Asteroids (from Greek 'star' and 'like, in form') are a class of Small Solar System Bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones. These terms have historically been applied to any astronomical object orbiting the Sun that did not show the disk of a planet and was not observed to have the characteristics of an active comet, but as small objects in the outer Solar System were discovered, their volatile-based surfaces were found to more closely resemble comets, and so were often distinguished from traditional asteroids. Thus the term ''asteroid'' has come increasingly to refer specifically to the small rocky–icy and metallic bodies of the inner Solar System out to the orbit of Jupiter. They are grouped with the outer bodies—centaurs, Neptune trojans, and trans-Neptunian objects—as minor planets, which is the term preferred in astronomical circles. This article will restrict the use of the term 'asteroid' to the minor planets of the inner Solar System.

There are millions of asteroids, and like most other Small Solar System Bodies, asteroids are thought to be the often shattered remnants of planetesimals, bodies within the young Sun’s solar nebula that never grew large enough to become planets. A large majority of known asteroids orbit in the asteroid belt between the orbits of Mars and Jupiter or co-orbital with Jupiter (the Jupiter Trojans). However, other orbital families exist with significant populations, including the near-Earth asteroids. Individual asteroids are classified by their characteristic spectra, with the majority falling into three main groups: C-type, S-type, and M-type. These were named after and are generally identified with carbon-rich, stony, and metallic compositions, respectively.

Naming

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

Symbols

The first asteroids to be discovered were assigned iconic symbols like the ones traditionally used to designate the planets. By 1855 there were two dozen asteroid symbols, which often occurred in several variants.

Old planetary symbol of Ceresx20px|Variant symbol of Ceresx20px|Other sickle variant symbol of Ceres || Ceres' scythe, reversed to double as the letter ''C''Old symbol of Pallasx20px|Variant symbol of Pallas|| Athena's (Pallas') spearOld symbol of Junox20px|Other symbol of Junox20px|| A star mounted on a scepter, for Juno, the Queen of HeavenImage:6 Hebe Astronomical Symbol.svgImage:9 Metis symbol.svgFile:11 Parthenope symbol alternate.svgAstronomical symbol of 13 Egeria || A shield, symbol of Egeria's protection, and a starAstronomical symbol of 14 Irene ||A dove carrying an olive-branch (symbol of ''irene'' 'peace') with a star on its head, or an olive branch, a flag of truce, and a starFile:19 Fortuna symbol.svgImage:28 Bellona symbol.svgImage:37 Fides symbol.svg

Asteroid	Symbol
Ceres (dwarf planet)	Ceres
2 Pallas
3 Juno
4 Vesta
5 Astraea
6 Hebe	x20px|| Hebe's cup
7 Iris	Image:7 Iris Astronomical Symbol.svg>x20px
8 Flora	Image:8 Flora Astronomical Symbol.svg>x20px
9 Metis	x20px|| The eye of wisdom and a star
10 Hygiea
11 Parthenope	x20pxx20px|| A harp, or a fish and a star;symbols of the sirens
12 Victoria	Image:12 Victoria symbol.svg>x20px
13 Egeria
14 Irene
15 Eunomia	Image:15 Eunomia symbol.svg>x20px
16 Psyche	File:16 Psyche symbol.svg>x20px
17 Thetis	Image:17 Thetis symbol.png>x20px
18 Melpomene	Image:18 Melpomene symbol.svg>x20px
19 Fortuna	x20px || The wheel of fortune and a star
26 Proserpina	Image:26 Proserpina symbol.svg>x20px
28 Bellona	x20px|| Bellona's whip and lance
29 Amphitrite	Image:29 Amphitrite symbol.svg>x20px
35 Leukothea	Image:35 Leukothea symbol.png>x20px
37 Fides	x20px|| The cross of faith (''fides'')

Johann Franz Encke made a major change in the 1854 edition of the ''Berliner Astronomisches Jahrbuch'' (BAJ, ''Berlin Astronomical Yearbook''). He introduced a disk (circle), a traditional symbol for a star, as the generic symbol for an asteroid. The circle was then numbered in order of discovery to indicate a specific asteroid, though he assigned ① to the fifth, Astraea, the first four continuing with their existing symbols. The numbered-circle convention was quickly adopted by the astronomical community, and no iconic symbols were created after 1855. That year Astraea's number was bumped up to ⑤, but Ceres through Vesta would not be listed by their numbers until the 1867 edition. The circle would become a pair of parentheses, and the parentheses sometimes omitted altogether over the next few decades, leading to the modern convention.

Discovery

The first asteroid to be discovered, Ceres, was found in 1801 by Giuseppe Piazzi, and 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", from Greek ''αστεροειδής'', ''asteroeidēs'' 'star-like, star-shaped', from ancient Greek ''αστήρ'', ''astēr'' 'star, planet'. In the early second half of the nineteenth century, the terms "asteroid" and "planet" (not always qualified as "minor") were still used interchangeably; for example, the ''Annual of Scientific Discovery for 1871'', page 316, reads "Professor J. Watson has been awarded by the Paris Academy of Sciences, the astronomical prize, Lalande foundation, for the discovery of eight new asteroids in one year. The planet Lydia (No. 110), discovered by M. Borelly at the Marseilles Observatory [...] M. Borelly had previously discovered two planets bearing the numbers 91 and 99 in the system of asteroids revolving between Mars and Jupiter".

Historical methods

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

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

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

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

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

In 1891, however, Max Wolf pioneered the use of astrophotography to detect asteroids, which appeared as short streaks on long-exposure photographic plates. This dramatically increased the rate of detection compared with earlier visual methods: Wolf alone discovered 248 asteroids, beginning with 323 Brucia, whereas only slightly more than 300 had been discovered up to that point. It was known that there were many more, but most astronomers did not bother with them, calling them "vermin of the skies", a phrase due to Edmund 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 four-step process. First, a region of the sky was photographed by a wide-field telescope, or Astrograph. Pairs of photographs were taken, typically one hour apart. Multiple pairs could be taken over a series of days. Second, the two films of the same region were viewed under a stereoscope. Any body in orbit around the Sun would move slightly between the pair of films. Under the stereoscope, the image of the body would seem to float slightly above the background of stars. Third, once a moving body was identified, its location would be measured precisely using a digitizing microscope. The location would be measured relative to known star locations.

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

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

[[Image:Asteroid 2004 FH.gif|framed|right| 2004 FH is the center dot being followed by the sequence; the object that flashes by during the clip is an artificial satellite.]] There is increasing interest in identifying asteroids whose orbits cross Earth's, and that could, given enough time, collide with Earth (see Earth-crosser asteroids). The three most important groups of near-Earth asteroids are the Apollos, Amors, and Atens. Various asteroid deflection strategies have been proposed, as early as the 1960s.

