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253 Mathilde, a C-type asteroid measuring about 50 km across. Photograph taken in 1997 by the NEAR Shoemaker probe.

Asteroids, also called minor planets or planetoids, are Solar System bodies smaller than planets but larger than meteoroids (which are commonly defined as being 10 meters across or less), and that are not comets. The distinction between asteroids and comets is made on visual appearance when discovered: comets must show a perceptible coma (a fuzzy "atmosphere"), while asteroids do not.

Asteroids vary greatly in size, from a few hundreds of kilometres in diameter down to rocks just tens of metres across. A few of the largest are roughly spherical and are very much like miniature planets. The vast majority, however, are much smaller and are irregularly shaped. The physical composition of asteroids is varied and in many cases poorly understood. Some are solid rocky bodies, with a greater or lesser metallic content, while others are piles of rubble held together loosely by gravity. Only one asteroid—Vesta—is visible to the naked eye, and this only in very dark skies when it is favourably positioned.

The first named minor planet, Ceres, was discovered in 1801 by Giuseppe Piazzi, and was originally considered a new planet. This was followed by the discovery of other similar bodies, which with the equipment of the time appeared to be points of light, like stars, showing little or no planetary disc (though readily distinguishable from stars due to their apparent motions). This prompted the astronomer Sir William Herschel to propose the term "asteroid", from Greek αστεροειδής, asteroeidēs = star-like, star-shaped, from ancient Greek Aστήρ, astēr = star.

The vast majority of known asteroids are found within the main asteroid belt, between the orbits of Mars and Jupiter, generally in relatively low- eccentricity (i.e., not very elongated) orbits. This belt is estimated to contain more than 750,000 asteroids larger than 1 kilometer across, and millions of smaller ones. It is thought that these asteroids are remnants of the protoplanetary disk, and in this region the accretion of planetesimals into planets during the formative period of the solar system was prevented by large gravitational perturbations by Jupiter. Some asteroids have moons or are found in co-orbiting pairs known as binary systems. Minor planets have more recently been found to cross the orbits of planets, from Mercury to Neptune—with hundreds of trans-Neptunian objects ( TNOs) now known to exist well past Neptune's orbit. (Using indirect methods, the total number of TNOs has been estimated in the hundreds of millions or even billions.)

Asteroids are given a provisional designation by year in the order of discovery, and a designation (a sequential number) and name if their existence is well established and an orbit has been determined.


The term "asteroid" is used to describe any of a diverse group of small celestial bodies orbiting the Sun—traditionally in the inner Solar System, since those were the only ones known. In English it is the most commonly used word for a minor planet, which was the term preferred by the International Astronomical Union (IAU) prior to 2006. Other languages prefer "planetoid" (Greek for "planet-like"). The word " planetesimal" has a similar meaning, but often refers specifically to small bodies that existed at the time the Solar System was forming. The term "planetule" was coined by the geologist Conybeare to describe minor planets, but is not in common use.

Traditionally, small bodies orbiting the Sun were classified as asteroids, comets or meteoroids, with anything smaller than, say, ten metres across being called a meteoroid. The main difference between an asteroid and a comet is that a comet shows a coma due to sublimation of near surface ices by solar radiation. A few objects have ended up being dual-listed because they were first classified as minor planets but later showed evidence of cometary activity. Conversely, some (perhaps all) comets eventually are depleted of their volatile ices and then appear as point-like objects, i.e. asteroids. A further distinction is that comets typically have more eccentric orbits than asteroids (though some objects classified as asteroids also have notably eccentric orbits).

In recent years, the situation has been complicated by the discovery of trans-Neptunian objects (TNOs). These inhabit the cold outer reaches of the Solar System where ices remain solid and comet-like bodies are not expected to exhibit much cometary activity. The innermost of these are the Kuiper Belt Objects (KBOs), called "objects" partly to avoid the need to classify them as asteroids or comets. KBOs are believed to be predominantly comet-like in composition, though some may be more akin to asteroids. Furthermore, they do not necessarily have the highly eccentric orbits usually associated with comets, and there are significant numbers very much larger than traditional comet nuclei. The much more distant Oort cloud is also hypothesised to be a reservoir of dormant comets.

