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Concept
Asteroids was conceived by Lyle Rains and programmed by Ed Logg with collaborations from other Atari staff. Logg was impressed with the Atari Video Computer System later called the Atari 2600, and he joined Atari's coinop division to work on Dirt Bike, which was never released due to an unsuccessful field test. Paul Mancuso joined the development team as Asteroids technician and engineer Howard Delman contributed to the hardware. During a meeting in April 1979, Rains discussed Planet Grab, a multiplayer arcade game later renamed to Cosmos. Logg did not know the name of the game, thinking Computer Space as "the inspiration for the twodimensional approach". Rains conceived of Asteroids as a mixture of Computer Space and Space Invaders, combining the twodimensional approach of Computer Space with Space Invaders addictive gameplay of "completion" and "eliminate all threats". The unfinished game featured a giant, indestructible asteroid, so Rains asked Logg "Well, why dont we have a game where you shoot |
the rocks and blow them up?" In response, Logg described a similar concept where the player selectively shoots at rocks that break into smaller pieces. Both agreed on the concept.
Hardware
Asteroids was implemented on hardware developed by Delman and is a vector game, in which the graphics are composed of lines drawn on a vector monitor. Rains initially wanted the game done in raster graphics, but Logg, experienced in vector graphics, suggested an XY monitor because the high image quality would permit precise aiming. The hardware is chiefly a MOS 6502 executing the game program, and QuadraScan, a highresolution vector graphics processor developed by Atari and referred to as an "XY display system" and the "Digital Vector Generator DVG".
The original design concepts for QuadraScan came out of Cyan Engineering, Atari's offcampus research lab in Grass Valley, California, in 1978. Cyan gave it to Delman, who finished the design and first used it for Lunar Lander. Logg received Delman's modified board with five b |
uttons, 13 sound effects, and additional RAM, and he used it to develop Asteroids. The size of the board was 4 by 4 inches, and it was "linked up" to a monitor.
Implementation
Logg modeled the player's ship, the fivebutton control scheme, and the game physics after Spacewar!, which he had played as a student at the University of California, Berkeley, but made several changes to improve playability. The ship was programmed into the hardware and rendered by the monitor, and it was configured to move with thrust and inertia. The hyperspace button was not placed near Logg's right thumb, which he was dissatisfied with, as he had a problem "taking his hand off the thrust button". Drawings of asteroids in various shapes were incorporated into the game. Logg copied the idea of a high score table with initials from Exidy's Star Fire.
The two saucers were formulated to be different from each other. A steadily decreasing timer shortens intervals between saucer attacks to keep the player from not shooting asteroids and |
saucers. A "heartbeat" soundtrack quickens as the game progresses. The game does not have a sound chip. Delman created a hardware circuit for 13 sound effects by hand which was wired onto the board.
A prototype of Asteroids was well received by several Atari staff and engineers, who "wandered between labs, passing comment and stopping to play as they went". Logg was often asked when he would be leaving by employees eager to play the prototype, so he created a second prototype for staff to play. Atari tested the game in arcades in Sacramento, California, and also observed players during focus group sessions at Atari. Players used to Spacewar! struggled to maintain grip on the thrust button and requested a joystick; players accustomed to Space Invaders noted they get no break in the game. Logg and other engineers observed proceedings and documented comments in four pages.
Asteroids slows down as the player gains 50100 lives, because there is no limit to the number of lives displayed. The player can "lose" th |
e game after more than 250 lives are collected.
Ports
Asteroids was released for the Atari VCS later renamed the Atari 2600 and Atari 8bit family in 1981, then the Atari 7800 in 1986. A port for the Atari 5200, identical to the Atari 8bit computer version, was in development in 1982, but was not published. The Atari 7800 version was a launch title and includes cooperative play; the asteroids have colorful textures and the "heartbeat" sound effect remains intact.
Programmers Brad Stewart and Bob Smith were unable to fit the Atari VCS port into a 4 KB cartridge. It became the first game for the console to use bank switching, a technique that increases ROM size from 4 KB to 8 KB.
Reception
Asteroids was immediately successful upon release. It displaced Space Invaders by popularity in the United States and became Atari's best selling arcade game of all time, with over 70,000 units sold. Atari earned an estimated 150 million in sales from the game, and arcade operators earned a further 500 million from coin dr |
ops. Atari had been in the process of manufacturing another vector game, Lunar Lander, but demand for Asteroids was so high "that several hundred Asteroids games were shipped in Lunar Lander cabinets". Asteroids was so popular that some video arcade operators had to install large boxes to hold the number of coins spent by players. It replaced Space Invaders at the top of the US RePlay amusement arcade charts in April 1980, though Space Invaders remained the top game at street locations. Asteroids went on to become the highestgrossing arcade video game of 1980 in the United States, dethroning Space Invaders. It shipped 70,000 arcade units worldwide in 1980, including over 60,000 sold in the United States that year, and grossed about worldwide adjusted for inflation by 1980. The game remained at the top of the US RePlay charts through March 1981. However, the game did not perform as well overseas in Europe and Asia. It sold 30,000 arcade units overseas, for a total of 100,000 arcade units sold worldwide. Atar |
i manufactured 76,312 units from its US and Ireland plants, including 21,394 Asteroids Deluxe units. It was a commercial failure in Japan when it released there in 1980, partly due to its complex controls and partly due to the Japanese market beginning to lose interest in space shoot 'em ups at the time.
Asteroids received positive reviews from video game critics and has been regarded as Logg's magnum opus. Richard A. Edwards reviewed the 1981 Asteroids home cartridge in The Space Gamer No. 46. Edwards commented that "this home cartridge is a virtual duplicate of the everpopular Atari arcade game. ... If blasting asteroids is the thing you want to do then this is the game, but at this price I can't wholeheartedly recommend it". Video Games Player magazine reviewed the Atari VCS version, rating the graphics and sound a B, while giving the game an overall B rating. Electronic Fun with Computers Games magazine gave the Atari VCS version an A rating.
William Cassidy, writing for GameSpy's "Classic Gaming", not |
iced its innovations, including being one of the first video games to track initials and allow players to enter their initials for appearing in the top 10 high scores, and commented, "the vector graphics fit the futuristic outer space theme very well". In 1996, Next Generation listed it as number 39 on their "Top 100 Games of All Time", particularly lauding the control dynamics which require "the constant juggling of speed, positioning, and direction". In 1999, Next Generation listed Asteroids as number 29 on their "Top 50 Games of All Time", commenting that "Asteroid was a classic the day it was released, and it has never lost any of its appeal". Asteroids was ranked fourth on Retro Gamers list of "Top 25 Arcade Games"; the Retro Gamer staff cited its simplicity and the lack of a proper ending as allowances of revisiting the game. In 2012, Asteroids was listed on Time AllTime 100 greatest video games list. Entertainment Weekly named Asteroids one of the top ten games for the Atari 2600 in 2013. It was added |
to the Museum of Modern Art's collection of video games. In 2021, The Guardian listed Asteroids as the second greatest video game of the 1970s, just below Galaxian 1979. By contrast, in March 1983 the Atari 8bit port of Asteroids won sixth place in Softlines Dog of the Year awards "for badness in computer games", Atari division, based on reader submissions.
Usage of the names of Saturday Night Live characters "Mr. Bill" and "Sluggo" to refer to the saucers in an Esquire article about the game led to Logg receiving a cease and desist letter from a lawyer with the "Mr. Bill Trademark".
Legacy
Arcade sequels
Released in 1981, Asteroids Deluxe was the first sequel to Asteroids. Dave Shepperd edited the code and made enhancements to the game without Logg's involvement. The onscreen objects are tinted blue, and hyperspace is replaced by a shield that depletes when used. The asteroids rotate, and new "killer satellite" enemies break into smaller ships that home in on the player's position. The arcade machine's mo |
nitor displays vector graphics overlaying a holographic backdrop. The game is more difficult than the original and enables saucers to shoot across the screen boundary, eliminating the lurking strategy for high scores in the original.
It was followed by Owen Rubin's Space Duel in 1982, featuring colorful geometric shapes and coop multiplayer gameplay.
In 1987's Blasteroids, Ed Rotberg added "powerups, ship morphing, branching levels, bosses, and the ability to dock your ships in multiplayer for added firepower". Blasteroids uses raster graphics instead of vectors.
Rereleases
The game is half of the Atari Lynx pairing Super Asteroids Missile Command, and included in the 1993 Microsoft Arcade compilation.
Activision published an enhanced version of Asteroids for the PlayStation 1998, Nintendo 64 1999, Microsoft Windows 1998, Game Boy Color 1999, and Macintosh 2000. The Atari Flashback series of dedicated video game consoles have included both the 2600 and the arcade versions of Asteroids.
Published by Cra |
ve Entertainment on December 14, 1999, Asteroids Hyper 64 made the ship and asteroids 3D and added new weapons and a multiplayer mode.
A technical demo of Asteroids was developed by iThink for the Atari Jaguar but was never released. Unofficially referred to as Asteroids 2000, it was demonstrated at EJagFest 2000.
In 2001, Infogrames released Atari Anniversary Edition for the Dreamcast, PlayStation, and Microsoft Windows. Developed by Digital Eclipse, it includes emulated versions of Asteroids and other games. The arcade and Atari 2600 versions of Asteroids were included in Atari Anthology for both Xbox and PlayStation 2.