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

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

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

The Lincoln Near-Earth Asteroid Research (LINEAR) team

The Near-Earth Asteroid Tracking (NEAT) team

Spacewatch

The Lowell Observatory Near-Earth-Object Search (LONEOS) team

The Catalina Sky Survey (CSS)

The Campo Imperatore Near-Earth Objects Survey (CINEOS) team

The Japanese Spaceguard Association

The Asiago-DLR Asteroid Survey (ADAS)

The LINEAR system alone has discovered 121,346 asteroids, as of March, 2011. Among all the automated systems, 4711 near-Earth asteroids have been discovered including over 600 more than in diameter.

== Terminology ==

Traditionally, small bodies orbiting the Sun were classified as asteroids, comets or meteoroids, with anything smaller than ten metres across being called a meteoroid. The term "asteroid" is ill-defined. It never had a formal definition, with the broader term minor planet being preferred by the International Astronomical Union from 1853 on. In 2006, the term "Small Solar System Body" was introduced to cover both most minor planets and comets. Other languages prefer "planetoid" (Greek for "planet-like"), and this term is occasionally used in English for the larger asteroids. The word "planetesimal" has a similar meaning, but refers specifically to the small building blocks of the planets that existed when the Solar System was forming. The term "planetule" was coined by the geologist William Daniel Conybeare to describe minor planets, but is not in common use. The three largest objects in the asteroid belt, Ceres, 2 Pallas, and 4 Vesta, grew to the stage of protoplanets. Ceres has been designated as a dwarf planet, the only one in the inner Solar System.

When found, asteroids were seen as a class of objects distinct from comets, and there was no unified term for the two until "Small Solar System Body" was coined in 2006. The main difference between an asteroid and a comet is that a comet shows a coma due to sublimation of near surface ices by solar radiation. A few objects have ended up being dual-listed because they were first classified as minor planets but later showed evidence of cometary activity. Conversely, some (perhaps all) comets are eventually depleted of their surface volatile ices and become asteroids. A further distinction is that comets typically have more eccentric orbits than most asteroids; most "asteroids" with notably eccentric orbits are probably dormant or extinct comets.

For almost two centuries, from the discovery of Ceres in 1801 until the discovery of the first centaur, 2060 Chiron, in 1977, all known asteroids spent most of their time at or within the orbit of Jupiter, though a few such as 944 Hidalgo ventured far beyond Jupiter for part of their orbit. When astronomers started finding more small bodies that permanently resided further out than Jupiter, now called centaurs, they numbered them among the traditional asteroids, though there was debate over whether they should be classified as asteroids or as a new type of object. Then, when the first trans-Neptunian object, 1992 QB1, was discovered in 1992, and especially when large numbers of similar objects started turning up, new terms were invented to sidestep the issue: Kuiper-belt object, trans-Neptunian object, scattered-disc object, and so on. These inhabit the cold outer reaches of the Solar System where ices remain solid and comet-like bodies are not expected to exhibit much cometary activity; if centaurs or trans-Neptunian objects were to venture close to the Sun, their volatile ices would sublimate, and traditional approaches would classify them as comets and not asteroids.

The innermost of these are the Kuiper-belt objects, called "objects" partly to avoid the need to classify them as asteroids or comets. They are believed to be predominantly comet-like 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, suggesting "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 near-Earth objects.

When the IAU introduced the class Small Solar System Bodies in 2006 to include most objects previously classified as minor planets and comets, they created the class of dwarf planets for the largest minor planets—those that have enough mass to have become ellipsoidal under their own gravity. According to the IAU, "the term 'minor planet' may still be used, but generally the term 'Small Solar System Body' will be preferred." Currently only the largest object in the asteroid belt, Ceres, at about across, has been placed in the dwarf planet category, although there are several large asteroids (Vesta, Pallas, and Hygiea) that may be classified as dwarf planets when their shapes are better known.

Formation

It is believed that planetesimals in the asteroid belt evolved much like the rest of the solar nebula until Jupiter neared its current mass, at which point excitation from orbital resonances with Jupiter ejected over 99% of planetesimals in the belt. Simulations and a discontinuity in spin rate and spectral properties suggest that asteroids larger than approximately 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 Kuiper-belt objects are captured in the outer asteroid belt, at distances greater than 2.6 AU. Most were later ejected by Jupiter, but those that remained may be the D-type asteroids, and possibly include Ceres.

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 low-eccentricity (i.e., not very elongated) orbits. This belt is now estimated to contain between 1.1 and 1.9 million asteroids larger than 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

Trojan asteroids are a population that share an orbit with a larger planet or moon, but do not collide with it because they orbit in one of the two Lagrangian points of stability, L4 and L5, which lie 60° ahead of and behind the larger body.

The most significant population of Trojan asteroids are the Jupiter Trojans. Although fewer Jupiter Trojans have been discovered as of 2010, it is thought that they are as numerous as the asteroids in the asteroid belt.

A couple trojans have also been found orbiting with Mars.

Near-Earth asteroids

Near-Earth asteroids, or NEA's, are asteroids that have orbits that pass close to that of Earth. Asteroids that actually cross the Earth's orbital path are known as ''Earth-crossers''. As of May 2010, 7,075 near-Earth asteroids are known and the number over one kilometre in diameter is estimated to be 500–1,000.

Characteristics

Size distribution

Objects in the asteroid belt vary greatly in size, from almost 1000 kilometres for the largest down to rocks just tens of metres 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 surviving planetesimals or fragments of larger bodies.

[[image:masses of asteroids vs main belt.png|thumb|left|The relative masses of the nine largest main-belt asteroids for which precise data is available, compared to the remaining mass of the asteroid belt. ]]

The dwarf planet Ceres is the largest object in the asteroid belt, with a diameter of . The next largest are the asteroids 2 Pallas and 4 Vesta, both with diameters of just over . Vesta is the only main-belt asteroid that can, on occasion, become visible to the naked eye. On some rare occasions, a near-Earth asteroid may briefly become visible without technical aid; see 99942 Apophis.

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

The number of asteroids decreases markedly with size. Although this generally follows a power law, there are 'bumps' at 5 km and 100 km, where more asteroids than expected from a logarithmic distribution are found.

{|class=wikitable align=center |+Approximate number of asteroids N larger than diameter D |- !D |100 m || 300 m || 500 m || 1 km || 3 km || 5 km || 10 km || 30 km || 50 km || 100 km || 200 km || 300 km || 500 km || 900 km |- !N | ~25,000,000 || 4,000,000 || 2,000,000 || 750,000 || 200,000 || 90,000 || 10,000 || 1100 || 600 || 200 || 30 || 5 || 3 || 1 |}

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 nickel-iron 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. Most of the smaller asteroids are thought to be piles of rubble held together loosely by gravity, though the largest are probably solid. Some asteroids have moons or are co-orbiting binaries: Rubble piles, moons, binaries, and scattered asteroid families are believed to be the results of collisions that disrupted a parent asteroid.