Other recent observations, such as the analysis of the cometary dust collected by the Stardust probe, are increasingly blurring the distinction between comets and asteroids, suggesting "a continuum between asteroids and comets" rather than a sharp dividing line.

In late August 2006, the IAU introduced the class small solar system bodies (SSSB) to include most objects previously classified as minor planets and comets. At the same time, the class dwarf planets was created for the largest minor planets—those which have sufficient mass to have become more-or-less spherical under their own gravity. According to the IAU, "the term 'minor planet' may still be used, but generally the term 'small solar system body' will be preferred." Currently only the largest object in the asteroid belt, Ceres, at about 950 km across, is in the dwarf planet category, although there are several relatively large near-spherical asteroids (Vesta, Pallas and Hygiea) that may be reclassified as dwarf planets in future.

Distribution within the Solar System

The Main asteroid belt (white) and the Trojan asteroids (green)

Hundreds of thousands of asteroids have been discovered within the Solar System, with the rate of discovery currently running at around 5,000 per month. Of the more than 400,000 registered minor planets, 187,745 have orbits known well enough to be assigned permanent official numbers. Of these, 14,525 have official names. The lowest-numbered, unnamed minor planet is (3708) 1974 FV1; the highest-numbered named minor planet is 181627 Philgeluck. Current estimates put the total number of asteroids above 1 km in diameter in the Solar System to be between 1.1 and 1.9 million. Ceres, with diameters of 975 × 909 km, was once considered the largest asteroid in the inner solar system, but it has since been recategorized as a dwarf planet. That distinction now falls to 2 Pallas and 4 Vesta; both have diameters of about 500 km. Normally Vesta is the only main belt asteroid that can, on occasion, become visible to the naked eye. However, on some very rare occasions, a near-Earth asteroid may briefly become visible without technical aid; see 99942 Apophis.

Left to right: 4 Vesta, 1 Ceres, Earth's Moon

The mass of all the objects of the Main asteroid belt, lying between the orbits of Mars and Jupiter, is estimated to be about 3.0-3.6×1021 kg, or about 4 percent of the mass of the Moon. Of this, Ceres comprises 0.95×1021 kg, some 32 percent of the total. Adding in the next three most massive asteroids, 4 Vesta (9%), 2 Pallas (7%), and 10 Hygiea (3%), brings this figure up to 51%; while the three after that, 511 Davida (1.2%), 704 Interamnia (1.0%), and 52 Europa (0.9%), only add another 3% to the total mass. The number of asteroids then increases rapidly as their individual masses decrease.

Various classes of asteroid have been discovered outside the main asteroid belt. Near-Earth asteroids have orbits in the vicinity of Earth's orbit. Trojan asteroids are gravitationally locked into synchronisation with a planet, either leading or trailing the planet in its orbit. The majority of Trojans are associated with Jupiter, but a few have been found orbiting with Mars or Neptune. Asteroids orbiting between Jupiter and Neptune are called Centaurs, and beyond this lie swarms of trans-Neptunian objects. A group of asteroids called Vulcanoids are hypothesised by some to lie very close to the Sun, within the orbit of Mercury, but none has so far been found.


Asteroids are commonly classified according to two criteria: the characteristics of their orbits, and features of their reflectance spectrum.

Orbit groups and families

Many asteroids have been placed in groups and families based on their orbital characteristics. Apart from the broadest divisions, it is customary to name a group of asteroids after the first member of that group to be discovered. Groups are relatively loose dynamical associations, whereas families are much tighter and result from the catastrophic break-up of a large parent asteroid sometime in the past. Families have only been recognized within the main asteroid belt. They were first recognised by Kiyotsugu Hirayama in 1918 and are often called Hirayama families in his honour.

About 30% to 35% of the bodies in the main belt belong to dynamical families each thought to have a common origin in a past collision between asteroids. A family has also been associated with the Trans-Neptunian Object (136108) 2003 EL61.

