
[From Greek asteroeidēs, starlike : astēr, star + -oeidēs, -oid.]
| No. | Name | Year | q | a | P | e | i | D | A |
| 1 | Ceres | 1801 | 2.55 | 2.99 | 4.610 | 0.079 | 10.60 | 933 | 0.054 |
| 2 | Pallas | 1802 | 2.11 | 3.42 | 4.607 | 0.237 | 34.85 | 608 | 0.074 |
| 3 | Juno | 1804 | 1.98 | 3.35 | 4.358 | 0.257 | 13.00 | 288 | 0.151 |
| 4 | Vesta | 1807 | 2.15 | 2.57 | 3.630 | 0.089 | 7.14 | 538 | 0.229 |
| 5 | Astraea | 1845 | 2.10 | 3.06 | 4.139 | 0.187 | 5.34 | 117 | 0.140 |
| 6 | Hebe | 1847 | 1.93 | 2.92 | 3.778 | 0.203 | 14.77 | 195 | 0.164 |
| 7 | Iris | 1847 | 1.84 | 2.94 | 3.686 | 0.230 | 5.50 | 209 | 0.154 |
| 8 | Flora | 1847 | 1.86 | 2.55 | 3.267 | 0.156 | 5.89 | 151 | 0.144 |
| 9 | Metis | 1848 | 2.09 | 2.68 | 3.684 | 0.123 | 5.58 | 151 | 0.139 |
| 10 | Hygeia | 1849 | 2.84 | 3.46 | 5.593 | 0.100 | 3.81 | 450 | 0.041 |
| 11 | Parthenope | 1850 | 2.20 | 2.70 | 3.840 | 0.102 | 4.62 | 150 | 0.126 |
| 12 | Victoria | 1850 | 1.82 | 2.85 | 3.568 | 0.218 | 8.37 | 126 | 0.114 |
| 13 | Egeria | 1850 | 2.36 | 2.80 | 4.135 | 0.085 | 16.53 | 224 | 0.041 |
| 14 | Irene | 1851 | 2.16 | 3.01 | 4.163 | 0.164 | 9.13 | 158 | 0.162 |
| 15 | Eunomia | 1851 | 2.15 | 3.14 | 4.300 | 0.188 | 11.73 | 272 | 0.155 |
| 16 | Psyche | 1851 | 2.53 | 3.32 | 5.000 | 0.135 | 3.09 | 250 | 0.093 |
| 17 | Thetis | 1852 | 2.13 | 3.52 | 3.880 | 0.138 | 5.59 | 109 | 0.103 |
| 18 | Melpomene | 1852 | 1.80 | 2.80 | 3.480 | 0.218 | 10.14 | 150 | 0.144 |
| 19 | Fortuna | 1852 | 2.06 | 2.83 | 3.816 | 0.158 | 1.56 | 215 | 0.032 |
| 20 | Massalia | 1852 | 2.06 | 2.76 | 3.737 | 0.145 | 0.70 | 131 | 0.164 |
| No. | Name | Diameter (km) |
| 1 | Ceres | 933 |
| 2 | Pallas | 608 |
| 4 | Vesta | 538 |
| 10 | Hygeia | 450 |
| 31 | Euphrosyne | 370 |
| 704 | Interamnia | 350 |
| 511 | Davida | 323 |
| 65 | Cybele | 309 |
| 52 | Europa | 289 |
| 3 | Juno | 288 |
For more information on asteroid, visit Britannica.com.
One of the many thousands of small planets (minor planets) revolving around the Sun, mainly between the orbits of Mars and Jupiter. Newly discovered asteroids are assigned a catalog number and name (such as 433 Eros) only after they are observed often enough to compute an accurate orbit. There are over 73,000 cataloged asteroids. See also Planet.
The majority of asteroids have semi-major axes (mean distances to the Sun; symbolized a) between 2.2 and 3.2 astronomical units (1 AU = distance from Earth to the Sun = 1.496 × 108 km = 9.3 × 107 mi). However, numerous small asteroids orbit between Venus and Mars, and two large groups, the Trojan asteroids, orbit at Jupiter's distance from the Sun. See also Trojan asteroids.
In 1992, the first of the trans-Neptunian “asteroids” was discovered. Called Kuiper Belt objects (KBOs), about 900 had been found by mid-2004. They represent a population of bodies much more numerous than the main-belt or Trojan asteroids, but are more properly thought of as comets. There are also a modest number of minor planets orbiting the Sun in temporary orbits beyond Jupiter but well inside the Kuiper Belt; they are termed Centaurs. See also Comet; Kuiper Belt.
Most asteroid orbits are more elliptical and inclined to the plane of the ecliptic than the orbits of major planets. A number of small asteroids (Amor objects) cross, but do not intersect, the orbit of Mars, and a few even cross the Earth's orbit (Apollo objects) or orbit inside the Earth's orbit (Aten objects).