Asteroids contain traces of amino-acids and other organic compounds, and some speculate that asteroid impacts may have seeded the early Earth with the chemicals necessary to initiate life, or may have even brought life itself to Earth. (See also panspermia.) In August 2011, a report, based on NASA studies with meteorites found on Earth, was published suggesting DNA and RNA components (adenine, guanine and related organic molecules) may have been formed on asteroids and comets in outer space.

Only one asteroid, 4 Vesta, which has a reflective surface, is normally visible to the naked eye, and this only in very dark skies when it is favorably positioned. Rarely, small asteroids passing close to Earth may be naked-eye visible for a short time.

Composition is calculated from three primary sources: albedo, surface spectrum, and density. The last can only be determined accurately by observing the orbits of moons the asteroid might have. So far, every asteroid with moons has turned out to be a rubble pile, a loose conglomeration of rock and metal that may be half empty space by volume. The investigated asteroids are as large as 280 km in diameter, and include 121 Hermione (268×186×183 km), and 87 Sylvia (384×262×232 km). Only half a dozen asteroids are larger than 87 Sylvia, though none of them have moons; however, some smaller asteroids are thought to be more massive, suggesting they may not have been disrupted, and indeed 511 Davida, the same size as Sylvia to within measurement error, is estimated to be two and a half times as massive, though this is highly uncertain. The fact that such large asteroids as Sylvia can be rubble piles, presumably due to disruptive impacts, has important consequences for the formation of the Solar system: Computer simulations of collisions involving solid bodies show them destroying each other as often as merging, but colliding rubble piles are more likely to merge. This means that the cores of the planets could have formed relatively quickly.

Surface features

Most asteroids outside the big four (Ceres, Pallas, Vesta, and Hygiea) are likely to be broadly similar is appearance, if irregular in shape. 50-km 253 Mathilde (shown at right) is a rubble pile saturated with craters with diameters the size of the asteroid's radius, and Earth-based observations of 300-km 511 Davida, one of the largest asteroids after the big four, reveal a similarly angular profile, suggesting it is also saturated with radius-size craters. Medium-sized asteroids such as Mathilde and 243 Ida that have been observed up close also reveal a deep regolith covering the surface. Of the big four, Pallas and Hygiea are practically unknown. Vesta has compression fractures encircling a radius-size crater at its south pole but is otherwise a spheroid. Ceres seems quite different in the glimpses Hubble has provided, with surface features that are unlikely to be due to simple craters and impact basins, but details will not be known until ''Dawn'' arrives in 2015.

Classification

Asteroids are commonly classified 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 associations, whereas families are tighter and result from the catastrophic break-up of a large parent asteroid sometime in the past. Families have only been recognized within the asteroid belt. They were first recognised by Kiyotsugu Hirayama in 1918 and are often called Hirayama families in his honor.

About 30% to 35% of the bodies in the asteroid belt belong to dynamical families each thought to have a common origin in a past collision between asteroids. A family has also been associated with the plutoid dwarf planet .

Quasi-satellites and horseshoe objects

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

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

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

Spectral classification

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

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

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

Problems

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 is in use. This has led to significant confusion. While asteroids of different spectral classifications are likely to be composed of different materials, there are no assurances that asteroids within the same taxonomic class are composed of similar materials.

At present, the spectral classification based on several coarse resolution spectroscopic surveys in the 1990s is still the standard. Scientists cannot agree on a better taxonomic system, largely due to the difficulty of obtaining detailed measurements consistently for a large sample of asteroids (e.g. finer resolution spectra, or non-spectral data such as densities would be very useful).

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 ground-based telescopes and the Earth-orbiting Hubble Space Telescope can resolve a small amount of detail on the surfaces of the largest asteroids, but even these mostly remain little more than fuzzy blobs. Limited information about the shapes and compositions of asteroids can be inferred from their light curves (their variation in brightness as they rotate) and their spectral properties, and asteroid sizes can be estimated by timing the lengths of star occulations (when an asteroid passes directly in front of a star). Radar imaging can yield good information about asteroid shapes and orbital and rotational parameters, especially for near-Earth asteroids. In terms of delta v and propellant requirements, NEOs are more easily accessible than the Moon.

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

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

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

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

In September 2005, the Japanese Hayabusa probe started studying 25143 Itokawa in detail and was plagued with difficulties, but returned samples of its surface to earth on June 13, 2010.

The European Rosetta probe (launched in 2004) flew by 2867 Šteins in 2008 and 21 Lutetia, the second-largest asteroid visited to date, in 2010.

In September 2007, NASA launched the Dawn Mission, which started orbiting the protoplanet 4 Vesta in July 2011, and is to orbit 1 Ceres in 2015. 4 Vesta is the largest asteroid visited to date.

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

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 spaceships traveling between two other points, and as a threat to life on Earth by potential impact.

See also

Asteroid belt

Asteroid mining

Mission Marco Polo

Rosetta probe

BOOTES (Burst Observer and Optical Transient Exploring System)

Category:Asteroid groups and families

Category:Asteroids

Category:Binary asteroids

Centaur (minor planet)

Dwarf planet

Impact event

Asteroid deflection strategies

List of asteroids named after people

List of asteroids named after places

List of minor planets

List of notable asteroids

Meanings of asteroid names

Mesoplanet

Minor planet

Minor Planet Center

Near-Earth object

Pioneer 10 space probe

Pronunciation of asteroid names

Notes

References

External links

Rocks from the Main Belt asteroids

Alphabetical list of minor planet names (ASCII) (Minor Planet Center)

Near Earth Asteroid Tracking (NEAT)

Asteroids Page at NASA's Solar System Exploration

Asteroid Simulator with Moon and Earth

Everything you wanted to know about comets and asteroids – Provided by ''New Scientist''.

Alphabetical and numerical lists of minor planet names (Unicode) (Institute of Applied Astronomy)

Future Asteroid Interception Research

Near Earth Objects Dynamic Site

Asteroids Dynamic Site Up-to-date osculating orbital elements and proper orbital elements University of Pisa, Italy.

JPL small bodies database Current down-loadable ASCII table of orbit data and absolute mags H for over 200000 asteroids, sorted by number. Caltech/JPL.

Asteroid naming statistics

Spaceguard UK

Large amount of information on asteroid groups collected by Gérard Faure, translation Richard Miles.

1908 Siberian asteroid

Committee on Small Body Nomenclature

List of minor planet orbital groupings and families from ProjectPluto

Cunningham, Clifford, "Introduction to Asteroids: The Next Frontier", ISBN 0-943396-16-6

James L. Hilton: When Did the Asteroids Become Minor Planets?

Kirkwood, Daniel; ''Relations between the Motions of some of the Minor Planets'' (1874).

Schmadel, L.D. (2003). ''Dictionary of Minor Planet Names.'' 5th ed. IAU/Springer-Verlag: Heidelberg.