Quasi-satellites and horseshoe objects

Some asteroids have unusual horseshoe orbits that are co-orbital with the Earth or some other planet. Examples are 3753 Cruithne and 2002 AA29. The first instance of this type of orbital arrangement was discovered between Saturn's moons Epimetheus and Janus.

Sometimes these horseshoe objects temporarily become quasi-satellites for a few decades or a few hundred years, before returning to their prior status. Both Earth and Venus are known to have quasi-satellites.

Such objects, if associated with Earth or Venus or even hypothetically Mercury are a special class of Aten asteroids. However, such objects could be associated with outer planets as well.

Spectral classification

This picture of 433 Eros shows the view looking from one end of the asteroid across the gouge on its underside and toward the opposite end. Features as small as 35 m across can be seen.

In 1975, an asteroid taxonomic system based on colour, albedo, and spectral shape was developed by Clark R. Chapman, David Morrison, and Ben Zellner. These properties are thought to correspond to the composition of the asteroid's surface material. The original classification system had three categories: C-types for dark carbonaceous objects (75% of known asteroids), S-types for stony (silicaceous) objects (17% of known asteroids) and U for those that did not fit into either C or S. This classification has since been expanded to include a number of other asteroid types. The number of types continues to grow as more asteroids are studied.

The two most widely used taxonomies currently used are the Tholen classification and SMASS classification. The former was proposed in 1984 by David J. Tholen, and was based on data collected from an eight-colour asteroid survey performed in the 1980s. This resulted in 14 asteroid categories. In 2002, the Small Main-Belt Asteroid Spectroscopic Survey resulted in a modified version of the Tholen taxonomy with 24 different types. Both systems have three broad categories of C, S, and X asteroids, where X consists of mostly metallic asteroids, such as the M-type. There are also a number of smaller classes.

Note that the proportion of known asteroids falling into the various spectral types does not necessarily reflect the proportion of all asteroids that are of that type; some types are easier to detect than others, biasing the totals.

Problems with spectral classification

Originally, spectral designations were based on inferences of an asteroid's composition. However, the correspondence between spectral class and composition is not always very good, and there are a variety of classifications in use. This has led to significant confusion. While asteroids of different spectral classifications are likely to be composed of different materials, there are no assurances that asteroids within the same taxonomic class are composed of similar materials.

At present, the spectral classification based on several coarse resolution spectroscopic surveys in the 1990s is still the standard. Scientists have been unable to agree on a better taxonomic system, largely due to the difficulty of obtaining detailed measurements consistently for a large sample of asteroids (e.g. finer resolution spectra, or non-spectral data such as densities would be very useful).


243 Ida and its moon Dactyl, the first satellite of an asteroid to be discovered.

Historical methods

Asteroid discovery methods have dramatically improved over the past two centuries.

In the last years of the 18th century, Baron Franz Xaver von Zach organized a group of 24 astronomers to search the sky for the missing planet predicted at about 2.8 AU from the Sun by the Titius-Bode law, partly as a consequence of the discovery, by Sir William Herschel in 1781, of the planet Uranus at the distance predicted by the law. This task required that hand-drawn sky charts be prepared for all stars in the zodiacal band down to an agreed-upon limit of faintness. On subsequent nights, the sky would be charted again and any moving object would, hopefully, be spotted. The expected motion of the missing planet was about 30 seconds of arc per hour, readily discernible by observers.

Ironically, the first asteroid, 1 Ceres, was not discovered by a member of the group, but rather by accident in 1801 by Giuseppe Piazzi, director of the observatory of Palermo in Sicily. He discovered a new star-like object in Taurus and followed the displacement of this object during several nights. His colleague, Carl Friedrich Gauss, used these observations to determine the exact distance from this unknown object to the Earth. Gauss' calculations placed the object between the planets Mars and Jupiter. Piazzi named it after Ceres, the Roman goddess of agriculture.

Three other asteroids ( 2 Pallas, 3 Juno, and 4 Vesta) were discovered over the next few years, with Vesta found in 1807. After eight more years of fruitless searches, most astronomers assumed that there were no more and abandoned any further searches.