Improvements in radar technology make it possible to image small asteroids that pass close to the Earth almost as well as by spacecraft flybys. For more distant asteroids the chief technique used to measure asteroid diameters is radiometry, which compares the brightness of reflected visible sunlight from an asteroid with the brightness of the asteroid's emitted thermal radiation in the infrared. See also Albedo; Infrared astronomy; Occultation.
There are about 30 asteroids larger than 124 mi (200 km) in diameter; about 75% of them are soot black (geometric albedos of 3–5%). Asteroids are much more numerous at smaller sizes, generally following a size distribution characteristic of fragmentation processes, as would be expected if the asteroids were smashing into each other. Indeed, there are so many large asteroids confined in the volume of the asteroid belt that collisions sufficient to fragment all but the larger asteroids occur every few billion years, and much more often for smaller ones. Thus all asteroids have been extensively battered and many are collisional fragments.
Spectra of sunlight reflected from asteroids have shapes, including absorption bands, characteristic of different rock-forming minerals. Combined with the albedo data from radiometry, the spectral colors of surfaces of over 2000 asteroids show that more than three-quarters of them have very low albedos and are composed of carbon-rich material (often with hydrated, or water-rich, minerals). The black asteroids located in the middle and outer parts of the belt (called C type) resemble carbonaceous meteorites, which are believed to be among the most primitive materials in the solar system, little altered since the planets were forming. The black asteroids near the outer edge of the main belt have a reddish tinge and are not represented by known meteorites on the Earth; they are called P types, and may be even richer in organic components. Still farther out, many of the Trojans are even redder and more mysterious; they are termed D types. Closer to the inner edge of the belt, most asteroids are so-called S types, characterized by moderately high albedos and by absorption bands due to the common silicate minerals pyroxene and olivine. They also contain considerable metal, and probably are akin to either the stony-iron meteorites or the ordinary chondritic meteorites. The general progression of asteroid compositions is thought to reflect the variation with distance from the Sun in the composition of the original nebular dust from which the planets were formed. See also Cosmochemistry; Meteorite; Solar system.
Apollo, Amor, and Aten asteroids are of special interest, particularly because they stand a chance of striking the Earth. Indeed, Meteor Crater (Arizona), and other craters on the Earth and the Moon, testify to the potential for collisions with near-Earth asteroids. Many scientists believe that just such a collision 6.5 × 107 years ago rendered most species of life, including the dinosaurs, extinct. A huge, eroded crater of that age on the Yucatán peninsula in Mexico must have been caused by the impact of an asteroid or comet about 10 mi (16 km) in diameter. In 1908, a small asteroid, perhaps 160 ft (50 m) across, exploded over the Tunguska region of Siberia with energy equivalent to 15 megatons of TNT. Only about one-third of the potentially threatening objects have been discovered so far.
After some tens of millions of years, most of the current crop of near-Earth asteroids will have struck the Earth, the Moon, the Sun, or one of the other inner planets, or will have been ejected from the solar system. Most are probably fragments of main-belt asteroids, traveling in chaotic orbits, just like their smaller cousins, the meteorites.
Current cosmogonical models for the origin of planets involve accretion from myriads of asteroidlike planetesimals. It is likely that asteroids are a remnant of the planetesimals that failed to accrete into a planet between Mars and Jupiter. Perhaps bombardment of the asteroid zone by large planetesimals scattered from massive, nearby Jupiter increased the relative velocities of asteroids to the present value of 3 mi/s (5 km/s) so that asteroids fragment rather than accrete when they meet each other. Instead of forming a planet, the asteroids have been smashing each other to bits.
Evidently some asteroids of primitive, nonvolatile solar composition were heated within the first few hundred million years after the origin of the solar system, perhaps by the solar wind or extinct radionuclides, and they melted. While the unmelted, weak, C-type asteroids may have been depleted by a large factor by collisions, most of the strong stony-iron cores of the melted proto-asteroids have survived; perhaps they are among the M- and S-type asteroids observed today. The asteroids still collide and fragment, occasionally spraying the inner solar system with chips that produce craters or fall as meteorites.
The orbits of most main-belt asteroids lie partially between the orbits of Mars and Jupiter. These asteroids, also known as cisjovian asteroids, are divided into subgroups named for the main asteroid in the grouping: Hungarias, Floras, Phocaea, Koronis, Eos, Themis, Cybeles, and Hildas. The near-earth asteroids, which closely approach the earth, are classed as Atens (with orbits between the earth and the sun), Apollos (with orbits similar to that of the earth), and Amors (with orbits between the earth and Mars). Asteroids that share a planet's orbit are known as Trojan asteroids. First found in Jupiter's orbit, Trojan asteroids have also been found in the orbits of Mars, Neptune, and earth. Centaurs are asteroids with orbits in the outer solar system.