Asteroid articles in Planetary Science Research Discoveries

Catalogue of the Solar System Small Bodies Orbital Evolution

Category:Spaceflight

af:Asteroïde als:Asteroid ar:كويكب an:Asteroide ast:Asteroide az:Asteroid bn:গ্রহাণু zh-min-nan:Sió-he̍k-chheⁿ be:Астэроід be-x-old:Астэроід bar:Asteroid bs:Asteroidi br:Asteroidenn bg:Астероид ca:Asteroide cv:Астероид cs:Asteroid cy:Asteroid da:Asteroide de:Asteroid et:Asteroid el:Αστεροειδής eml:Asteròid es:Asteroide eo:Asteroido eu:Asteroide fa:سیارک hif:Chhota tara fr:Astéroïde fy:Asteroïde ga:Astaróideach gl:Asteroide ko:소행성 hi:क्षुद्रग्रह hr:Asteroidi io:Asteroido-zono id:Asteroid ia:Asteroide os:Астероид is:Smástirni it:Asteroide he:אסטרואיד jv:Asteroid kn:ಕ್ಷುದ್ರ ಗ್ರಹ pam:Asteroid krc:Астероид ka:მცირე ცთომილები kk:Астероид ku:Asteroîd sw:Asteroidi ht:Astewoyid la:Asteroides lv:Asteroīds lb:Asteroid lt:Asteroidas li:Planetoïed hu:Kisbolygó mk:Астероид ml:ഛിന്നഗ്രഹം mt:Asterojde ms:Asteroid nl:Planetoïde ja:小惑星 no:Asteroide nn:Asteroide oc:Asteroïde uz:Asteroid pnb:تارے ورگا nds:Asteroid pl:Planetoida pt:Asteroide ksh:Asteroid ro:Asteroid qu:Puriq quyllurcha rue:Астероід ru:Астероид sah:Астероид stq:Planetoiden sq:Asteroidet scn:Astiròidi simple:Asteroid sk:Asteroid sl:Asteroid sr:Астероид sh:Asteroid su:Astéroid fi:Asteroidi sv:Asteroid tl:Asteroyd ta:சிறுகோள் tt:Asteroidlar th:ดาวเคราะห์น้อย tr:Asteroyit tk:Asteroid uk:Астероїд ur:نجمانی vec:Asteroide vi:Tiểu hành tinh vls:Asteroïde war:Asteroyd yi:אסטערויד zh-yue:小行星 bat-smg:Asteruoids zh:小行星

name	Philip Cary Plait
birth date
birth place	Washington, D. C.
death date
residence	Boulder, Colorado
citizenship	United States of America
nationality	American
fields	Physics, astronomy, science communication
workplaces	Sonoma State University
signature
spouse	}}

Dr. Philip Cary Plait (a.k.a. The Bad Astronomer) is an American astronomer and skeptic who runs the website BadAstronomy.com. He formerly worked at the physics and astronomy department at Sonoma State University. In early 2007, he resigned from his job to write ''Death from the Skies''. On August 4, 2008, he became President of the James Randi Educational Foundation. He served in that position until January 1, 2010, when he was succeeded by noted skeptic D. J. Grothe.

Biography

Plait grew up in the Washington, D.C. area. He received his Ph.D. in astronomy from the University of Virginia in 1994 with a thesis on SN 1987A, which he studied with the Supernova Intensive Study (SINS). He first worked with the COBE satellite and then with the STIS on the Hubble Space Telescope for five years. He currently resides in Boulder, Colorado and writes full time, but often hosts special events and serves as an adviser and commentator in several capacities, including events focusing on skepticism.

Work

Plait performed web-based public outreach for the Gamma-ray Large Area Space Telescope (renamed Fermi Gamma-ray Space Telescope upon launch in 2008) and other NASA-funded missions while at Sonoma State University from 2000 to 2007. Prior to that, during the 1990s, he was part of the Hubble Space Telescope team at NASA Goddard Space Flight Center, working largely on the Space Telescope Imaging Spectrograph.

Phil Plait has been featured on TV and on such websites as Space.com. He has also appeared on two Penn & Teller: Bullshit! episodes. On season 3 he argued against the Apollo moon landing hoax accusations and on season 7 against astrology. His work has also appeared in the ''Encyclopedia Britannica Yearbook of Science and the Future'' and ''Astronomy'' magazine. He has a regular column in Night Sky magazine (an offshoot of ''Sky and Telescope'') called "Straight Talk", which has appeared since the magazine's first issue.

Plait is also a frequent guest on the SETI Institute's weekly science radio show ''Are We Alone?'' hosted by Seth Shostak and Molly Bentley. He can be heard on many ''Are We Alone?'' installments debunking junk science and waxing poetic on the sublime nature of the universe.

His first book, ''Bad Astronomy: Misconceptions and Misuses Revealed, from Astrology to the Moon Landing "Hoax"'' (Wiley, 2002), deals with much the same subject matter as his website. His second book, ''Death from the Skies'', details multiple ways astronomical events could wipe out life on Earth and was released in October 2008. ''Bad Astronomy'' was the first in a science book series on myths and misconceptions that includes ''Bad Medicine'' (Wiley, 2003).

In March 2008, Plait had an asteroid named after him by the late astronomer Jeff Medkeff. Asteroid 2000 WG11 was renamed 165347 Philplait.

Badastronomy.com

Badastronomy.com is a website dedicated to clearing up public misconceptions about astronomy and space science in movies, the news, print, and on the Internet. Plait also debunks several pseudoscientific theories related to space and astronomy, such as Planet X, Richard Hoagland's theories, and most famously, the moon landing "hoax". In March 2006, ''Science'' magazine's NetWatch feature lauded the Bad Astronomy website, praising in particular the Bad Astronomy Blog which Plait began in March 2005. This blog was also a finalist for the 2006 Weblog Award (the "Bloggie"), in the "best topical weblog" category since the Bloggies, like many blog awards, do not have a specific science category. Plait used to write the occasional science article for ''The Huffington Post''. However he claims that he was "embarrassed" by this and now criticizes them for their stance on anti-vaccination and New Age antiscience.

On January 28, 2007, a new segment of the blog called ''Q & BA'' was announced. The feature consists of a video log where questions sent by the blog's readers are answered. New videos were originally posted every Monday, but because of time management issues, Plait decided to temporarily stop making new videos.

In early September 2005, the Bad Astronomy message boards were merged with the Universe Today message boards to form a more robust, combined site, titled ''BAUT forum'', standing for the "Bad Astronomy and Universe Today Message Board." On July 1, 2008, the Bad Astronomy blog became integrated with ''Discover Magazine''.

Plait is interviewed on the Monster Talk podcast, with Blake Smith, Karen Stollznow, and Ben Radford. Plait talks Martian bacteria, Godzilla, and about the movie Five Million Years to Earth.

Discovery Channel Series

On July 22, 2010, Discovery Channel released a short video on YouTube of an upcoming television series called Phil Plait's ''Bad Universe''. Plait later announced it was a three-part documentary series first airing in the United States August 29, 2010.

Personal Life

Plait is married and has one daughter.