Released on November 28, 2007, the Xbox Live Arcade port of Asteroids has revamped HD graphics along with an added intense "throttle monkey" mode. The arcade and 2600 versions were made available through Microsofts Game Room service in 2010. Glu Mobile released an enhanced mobile phone port.
Asteroids is included on Atari Greatest Hits Volume 1 for the Nintendo DS.
An updated version of |
the game was announced in 2018 for the Intellivision Amico.
Both the Atari 2600 and Atari 7800 versions of the game was included on Atari Collection 1 and 2 in 2020 for the Evercade.
Clones
Quality Software's Asteroids in Space 1980 was one of the best selling games for the Apple II and voted one of the most popular software titles of 197880 by Softalk magazine.
In December 1981, Byte reviewed eight Asteroids clones for home computers. Three other Apple II Asteroids clones were reviewed together in the 1982 Creative Computing Software Buyers Guide The Asteroid Field, Asteron, and AppleOids. In the last of these, the asteroids are in the shape of apples. Two independent clones, Asteroid for the Apple II and Fasteroids for TRS80, were renamed to Planetoids and sold by Adventure International. Others clones include Acornsoft's Meteors, Moons of Jupiter for the VIC20, and MineStorm for the Vectrex.
The Mattel Intellivision game Meteor! , an Asteroids clone, was cancelled to avoid a lawsuit, and was reworked a |
s Astrosmash. The game borrows elements from Asteroids and Space Invaders.
Elon Musk, when he was a 12 yearold child, programmed a space shoot 'em up game inspired by Space Invaders and Asteroids, called Blastar, which was published for the Commodore VIC20 in 1984.
World records
On February 6, 1982, Leo Daniels of Carolina Beach, North Carolina, set a world record score of 40,101,910 points. On November 13 of the same year, 15yearold Scott Safran of Cherry Hill, New Jersey, set a new record at 41,336,440 points. In 1998, to congratulate Safran on his accomplishment, the Twin Galaxies Intergalactic Scoreboard searched for him for four years until 2002, when it was discovered that he had died in an accident in 1989. In a ceremony in Philadelphia on April 27, 2002, Walter Day of Twin Galaxies presented an award to the surviving members of Safran's family, commemorating his achievement. On April 5, 2010, John McAllister broke Safran's record with a high score of 41,838,740 in a 58hour Internet livestream.
So |
me claim that the true world record for Asteroids was set in a laundromat in Hyde Park, New York, from June 30 to July 3, 1982, and that details of the score of over 48 million were published in the July 4th edition of the Poughkeepsie Journal.
References
External links
at Atari
1979 video games
Arcade video games
Atari 2600 games
Atari 7800 games
Atari 8bit family games
Atari arcade games
Atari Lynx games
Cancelled Atari 5200 games
Cancelled Atari Jaguar games
Ed Logg games
Game Boy games
Game Boy Color games
Multidirectional shooters
Multiplayer and singleplayer video games
Science fiction video games
Sega arcade games
Taito arcade games
Xbox 360 games
Xbox 360 Live Arcade games
Vector arcade video games
Video games developed in the United States |
Asparagales asparagoid lilies is an order of plants in modern classification systems such as the Angiosperm Phylogeny Group APG and the Angiosperm Phylogeny Web. The order takes its name from the type family Asparagaceae and is placed in the monocots amongst the lilioid monocots. The order has only recently been recognized in classification systems. It was first put forward by Huber in 1977 and later taken up in the Dahlgren system of 1985 and then the APG in 1998, 2003 and 2009. Before this, many of its families were assigned to the old order Liliales, a very large order containing almost all monocots with colorful tepals and lacking starch in their endosperm. DNA sequence analysis indicated that many of the taxa previously included in Liliales should actually be redistributed over three orders, Liliales, Asparagales, and Dioscoreales. The boundaries of the Asparagales and of its families have undergone a series of changes in recent years; future research may lead to further changes and ultimately greater st |
ability. In the APG circumscription, Asparagales is the largest order of monocots with 14 families, 1,122 genera, and about 36,000 species.
The order is clearly circumscribed on the basis of molecular phylogenetics, but it is difficult to define morphologically since its members are structurally diverse. Most species of Asparagales are herbaceous perennials, although some are climbers and some are treelike. The order also contains many geophytes bulbs, corms, and various kinds of tuber. According to telomere sequence, at least two evolutionary switchpoints happened within the order. The basal sequence is formed by TTTAGGG like in the majority of higher plants. Basal motif was changed to vertebratelike TTAGGG and finally, the most divergent motif CTCGGTTATGGG appears in Allium. One of the defining characteristics synapomorphies of the order is the presence of phytomelanin, a black pigment present in the seed coat, creating a dark crust. Phytomelanin is found in most families of the Asparagales although not in |
Orchidaceae, thought to be the sistergroup of the rest of the order.
The leaves of almost all species form a tight rosette, either at the base of the plant or at the end of the stem, but occasionally along the stem. The flowers are not particularly distinctive, being 'lily type', with six tepals and up to six stamina.
The order is thought to have first diverged from other related monocots some 120130 million years ago early in the Cretaceous period, although given the difficulty in classifying the families involved, estimates are likely to be uncertain.
From an economic point of view, the order Asparagales is second in importance within the monocots only to the order Poales which includes grasses and cereals. Species are used as food and flavourings e.g. onion, garlic, leek, asparagus, vanilla, saffron, in medicinal or cosmetic applications Aloe, as cut flowers e.g. freesia, gladiolus, iris, orchids, and as garden ornamentals e.g. day lilies, lily of the valley, Agapanthus.
Description
Although most spe |
cies in the order are herbaceous, some no more than 15 cm high, there are a number of climbers e.g., some species of Asparagus, as well as several genera forming trees e.g. Agave, Cordyline, Yucca, Dracaena, Aloe , which can exceed 10 m in height. Succulent genera occur in several families e.g. Aloe.
Almost all species have a tight cluster of leaves a rosette, either at the base of the plant or at the end of a moreorless woody stem as with Yucca. In some cases, the leaves are produced along the stem. The flowers are in the main not particularly distinctive, being of a general 'lily type', with six tepals, either free or fused from the base and up to six stamina. They are frequently clustered at the end of the plant stem.
The Asparagales are generally distinguished from the Liliales by the lack of markings on the tepals, the presence of septal nectaries in the ovaries, rather than the bases of the tepals or stamen filaments, and the presence of secondary growth. They are generally geophytes, but with linear |
leaves, and a lack of fine reticular venation.
The seeds characteristically have the external epidermis either obliterated in most species bearing fleshy fruit, or if present, have a layer of black carbonaceous phytomelanin in species with dry fruits nuts. The inner part of the seed coat is generally collapsed, in contrast to Liliales whose seeds have a well developed outer epidermis, lack phytomelanin, and usually display a cellular inner layer.
The orders which have been separated from the old Liliales are difficult to characterize. No single morphological character appears to be diagnostic of the order Asparagales.
The flowers of Asparagales are of a general type among the lilioid monocots. Compared to Liliales, they usually have plain tepals without markings in the form of dots. If nectaries are present, they are in the septa of the ovaries rather than at the base of the tepals or stamens.
Those species which have relatively large dry seeds have a dark, crustlike crustose outer layer containing the pi |
gment phytomelan. However, some species with hairy seeds e.g. Eriospermum, family Asparagaceae s.l., berries e.g. Maianthemum, family Asparagaceae s.l., or highly reduced seeds e.g. orchids lack this dark pigment in their seed coats. Phytomelan is not unique to Asparagales i.e. it is not a synapomorphy but it is common within the order and rare outside it. The inner portion of the seed coat is usually completely collapsed. In contrast, the morphologically similar seeds of Liliales have no phytomelan, and usually retain a cellular structure in the inner portion of the seed coat.
Most monocots are unable to thicken their stems once they have formed, since they lack the cylindrical meristem present in other angiosperm groups. Asparagales have a method of secondary thickening which is otherwise only found in Dioscorea in the monocot order Disoscoreales. In a process called 'anomalous secondary growth', they are able to create new vascular bundles around which thickening growth occurs. Agave, Yucca, Aloe, Dracaen |
a, Nolina and Cordyline can become massive trees, albeit not of the height of the tallest dicots, and with less branching. Other genera in the order, such as Lomandra and Aphyllanthes, have the same type of secondary growth but confined to their underground stems.
Microsporogenesis part of pollen formation distinguishes some members of Asparagales from Liliales. Microsporogenesis involves a cell dividing twice meiotically to form four daughter cells. There are two kinds of microsporogenesis successive and simultaneous although intermediates exist. In successive microsporogenesis, walls are laid down separating the daughter cells after each division. In simultaneous microsporogenesis, there is no wall formation until all four cell nuclei are present. Liliales all have successive microsporogenesis, which is thought to be the primitive condition in monocots. It seems that when the Asparagales first diverged they developed simultaneous microsporogenesis, which the 'lower' Asparagale families retain. However, the |
'core' Asparagales see Phylogenetics have reverted to successive microsporogenesis.
The Asparagales appear to be unified by a mutation affecting their telomeres a region of repetitive DNA at the end of a chromosome. The typical 'Arabidopsistype' sequence of bases has been fully or partially replaced by other sequences, with the 'humantype' predominating.