Asteroids are also classified by composition and albedo, most being one of three types. The majority (C-type) are similar to carbon-chrondite meteorites with approximately the same composition as the sun (excluding hydrogen) and are relatively dark. Those with a composition of nickel iron mixed with silicates of iron and magnesium (S-type) are relatively bright. The M-type are composed of nickel iron and are bright. Some of the Trojan asteroids appear to be captured comets, composed of ice and dirt, rather than rocky asteroids.
Toward the end of the 18th cent. astronomers were searching for a planet whose orbit should, according to Bode's law, have an average distance from the sun of 2.8 AU On Jan. 1, 1801, G. Piazzi discovered Ceres while studying the sky in the constellation Taurus; Ceres was later found to have an orbit very near that predicted by Bode's law. Ceres and the asteroids Juno, Pallas, and Vesta, which were discovered soon (1802-7) after Ceres, were initially regarded as planets by many astronomers, a view that was not overturned until additional asteroids were identified in the 1840s and 50s. By 1890 more than 300 asteroids had been discovered by visual means. In 1891, Max Wolf introduced the method of identifying an asteroid by the record of its path on an exposed photographic plate; it appears as a short line in a time exposure, rather than as the sharp point of a star. Brucia was the first asteroid discovered by this method. A more modern approach uses two photographs taken less than an hour apart and examined through a stereomicroscope that allows the asteroid to appear suspended above the background of stars. Still more modern techniques were employed in the discoveries of Ixion, found in 2001 using virtual telescope techniques, and of Quaoar, found in 2002 using photographs taken with the Hubble Space Telescope.
More than 1,200 asteroids larger than about 500 ft (150 m) in diameter have been identified that regularly intersect the orbit of the earth at a distance of 0.5 AU or less (half the distance to the sun), and over geologic time asteroids in similar orbits have struck the earth. Hermes, discovered in 1937 and subsequently lost until 2003 when it was identified as a pair of asteroids, comes within 378,000 mi (608,000 km), and Eros comes within 14 million mi (22 million km). More recently, a small asteroid provisionally designated 2002 MN, 150-360 ft (45-110 m) in diameter, passed within 75,000 mi (121,000 km) of the earth-about a third of the distance to the moon-in 2002. Astronomers have observed about several hundred small asteroids, most measuring less than 55 yd (50 m) across, in near-earth orbits that are spread thinly between the earth and Mars. Many of these small asteroids have orbits that intersect the earth's. In 2009 an apparent asteroid impact was observed on Jupiter; estimated to be 0.3 mi (.5 km) wide, the object would have caused widespread devastation had it struck the earth.
Asteroids have been implicated in several mass extinctions of large numbers of animal and plant species in the past. From evidence found in sediments, Luis Walter Alvarez and others hypothesized that the great mass extinction of the dinosaurs and other species 65 million years ago, at the end of the Cretaceous period, was caused by the atmospheric and climatic effects of an asteroid impact; a possible crater exists in the Yucatán region of Mexico. In 1992, scientists reported that the appearance of patterns of shattered quartz crystals imbedded in Triassic shale and other fossil evidence suggest that another major mass extinction, about 200 million years ago, was caused by three closely spaced asteroid impacts.
The origin of asteroids is unclear; one theory claims that they were formed from material that could not condense into a single planet because of perturbation effects involving Jupiter. Some asteroids are actually nuclei of comets that are no longer active.
The space probe Galileo, which passed near and photographed Gaspra (1991) and Ida (1993), provided the first close images of an asteroid. The pictures revealed that Ida has a natural satellite, Dactyl. Ida, in the main asteroid belt between Mars and Jupiter, is about 35 mi (56 km) long and 15 mi (24 km) in diameter. Its tiny moon is about a mile (1.6 km) in diameter and orbits about 60 mi (97 km) above Ida. Since then several other asteroids have been found to have companions, leading astronomers to believe that it may not be uncommon. The probe NEAR-Shoemaker examined Mathilde (1997) on its way to rendezvous (1999) and orbit (2000) Eros. After providing the most information ever obtained about an asteroid (measurements of size, shape, mass, and gravitational field; elemental and mineral composition of the surface; topographic mapping; and measurement of the magnetic field and its interaction with the solar wind), NEAR-Shoemaker made an originally unplanned landing on Eros in 2001, returning close-up images as it descended and data about surface composition. In 1999 the probe Deep Space 1 accomplished the then closest-ever flyby of an asteroid, coming within 16 mi (26 km) of the surface of Braille; spectroscopic data suggests that Braille broke off from Vesta millions of years ago. Other probes have since studied asteroids, including Rosetta, Hayabusa, which returned with samples of the asteroid Itokawa in 2010, and Dawn, which began a year in orbit around Vesta in 2011.
