Asteroid

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253 Mathilde, a C-type asteroid.

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Asteroids, also called minor planets or planetoids, are a class of solid astronomical objects smaller than planets. The term asteroid is generally used to indicate a diverse group of small celestial bodies in the solar system that orbit the Sun. It means "star-like" in Greek, and in English is the most commonly used word for a minor planet, which is the term preferred by the International Astronomical Union. Other languages prefer planetoid (Greek for "planet-like") because it more or less describes what they are. In late August 2006, the IAU introduced the class "small solar system bodies" (SSSB) to include most objects thus far classified as minor planets and comets. At the same time, the class "dwarf planets" was created for the largest minor planets.

The first asteroid to be discovered, Ceres, is the largest asteroid known to date and is now classified as a dwarf planet. All others are now classified as small solar system bodies. The vast majority of asteroids are found within the main asteroid belt, with elliptical orbits between those of Mars and Jupiter. It is thought that these asteroids are remnants of the protoplanetary disc, 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.

[edit] In 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 at the present rate of discovery around 5,000 per month. Of the more than 400,000 registered minor planets, 181,699 have orbits known well enough to be assigned permanent official numbers.^[1][2] Of these, 14,366 have official names.^[3] The lowest-numbered, unnamed minor planet is (3708) 1974 FV₁;^[4] the highest-numbered named minor planet is 175629 Lambertini.^[5] 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.^[6] The largest asteroid in the inner solar system is 1 Ceres, with diameters of 975 × 909 km. Two other large inner solar system belt asteroids are 2 Pallas and 4 Vesta; both have diameters of ~500 km. Vesta is the only main belt asteroid that is sometimes visible to the naked eye (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 asteroids of the Main Belt 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.^[7][8] 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 3 Juno (0.9%), only add another 3% to the total mass. The number of asteroids then increases rapidly as their individual masses decrease.

See also: list of noteworthy asteroids, list of asteroids, and comets

[edit] Classification

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

[edit] Orbit groups and families

Main articles: Asteroid family and Minor planet

Many asteroids have been placed in groups and families based on their orbital characteristics. 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.^[9]

For a full listing of known asteroid groups and families, see minor planet and asteroid family.

[edit] Spectral classification

The following text needs to be harmonized with text in the article Asteroid spectral types. (See e.g. Wikipedia:Summary style.)

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.^[10] These properties are thought to correspond to the composition of the asteroid's surface material. Originally, they classified only three types of asteroids, based on meteoroid types:

• C-type asteroids - carbonaceous, 75% of known asteroids
• S-type asteroids - silicaceous, 17% of known asteroids
• M-type asteroids - metallic, 8% of known asteroids

This list has since been expanded to include a number of other asteroid types. The number of types continues to grow as more asteroids are studied. See Asteroid spectral types for more detail or Category:Asteroid spectral classes for a list.

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.

[edit] Problems with spectral classification

Originally, spectral designations were based on inferences of an asteroid's composition:^[11]

• C - Carbonaceous
• S - Silicaceous
• M - Metallic

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).

[edit] Discovery

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

[edit] Historical methods

Asteroid discovery methods have drastically 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 drastically 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".

[edit] 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.^[12]

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 Brian Marsden of the Minor Planet Center, where computer programs that 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 honor of naming the object subject to the approval of the International Astronomical Union.

[edit] 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:^[13]

• 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.^[14] Between all of the automated systems, 4711 near-Earth asteroids have been discovered^[15] including over 600 more than 1 km in diameter.

[edit] Naming

[edit] Overview: naming conventions

A newly discovered asteroid is given a provisional designation consisting of the year of discovery and an alphanumeric code (such as 2002 AT₄). Once its 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 discovering 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.

[edit] Numbering

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.

[edit] Sources for names

Main article: Meanings of asteroid names

The first few asteroids were named after figures from Graeco-Roman mythology, but as such names started to dwindle the names of famous people, literary characters, discoverer's wives, children, and even television characters were used.

The first asteroid to be given a non-mythological name was 20 Massalia, named after the city of Marseilles. For some time only female (or feminized) names were used; Alexander von Humboldt was the first man to have an asteroid named after him, but his name was feminized to 54 Alexandra. This unspoken tradition lasted until 334 Chicago was named; even then, oddly feminised names show up in the list for years afterward.

As the number of asteroids began to run into the hundreds, and eventually the thousands, discoverers began to give them increasingly frivolous names. The first hints of this were 482 Petrina and 483 Seppina, named after the discoverer's pet dogs. However, there was little controversy about this until 1971, upon the naming of 2309 Mr. Spock (which was not even named after the Star Trek character, but after the discoverer's cat who supposedly bore a resemblance to him). Although the IAU subsequently banned pet names as sources, eccentric asteroid names are still being proposed and accepted, such as 4321 Zero, 6042 Cheshirecat, 9007 James Bond, 13579 Allodd, 24680 Alleven, or 26858 Misterrogers.

[edit] 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.^[16][17] A few objects are also cross-listed as both comets and asteroids, such as 4015 Wilson-Harrington and 107P/Wilson-Harrington.

[edit] Symbols

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).^[18]

Asteroid	Symbol
Ceres
2 Pallas
3 Juno
4 Vesta
5 Astraea
6 Hebe
7 Iris
8 Flora
9 Metis
10 Hygiea
11 Parthenope
12 Victoria
13 Egeria	Never assigned.
14 Irene	"A dove carrying an olive-branch, with a star on its head," never drawn.[19]
15 Eunomia
28 Bellona
35 Leukothea
37 Fides

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,^[20] 35 Leukothea,^[21] and 37 Fides^[22]) 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.^[19]

[edit] Exploration

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 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.

[edit] In fiction

Main article: Asteroids 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.

[edit] See also

[edit] References

[edit] External links

Wikimedia Commons has media related to:

Asteroids

• Alphabetical list of minor planet names (ASCII) (Minor Planet Center)
• Near Earth Asteroid Tracking (NEAT)
• Asteroids Page at NASA's Solar System Exploration
• When Did the Asteroids Become Minor Planets?
• Asteroid Simulator with Moon and Earth
• Everything you wanted to know about comets and asteroids - Provided by New Scientist.
• Alphabetical and numerical lists of minor planet names (Unicode) (Institute of Applied Astronomy)
• Known Asteroid Impacts & Their Effects
• Assyrian clay tablet points to 'Sodom and Gomorrah' asteroid
• Future Asteroid Interception Research
• Near Earth Objects Dynamic Site
• Asteroids Dynamic Site Up-to date osculating orbital elements and proper orbital elements
• Asteroid naming statistics
• Spaceguard UK
• Large amount of information on asteroid groups collected by Gérard Faure, translation Richard Miles.
• Near Earth Objects and Asteroids: Are We Whistling in the Dark?
• 1908 Siberian asteroid

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