Other apomorphic characters of the order according to Stevens are the presence of chelidonic acid, anthers longer than wide, tapetal cells bi to tetranuclear, tegmen not persistent, endosperm helobial, and loss of mitochondrial gene sdh3.
Taxonomy
As circumscribed within the Angiosperm Phylogeny Group system Asparagales is the largest order within the monocotyledons, with 14 families, 1,122 genera and about 25,00042,000 species, thus accounting for about 50 of all monocots and 1015 of the flowering plants angiosperms. The attribution of botanical authority for the name Asparagales belongs to Johann Heinrich Friedrich Link 17671851 who coined the word 'A |
sparaginae' in 1829 for a higher order taxon that included Asparagus although Adanson and Jussieau had also done so earlier see History. Earlier circumscriptions of Asparagales attributed the name to Bromhead 1838, who had been the first to use the term 'Asparagales'.
History
PreDarwinian
The type genus, Asparagus, from which the name of the order is derived, was described by Carl Linnaeus in 1753, with ten species. He placed Asparagus within the Hexandria Monogynia six stamens, one carpel in his sexual classification in the Species Plantarum. The majority of taxa now considered to constitute Asparagales have historically been placed within the very large and diverse family, Liliaceae. The family Liliaceae was first described by Michel Adanson in 1763, and in his taxonomic scheme he created eight sections within it, including the Asparagi with Asparagus and three other genera. The system of organising genera into families is generally credited to Antoine Laurent de Jussieu who formally described both the L |
iliaceae and the type family of Asparagales, the Asparagaceae, as Lilia and Asparagi, respectively, in 1789. Jussieu established the hierarchical system of taxonomy phylogeny, placing Asparagus and related genera within a division of Monocotyledons, a class III of Stamina Perigynia and 'order' Asparagi, divided into three subfamilies. The use of the term Ordo order at that time was closer to what we now understand as Family, rather than Order. In creating his scheme he used a modified form of Linnaeus' sexual classification but using the respective topography of stamens to carpels rather than just their numbers. While De Jussieu's Stamina Perigynia also included a number of 'orders' that would eventually form families within the Asparagales such as the Asphodeli Asphodelaceae, Narcissi Amaryllidaceae and Irides Iridaceae, the remainder are now allocated to other orders. Jussieu's Asparagi soon came to be referred to as Asparagaces in the French literature Latin Asparagaceae. Meanwhile, the 'Narcissi' had been |
renamed as the 'Amaryllides' Amaryllideae in 1805, by Jean Henri Jaume SaintHilaire, using Amaryllis as the type species rather than Narcissus, and thus has the authority attribution for Amaryllidaceae. In 1810, Brown proposed that a subgroup of Liliaceae be distinguished on the basis of the position of the ovaries and be referred to as Amaryllideae and in 1813 de Candolle described Liliaces Juss. and Amaryllides Brown as two quite separate families.
The literature on the organisation of genera into families and higher ranks became available in the English language with Samuel Frederick Gray's A natural arrangement of British plants 1821. Gray used a combination of Linnaeus' sexual classification and Jussieu's natural classification to group together a number of families having in common six equal stamens, a single style and a perianth that was simple and petaloid, but did not use formal names for these higher ranks. Within the grouping he separated families by the characteristics of their fruit and seed. H |
e treated groups of genera with these characteristics as separate families, such as Amaryllideae, Liliaceae, Asphodeleae and Asparageae.
The circumscription of Asparagales has been a source of difficulty for many botanists from the time of John Lindley 1846, the other important British taxonomist of the early nineteenth century. In his first taxonomic work, An Introduction to the Natural System of Botany 1830 he partly followed Jussieu by describing a subclass he called Endogenae, or Monocotyledonous Plants preserving de Candolle's Endogen phanerogam divided into two tribes, the Petaloidea and Glumaceae. He divided the former, often referred to as petaloid monocots, into 32 orders, including the Liliaceae defined narrowly, but also most of the families considered to make up the Asparagales today, including the Amaryllideae.
By 1846, in his final scheme Lindley had greatly expanded and refined the treatment of the monocots, introducing both an intermediate ranking Alliances and tribes within orders i.e. fami |
lies. Lindley placed the Liliaceae within the Liliales, but saw it as a paraphyletic "catchall" family, being all Liliales not included in the other orders, but hoped that the future would reveal some characteristic that would group them better. The order Liliales was very large and had become a used to include almost all monocotyledons with colourful tepals and without starch in their endosperm the lilioid monocots. The Liliales was difficult to divide into families because morphological characters were not present in patterns that clearly demarcated groups. This kept the Liliaceae separate from the Amaryllidaceae Narcissales. Of these Liliaceae was divided into eleven tribes with 133 genera and Amaryllidaceae into four tribes with 68 genera, yet both contained many genera that would eventually segregate to each other's contemporary orders Liliales and Asparagales respectively. The Liliaceae would be reduced to a small 'core' represented by the tribe Tulipae, while large groups such Scilleae and Asparagae wo |
uld become part of Asparagales either as part of the Amaryllidaceae or as separate families. While of the Amaryllidaceae, the Agaveae would be part of Asparagaceae but the Alstroemeriae would become a family within the Liliales.
The number of known genera and species continued to grow and by the time of the next major British classification, that of the Bentham Hooker system in 1883 published in Latin several of Lindley's other families had been absorbed into the Liliaceae. They used the term 'series' to indicate suprafamilial rank, with seven series of monocotyledons including Glumaceae, but did not use Lindley's terms for these. However they did place the Liliaceous and Amaryllidaceous genera into separate series. The Liliaceae were placed in series Coronariae, while the Amaryllideae were placed in series Epigynae. The Liliaceae now consisted of twenty tribes including Tulipeae, Scilleae and Asparageae, and the Amaryllideae of five including Agaveae and Alstroemerieae. An important addition to the treatme |
nt of the Liliaceae was the recognition of the Allieae as a distinct tribe that would eventually find its way to the Asparagales as the subfamily Allioideae of the Amaryllidaceae.
PostDarwinian
The appearance of Charles Darwin's Origin of Species in 1859 changed the way that taxonomists considered plant classification, incorporating evolutionary information into their schemata. The Darwinian approach led to the concept of phylogeny treelike structure in assembling classification systems, starting with Eichler. Eichler, having established a hierarchical system in which the flowering plants angiosperms were divided into monocotyledons and dicotyledons, further divided into former into seven orders. Within the Liliiflorae were seven families, including Liliaceae and Amaryllidaceae. Liliaceae included Allium and Ornithogalum modern Allioideae and Asparagus.
Engler, in his system developed Eichler's ideas into a much more elaborate scheme which he treated in a number of works including Die Natrlichen Pflanzenfa |
milien Engler and Prantl 1888 and Syllabus der Pflanzenfamilien 18921924. In his treatment of Liliiflorae the Liliineae were a suborder which included both families Liliaceae and Amaryllidaceae. The Liliaceae had eight subfamilies and the Amaryllidaceae four. In this rearrangement of Liliaceae, with fewer subdivisions, the core Liliales were represented as subfamily Lilioideae with Tulipae and Scilleae as tribes, the Asparagae were represented as Asparagoideae and the Allioideae was preserved, representing the alliaceous genera. Allieae, Agapantheae and Gilliesieae were the three tribes within this subfamily. In the Amaryllidacea, there was little change from the Bentham Hooker. A similar approach was adopted by Wettstein.
Twentieth century
In the twentieth century the Wettstein system 19011935 placed many of the taxa in an order called 'Liliiflorae'. Next Johannes Paulus Lotsy 1911 proposed dividing the Liliiflorae into a number of smaller families including Asparagaceae. Then Herbert Huber 1969, 1977, fo |
llowing Lotsy's example, proposed that the Liliiflorae be split into four groups including the 'Asparagoid' Liliiflorae.
The widely used Cronquist system 19681988 used the very broadly defined order Liliales.
These various proposals to separate small groups of genera into more homogeneous families made little impact till that of Dahlgren 1985 incorporating new information including synapomorphy. Dahlgren developed Huber's ideas further and popularised them, with a major deconstruction of existing families into smaller units. They created a new order, calling it Asparagales. This was one of five orders within the superorder Liliiflorae. Where Cronquist saw one family, Dahlgren saw forty distributed over three orders predominantly Liliales and Asparagales.
Over the 1980s, in the context of a more general review of the classification of angiosperms, the Liliaceae were subjected to more intense scrutiny. By the end of that decade, the Royal Botanic Gardens at Kew, the British Museum of Natural History and the E |
dinburgh Botanical Gardens formed a committee to examine the possibility of separating the family at least for the organization of their herbaria. That committee finally recommended that 24 new families be created in the place of the original broad Liliaceae, largely by elevating subfamilies to the rank of separate families.
The order Asparagales as currently circumscribed has only recently been recognized in classification systems, through the advent of phylogenetics. The 1990s saw considerable progress in plant phylogeny and phylogenetic theory, enabling a phylogenetic tree to be constructed for all of the flowering plants. The establishment of major new clades necessitated a departure from the older but widely used classifications such as Cronquist and Thorne based largely on morphology rather than genetic data. This complicated discussion about plant evolution and necessitated a major restructuring. rbcL gene sequencing and cladistic analysis of monocots had redefined the Liliales in 1995. from four morp |
hological orders sensu Dahlgren. The largest clade representing the Liliaceae, all previously included in Liliales, but including both the Calochortaceae and Liliaceae sensu Tamura. This redefined family, that became referred to as core Liliales, but corresponded to the emerging circumscription of the Angiosperm Phylogeny Group 1998.