Bibliography
See T. Gehrels and M. S. Matthews, ed., Hazards Due to Comets and Asteroids (1995); J. S. Lewis, Rain of Iron and Ice: The Very Real Threat of Comet and Asteroid Bombardment (1997); C. T. Russell, ed., The Near Earth Asteroid Rendezvous Mission (1998); N. F. Michelson, The Asteroid Ephemeris 1900-2050 (1999).
A small, mostly rocky body orbiting the Sun. Asteroids range in size from 1000 kilometers in diameter to tiny objects you could hold in your hand. Most asteroids orbit the Sun between Mars and Jupiter, and are the source of most meteorites.
A small planet that revolves around the sun. The largest asteroid is only about six hundred miles in diameter. (See asteroid belt.)

Asteroids (from Greek ἀστεροειδής - asteroeidēs, "star-like",[1] from ἀστήρ "star" and εἶδος "like, in form") are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones. These terms have historically been applied to any astronomical object orbiting the Sun that did not show the disk of a planet and was not observed to have the characteristics of an active comet, but as small objects in the outer Solar System were discovered, their volatile-based surfaces were found to more closely resemble comets, and so were often distinguished from traditional asteroids.[2] Thus the term asteroid has come increasingly to refer specifically to the small bodies of the inner Solar System out to the orbit of Jupiter, which are usually rocky or metallic. They are grouped with the outer bodies—centaurs, Neptune trojans, and trans-Neptunian objects—as minor planets, which is the term preferred in astronomical circles.[3] This article will restrict the use of the term 'asteroid' to the minor planets of the inner Solar System.
There are millions of asteroids, many thought to be the shattered remnants of planetesimals, bodies within the young Sun’s solar nebula that never grew large enough to become planets.[4] A large majority of known asteroids orbit in the asteroid belt between the orbits of Mars and Jupiter or co-orbital with Jupiter (the Jupiter Trojans). However, other orbital families exist with significant populations, including the near-Earth asteroids. Individual asteroids are classified by their characteristic spectra, with the majority falling into three main groups: C-type, S-type, and M-type. These were named after and are generally identified with carbon-rich, stony, and metallic compositions, respectively.
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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.
The first asteroids to be discovered were assigned iconic symbols like the ones traditionally used to designate the planets. By 1855 there were two dozen asteroid symbols, which often occurred in several variants.[5]
| Asteroid | Symbol | |
|---|---|---|
| Ceres | ⚳ |
Ceres' scythe, reversed to double as the letter C |
| 2 Pallas | ⚴ |
Athena's (Pallas') spear |
| 3 Juno | ⚵ |
A star mounted on a scepter, for Juno, the Queen of Heaven |
| 4 Vesta | ⚶ |
The altar and sacred fire of Vesta |
| 5 Astraea | A scale, or an inverted anchor, symbols of justice |
|
| 6 Hebe | Hebe's cup | |
| 7 Iris | A rainbow (iris) and a star | |
| 8 Flora | A flower (flora) (spec. the Rose of England) |
|
| 9 Metis | The eye of wisdom and a star | |
| 10 Hygiea | Hygiea's serpent and a star, or the Rod of Asclepius | |
| 11 Parthenope | A harp, or a fish and a star; symbols of the sirens |
|
| 12 Victoria | The laurels of victory and a star | |
| 13 Egeria | A shield, symbol of Egeria's protection, and a star |
|
| 14 Irene | A dove carrying an olive-branch (symbol of irene 'peace') with a star on its head,[6] or an olive branch, a flag of truce, and a star |
|
| 15 Eunomia | A heart, symbol of good order (eunomia), and a star |
|
| 16 Psyche | A butterfly's wing, symbol of the soul (psyche), and a star |
|
| 17 Thetis | A dolphin, symbol of Thetis, and a star | |
| 18 Melpomene | The dagger of Melpomene, and a star | |
| 19 Fortuna | The wheel of fortune and a star | |
| 26 Proserpina | Proserpina's pomegranate | |
| 28 Bellona | Bellona's whip and lance[7] | |
| 29 Amphitrite | The shell of Amphitrite and a star | |
| 35 Leukothea | A lighthouse beacon, symbol of Leucothea[8] |
|
| 37 Fides | The cross of faith (fides)[9] | |
In 1851,[10] Johann Franz Encke made a major change in the upcoming 1854 edition of the Berliner Astronomisches Jahrbuch (BAJ, Berlin Astronomical Yearbook). He introduced a disk (circle), a traditional symbol for a star, as the generic symbol for an asteroid. The circle was then numbered in order of discovery to indicate a specific asteroid, though he assigned ① to the fifth, Astraea, the first four continuing with their existing symbols. The numbered-circle convention was quickly adopted by the astronomical community, and no iconic symbols were created after 1855.[11] That year Astraea's number was bumped up to ⑤, but Ceres through Vesta would not be listed by their numbers until the 1867 edition. The circle would become a pair of parentheses, and the parentheses sometimes omitted altogether over the next few decades, leading to the modern convention.[6]
The first asteroid to be discovered, Ceres, was found in 1801 by Giuseppe Piazzi, and was originally considered to be a new planet.[note 1] 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 αστήρ, astēr 'star, planet'. In the early second half of the nineteenth century, the terms "asteroid" and "planet" (not always qualified as "minor") were still used interchangeably; for example, the Annual of Scientific Discovery for 1871, page 316, reads "Professor J. Watson has been awarded by the Paris Academy of Sciences, the astronomical prize, Lalande foundation, for the discovery of eight new asteroids in one year. The planet Lydia (No. 110), discovered by M. Borelly at the Marseilles Observatory [...] M. Borelly had previously discovered two planets bearing the numbers 91 and 99 in the system of asteroids revolving between Mars and Jupiter".