Recognition

The BA blog was a co-winner of best science blog in the 2007 Weblog Awards. Time.com's 25 Best blogs 2009 The Committee for Skeptical Inquiry lists Plait as one of their fellows.

Books

Philip Plait (2008). ''Death from the Skies!: These are the Ways the World Will End'' Viking Press, ISBN 9780670019977.

Philip Plait (2002). ''Bad Astronomy: Misconceptions and Misuses Revealed, from Astrology to the Moon Landing "Hoax"''. John Wiley & Sons, ISBN 0-471-40976-6.

References

External links

Bad Astronomy Website

Bad Astronomy Blog

Bad Astronomy and Universe Today Message Board

Collection of Phil Plait audio files

"Science is under attack", by Philip Plait

NetWatch, ''Science'' 31 March 2006: Vol. 311. no. 5769, p. 1841.

Audio - Phil Plait/Pamela Gay Astronomy Cast Death From The Skies - Interview With Phil Plait

Interview with Phil on the American Freethought Podcast

Interview with Dr. Phil Plait on Skeptiko

Category:American astronomers Category:Living people Category:American skeptics Category:Science bloggers Category:Space advocates Category:University of Michigan alumni Category:University of Virginia alumni Category:Year of birth missing (living people) Category:American science writers

es:Phil Plait fr:Philip Plait no:Phil Plait sv:Phil Plait

name	Myocardial infarction
diseasesdb	8664
icd10	-
icd9
medlineplus	000195
emedicinesubj	med
emedicinetopic	1567
emedicine mult
meshid	D009203 }}

Myocardial infarction (MI) or acute myocardial infarction (AMI), commonly known as a heart attack, results from the interruption of blood supply to a part of the heart, causing heart cells to die. This is most commonly due to occlusion (blockage) of a coronary artery following the rupture of a vulnerable atherosclerotic plaque, which is an unstable collection of lipids (cholesterol and fatty acids) and white blood cells (especially macrophages) in the wall of an artery. The resulting ischemia (restriction in blood supply) and ensuing oxygen shortage, if left untreated for a sufficient period of time, can cause damage or death (''infarction'') of heart muscle tissue (''myocardium'').

Classical symptoms of acute myocardial infarction include sudden chest pain (typically radiating to the left arm or left side of the neck), shortness of breath, nausea, vomiting, palpitations, sweating, and anxiety (often described as a sense of impending doom). Women may experience fewer typical symptoms than men, most commonly shortness of breath, weakness, a feeling of indigestion, and fatigue. Approximately one-quarter of all myocardial infarctions are "silent", that is without chest pain or other symptoms.

Among the diagnostic tests available to detect heart muscle damage are an electrocardiogram (ECG), echocardiography, cardiac MRI and various blood tests. The most often used blood markers are the creatine kinase-MB (CK-MB) fraction and the troponin levels. Immediate treatment for suspected acute myocardial infarction includes oxygen, aspirin, and sublingual nitroglycerin.

Most cases of STEMI (ST elevation MI) are treated with thrombolysis or percutaneous coronary intervention (PCI). NSTEMI (non-ST elevation MI) should be managed with medication, although PCI is often performed during hospital admission. In people who have multiple blockages and who are relatively stable, or in a few emergency cases, bypass surgery may be an option, especially in diabetics.

Ischemic heart disease (which includes myocardial infarction, angina pectoris and heart failure when preceded by myocardial infarction) was the leading cause of death for both men and women worldwide in 2004. Important risk factors are previous cardiovascular disease, older age, tobacco smoking, high blood levels of certain lipids (triglycerides, low-density lipoprotein) and low levels of high density lipoprotein (HDL), diabetes, high blood pressure, obesity, chronic kidney disease, heart failure, excessive alcohol consumption, the abuse of certain drugs (such as cocaine and methamphetamine), and chronic high stress levels.

Classification

There are two basic types of acute myocardial infarction: Transmural: associated with atherosclerosis involving major coronary artery. It can be subclassified into anterior, posterior, inferior, lateral or septal. Transmural infarcts extend through the whole thickness of the heart muscle and are usually a result of complete occlusion of the area's blood supply. Subendocardial: involving a small area in the subendocardial wall of the left ventricle, ventricular septum, or papillary muscles. The subendocardial area is particularly susceptible to ischemia.

In the clinical context, a myocardial infarction can be further subclassified into a ST elevation MI (STEMI) versus a non-ST elevation MI (non-STEMI) based on ECG changes.

The phrase ''heart attack'' is sometimes used incorrectly to describe sudden cardiac death, which may or may not be the result of acute myocardial infarction. A heart attack is different from, but can be the cause of cardiac arrest, which is the stopping of the heartbeat, and cardiac arrhythmia, an abnormal heartbeat. It is also distinct from heart failure, in which the pumping action of the heart is impaired; however severe myocardial infarction may lead to heart failure.

A 2007 consensus document classifies myocardial infarction into five main types:

Type 1 – Spontaneous myocardial infarction related to ischaemia due to a primary coronary event such as plaque erosion and/or rupture, fissuring, or dissection

Type 2 – Myocardial infarction secondary to ischaemia due to either increased oxygen demand or decreased supply, e.g. coronary artery spasm, coronary embolism, anaemia, arrhythmias, hypertension, or hypotension

Type 3 – Sudden unexpected cardiac death, including cardiac arrest, often with symptoms suggestive of myocardial ischaemia, accompanied by new ST elevation, or new LBBB, or evidence of fresh thrombus in a coronary artery by angiography and/or at autopsy, but death occurring before blood samples could be obtained, or at a time before the appearance of cardiac biomarkers in the blood

Type 4 – Associated with coronary angioplasty or stents:

* Type 4a – Myocardial infarction associated with PCI

* Type 4b – Myocardial infarction associated with stent thrombosis as documented by angiography or at autopsy

Type 5 – Myocardial infarction associated with CABG

Signs and symptoms

The onset of symptoms in myocardial infarction (MI) is usually gradual, over several minutes, and rarely instantaneous. Chest pain is the most common symptom of acute myocardial infarction and is often described as a sensation of tightness, pressure, or squeezing. Chest pain due to ischemia (a lack of blood and hence oxygen supply) of the heart muscle is termed angina pectoris. Pain radiates most often to the left arm, but may also radiate to the lower jaw, neck, right arm, back, and epigastrium, where it may mimic heartburn. Levine's sign, in which the patient localizes the chest pain by clenching their fist over the sternum, has classically been thought to be predictive of cardiac chest pain, although a prospective observational study showed that it had a poor positive predictive value.

Shortness of breath (dyspnea) occurs when the damage to the heart limits the output of the left ventricle, causing left ventricular failure and consequent pulmonary edema. Other symptoms include diaphoresis (an excessive form of sweating), weakness, light-headedness, nausea, vomiting, and palpitations. These symptoms are likely induced by a massive surge of catecholamines from the sympathetic nervous system which occurs in response to pain and the hemodynamic abnormalities that result from cardiac dysfunction. Loss of consciousness (due to inadequate cerebral perfusion and cardiogenic shock) and sudden death (frequently due to the development of ventricular fibrillation) can occur in myocardial infarctions.