Phylogeny and APG system
The 2009 revision of the Angiosperm Phylogeny Group system, APG III, places the order in the clade monocots.
From the Dahlgren system of 1985 onwards, studies based mainly on morphology had identified the Asparagales as a distinct group, but had also included groups now located in Liliales, Pandanales and Zingiberales. Research in the 21st century has supported the monophyly of Asparagales, based on morphology, 18S rDNA, and other DNA sequences, although some phylogenetic reconstructions based on molecular data have suggested that Asparagales may be paraphyletic, with Orchidaceae separated from the rest. Within the monocots, Asparagales is the sister gr |
oup of the commelinid clade.
This cladogram shows the placement of Asparagales within the orders of Lilianae sensu Chase Reveal monocots based on molecular phylogenetic evidence. The lilioid monocot orders are bracketed, namely Petrosaviales, Dioscoreales, Pandanales, Liliales and Asparagales. These constitute a paraphyletic assemblage, that is groups with a common ancestor that do not include all direct descendants in this case commelinids as the sister group to Asparagales; to form a clade, all the groups joined by thick lines would need to be included. While Acorales and Alismatales have been collectively referred to as "alismatid monocots" basal or early branching monocots, the remaining clades lilioid and commelinid monocots have been referred to as the "core monocots". The relationship between the orders with the exception of the two sister orders is pectinate, that is diverging in succession from the line that leads to the commelinids. Numbers indicate crown group most recent common ancestor of the s |
ampled species of the clade of interest divergence times in mya million years ago.
Subdivision
A phylogenetic tree for the Asparagales, generally to family level, but including groups which were recently and widely treated as families but which are now reduced to subfamily rank, is shown below.
The tree shown above can be divided into a basal paraphyletic group, the 'lower Asparagales asparagoids', from Orchidaceae to Asphodelaceae, and a wellsupported monophyletic group of 'core Asparagales' higher asparagoids, comprising the two largest families, Amaryllidaceae sensu lato and Asparagaceae sensu lato.
Two differences between these two groups although with exceptions are the mode of microsporogenesis and the position of the ovary. The 'lower Asparagales' typically have simultaneous microsporogenesis i.e. cell walls develop only after both meiotic divisions, which appears to be an apomorphy within the monocots, whereas the 'core Asparagales' have reverted to successive microsporogenesis i.e. cell walls dev |
elop after each division. The 'lower Asparagales' typically have an inferior ovary, whereas the 'core Asparagales' have reverted to a superior ovary. A 2002 morphological study by Rudall treated possessing an inferior ovary as a synapomorphy of the Asparagales, stating that reversions to a superior ovary in the 'core Asparagales' could be associated with the presence of nectaries below the ovaries. However, Stevens notes that superior ovaries are distributed among the 'lower Asparagales' in such a way that it is not clear where to place the evolution of different ovary morphologies. The position of the ovary seems a much more flexible character here and in other angiosperms than previously thought.
Changes to family structure in APG III
The APG III system when it was published in 2009, greatly expanded the families Xanthorrhoeaceae, Amaryllidaceae, and Asparagaceae. Thirteen of the families of the earlier APG II system were thereby reduced to subfamilies within these three families. The expanded Xanthorrhoe |
aceae is now called "Asphodelaceae". The APG II families left and their equivalent APG III subfamilies right are as follows
Structure of Asparagales
Orchid clade
Orchidaceae is possibly the largest family of all angiosperms only Asteraceae might or might not be more speciose and hence by far the largest in the order. The Dahlgren system recognized three families of orchids, but DNA sequence analysis later showed that these families are polyphyletic and so should be combined. Several studies suggest with high bootstrap support that Orchidaceae is the sister of the rest of the Asparagales. Other studies have placed the orchids differently in the phylogenetic tree, generally among the BoryaceaeHypoxidaceae clade. The position of Orchidaceae shown above seems the best current hypothesis, but cannot be taken as confirmed.
Orchids have simultaneous microsporogenesis and inferior ovaries, two characters that are typical of the 'lower Asparagales'. However, their nectaries are rarely in the septa of the ovaries, |
and most orchids have dustlike seeds, atypical of the rest of the order. Some members of Vanilloideae and Cypripedioideae have crustose seeds, probably associated with dispersal by birds and mammals that are attracted by fermenting fleshy fruit releasing fragrant compounds, e.g. vanilla.
In terms of the number of species, Orchidaceae diversification is remarkable. However, although the other Asparagales may be less rich in species, they are more variable morphologically, including treelike forms.
Boryaceae to Hypoxidaceae
The four families excluding Boryaceae form a wellsupported clade in studies based on DNA sequence analysis. All four contain relatively few species, and it has been suggested that they be combined into one family under the name Hypoxidaceae sensu lato. The relationship between Boryaceae which includes only two genera, Borya and Alania, and other Asparagales has remained unclear for a long time. The Boryaceae are mycorrhizal, but not in the same way as orchids. Morphological studies have s |
uggested a close relationship between Boryaceae and Blandfordiaceae. There is relatively low support for the position of Boryaceae in the tree shown above.
Ixioliriaceae to Xeronemataceae
The relationship shown between Ixioliriaceae and Tecophilaeaceae is still unclear. Some studies have supported a clade of these two families, others have not. The position of Doryanthaceae has also varied, with support for the position shown above, but also support for other positions.
The clade from Iridaceae upwards appears to have stronger support. All have some genetic characteristics in common, having lost Arabidopsistype telomeres. Iridaceae is distinctive among the Asparagales in the unique structure of the inflorescence a rhipidium, the combination of an inferior ovary and three stamens, and the common occurrence of unifacial leaves whereas bifacial leaves are the norm in other Asparagales.
Members of the clade from Iridaceae upwards have infralocular septal nectaries, which Rudall interpreted as a driver towards |
secondarily superior ovaries.
Asphodelaceae 'core Asparagales'
The next node in the tree Xanthorrhoeaceae sensu lato the 'core Asparagales' has strong support. 'Anomalous' secondary thickening occurs among this clade, e.g. in Xanthorrhoea family Asphodelaceae and Dracaena family Asparagaceae sensu lato, with species reaching treelike proportions.
The 'core Asparagales', comprising Amaryllidaceae sensu lato and Asparagaceae sensu lato, are a strongly supported clade, as are clades for each of the families. Relationships within these broadly defined families appear less clear, particularly within the Asparagaceae sensu lato. Stevens notes that most of its subfamilies are difficult to recognize, and that significantly different divisions have been used in the past, so that the use of a broadly defined family to refer to the entire clade is justified. Thus the relationships among subfamilies shown above, based on APWeb , is somewhat uncertain.
Evolution
Several studies have attempted to date the evolution of |
the Asparagales, based on phylogenetic evidence. Earlier studies generally give younger dates than more recent studies, which have been preferred in the table below.
A 2009 study suggests that the Asparagales have the highest diversification rate in the monocots, about the same as the order Poales, although in both orders the rate is little over half that of the eudicot order Lamiales, the clade with the highest rate.
Comparison of family structures
The taxonomic diversity of the monocotyledons is described in detail by Kubitzki. Uptodate information on the Asparagales can be found on the Angiosperm Phylogeny Website.
The APG III system's family circumscriptions are being used as the basis of the Kewhosted World Checklist of Selected Plant Families. With this circumscription, the order consists of 14 families Dahlgren had 31 with approximately 1120 genera and 26000 species.
Order Asparagales Link
Family Amaryllidaceae J.St.Hil. including Agapanthaceae F.Voigt, Alliaceae Borkh.
Family Asparagaceae J |
uss. including Agavaceae Dumort. which includes Anemarrhenaceae, Anthericaceae, Behniaceae and Herreriaceae, Aphyllanthaceae Burnett, Hesperocallidaceae Traub, Hyacinthaceae Batsch ex Borkh., Laxmanniaceae Bubani, Ruscaceae M.Roem. which includes Convallariaceae and Themidaceae Salisb.
Family Asteliaceae Dumort.
Family Blandfordiaceae R.Dahlgren Clifford
Family Boryaceae M.W. Chase, Rudall Conran
Family Doryanthaceae R.Dahlgren Clifford
Family Hypoxidaceae R.Br.
Family Iridaceae Juss.
Family Ixioliriaceae Nakai
Family Lanariaceae R.Dahlgren A.E.van Wyk
Family Orchidaceae Juss.
Family Tecophilaeaceae Leyb.
Family Xanthorrhoeaceae Dumort. including Asphodelaceae Juss. and Hemerocallidaceae R.Br., now Asphodelaceae Juss.
Family Xeronemataceae M.W.Chase, Rudall M.F.Fay
The earlier 2003 version, APG II, allowed 'bracketed' families, i.e. families which could either be segregated from more comprehensive families or could be included in them. These are the families given under "including" in the li |
st above. APG III does not allow bracketed families, requiring the use of the more comprehensive family; otherwise the circumscription of the Asparagales is unchanged. A separate paper accompanying the publication of the 2009 APG III system provided subfamilies to accommodate the families which were discontinued. The first APG system of 1998 contained some extra families, included in square brackets in the list above.