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 because 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.
The first object, 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 find 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 earlier visual methods: Wolf alone discovered 248 asteroids, beginning with 323 Brucia, whereas only slightly more than 300 had been discovered up to that point. It was known that there were many more, but most astronomers did not bother with them[citation needed], calling them "vermin of the skies", a phrase due to Edmund Weiss.[12] Even a century later, only a few thousand asteroids were identified, numbered and named.
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 or plates 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 seem 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.[13]
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 half-month of discovery, and finally a letter and a number indicating the discovery's sequential number (example: 1998 FJ74).
The last 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 earlier 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 honor of naming the object subject to the approval of the International Astronomical Union.
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 alarm: the increasing acceptance of Walter Alvarez' hypothesis that an impact event resulted in the Cretaceous-Paleogene 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 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:[14]
The LINEAR system alone has discovered 121,346 asteroids, as of March, 2011.[15] Among all the automated systems, 4711 near-Earth asteroids have been discovered[16] including over 600 more than 1 km (0.6 mi) in diameter.
Traditionally, small bodies orbiting the Sun were classified as asteroids, comets or meteoroids, with anything smaller than ten metres across being called a meteoroid.[17] The term "asteroid" is ill-defined. It never had a formal definition, with the broader term minor planet being preferred by the International Astronomical Union from 1853 on. In 2006, the term "small Solar System body" was introduced to cover both most minor planets and comets.[18] Other languages prefer "planetoid" (Greek for "planet-like"), and this term is occasionally used in English for the larger asteroids. The word "planetesimal" has a similar meaning, but refers specifically to the small building blocks of the planets that existed when the Solar System was forming. The term "planetule" was coined by the geologist William Daniel Conybeare to describe minor planets,[19] but is not in common use. The three largest objects in the asteroid belt, Ceres, 2 Pallas, and 4 Vesta, grew to the stage of protoplanets. Ceres has been classified as a dwarf planet, the only one in the inner Solar System.
When found, asteroids were seen as a class of objects distinct from comets, and there was no unified term for the two until "small Solar System body" was coined in 2006. 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 are eventually depleted of their surface volatile ices and become asteroids. A further distinction is that comets typically have more eccentric orbits than most asteroids; most "asteroids" with notably eccentric orbits are probably dormant or extinct comets.[20]
For almost two centuries, from the discovery of Ceres in 1801 until the discovery of the first centaur, 2060 Chiron, in 1977, all known asteroids spent most of their time at or within the orbit of Jupiter, though a few such as 944 Hidalgo ventured far beyond Jupiter for part of their orbit. When astronomers started finding more small bodies that permanently resided further out than Jupiter, now called centaurs, they numbered them among the traditional asteroids, though there was debate over whether they should be classified as asteroids or as a new type of object. Then, when the first trans-Neptunian object, 1992 QB1, was discovered in 1992, and especially when large numbers of similar objects started turning up, new terms were invented to sidestep the issue: Kuiper-belt object, trans-Neptunian object, scattered-disc object, and so on. 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; if centaurs or trans-Neptunian objects were to venture close to the Sun, their volatile ices would sublimate, and traditional approaches would classify them as comets and not asteroids.
The innermost of these are the Kuiper-belt objects, called "objects" partly to avoid the need to classify them as asteroids or comets.[21] They are believed to be predominantly comet-like in composition, though some may be more akin to asteroids.[22] Furthermore, most do not have the highly eccentric orbits associated with comets, and the ones so far discovered are larger than traditional comet nuclei. (The much more distant Oort cloud is hypothesized to be the main 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,[23] suggesting "a continuum between asteroids and comets" rather than a sharp dividing line.[24]
The minor planets beyond Jupiter's orbit are sometimes also called "asteroids", especially in popular presentations.[25] However, it is becoming increasingly common for the term "asteroid" to be restricted to minor planets of the inner Solar System.[21] Therefore, this article will restrict itself for the most part to the classical asteroids: objects of the asteroid belt, Jupiter trojans, and near-Earth objects.