However, Karl Ludwig Hencke persisted, and began searching for more asteroids in 1830. Fifteen years later, he found 5 Astraea, the first new asteroid in 38 years. He also found 6 Hebe less than two years later. After this, other astronomers joined in the search and at least one new asteroid was discovered every year after that (except the wartime year 1945). Notable asteroid hunters of this early era were J. R. Hind, Annibale de Gasparis, Robert Luther, H. M. S. Goldschmidt, Jean Chacornac, James Ferguson, Norman Robert Pogson, E. W. Tempel, J. C. Watson, C. H. F. Peters, A. Borrelly, J. Palisa, the Henry brothers and Auguste Charlois.

In 1891, however, Max Wolf pioneered the use of astrophotography to detect asteroids, which appeared as short streaks on long-exposure photographic plates. This dramatically increased the rate of detection compared with previous visual methods: Wolf alone discovered 248 asteroids, beginning with 323 Brucia, whereas only slightly more than 300 had been discovered up to that point. Still, a century later, only a few thousand asteroids were identified, numbered and named. It was known that there were many more, but most astronomers did not bother with them, calling them "vermin of the skies".

Manual methods of the 1900s and modern reporting

Until 1998, asteroids were discovered by a four-step process. First, a region of the sky was photographed by a wide-field telescope, or Astrograph. Pairs of photographs were taken, typically one hour apart. Multiple pairs could be taken over a series of days. Second, the two films of the same region were viewed under a stereoscope. Any body in orbit around the Sun would move slightly between the pair of films. Under the stereoscope, the image of the body would appear to float slightly above the background of stars. Third, once a moving body was identified, its location would be measured precisely using a digitizing microscope. The location would be measured relative to known star locations.

These first three steps do not constitute asteroid discovery: the observer has only found an apparition, which gets a provisional designation, made up of the year of discovery, a letter representing the week of discovery, and finally a letter and a number indicating the discovery's sequential number (example: 1998 FJ74).

The final step of discovery is to send the locations and time of observations to the Minor Planet Center, where computer programs determine whether an apparition ties together previous apparitions into a single orbit. If so, the object receives a catalogue number and the observer of the first apparition with a calculated orbit is declared the discoverer, and granted the honour of naming the object subject to the approval of the International Astronomical Union.

Computerized methods

2004 FH is the centre dot being followed by the sequence; the object that flashes by during the clip is an artificial satellite.

There is increasing interest in identifying asteroids whose orbits cross Earth's, and that could, given enough time, collide with Earth (see Earth-crosser asteroids). The three most important groups of near-Earth asteroids are the Apollos, Amors, and Atens. Various asteroid deflection strategies have been proposed, as early as the 1960s.

The near-Earth asteroid 433 Eros had been discovered as long ago as 1898, and the 1930s brought a flurry of similar objects. In order of discovery, these were: 1221 Amor, 1862 Apollo, 2101 Adonis, and finally 69230 Hermes, which approached within 0.005 AU of the Earth in 1937. Astronomers began to realize the possibilities of Earth impact.

Two events in later decades increased the level of alarm: the increasing acceptance of Walter Alvarez' hypothesis that an impact event resulted in the Cretaceous-Tertiary extinction, and the 1994 observation of Comet Shoemaker-Levy 9 crashing into Jupiter. The U.S. military also declassified the information that its military satellites, built to detect nuclear explosions, had detected hundreds of upper-atmosphere impacts by objects ranging from one to 10 metres across.