Two older systems which use the order Asparagales are the Dahlgren system and the Kubitzki system. The families included in the circumscriptions of the order in these two systems are shown in the first and second columns of the table below. The equivalent family in the modern APG III system see below is shown in the third column. Note that although these systems may use the same name for a family, the genera which it includes may be different, so the equivalence between systems is only approximate in some cases.
Uses
The Asparagales include many important crop plants and ornamental plants. Cr |
ops include Allium, Asparagus and Vanilla, while ornamentals include irises, hyacinths and orchids.
See also
Taxonomy of Liliaceae
Notes
References
Bibliography
Books
Contents
Chapters
, In .
, in
, in
, in
Articles
APG
Historical sources
Digital edition by the University and State Library Dsseldorf
1st ed. 19011908; 2nd ed. 19101911; 3rd ed. 19231924; 4th ed. 19331935
Websites
Families included in the checklist
Reference materials
External links
Biodiversity Heritage Library
Angiosperm orders
Extant Late Cretaceous first appearances |
The Alismatales alismatids are an order of flowering plants including about 4500 species. Plants assigned to this order are mostly tropical or aquatic. Some grow in fresh water, some in marine habitats.
Description
The Alismatales comprise herbaceous flowering plants of often aquatic and marshy habitats, and the only monocots known to have green embryos other than the Amaryllidaceae. They also include the only marine angiosperms growing completely submerged, the seagrasses. The flowers are usually arranged in inflorescences, and the mature seeds lack endosperm.
Both marine and freshwater forms include those with staminate flowers that detach from the parent plant and float to the surface. There they can pollinate carpellate flowers floating on the surface via long pedicels. In others, pollination occurs underwater, where pollen may form elongated strands, increasing chance of success. Most aquatic species have a totally submerged juvenile phase, and flowers are either floating or emergent. Vegetation may be |
totally submersed, have floating leaves, or protrude from the water. Collectively, they are commonly known as "water plantain".
Taxonomy
The Alismatales contain about 165 genera in 13 families, with a cosmopolitan distribution. Phylogenetically, they are basal monocots, diverging early in evolution relative to the lilioid and commelinid monocot lineages. Together with the Acorales, the Alismatales are referred to informally as the alismatid monocots.
Early systems
The Cronquist system 1981 places the Alismatales in subclass Alismatidae, class Liliopsida monocotyledons and includes only three families as shown
Alismataceae
Butomaceae
Limnocharitaceae
Cronquist's subclass Alismatidae conformed fairly closely to the order Alismatales as defined by APG, minus the Araceae.
The Dahlgren system places the Alismatales in the superorder Alismatanae in the subclass Liliidae monocotyledons in the class Magnoliopsida angiosperms with the following families included
Alismataceae
Aponogetonaceae
Butomaceae
|
Hydrocharitaceae
Limnocharitaceae
In Tahktajan's classification 1997, the order Alismatales contains only the Alismataceae and Limnocharitaceae, making it equivalent to the Alismataceae as revised in APGIII. Other families included in the Alismatates as currently defined are here distributed among 10 additional orders, all of which are assigned, with the following exception, to the Subclass Alismatidae. Araceae in Tahktajan 1997 is assigned to the Arales and placed in the Subclass Aridae; Tofieldiaceae to the Melanthiales and placed in the Liliidae.
Angiosperm Phylogeny Group
The Angiosperm Phylogeny Group system APG of 1998 and APG II 2003 assigned the Alismatales to the monocots, which may be thought of as an unranked clade containing the families listed below. The biggest departure from earlier systems see below is the inclusion of family Araceae. By its inclusion, the order has grown enormously in number of species. The family Araceae alone accounts for about a hundred genera, totaling over two thou |
sand species. The rest of the families together contain only about five hundred species, many of which are in very small families.
The APG III system 2009 differs only in that the Limnocharitaceae are combined with the Alismataceae; it was also suggested that the genus Maundia of the Juncaginaceae could be separated into a monogeneric family, the Maundiaceae, but the authors noted that more study was necessary before the Maundiaceae could be recognized.
order Alismatales sensu APG III
family Alismataceae including Limnocharitaceae
family Aponogetonaceae
family Araceae
family Butomaceae
family Cymodoceaceae
family Hydrocharitaceae
family Juncaginaceae
family Posidoniaceae
family Potamogetonaceae
family Ruppiaceae
family Scheuchzeriaceae
family Tofieldiaceae
family Zosteraceae
In APG IV 2016, it was decided that evidence was sufficient to elevate Maundia to family level as the monogeneric Maundiaceae. The authors considered including a number of the smaller orders within the Juncaginaceae, but |
an online survey of botanists and other users found little support for this "lumping" approach. Consequently, the family structure for APG IV is
family Alismataceae including Limnocharitaceae
family Aponogetonaceae
family Araceae
family Butomaceae
family Cymodoceaceae
family Hydrocharitaceae
family Juncaginaceae
family Maundiaceae
family Posidoniaceae
family Potamogetonaceae
family Ruppiaceae
family Scheuchzeriaceae
family Tofieldiaceae
family Zosteraceae
Phylogeny
Cladogram showing the orders of monocots Lilianae sensu Chase Reveal based on molecular phylogenetic evidence
References
Further reading
B. C. J. du Mortier 1829. Analyse des Familles de Plantes avec l'indication des principaux genres qui s'y rattachent. Imprimerie de J. Casterman, Tournay
W. S. Judd, C. S. Campbell, E. A. Kellogg, P. F. Stevens, M. J. Donoghue, 2002. Plant Systematics A Phylogenetic Approach, 2nd edition. Sinauer Associates, Sunderland, Massachusetts .
, in
External links
Angiosperm order |
s |
The Apiales are an order of flowering plants. The families are those recognized in the APG III system. This is typical of the newer classifications, though there is some slight variation and in particular, the Torriceliaceae may be divided.
Under this definition, wellknown members include carrots, celery, parsley, and Hedera helix English ivy.
The order Apiales is placed within the asterid group of eudicots as circumscribed by the APG III system. Within the asterids, Apiales belongs to an unranked group called the campanulids, and within the campanulids, it belongs to a clade known in phylogenetic nomenclature as Apiidae. In 2010, a subclade of Apiidae named Dipsapiidae was defined to consist of the three orders Apiales, Paracryphiales, and Dipsacales.
Taxonomy
Under the Cronquist system, only the Apiaceae and Araliaceae were included here, and the restricted order was placed among the rosids rather than the asterids. The Pittosporaceae were placed within the Rosales, and many of the other forms within the |
family Cornaceae. Pennantia was in the family Icacinaceae. In the classification system of Dahlgren the families Apiaceae and Araliaceae were placed in the order Ariales, in the superorder Araliiflorae also called Aralianae.
The present understanding of the Apiales is fairly recent and is based upon comparison of DNA sequences by phylogenetic methods. The circumscriptions of some of the families have changed. In 2009, one of the subfamilies of Araliaceae was shown to be polyphyletic.
Gynoecia
The largest and obviously closely related families of Apiales are Araliaceae, Myodocarpaceae and
Apiaceae, which resemble each other in the structure of their gynoecia. In this respect however, the Pittosporaceae is notably distinct from them.
Typical syncarpous gynoecia exhibit four vertical zones, determined by the extent of fusion of the carpels. In most plants the synascidiate i.e. "united bottleshaped" and symplicate zones are fertile and bear the ovules. Each of the first three families possess mainly bi or mul |
tilocular ovaries in a gynoecium with a long synascidiate, but very short symplicate zone, where the ovules are inserted at their transition, the socalled crosszone or "Querzone".
In gynoecia of the Pittosporaceae, the symplicate is much longer than the synascidiate zone, and the ovules are arranged along the first. Members of the latter family consequently have unilocular ovaries with a single cavity between adjacent carpels.
References
Angiosperm orders
Taxa named by Takenoshin Nakai |
Asterales is an order of dicotyledonous flowering plants that includes the large family Asteraceae or Compositae known for composite flowers made of florets, and ten families related to the Asteraceae. While asterids in general are characterized by fused petals, composite flowers consisting of many florets create the false appearance of separate petals as found in the rosids.
The order is cosmopolitan plants found throughout most of the world including desert and frigid zones, and includes mostly herbaceous species, although a small number of trees such as the Lobelia deckenii, the giant lobelia, and Dendrosenecio, giant groundsels and shrubs are also present.
Asterales are organisms that seem to have evolved from one common ancestor. Asterales share characteristics on morphological and biochemical levels. Synapomorphies a character that is shared by two or more groups through evolutionary development include the presence in the plants of oligosaccharide inulin, a nutrient storage molecule used instead of |
starch; and unique stamen morphology. The stamens are usually found around the style, either aggregated densely or fused into a tube, probably an adaptation in association with the plunger brush; or secondary pollination that is common among the families of the order, wherein pollen is collected and stored on the length of the pistil.
Taxonomy
The name and order Asterales is botanically venerable, dating back to at least 1926 in the Hutchinson system of plant taxonomy when it contained only five families, of which only two are retained in the APG III classification. Under the Cronquist system of taxonomic classification of flowering plants, Asteraceae was the only family in the group, but newer systems such as APG II and APG III have expanded it to 11. In the classification system of Dahlgren the Asterales were in the superorder Asteriflorae also called Asteranae.