When the IAU introduced the class small Solar System bodies in 2006 to include most objects previously classified as minor planets and comets, they created the class of dwarf planets for the largest minor planets—those that have enough mass to have become ellipsoidal 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."[26] Currently only the largest object in the asteroid belt, Ceres, at about 950 km (590 mi) across, has been placed in the dwarf planet category, although there are several large asteroids (Vesta, Pallas, and Hygiea) that may be classified as dwarf planets when their shapes are better known.[27]
It is believed that planetesimals in the asteroid belt evolved much like the rest of the solar nebula until Jupiter neared its current mass, at which point excitation from orbital resonances with Jupiter ejected over 99% of planetesimals in the belt. Simulations and a discontinuity in spin rate and spectral properties suggest that asteroids larger than approximately 120 km (75 mi) in diameter accreted during that early era, whereas smaller bodies are fragments from collisions between asteroids during or after the Jovian disruption.[28] Ceres and Vesta grew large enough to melt and differentiate, with heavy metallic elements sinking to the core, leaving rocky minerals in the crust.[29]
In the Nice model, many Kuiper-belt objects are captured in the outer asteroid belt, at distances greater than 2.6 AU. Most were later ejected by Jupiter, but those that remained may be the D-type asteroids, and possibly include Ceres.[30]
Various dynamical groups of asteroids have been discovered orbiting in the inner Solar System. Their orbits are perturbed by the gravity of other bodies in the Solar System and by the Yarkovsky effect. Significant populations include:
The majority of known asteroids orbit within the asteroid belt between the orbits of Mars and Jupiter, generally in relatively low-eccentricity (i.e., not very elongated) orbits. This belt is now estimated to contain between 1.1 and 1.9 million asteroids larger than 1 km (0.6 mi) in diameter,[31] and millions of smaller ones.[32] These asteroids may be 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.
Trojan asteroids are a population that share an orbit with a larger planet or moon, but do not collide with it because they orbit in one of the two Lagrangian points of stability, L4 and L5, which lie 60° ahead of and behind the larger body.
The most significant population of Trojan asteroids are the Jupiter Trojans. Although fewer Jupiter Trojans have been discovered as of 2010, it is thought that they are as numerous as the asteroids in the asteroid belt.
A couple of trojans have also been found orbiting with Mars.[note 2]
Near-Earth asteroids, or NEAs, are asteroids that have orbits that pass close to that of Earth. Asteroids that actually cross the Earth's orbital path are known as Earth-crossers. As of May 2010, 7,075 near-Earth asteroids are known and the number over one kilometre in diameter is estimated to be 500–1,000.
Asteroids vary greatly in size, from almost 1000 kilometres for the largest down to rocks just tens of metres across.[note 3] The three largest are very much like miniature planets: they are roughly spherical, have at least partly differentiated interiors,[33] and are thought to be surviving protoplanets. The vast majority, however, are much smaller and are irregularly shaped; they are thought to be either surviving planetesimals or fragments of larger bodies.
The dwarf planet Ceres is by far the largest asteroid, with a diameter of 975 km (610 mi). The next largest are 2 Pallas and 4 Vesta, both with diameters of just over 500 km (300 mi). Vesta is the only main-belt asteroid that can, on occasion, be visible to the naked eye. On some rare occasions, a near-Earth asteroid may briefly become visible without technical aid; see 99942 Apophis.
The mass of all the objects of the asteroid belt, lying between the orbits of Mars and Jupiter, is estimated to be about 2.8-3.2×1021 kg, or about 4 percent of the mass of the Moon. Of this, Ceres comprises 0.95×1021 kg, a third of the total.[34] Adding in the next three most massive objects, Vesta (9%), Pallas (7%), and 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.
The number of asteroids decreases markedly with size. Although this generally follows a power law, there are 'bumps' at 5 km and 100 km, where more asteroids than expected from a logarithmic distribution are found.[35]
| D | 100 m | 300 m | 500 m | 1 km | 3 km | 5 km | 10 km | 30 km | 50 km | 100 km | 200 km | 300 km | 500 km | 900 km |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| N | ~25,000,000 | 4,000,000 | 2,000,000 | 750,000 | 200,000 | 90,000 | 10,000 | 1,100 | 600 | 200 | 30 | 5 | 3 | 1 |
|
all others
|
Although their location in the asteroid belt excludes them from planet status, the four largest objects, Ceres, Vesta, Pallas, and Hygiea, are remnant protoplanets that share many characteristics common to planets, and are atypical compared to the majority of "potato"-shaped asteroids.