Women and older patients report atypical symptoms more frequently than their male and younger counterparts. Women also report more numerous symptoms compared with men (2.6 on average vs 1.8 symptoms in men). The most common symptoms of MI in women include dyspnea (shortness of breath), weakness, and fatigue. Fatigue, sleep disturbances, and dyspnea have been reported as frequently occurring symptoms that may manifest as long as one month before the actual clinically manifested ischemic event. In women, chest pain may be less predictive of coronary ischemia than in men.

Approximately one-fourth of all myocardial infarctions are silent, without chest pain or other symptoms. These cases can be discovered later on electrocardiograms, using blood enzyme tests or at autopsy without a prior history of related complaints. A silent course is more common in the elderly, in patients with diabetes mellitus and after heart transplantation, probably because the donor heart is not fully innervated by the nervous system of the recipient. In diabetics, differences in pain threshold, autonomic neuropathy, and psychological factors have been cited as possible explanations for the lack of symptoms.

Any group of symptoms compatible with a sudden interruption of the blood flow to the heart are called an acute coronary syndrome.

The differential diagnosis includes other catastrophic causes of chest pain, such as pulmonary embolism, aortic dissection, pericardial effusion causing cardiac tamponade, tension pneumothorax, and esophageal rupture. Other non-catastrophic differentials include gastroesophageal reflux and Tietze's syndrome.

Causes

Heart attack rates are higher in association with intense exertion, be it psychological stress or physical exertion, especially if the exertion is more intense than the individual usually performs. The period of intense exercise and subsequent recovery is associated with about a 6-fold higher myocardial infarction rate (compared with other more relaxed time frames) for people who are physically very fit. For those in poor physical condition, the rate differential is over 35-fold higher. One observed mechanism for this phenomenon is the increased arterial pulse pressure stretching and relaxation of arteries with each heart beat, which, as has been observed with intravascular ultrasound, increases mechanical "shear stress" on atheromas and the likelihood of plaque rupture.

Acute severe infection, such as pneumonia, can trigger myocardial infarction. A more controversial link is that between ''Chlamydophila pneumoniae'' infection and atherosclerosis. While this intracellular organism has been demonstrated in atherosclerotic plaques, evidence is inconclusive as to whether it can be considered a causative factor. Treatment with antibiotics in patients with proven atherosclerosis has not demonstrated a decreased risk of heart attacks or other coronary vascular diseases.

There is an association of an increased incidence of a heart attack in the morning hours, more specifically around 9 a.m. Some investigators have noticed that the ability of platelets to aggregate varies according to a circadian rhythm, although they have not proven causation.

Risk factors

In general, risk factors for atherosclerosis are risk factors for myocardial infarction:

Age — Men acquire an independent risk factor at age 45, Women acquire an independent risk factor at age 55; in addition, individuals acquire another independent risk factor if they have a first-degree male relative (brother, father) who suffered a coronary vascular event at or before age 55. Another independent risk factor is acquired if one has a first-degree female relative (mother, sister) who suffered a coronary vascular event at age 65 or younger.

Males are more at risk than females.

Diabetes (with or without insulin resistance)

High blood pressure

Dyslipidemia/hypercholesterolemia (abnormal levels of lipoproteins in the blood), particularly high low-density lipoprotein, low high-density lipoprotein and high Triglycerides

Tobacco smoking including secondhand smoke

Air pollution

Family history of ischaemic heart disease (IHD)

Obesity (defined by a body mass index of more than 30 kg/m², or alternatively by waist circumference or waist-hip ratio).

Hyperhomocysteinemia (high homocysteine)

Stress — Occupations with high stress index are known to have susceptibility for atherosclerosis

Alcohol — Studies show that prolonged exposure to high quantities of alcohol can increase the risk of heart attack

Many of these risk factors are modifiable, so many heart attacks can be prevented by maintaining a healthier lifestyle. Physical activity, for example, is associated with a lower risk profile. Non-modifiable risk factors include age, sex, and family history of an early heart attack (before the age of 60), which is thought of as reflecting a genetic predisposition.

Socioeconomic factors such as a shorter education and lower income (particularly in women), and unmarried cohabitation are also correlated with a higher risk of MI. To understand epidemiological study results, it is important to note that many factors associated with MI mediate their risk via other factors. For example, the effect of education is partially based on its effect on income and marital status.

Women who use combined oral contraceptive pills have a modestly increased risk of myocardial infarction, especially in the presence of other risk factors, such as smoking.

Inflammation is known to be an important step in the process of atherosclerotic plaque formation. C-reactive protein (CRP) is a sensitive but non-specific marker for inflammation. Elevated CRP blood levels, especially measured with high-sensitivity assays, can predict the risk of MI, as well as stroke and development of diabetes. Moreover, some drugs for MI might also reduce CRP levels. The use of high-sensitivity CRP assays as a means of screening the general population is advised against, but it may be used optionally at the physician's discretion in patients who already present with other risk factors or known coronary artery disease. Whether CRP plays a direct role in atherosclerosis remains uncertain.

Inflammation in periodontal disease may be linked to coronary heart disease, and, since periodontitis is very common, this could have great consequences for public health. Serological studies measuring antibody levels against typical periodontitis-causing bacteria found that such antibodies were more present in subjects with coronary heart disease. Periodontitis tends to increase blood levels of CRP, fibrinogen and cytokines; thus, periodontitis may mediate its effect on MI risk via other risk factors. Preclinical research suggests that periodontal bacteria can promote aggregation of platelets and promote the formation of foam cells. A role for specific periodontal bacteria has been suggested but remains to be established. There is some evidence that influenza may trigger an acute myocardial infarction.

Baldness, hair greying, a diagonal earlobe crease (Frank's sign) and possibly other skin features have been suggested as independent risk factors for MI. Their role remains controversial; a common denominator of these signs and the risk of MI is supposed, possibly genetic.

Calcium deposition is another part of atherosclerotic plaque formation. Calcium deposits in the coronary arteries can be detected with CT scans. Several studies have shown that coronary calcium can provide predictive information beyond that of classical risk factors.

The European Society of Cardiology and the European Association for Cardiovascular Prevention and Rehabilitation have developed an interactive tool for prediction and managing the risk of heart attack and stroke in Europe. HeartScore is aimed at supporting clinicians in optimising individual cardiovascular risk reduction. The Heartscore Programme is available in 12 languages and offers web-based or PC version.