The order Asterales currently includes 11 families, the largest of which are the Asteraceae, with about 25,000 species, and the Campanulaceae " |
bellflowers", with about 2,000 species. The remaining families count together for less than 1500 species. The two large families are cosmopolitan, with many of their species found in the Northern Hemisphere, and the smaller families are usually confined to Australia and the adjacent areas, or sometimes South America.
Only the Asteraceae have composite flower heads; the other families do not, but share other characteristics such as storage of inulin that define the 11 families as more closely related to each other than to other plant families or orders such as the rosids.
The phylogenetic tree according to APG III for the Campanulid clade is as below.
Biogeography
The core Asterales are Stylidiaceae six genera, APA clade Alseuosmiaceae, Phellinaceae and Argophyllaceae, together 7 genera, MGCA clade Menyanthaceae, Goodeniaceae, Calyceraceae, in total twenty genera, and Asteraceae about sixteen hundred genera. Other Asterales are Rousseaceae four genera, Campanulaceae eighty four genera and Pentaphragmataceae |
one genus.
All Asterales families are represented in the Southern Hemisphere; however, Asteraceae and Campanulaceae are cosmopolitan and Menyanthaceae nearly so.
Evolution
Although most extant species of Asteraceae are herbaceous, the examination of the basal members in the family suggests that the common ancestor of the family was an arborescent plant, a tree or shrub, perhaps adapted to dry conditions, radiating from South America. Less can be said about the Asterales themselves with certainty, although since several families in Asterales contain trees, the ancestral member is most likely to have been a tree or shrub.
Because all clades are represented in the southern hemisphere but many not in the northern hemisphere, it is natural to conjecture that there is a common southern origin to them. Asterales are angiosperms, flowering plants that appeared about 140 million years ago. The Asterales order probably originated in the Cretaceous 145 66 Mya on the supercontinent Gondwana which broke up from 184 |
80 Mya, forming the area that is now Australia, South America, Africa, India and Antarctica.
Asterales contain about 14 of eudicot diversity. From an analysis of relationships and diversities within the Asterales and with their superorders, estimates of the age of the beginning of the Asterales have been made, which range from 116 Mya to 82Mya. However few fossils have been found, of the MenyanthaceaeAsteraceae clade in the Oligocene, about 29 Mya.
Fossil evidence of the Asterales is rare and belongs to rather recent epochs, so the precise estimation of the order's age is quite difficult. An Oligocene 34 23 Mya pollen is known for Asteraceae and Goodeniaceae, and seeds from Oligocene and Miocene 23 5.3 Mya are known for Menyanthaceae and Campanulaceae respectively.
Economic importance
The Asterales, by dint of being a superset of the family Asteraceae, include some species grown for food, including the sunflower Helianthus annuus, lettuce Lactuca sativa and chicory Cichorium. Many are also used as spic |
es and traditional medicines.
Asterales are common plants and have many known uses. For example, pyrethrum derived from Old World members of the genus Chrysanthemum is a natural insecticide with minimal environmental impact. Wormwood, derived from a genus that includes the sagebrush, is used as a source of flavoring for absinthe, a bitter classical liquor of European origin.
References
Citations
General references
W. S. Judd, C. S. Campbell, E. A. Kellogg, P. F. Stevens, M. J. Donoghue 2002. Plant Systematics A Phylogenetic Approach, 2nd edition. pp. 476486 Asterales. Sinauer Associates, Sunderland, Massachusetts. .
External links
Angiosperm orders |
An asteroid is a minor planet of the inner Solar System. Historically, these terms have been applied to any astronomical object orbiting the Sun that did not resolve into a disc in a telescope and was not observed to have characteristics of an active comet such as a tail. As minor planets in the outer Solar System were discovered that were found to have volatilerich surfaces similar to comets, these came to be distinguished from the objects found in the main asteroid belt. Thus the term "asteroid" now generally refers to the minor planets of the inner Solar System, including those coorbital with Jupiter. Larger asteroids are often called planetoids.
Overview
Millions of asteroids exist many are shattered remnants of planetesimals, bodies within the young Sun's solar nebula that never grew large enough to become planets. The vast majority of known asteroids orbit within the main asteroid belt located between the orbits of Mars and Jupiter, or are coorbital with Jupiter the Jupiter trojans. However, other orbi |
tal families exist with significant populations, including the nearEarth objects. Individual asteroids are classified by their characteristic spectra, with the majority falling into three main groups Ctype, Mtype, and Stype. These were named after and are generally identified with carbonrich, metallic, and silicate stony compositions, respectively. The sizes of asteroids varies greatly; the largest, Ceres, is almost across and massive enough to qualify as a dwarf planet.
Asteroids are somewhat arbitrarily differentiated from comets and meteoroids. In the case of comets, the difference is one of composition while asteroids are mainly composed of mineral and rock, comets are primarily composed of dust and ice. Furthermore, asteroids formed closer to the sun, preventing the development of cometary ice. The difference between asteroids and meteoroids is mainly one of size meteoroids have a diameter of one meter or less, whereas asteroids have a diameter of greater than one meter. Finally, meteoroids can be comp |
osed of either cometary or asteroidal materials.
Only one asteroid, 4 Vesta, which has a relatively reflective surface, is normally visible to the naked eye, and this is only in very dark skies when it is favorably positioned. Rarely, small asteroids passing close to Earth may be visible to the naked eye for a short time. , the Minor Planet Center had data on 930,000 minor planets in the inner and outer Solar System, of which about 545,000 had enough information to be given numbered designations.
The United Nations declared 30 June as International Asteroid Day to educate the public about asteroids. The date of International Asteroid Day commemorates the anniversary of the Tunguska asteroid impact over Siberia, Russian Federation, on 30 June 1908.
In April 2018, the B612 Foundation reported "It is 100 percent certain we'll be hit by a devastating asteroid, but we're not 100 percent sure when." Also in 2018, physicist Stephen Hawking,
in his final book Brief Answers to the Big Questions, considered an aster |
oid collision to be the biggest threat to the planet. In June 2018, the US National Science and Technology Council warned that America is unprepared for an asteroid impact event, and has developed and released the "National NearEarth Object Preparedness Strategy Action Plan" to better prepare. According to expert testimony in the United States Congress in 2013, NASA would require at least five years of preparation before a mission to intercept an asteroid could be launched.
Discovery
The first asteroid to be discovered, Ceres, was originally considered to be 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", coined in Greek as , or asteroeids, meaning 'starlike, starshaped', and derived from the Ancient Greek ast |
r '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.
Discovery timeline
10 by 1849
1 Ceres, 1801
2 Pallas 1802
3 Juno 1804
4 Vesta 1807
5 Astraea 1845
in 1846, planet Neptune was discovered
6 Hebe July 1847
7 Iris August 1847
8 Flora October 1847
9 Metis 25 April 1848
10 Hygiea 12 April 1849 tenth asteroid discovered
100 asteroids by 1868
1,000 by 1921
10,000 by 1989
100,000 by 2005
1,000,000 by 2020
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 TitiusBode 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 handdrawn sky charts be pr |
epared for all stars in the zodiacal band down to an agreedupon 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 starlike object in Taurus and followed the displacement of this object during several nights. Later that year, Carl Friedrich Gauss used these observations to calculate the orbit of this unknown object, which was found to be 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 w |
ere 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, A. de Gasparis, R. Luther, H.M.S. Goldschmidt, J. Chacornac, J. Ferguson, N.R. Pogson, E.W. Tempel, J.C. Watson, C.H.F. Peters, A. Borrelly, J. Palisa, the Henry brothers and A. Charlois.
In 1891, Max Wolf pioneered the use of astrophotography to detect asteroids, which appeared as short streaks on longexposure 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 poi |
nt. It was known that there were many more, but most astronomers did not bother with them, some calling them "vermin of the skies", a phrase variously attributed to E. Suess and E. Weiss. Even a century later, only a few thousand asteroids were identified, numbered and named.
Manual methods of the 1900s and modern reporting
Until 1998, asteroids were discovered by a fourstep process. First, a region of the sky was photographed by a widefield 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 l |
ocations.
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 halfmonth of discovery, and finally a letter and a number indicating the discovery's sequential number example .
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.
Computerized methods
There is increasing interest in identifying asteroids whose orbits cross Earth's, and that could, given enough time, collide with Earth see Earthcrosser asteroids. The three most important groups of nearEarth a |
steroids are the Apollos, Amors, and Atens. Various asteroid deflection strategies have been proposed, as early as the 1960s.
The nearEarth 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 Earth in 1937. Astronomers began to realize the possibilities of Earth impact.
Two events in later decades increased the alarm the increasing acceptance of the Alvarez hypothesis that an impact event resulted in the CretaceousPaleogene extinction, and the 1994 observation of Comet ShoemakerLevy 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 upperatmosphere impacts by objects ranging from one to ten meters across.
All these considerations helped spur the launch of highly efficient surveys that consist of chargeco |
upled device CCD cameras and computers directly connected to telescopes. , it was estimated that 89 to 96 of nearEarth asteroids one kilometer or larger in diameter had been discovered. A list of teams using such systems includes
Lincoln NearEarth Asteroid Research LINEAR
NearEarth Asteroid Tracking NEAT
Spacewatch
Lowell Observatory NearEarthObject Search LONEOS
Catalina Sky Survey CSS
PanSTARRS
NEOWISE
Asteroid Terrestrialimpact Last Alert System ATLAS
Campo Imperatore NearEarth Object Survey CINEOS
Japanese Spaceguard Association
AsiagoDLR Asteroid Survey ADAS
, the LINEAR system alone has discovered 147,132 asteroids. Among all the surveys, 19,266 nearEarth asteroids have been discovered including almost 900 more than in diameter.