| Attributes of protoplanetary asteroids | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Name | Orbital radius (AU) |
Orbital period (years) |
Inclination to ecliptic |
Orbital eccentricity |
Diameter (km) |
Diameter (% of Moon) |
Mass (×1018 kg) |
Mass (% of Ceres) |
Rotation period (hr) |
Axial tilt | Surface temperature |
| Vesta | 2.36 | 3.63 | 7.1° | 0.089 | 573×557×446 (mean 525) |
15% | 260 | 28% | 5.34 | 29° | 85–270 K |
| Ceres | 2.77 | 4.60 | 10.6° | 0.079 | 975×975×909 (mean 952) |
28% | 940 | 100% | 9.07 | ≈ 3° | 167 K |
| Pallas | 2.77 | 4.62 | 34.8° | 0.231 | 580×555×500 (mean 545) |
16% | 210 | 22% | 7.81 | ≈ 80° | 164 K |
| Hygiea | 3.14 | 5.56 | 3.8° | 0.117 | 530×407×370 (mean 430) |
12% | 87 | 9% | 27.6 | ≈ 60° | 164 K |
Ceres is the only asteroid large enough for its gravity to force it into a spheroidal shape, and so, according to the IAU's 2006 resolution on the definition of a planet, it has been classified as a dwarf planet.[38] Vesta may eventually be so classified as well. Ceres has a much higher absolute magnitude than the other asteroids, of around 3.32,[39] and may possess a surface layer of ice.[40] Like the planets, Ceres is differentiated: it has a crust, a mantle and a core.[40] Vesta, too, has a differentiated interior, though it formed inside the Solar System's frost line, and so is devoid of water;[41] its composition is mainly of basaltic rock such as olivine.[42] Pallas is unusual in that, like Uranus, it rotates on its side, with one pole regularly facing the Sun and the other facing away.[43] Its composition is similar to that of Ceres: high in carbon and silicon, and perhaps partially differentiated.[44] Hygiea is a carbonaceous asteroid and, unlike the other largest asteroids, lies relatively close to the plane of the ecliptic.[45]
Measurements of the rotation rates of large asteroids in the asteroid belt show that there is an upper limit. No asteroid with a diameter larger than 100 meters has a rotation period smaller than 2.2 hours. For asteroids rotating faster than approximately this rate, the inertia at the surface is greater than the gravitational force, so any loose surface material would be flung out. However, a solid object should be able to rotate much more rapidly. This suggests that most asteroids with a diameter over 100 meters are rubble piles formed through accumulation of debris after collisions between asteroids.[46]
The physical composition of asteroids is varied and in most cases poorly understood. Ceres appears to be composed of a rocky core covered by an icy mantle, where Vesta is thought to have a nickel-iron core, olivine mantle, and basaltic crust.[47] 10 Hygiea, however, which appears to have a uniformly primitive composition of carbonaceous chondrite, is thought to be the largest undifferentiated asteroid. Most of the smaller asteroids are thought to be piles of rubble held together loosely by gravity, though the largest are probably solid. Some asteroids have moons or are co-orbiting binaries: Rubble piles, moons, binaries, and scattered asteroid families are believed to be the results of collisions that disrupted a parent asteroid.
Asteroids contain traces of amino acids and other organic compounds, and some speculate that asteroid impacts may have seeded the early Earth with the chemicals necessary to initiate life, or may have even brought life itself to Earth. (See also panspermia.)[48] In August 2011, a report, based on NASA studies with meteorites found on Earth, was published suggesting DNA and RNA components (adenine, guanine and related organic molecules) may have been formed on asteroids and comets in outer space.[49][50][51]
Only one asteroid, 4 Vesta, which has a reflective surface, is normally visible to the naked eye, and this only in very dark skies when it is favorably positioned. Rarely, small asteroids passing close to Earth may be naked-eye visible for a short time.[52]
Composition is calculated from three primary sources: albedo, surface spectrum, and density. The last can only be determined accurately by observing the orbits of moons the asteroid might have. So far, every asteroid with moons has turned out to be a rubble pile, a loose conglomeration of rock and metal that may be half empty space by volume. The investigated asteroids are as large as 280 km in diameter, and include 121 Hermione (268×186×183 km), and 87 Sylvia (384×262×232 km). Only half a dozen asteroids are larger than 87 Sylvia, though none of them have moons; however, some smaller asteroids are thought to be more massive, suggesting they may not have been disrupted, and indeed 511 Davida, the same size as Sylvia to within measurement error, is estimated to be two and a half times as massive, though this is highly uncertain. The fact that such large asteroids as Sylvia can be rubble piles, presumably due to disruptive impacts, has important consequences for the formation of the Solar system: Computer simulations of collisions involving solid bodies show them destroying each other as often as merging, but colliding rubble piles are more likely to merge. This means that the cores of the planets could have formed relatively quickly.[53]
Most asteroids outside the big four (Ceres, Pallas, Vesta, and Hygiea) are likely to be broadly similar in appearance, if irregular in shape. 50-km 253 Mathilde (shown at right) is a rubble pile saturated with craters with diameters the size of the asteroid's radius, and Earth-based observations of 300-km 511 Davida, one of the largest asteroids after the big four, reveal a similarly angular profile, suggesting it is also saturated with radius-size craters.[54] Medium-sized asteroids such as Mathilde and 243 Ida that have been observed up close also reveal a deep regolith covering the surface. Of the big four, Pallas and Hygiea are practically unknown. Vesta has compression fractures encircling a radius-size crater at its south pole but is otherwise a spheroid. Ceres seems quite different in the glimpses Hubble has provided, with surface features that are unlikely to be due to simple craters and impact basins, but details will not be known until Dawn arrives in 2015.