Pathophysiology

Acute myocardial infarction refers to two subtypes of acute coronary syndrome, namely non-ST-elevated myocardial infarction and ST-elevated myocardial infarction, which are most frequently (but not always) a manifestation of coronary artery disease. The most common triggering event is the disruption of an atherosclerotic plaque in an epicardial coronary artery, which leads to a clotting cascade, sometimes resulting in total occlusion of the artery. Atherosclerosis is the gradual buildup of cholesterol and fibrous tissue in plaques in the wall of arteries (in this case, the coronary arteries), typically over decades. Blood stream column irregularities visible on angiography reflect artery lumen narrowing as a result of decades of advancing atherosclerosis. Plaques can become unstable, rupture, and additionally promote a thrombus (blood clot) that occludes the artery; this can occur in minutes. When a severe enough plaque rupture occurs in the coronary vasculature, it leads to myocardial infarction (necrosis of downstream myocardium).

If impaired blood flow to the heart lasts long enough, it triggers a process called the ischemic cascade; the heart cells in the territory of the occluded coronary artery die (chiefly through necrosis) and do not grow back. A collagen scar forms in its place. Recent studies indicate that another form of cell death called apoptosis also plays a role in the process of tissue damage subsequent to myocardial infarction. As a result, the patient's heart will be permanently damaged. This myocardial scarring also puts the patient at risk for potentially life threatening arrhythmias, and may result in the formation of a ventricular aneurysm that can rupture with catastrophic consequences.

Injured heart tissue conducts electrical impulses more slowly than normal heart tissue. The difference in conduction velocity between injured and uninjured tissue can trigger re-entry or a feedback loop that is believed to be the cause of many lethal arrhythmias. The most serious of these arrhythmias is ventricular fibrillation (''V-Fib''/VF), an extremely fast and chaotic heart rhythm that is the leading cause of sudden cardiac death. Another life-threatening arrhythmia is ventricular tachycardia (''V-Tach''/VT), which may or may not cause sudden cardiac death. However, ventricular tachycardia usually results in rapid heart rates that prevent the heart from pumping blood effectively. Cardiac output and blood pressure may fall to dangerous levels, which can lead to further coronary ischemia and extension of the infarct.

The cardiac defibrillator is a device that was specifically designed to terminate these potentially fatal arrhythmias. The device works by delivering an electrical shock to the patient in order to depolarize a critical mass of the heart muscle, in effect "rebooting" the heart. This therapy is time dependent, and the odds of successful defibrillation decline rapidly after the onset of cardiopulmonary arrest.

Diagnosis

The diagnosis of myocardial infarction can be made after assessing patient's complaints and physical status. ECG changes, coronary angiogram and levels of cardiac markers help to confirm the diagnosis. ECG gives valuable clues to identify the site of myocardial damage while coronary angiogram allows visualization of narrowing or obstructions in the heart vessels. At autopsy, a pathologist can diagnose a myocardial infarction based on anatomopathological findings.

A chest radiograph and routine blood tests may indicate complications or precipitating causes and are often performed upon arrival to an emergency department. New regional wall motion abnormalities on an echocardiogram are also suggestive of a myocardial infarction. Echo may be performed in equivocal cases by the on-call cardiologist. In stable patients whose symptoms have resolved by the time of evaluation, Technetium (99mTc) sestamibi (i.e. a "MIBI scan") or thallium-201 chloride can be used in nuclear medicine to visualize areas of reduced blood flow in conjunction with physiologic or pharmocologic stress. Thallium may also be used to determine viability of tissue, distinguishing whether non-functional myocardium is actually dead or merely in a state of hibernation or of being stunned.

WHO criteria formulated in 1979 have classically been used to diagnose MI; a patient is diagnosed with myocardial infarction if two (probable) or three (definite) of the following criteria are satisfied: # Clinical history of ischaemic type chest pain lasting for more than 20 minutes # Changes in serial ECG tracings # Rise and fall of serum cardiac biomarkers such as creatine kinase-MB fraction and troponin

The WHO criteria were refined in 2000 to give more prominence to cardiac biomarkers. According to the new guidelines, a cardiac troponin rise accompanied by either typical symptoms, pathological Q waves, ST elevation or depression, or coronary intervention is diagnostic of MI.

Prevention

The risk of a recurrent myocardial infarction decreases with strict blood pressure management and lifestyle changes, chiefly smoking cessation, regular exercise, a sensible diet for those with heart disease, and limitation of alcohol intake. People are usually commenced on several long-term medications post-MI, with the aim of preventing secondary cardiovascular events such as further myocardial infarctions, congestive heart failure or cerebrovascular accident (CVA). Unless contraindicated, such medications may include:

Antiplatelet drug therapy such as aspirin and/or clopidogrel should be continued to reduce the risk of plaque rupture and recurrent myocardial infarction. Aspirin is first-line, owing to its low cost and comparable efficacy, with clopidogrel reserved for patients intolerant of aspirin. The combination of clopidogrel and aspirin may further reduce risk of cardiovascular events, however the risk of hemorrhage is increased. Beta blocker therapy such as metoprolol or carvedilol should be commenced. These have been particularly beneficial in high-risk patients such as those with left ventricular dysfunction and/or continuing cardiac ischaemia. β-Blockers decrease mortality and morbidity. They also improve symptoms of cardiac ischemia in NSTEMI. ACE inhibitor therapy should be commenced 24–48 hours post-MI in hemodynamically-stable patients, particularly in patients with a history of MI, diabetes mellitus, hypertension, anterior location of infarct (as assessed by ECG), and/or evidence of left ventricular dysfunction. ACE inhibitors reduce mortality, the development of heart failure, and decrease ventricular remodelling post-MI. Statin therapy has been shown to reduce mortality and morbidity post-MI. The effects of statins may be more than their LDL lowering effects. The general consensus is that statins have plaque stabilization and multiple other ("pleiotropic") effects that may prevent myocardial infarction in addition to their effects on blood lipids. The aldosterone antagonist agent eplerenone has been shown to further reduce risk of cardiovascular death post-MI in patients with heart failure and left ventricular dysfunction, when used in conjunction with standard therapies above. Spironolactone is another option that is sometimes preferable to eplerenone due to cost. Evidence supports the consumption of polyunsaturated fats instead of saturated fats as a measure of decreasing coronary heart disease. Omega-3 fatty acids, commonly found in fish, have been shown to reduce mortality post-MI. While the mechanism by which these fatty acids decrease mortality is unknown, it has been postulated that the survival benefit is due to electrical stabilization and the prevention of ventricular fibrillation. However, further studies in a high-risk subset have not shown a clear-cut decrease in potentially fatal arrhythmias due to omega-3 fatty acids.

Blood donation may reduce the risk of heart disease for men, but the link has not been firmly established.

A Cochrane review found that giving heparin to people with heart conditions like unstable angina and some forms of heart attacks reduces the risk of having another heart attack. However, heparin also increases the chance of suffering from minor bleeding.

Management

An MI is a medical emergency that requires immediate medical attention. Treatment attempts to salvage as much myocardium as possible and to prevent further complications, hence the phrase "time is muscle". Oxygen, aspirin, and nitroglycerin may be administered. Morphine was classically used if nitroglycerin was not effective; however, it may increase mortality in the setting of NSTEMI. A 2009 and 2010 review of high flow oxygen in myocardial infarction found increased mortality and infarct size, calling into question the recommendation about its routine use. Other analgesics such as nitrous oxide are of unknown benefit. Percutaneous coronary intervention (PCI) or fibrinolysis are recommended in those with an STEMI.