Terminology
Traditionally, small bodies orbiting the Sun were classified as comets, asteroids, or meteoroids, with anything smaller than one meter across being called a meteoroid. Beech and Steel's 1995 paper proposed a meteoroid definition including size limits |
. The term "asteroid", from the Greek word for "starlike", never had a formal definition, with the broader term minor planet being preferred by the International Astronomical Union.
However, following the discovery of asteroids below ten meters in size, Rubin and Grossman's 2010 paper revised the previous definition of meteoroid to objects between 10 m and 1 meter in size in order to maintain the distinction between asteroids and meteoroids. The smallest asteroids discovered based on absolute magnitude H are with and with both with an estimated size of about 1 meter.
In 2006, the term "small Solar System body" was also introduced to cover both most minor planets and comets. Other languages prefer "planetoid" Greek for "planetlike", and this term is occasionally used in English especially for larger minor planets such as the dwarf planets as well as an alternative for asteroids since they are not starlike. 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, but is not in common use. The three largest objects in the asteroid belt, Ceres, Pallas, and Vesta, grew to the stage of protoplanets. Ceres is 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 nearsurface ices by solar radiation. A few objects have ended up being duallisted 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 asteroidlike. A further distinction is that comets typically have more eccentric orbits than most ast |
eroids; most "asteroids" with notably eccentric orbits are probably dormant or extinct comets.
For almost two centuries, from the discovery of Ceres in 1801 until the discovery of the first centaur, Chiron in 1977, all known asteroids spent most of their time at or within the orbit of Jupiter, though a few such as Hidalgo ventured far beyond Jupiter for part of their orbit. Those located between the orbits of Mars and Jupiter were known for many years simply as The Asteroids. 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 considered asteroids or as a new type of object. Then, when the first transNeptunian object other than Pluto, Albion, was discovered in 1992, and especially when large numbers of similar objects started turning up, new terms were invented to sidestep the issue Kuiperbelt object, transNeptunian object, scattereddis |
c object, and so on. These inhabit the cold outer reaches of the Solar System where ices remain solid and cometlike bodies are not expected to exhibit much cometary activity; if centaurs or transNeptunian 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 Kuiperbelt objects, called "objects" partly to avoid the need to classify them as asteroids or comets. They are thought to be predominantly cometlike in composition, though some may be more akin to asteroids. 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, s |
uggesting "a continuum between asteroids and comets" rather than a sharp dividing line.
The minor planets beyond Jupiter's orbit are sometimes also called "asteroids", especially in popular presentations. However, it is becoming increasingly common for the term "asteroid" to be restricted to minor planets of the inner Solar System. Therefore, this article will restrict itself for the most part to the classical asteroids objects of the asteroid belt, Jupiter trojans, and nearEarth 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." Currently only the largest object in the asteroid belt, Ceres, at about across, has been placed i |
n the dwarf planet category.
Formation
It is thought 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 in diameter accreted during that early era, whereas smaller bodies are fragments from collisions between asteroids during or after the Jovian disruption. Ceres and Vesta grew large enough to melt and differentiate, with heavy metallic elements sinking to the core, leaving rocky minerals in the crust.
In the Nice model, many Kuiperbelt 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 Dtype asteroids, and possibly include Ceres.
Distribution within the Solar System
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
Asteroid belt
The majority of known asteroids orbit within the asteroid belt between the orbits of Mars and Jupiter, generally in relatively loweccentricity i.e. not very elongated orbits. This belt is now estimated to contain between 1.1 and 1.9 million asteroids larger than in diameter, and millions of smaller ones. 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.
Trojans
Trojans are populations 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 populat |
ion of trojans are the Jupiter trojans. Although fewer Jupiter trojans have been discovered , it is thought that they are as numerous as the asteroids in the asteroid belt. Trojans have been found in the orbits of other planets, including Venus, Earth, Mars, Uranus, and Neptune.
NearEarth asteroids
NearEarth asteroids, or NEAs, are asteroids that have orbits that pass close to that of Earth. Asteroids that actually cross Earth's orbital path are known as Earthcrossers. , 14,464 nearEarth asteroids are known and approximately 9001,000 have a diameter of over one kilometer.
Characteristics
Size distribution
Asteroids vary greatly in size, from almost for the largest down to rocks just 1 meter across. The three largest are very much like miniature planets they are roughly spherical, have at least partly differentiated interiors, and are thought to be surviving protoplanets. The vast majority, however, are much smaller and are irregularly shaped; they are thought to be either battered planetesimals or fra |
gments of larger bodies.
The dwarf planet Ceres is by far the largest asteroid, with a diameter of . The next largest are 4 Vesta and 2 Pallas, both with diameters of just over . Vesta is the only mainbelt asteroid that can, on occasion, be visible to the naked eye. On some rare occasions, a nearEarth 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 in the range of , about 4 of the mass of the Moon. Of this, Ceres comprises , about a third of the total. Adding in the next three most massive objects, Vesta 9, Pallas 7, and Hygiea 3, brings this figure up to half, whereas the three mostmassive asteroids after that, 511 Davida 1.2, 704 Interamnia 1.0, and 52 Europa 0.9, constitute only another 3. The number of asteroids increases rapidly as their individual masses decrease.
The number of asteroids decreases markedly with size. Although this generally follows a power l |
aw, there are 'bumps' at and , where more asteroids than expected from a logarithmic distribution are found.
Largest asteroids
Although their location in the asteroid belt excludes them from planet status, the three largest objects, Ceres, Vesta, and Pallas, are intact protoplanets that share many characteristics common to planets, and are atypical compared to the majority of irregularly shaped asteroids. The fourthlargest asteroid, Hygiea, appears nearly spherical although it may have an undifferentiated interior, like the majority of asteroids. Between them, the four largest asteroids constitute half the mass of the asteroid belt.
Ceres is the only asteroid that appears to be plastic shape under its own gravity and hence the only one that is a likely dwarf planet. It has a much higher absolute magnitude than the other asteroids, of around 3.32, and may possess a surface layer of ice. Like the planets, Ceres is differentiated it has a crust, a mantle and a core. No meteorites from Ceres have been found o |
n Earth.
Vesta, too, has a differentiated interior, though it formed inside the Solar System's frost line, and so is devoid of water; its composition is mainly of basaltic rock with minerals such as olivine. Aside from the large crater at its southern pole, Rheasilvia, Vesta also has an ellipsoidal shape. Vesta is the parent body of the Vestian family and other Vtype asteroids, and is the source of the HED meteorites, which constitute 5 of all meteorites on Earth.
Pallas is unusual in that, like Uranus, it rotates on its side, with its axis of rotation tilted at high angles to its orbital plane. Its composition is similar to that of Ceres high in carbon and silicon, and perhaps partially differentiated. Pallas is the parent body of the Palladian family of asteroids.
Hygiea is the largest carbonaceous asteroid and, unlike the other largest asteroids, lies relatively close to the plane of the ecliptic. It is the largest member and presumed parent body of the Hygiean family of asteroids. Because there is n |
o sufficiently large crater on the surface to be the source of that family, as there is on Vesta, it is thought that Hygiea may have been completely disrupted in the collision that formed the Hygiean family and recoalesced after losing a bit less than 2 of its mass. Observations taken with the Very Large Telescope's SPHERE imager in 2017 and 2018, and announced in late 2019, revealed that Hygiea has a nearly spherical shape, which is consistent both with it being in hydrostatic equilibrium and thus a dwarf planet, or formerly being in hydrostatic equilibrium, or with being disrupted and recoalescing.
Rotation
Measurements of the rotation rates of large asteroids in the asteroid belt show that there is an upper limit. Very few asteroids with a diameter larger than 100 meters have a rotation period smaller than 2.2 hours. For asteroids rotating faster than approximately this rate, the inertial force at the surface is greater than the gravitational force, so any loose surface material would be flung out. Howev |
er, 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 the accumulation of debris after collisions between asteroids.
Composition
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 nickeliron core, olivine mantle, and basaltic crust. 10 Hygiea, however, which appears to have a uniformly primitive composition of carbonaceous chondrite, is thought to be the largest undifferentiated asteroid, though it may be a differentiated asteroid that was globally disrupted by an impact and then reassembled. Other asteroids appear to be the remnant cores or mantles of protoplanets, high in rock and metal Most small 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 coorbiting binaries |
Rubble piles, moons, binaries, and scattered asteroid families are thought to be the results of collisions that disrupted a parent asteroid, or, possibly, a planet.
In the main asteroid belt, there appear to be two primary populations of asteroid a dark, volatilerich population, consisting of the Ctype and Ptype asteroids, with albedos less that 0.10 and densities under , and a dense, volatilepoor population, consisting of the Stype and Mtype asteroids, with albedos over 0.15 and densities greater than 2.7. Within these populations, larger asteroids are denser, presumably due to compression. There appears to be minimal macroporosity interstitial vacuum in the score of asteroids with masses greater than .