Asteroids are commonly classified according to two criteria: the characteristics of their orbits, and features of their reflectance spectrum.
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 tighter and result from the catastrophic break-up of a large parent asteroid sometime in the past.[55] Families have only been recognized within the asteroid belt. They were first recognised by Kiyotsugu Hirayama in 1918 and are often called Hirayama families in his honor.
About 30% to 35% of the bodies in the asteroid 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 plutoid dwarf planet Haumea.
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 earlier 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.
In 1975, an asteroid taxonomic system based on colour, albedo, and spectral shape was developed by Clark R. Chapman, David Morrison, and Ben Zellner.[56] 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 many other asteroid types. The number of types continues to grow as more asteroids are studied.
The two most widely used taxonomies now 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-color asteroid survey performed in the 1980s. This resulted in 14 asteroid categories.[57] 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 several smaller classes.[58]
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.
Originally, spectral designations were based on inferences of an asteroid's composition.[59] However, the correspondence between spectral class and composition is not always very good, and a variety of classifications is 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 cannot agree on a better taxonomic system,[citation needed] 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).
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 and the Earth-orbiting Hubble Space Telescope can resolve a small amount of detail on the surfaces of the 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. In terms of delta v and propellant requirements, NEOs are more easily accessible than the Moon.[60]
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 later images from the Voyager probes of the small moons of the gas giants.
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 was plagued with difficulties, but returned samples of its surface to earth on June 13, 2010.
The European Rosetta probe (launched in 2004) flew by 2867 Šteins in 2008 and 21 Lutetia, the second-largest asteroid visited to date, in 2010.
In September 2007, NASA launched the Dawn Mission, which started orbiting the protoplanet 4 Vesta in July 2011, and is to orbit 1 Ceres in 2015. 4 Vesta is the largest asteroid visited to date.
In May 2011, NASA announced the OSIRIS-REx sample return mission to asteroid 1999 RQ36, and is expected to launch in 2016.
It has been suggested that asteroids might be used as a source of materials that 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.
Asteroids and the asteroid belt are a staple of science fiction stories. Asteroids play several potential roles in science fiction: as places human beings might colonize, resources for extracting minerals, hazards encountered by spaceships traveling between two other points, and as a threat to life on Earth by potential impact.
<http://ssd.jpl.nasa.gov/?asteroids>"We include Trojans (bodies captured in Jupiter's 4th and 5th Lagrange points), Centaurs (bodies in orbit between Jupiter and Neptune), and trans-Neptunian objects (orbiting beyond Neptune) in our definition of "asteroid" as used on this site, even though they may more correctly be called "minor planets" instead of asteroids."
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Dansk (Danish)
n. - asteroide, småplanet
adj. - stjerneformet
Nederlands (Dutch)
asteroïde, zeeaster, klein planeet
Français (French)
n. - astéroïde
adj. - de forme astéroïde
Deutsch (German)
n. - Asteroid, Planetoid
adj. - sternartig, (Bot.) asterblutig, seesternähnlich
Ελληνική (Greek)
n. - (αστρον.) αστεροειδής
Português (Portuguese)
n. - asteróide (m)
Español (Spanish)
n. - asteroide, planetoide
adj. - relativo a asteroides o planetoides
Svenska (Swedish)
n. - asteroid
中文(简体)(Chinese (Simplified))
小行星, 小游星, 海星, 星状的
中文(繁體)(Chinese (Traditional))
n. - 小行星, 小遊星, 海星
adj. - 星狀的
한국어 (Korean)
n. - 불가사리, 소행성
adj. - 별 모양의
العربيه (Arabic)
(الاسم) السيير : الكويكب أحد الكواكب السياره الصغيره الواقعه بين المريخ والمشتري, نجم البحر, على شكل نجمه, شبيه بنجمه البحر
עברית (Hebrew)
n. - גוף סלעי קטן הסובב את השמש בד"כ בין מסלולי מאדים וצדק, דג-הכוכב, בן-כוכב, אסטרואיד
adj. - דמוי כוכב
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