Prognosis

The prognosis post myocardial infarction varies greatly, depending on a person's health, the extent of the heart damage and the treatment given. For the period 2005–2008 in the United States, the median mortality at 30 days was 16.6% with a range from 10.9% to 24.9% depending on the hospital. Using variables available in the emergency room, people with a higher risk of adverse outcome can be identified. One study found that 0.4% of patients with a low-risk profile died after 90 days, whereas in high-risk people it was 21.1%.

Some of the more reproduced risk-stratifying factors include: age, hemodynamic parameters (such as heart failure, cardiac arrest on admission, systolic blood pressure, or Killip class of two or greater), ST-segment deviation, diabetes, serum creatinine, peripheral vascular disease and elevation of cardiac markers. Assessment of left ventricular ejection fraction may increase the predictive power. The prognostic importance of Q-waves is debated. Prognosis is significantly worsened if a mechanical complication such as papillary muscle or myocardial free wall rupture occur. Morbidity and mortality from myocardial infarction has improved over the years due to better treatment.

Complications

Complications may occur immediately following the heart attack (in the acute phase), or may need time to develop (a chronic problem). Acute complications may include heart failure if the damaged heart is no longer able to adequately pump blood around the body; aneurysm or rupture of the myocardium; mitral regurgitation, in particular if the infarction causes dysfunction of the papillary muscle; and arrhythmias, such as ventricular fibrillation, ventricular tachycardia, atrial fibrillation and heart block. Longer-term complications include heart failure, atrial fibrillation, and the increased risk of a second myocardial infarction.

Epidemiology

Myocardial infarction is a common presentation of ischemic heart disease. The WHO estimated in 2002, that 12.6 percent of worldwide deaths were from ischemic heart disease with it the leading cause of death in developed countries, and third to AIDS and lower respiratory infections in developing countries. Worldwide more than 3 million people have STEMIs and 4 million have NSTEMIs a year.

Coronary heart disease is responsible for 1 in 5 deaths in the United States. It is becoming more common in the developing world such that, in India, cardiovascular disease (CVD) is the leading cause of death. The deaths due to CVD in India were 32% of all deaths in 2007 and are expected to rise from 1.17 million in 1990 and 1.59 million in 2000 to 2.03 million in 2010. Although a relatively new epidemic in India, it has quickly become a major health issue with deaths due to CVD expected to double during 1985–2015. Mortality estimates due to CVD vary widely by state, ranging from 10% in Meghalaya to 49% in Punjab (percentage of all deaths). Punjab (49%), Goa (42%), Tamil Nadu (36%) and Andhra Pradesh (31%) have the highest CVD related mortality estimates. State-wise differences are correlated with prevalence of specific dietary risk factors in the states. Moderate physical exercise is associated with reduced incidence of CVD in India (those that exercise have less than half the risk of those that do not).

Legal implications

At common law, in general a myocardial infarction is a disease, but may sometimes be an injury. This has implications for no-fault insurance schemes such as workers' compensation. In general, a heart attack is not covered; however, it may be a work-related injury if it results, for example, from unusual emotional stress or unusual exertion. In addition, in some jurisdictions, heart attacks suffered by persons in particular occupations such as police officers may be classified as line-of-duty injuries by statute or policy. In some countries or states, a person having suffered from a myocardial infarction may be prevented from participating in activity that puts other people's lives at risk, for example driving a car or flying an airplane.

Research

Patients who receive stem cell treatment by coronary artery injections of stem cells derived from their own bone marrow after a myocardial infarction (MI) show improvements in left ventricular ejection fraction and end-diastolic volume not seen with placebo. The larger the initial infarct size, the greater the effect of the infusion. Clinical trials of progenitor cell infusion as a treatment approach to ST elevation MI are proceeding.

There are currently 3 biomaterial and tissue engineering approaches for the treatment of MI, but these are in an even earlier stage of medical research, so many questions and issues must be addressed before they can be applied to patients. The first involves polymeric left ventricular restraints in the prevention of heart failure. The second utilizes ''in vitro'' engineered cardiac tissue, which is subsequently implanted ''in vivo''. The final approach entails injecting cells and/or a scaffold into the myocardium to create ''in situ'' engineered cardiac tissue.

References

External links

American Heart Association's Heart Attack web site – Information and resources for preventing, recognizing and treating heart attack.

Heart Attack – overview of resources from MedlinePlus

Category:Aging-associated diseases Category:Causes of death Category:Ischemic heart diseases Category:Medical emergencies

ar:احتشاء عضل القلب an:Infarcto de miocardio be:Востры інфаркт міякарда be-x-old:Востры інфаркт міякарда bs:Infarkt miokarda bg:Инфаркт на миокарда ca:Infart miocardíac ceb:Atake sa kasingkasing cs:Infarkt myokardu cy:Trawiad ar y galon da:Akut myokardieinfarkt de:Myokardinfarkt dv:ހާރޓް އެޓޭކް ޖެހުން et:Müokardi infarkt el:Έμφραγμα του μυοκαρδίου es:Infarto agudo de miocardio eo:Korinfarkto eu:Miokardio infartu akutu fa:سکته قلبی fr:Infarctus du myocarde gu:હૃદયરોગનો હુમલો ko:심근 경색 hi:हृदयाघात hr:Infarkt miokarda id:Serangan jantung is:Hjartaáfall it:Infarto miocardico acuto he:התקף לב kn:ಹೃದಯಾಘಾತ kk:Миокард инфарктысы ku:Mirina masûlkeyên dil la:Infarctus cordis lv:Miokarda infarkts lt:Miokardo infarktas hu:Szívinfarktus mk:Срцев напад ml:ഹൃദയാഘാതം mr:हृदयाघात mn:Зүрхний шигдээс nl:Hartinfarct ja:心筋梗塞 no:Hjerteinfarkt nn:Hjarteåtak oc:Infart miocardiac pnb:دل دا دورہ pl:Zawał mięśnia sercowego pt:Infarto agudo do miocárdio ro:Infarct miocardic qu:Sunqu p'itiy ru:Острый инфаркт миокарда sq:Infarkti miokardial si:හෘදයාබාධය simple:Myocardial infarction sl:Miokardni infarkt sr:Срчани удар sh:Srčani udar fi:Sydäninfarkti sv:Hjärtinfarkt ta:மாரடைப்பு te:గుండెపోటు th:กล้ามเนื้อหัวใจตายเหตุขาดเลือด tr:Kalp krizi uk:Гострий інфаркт міокарда ur:احتشاء عضل قلب vi:Nhồi máu cơ tim war:Atake ha kasingkasing yi:הארץ אטאקע zh:心肌梗死