All of these considerations helped spur the launch of highly efficient automated systems that consist of Charge-Coupled Device ( CCD) cameras and computers directly connected to telescopes. Since 1998, a large majority of the asteroids have been discovered by such automated systems. A list of teams using such automated systems includes:

  • The Lincoln Near-Earth Asteroid Research (LINEAR) team
  • The Near-Earth Asteroid Tracking (NEAT) team
  • Spacewatch
  • The Lowell Observatory Near-Earth-Object Search (LONEOS) team
  • The Catalina Sky Survey (CSS)
  • The Campo Imperatore Near-Earth Objects Survey (CINEOS) team
  • The Japanese Spaceguard Association
  • The Asiago-DLR Asteroid Survey (ADAS)

The LINEAR system alone has discovered 84,764 asteroids, as of August 28, 2007. Between all of the automated systems, 4711 near-Earth asteroids have been discovered including over 600 more than 1 km in diameter.


Overview: naming conventions

A newly discovered asteroid is given a provisional designation (such as 2002 AT4) consisting of the year of discovery and an alphanumeric code indicating the half-month of discovery and the sequence within that half-month. Once an asteroid's orbit has been confirmed, it is given a number, and later may also be given a name (e.g. 433 Eros). The formal naming convention uses parentheses around the number (e.g. (433) Eros), but dropping the parentheses is quite common. Informally, it is common to drop the number altogether, or to drop it after the first mention when a name is repeated in running text.

Asteroids that have been given a number but not a name keep their provisional designation, e.g. (29075) 1950 DA. As modern discovery techniques are finding vast numbers of new asteroids, they are increasingly being left unnamed. The first asteroid to be left unnamed was for a long time (3360) 1981 VA, now 3360 Syrinx; as of November 2006, this distinction is now held by (3708) 1974 FV1. On rare occasions, a small body's provisional designation may become used as a name in itself: the still unnamed (15760) 1992 QB1 gave its name to a group of Kuiper belt objects which became known as cubewanos.


Asteroids are awarded with an official number once their orbits are confirmed. With the increasing rapidity of asteroid discovery, asteroids are currently being awarded six-figure numbers. The switch from five figures to six figures arrived with the publication of the Minor Planet Circular (MPC) of October 19, 2005, which saw the highest numbered asteroid jump from 99947 to 118161. This change caused a small Y2K-like crisis for various automated data services, since only five digits were allowed in most data formats for the asteroid number. Most services have now widened the asteroid number field. For those which did not, the problem has been addressed in some cases by having the leftmost digit (the ten-thousands place) use the alphabet as a digit extension. A=10, B=11,..., Z=35, a=36,..., z=61. A high number such as 120437 is thus cross-referenced as C0437 on some lists.

Special naming rules

Asteroid naming is not always a free-for-all: there are some types of asteroid for which rules have developed about the sources of names. For instance Centaurs (asteroids orbiting between Saturn and Neptune) are all named after mythological centaurs, Trojans after heroes from the Trojan War, and trans-Neptunian objects after underworld spirits.

Another well-established rule is that comets are named after their discoverer(s), whereas asteroids are not. One way to circumvent this rule has been for astronomers to exchange the courtesy of naming their discoveries after each other. A particular exception to this rule is 96747 Crespodasilva, which was named after its discoverer, Lucy d'Escoffier Crespo da Silva, because she died shortly after the discovery, at age 22. A few objects are also cross-listed as both comets and asteroids, such as 4015 Wilson-Harrington and 107P/Wilson-Harrington.


The first few asteroids discovered were assigned symbols like the ones traditionally used to designate Earth, the Moon, the Sun and planets. The symbols quickly became ungainly, hard to draw and recognise. By the end of 1851 there were 15 known asteroids, each (except one) with its own symbol(s).

Asteroid Symbol
Ceres Old planetary symbol of Ceres Variant symbol of Ceres Sickle variant symbol of Ceres Other sickle variant symbol of Ceres
2 Pallas Old symbol of Pallas Variant symbol of Pallas
3 Juno Old symbol of Juno Other symbol of Juno Symbol 3.jpg
4 Vesta Old symbol of Vesta Old planetary symbol of Vesta Modern astrological symbol of Vesta4 Vesta Unsimplified Symbol.svg
5 Astraea 5 Astraea Symbol.svg
6 Hebe 6 Hebe Astronomical Symbol.svg
7 Iris 7 Iris Astronomical Symbol.svg
8 Flora 8 Flora Astronomical Symbol.svg
9 Metis 9 Metis symbol.svg
10 Hygiea 10 Hygiea Astronomical Symbol.svg
11 Parthenope 11 Parthenope symbol.svg
12 Victoria 12 Victoria symbol.svg
13 Egeria Never assigned.
14 Irene "A dove carrying an olive-branch, with a star on its head," never drawn.
15 Eunomia 15 Eunomia symbol.svg
28 Bellona 28 Bellona symbol.svg
35 Leukothea 35 Leukothea symbol.png
37 Fides 37 Fides symbol.svg