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 also see panspermia. In August 2011, a report, based on NASA studies wit |
h 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.
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 268186183 km, and 87 Sylvia 384262232 km. Only half a dozen asteroids are larger than 87 Sylvia, though none of them have moons. The fact that such large asteroids as Sylvia may 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 mergin |
g, but colliding rubble piles are more likely to merge. This means that the cores of the planets could have formed relatively quickly.
On 7 October 2009, the presence of water ice was confirmed on the surface of 24 Themis using NASA's Infrared Telescope Facility. The surface of the asteroid appears completely covered in ice. As this ice layer is sublimating, it may be getting replenished by a reservoir of ice under the surface. Organic compounds were also detected on the surface. Scientists hypothesize that some of the first water brought to Earth was delivered by asteroid impacts after the collision that produced the Moon. The presence of ice on 24 Themis supports this theory.
In October 2013, water was detected on an extrasolar body for the first time, on an asteroid orbiting the white dwarf GD 61. On 22 January 2014, European Space Agency ESA scientists reported the detection, for the first definitive time, of water vapor on Ceres, the largest object in the asteroid belt. The detection was made by using |
the farinfrared abilities of the Herschel Space Observatory. The finding is unexpected because comets, not asteroids, are typically considered to "sprout jets and plumes". According to one of the scientists, "The lines are becoming more and more blurred between comets and asteroids."
In May 2016, significant asteroid data arising from the Widefield Infrared Survey Explorer and NEOWISE missions have been questioned. Although the early original criticism had not undergone peer review, a more recent peerreviewed study was subsequently published.
In November 2019, scientists reported detecting, for the first time, sugar molecules, including ribose, in meteorites, suggesting that chemical processes on asteroids can produce some fundamentally essential bioingredients important to life, and supporting the notion of an RNA world prior to a DNAbased origin of life on Earth, and possibly, as well, the notion of panspermia.
Acfer 049, a meteorite discovered in Algeria in 1990, was shown in 2019 to have ice fossils in |
side it the first direct evidence of water ice in the composition of asteroids.
Findings have shown that solar winds can react with the oxygen in the upper layer of the asteroids and create water. It has been estimated that every cubic metre of irradiated rock could contain up to 20 litres.
Surface features
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 31 mi 253 Mathilde is a rubble pile saturated with craters with diameters the size of the asteroid's radius, and Earthbased observations of 300 km 186 mi 511 Davida, one of the largest asteroids after the big four, reveal a similarly angular profile, suggesting it is also saturated with radiussize craters. Mediumsized 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 radiussize crate |
r 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 be expanded with the Dawn spacecraft, which entered Ceres orbit on 6 March 2015.
Color
Asteroids become darker and redder with age due to space weathering. However evidence suggests most of the color change occurs rapidly, in the first hundred thousand years, limiting the usefulness of spectral measurement for determining the age of asteroids.
Classification
Asteroids are commonly categorized according to two criteria the characteristics of their orbits, and features of their reflectance spectrum.
Orbital classification
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 asso |
ciations, whereas families are tighter and result from the catastrophic breakup of a large parent asteroid sometime in the past. Families are more common and easier to identify within the main asteroid belt, but several small families have been reported among the Jupiter trojans. Main belt families were first recognized by Kiyotsugu Hirayama in 1918 and are often called Hirayama families in his honor.
About 3035 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 .
Quasisatellites and horseshoe objects
Some asteroids have unusual horseshoe orbits that are coorbital with Earth or some other planet. Examples are 3753 Cruithne and . The first instance of this type of orbital arrangement was discovered between Saturn's moons Epimetheus and Janus.
Sometimes these horseshoe objects temporarily become quasisatellites for a few decades or a few hundred years, befo |
re returning to their earlier status. Both Earth and Venus are known to have quasisatellites.
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
In 1975, an asteroid taxonomic system based on color, albedo, and spectral shape was developed by Chapman, Morrison, and Zellner. These properties are thought to correspond to the composition of the asteroid's surface material. The original classification system had three categories Ctypes for dark carbonaceous objects 75 of known asteroids, Stypes 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 eightcolor asteroid survey performed in the 1980s. This resulted in 14 asteroid categories. In 2002, the Small MainBelt 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 Mtype. There are also several smaller classes.
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
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 a variety of classifications are in use. This has led to significant confusion. Although asteroids of different sp |
ectral classifications are likely to be composed of different materials, there are no assurances that asteroids within the same taxonomic class are composed of the same or similar materials.
Naming
A newly discovered asteroid is given a provisional designation such as consisting of the year of discovery and an alphanumeric code indicating the halfmonth of discovery and the sequence within that halfmonth. Once an asteroid's orbit has been confirmed, it is given a number, and later may also be given a name e.g. . 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. In addition, names can be proposed by the asteroid's discoverer, within guidelines established by the International Astronomical Union.
Symbols
The first asteroids to be discovered were assigned iconic symbols like the ones traditionally u |
sed to designate the planets. By 1855 there were two dozen asteroid symbols, which often occurred in multiple variants.
In 1851, after the fifteenth asteroid Eunomia had been discovered, 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 although he assigned to the fifth, Astraea, while continuing to designate the first four only with their existing iconic symbols. The numberedcircle convention was quickly adopted by astronomers, and the next asteroid to be discovered 16 Psyche, in 1852 was the first to be designated in that way at the time of its discovery. However, Psyche was given an iconic symbol as well, as were a few other asteroids discovered over the next few years see chart above. 20 Massalia was the first asteroid that was not |
assigned an iconic symbol, and no iconic symbols were created after the 1855 discovery of 37 Fides. That year Astraea's number was increased to , but the first four asteroids, Ceres to Vesta, were not listed by their numbers until the 1867 edition. The circle was soon abbreviated to a pair of parentheses, which were easier to typeset and sometimes omitted altogether over the next few decades, leading to the modern convention.
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 best modern groundbased telescopes and the Earthorbiting 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, an |
d asteroid sizes can be estimated by timing the lengths of star occultations 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 nearEarth asteroids. In terms of deltav and propellant requirements, NEOs are more easily accessible than the Moon.
The first closeup photographs of asteroidlike 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, potatolike 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 closeup 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 int |
o 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.
From September to November 2005, the Japanese Hayabusa probe studied 25143 Itokawa in detail and was plagued with difficulties, but returned samples of its surface to Earth on 13 June 2010.
The European Rosetta probe launched in 2004 flew by 2867 teins in 2008 and 21 Lutetia, the thirdlargest asteroid visited to date, in 2010.
In September 2007, NASA launched the Dawn spacecraft, which orbited 4 Vesta from July 2011 to September 2012, and has been orbiting the dwarf planet 1 Ceres since 2015. 4 Vesta is the secondlargest asteroid visited to date.
On 13 December 2012, China's lunar orbiter Chang'e 2 flew within of the asteroid 4179 Toutatis on an extended mission.
The Japan Aerospace Exploration Agency JAXA launched the Hayabusa2 probe in December 2014, and plans to return sample |
s from 162173 Ryugu in December 2020.
In June 2018, the US National Science and Technology Council warned that America is unprepared for an asteroid impact event, and has developed and released the "National NearEarth Object Preparedness Strategy Action Plan" to better prepare.
In September 2016, NASA launched the OSIRISREx sample return mission to asteroid 101955 Bennu, which it reached in December 2018. On May 10 2021, the probe departed the asteroid with a sample from its surface, and is expected to return to Earth on September 24 2023.
Planned and future missions
In early 2013, NASA announced the planning stages of a mission to capture a nearEarth asteroid and move it into lunar orbit where it could possibly be visited by astronauts and later impacted into the Moon. On 19 June 2014, NASA reported that asteroid 2011 MD was a prime candidate for capture by a robotic mission, perhaps in the early 2020s.
It has been suggested that asteroids might be used as a source of materials that may be rare or exha |
usted 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 the U.S. Discovery program the Psyche spacecraft proposal to 16 Psyche and Lucy spacecraft to Jupiter trojans made it to the semifinalist stage of mission selection.
In January 2017, Lucy and Psyche mission were both selected as NASA's Discovery Program missions 13 and 14 respectively.
In November 2021, NASA launched its Double Asteroid Redirection Test DART, a mission to test technology for defending Earth against potential asteroids or comets.
Location of Ceres within asteroid belt compared to other bodies of the Solar System
Fiction
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 spacecraft traveling between two other points, and as a threat to life on Earth or other inhabited planets, dwarf planets, and natural satellites by potential impact.
Gallery
See also
Oumuamua
Active asteroid
Amor asteroid
Apollo asteroid
Asteroid Day
Asteroid impact avoidance
Asteroids in astrology
Aten asteroid
Atira asteroid
BOOTES Burst Observer and Optical Transient Exploring System
CategoryAsteroid groups and families
CategoryAsteroids
CategoryBinary asteroids
Centaur minor planet
Chang'e 2 lunar orbiter
Constellation program
Dawn spacecraft
Dwarf planet
Impact event
List of asteroid close approaches to Earth
List of exceptional asteroids
List of impact craters on Earth
List of minor planets
List of minor planets named after people
List of minor planets named after places
List of possible impact structures on Earth
Lost minor planet
Marco Polo spacecraft
Meanings of minor planet names
Mesoplanet
Meteoroid
Minor planet
NearEarth object
NEOShield
NEOSS |
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