Observatory), California. Its orbit extends from beyond Mars to within that of Mercury; it can approach within 6.4 million km (4 million miles) of Earth. In June 1968 Icarus, the first aster- oid to be examined by radar, proved to have a diameter of about 0.8 km (0.5 miles), considerably smaller than previ- ous estimates, and a rotation period of about 2.5 hours.
Pallas is the second largest asteroid in the asteroid belt and the second such object to be discovered, by the German astronomer and physician Wilhelm Olbers on March 28, 1802, following the discovery of Ceres the year before. It is named after Pallas Athena, the Greek goddess of wisdom. Pallas’s orbital inclination of 34.8° is rather large, but its moderate orbital eccentricity (0.23), mean distance from the Sun of 2.77 AU (about 414 million km [257 million miles]), and orbital period of 4.61 years are typical for asteroids located between the orbits of Mars and Jupiter.