Johann Franz Encke made a major change in the Berliner Astronomisches Jahrbuch (BAJ, Berlin Astronomical Yearbook) for 1854. He introduced encircled numbers instead of symbols, although his numbering began with Astraea, the first four asteroids continuing to be denoted by their traditional symbols. This symbolic innovation was adopted very quickly by the astronomical community. The following year (1855), Astraea's number was bumped up to 5, but Ceres through Vesta would be listed by their numbers only in the 1867 edition. A few more asteroids ( 28 Bellona, 35 Leukothea, and 37 Fides) would be given symbols as well as using the numbering scheme. The circle would become a pair of parentheses, and the parentheses sometimes omitted altogether over the next few decades.


Until the age of space travel, objects in the asteroid belt were merely pinpricks of light in even the largest telescopes and their shapes and terrain remained a mystery. The best modern ground-based telescopes, as well as the Earth-orbiting Hubble Space Telescope, can resolve a small amount of detail on the surfaces of the very largest asteroids, but even these mostly remain little more than fuzzy blobs. Limited information about the shapes and compositions of asteroids can be inferred from their light curves (their variation in brightness as they rotate) and their spectral properties, and asteroid sizes can be estimated by timing the lengths of star occulations (when an asteroid passes directly in front of a star). Radar imaging can yield good information about asteroid shapes and orbital and rotational parameters, especially for near-Earth asteroids.

The first close-up photographs of asteroid-like objects were taken in 1971 when the Mariner 9 probe imaged Phobos and Deimos, the two small moons of Mars, which are probably captured asteroids. These images revealed the irregular, potato-like shapes of most asteroids, as did subsequent images from the Voyager probes of the small moons of the gas giants.

951 Gaspra, the first asteroid to be imaged in close up.

The first true asteroid to be photographed in close-up was 951 Gaspra in 1991, followed in 1993 by 243 Ida and its moon Dactyl, all of which were imaged by the Galileo probe en route to Jupiter.

The first dedicated asteroid probe was NEAR Shoemaker, which photographed 253 Mathilde in 1997, before entering into orbit around 433 Eros, finally landing on its surface in 2001.

Other asteroids briefly visited by spacecraft en route to other destinations include 9969 Braille (by Deep Space 1 in 1999), and 5535 Annefrank (by Stardust in 2002).

In September 2005, the Japanese Hayabusa probe started studying 25143 Itokawa in detail and may return samples of its surface to earth. The Hayabusa mission has been plagued with difficulties, including the failure of two of its three control wheels, rendering it difficult to maintain its orientation to the sun to collect solar energy. Following that, the next asteroid encounters will involve the European Rosetta probe (launched in 2004), which will study 2867 Šteins and 21 Lutetia in 2008 and 2010.

In September 2007, NASA launched the Dawn Mission, which will orbit the dwarf planet Ceres and the asteroid 4 Vesta in 2011-2015, with its mission possibly then extended to 2 Pallas.

It has been suggested that asteroids might be used in the future as a source of materials which may be rare or exhausted on earth ( asteroid mining), or materials for constructing space habitats (see Colonization of the asteroids). Materials that are heavy and expensive to launch from earth may someday be mined from asteroids and used for space manufacturing and construction.

In fiction

Asteroids and asteroid belts are a staple of science fiction stories. Asteroids play several potential roles in science fiction: as places which human beings might colonize; as resources for extracting minerals; as a hazard encountered by spaceships travelling between two other points; and as a threat to life on Earth due to potential impacts.

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