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A comet is an icy small Solar System body that, when passing close to the Sun, heats up and begins to outgas, displaying a visible atmosphere or coma, and sometimes also a tail. These phenomena are due to the effects of solar radiation and the solar wind upon the nucleus of the comet. Comet nuclei range from a few hundred metres to tens of kilometres across and are composed of loose collections of ice, dust, and small rocky particles. The coma and tail are much larger and, if sufficiently bright, may be seen from the Earth without the aid of a telescope. Comets have been observed and recorded since ancient times by many cultures.

Comets have a wide range of orbital periods, ranging from several years to potentially several millions of years. Short-period comets originate in the Kuiper belt or its associated scattered disc, which lie beyond the orbit of Neptune. Long-period comets are thought to originate in the Oort cloud, a spherical cloud of icy bodies extending from outside the Kuiper belt to halfway to the next nearest star. Long-period comets are directed towards the Sun from the Oort cloud by gravitational perturbations caused by passing stars and the galactic tide. Hyperbolic comets may pass once through the inner Solar System before being flung out to interstellar space.

Comets are distinguished from asteroids by the presence of an extended,

 

 

 

Comets – nucleus, coma and tail:

Top: 9P/Tempel (impactor collision: Deep

Impact), 67P/Churyumov–Gerasimenko

(Rosetta)

Middle: 17P/Holmes and its blue ionized

tail, 81P/Wild (Wild 2) visited by Stardust

Bottom: Hale–Bopp seen from Earth in

1997, C/2011 W3 (Lovejoy) imaged from

gravitationally unbound atmosphere surrounding their central nucleus. This atmosphere has parts termed the coma (the central part immediately surrounding the nucleus) and the tail (a typically linear section consisting of dust or gas blown out from the coma by the Sun's light pressure or outstreaming solar wind plasma). However, extinct comets that have passed close to the Sun many times have lost nearly all of their volatile ices and dust and may come to resemble small asteroids.[1] Asteroids are thought to have a different origin from comets, having formed inside the orbit of Jupiter rather than in the outer Solar System.[2][3] The discovery of main-belt comets and active centaurs has blurred the distinction between asteroids and comets.

As of November 2014 there are 5,253 known comets,[4] a number that is steadily increasing. However, this represents only a tiny fraction of the total potential comet population, as the reservoir of comet-like bodies in the outer Solar System (in the Oort cloud) is estimated to be one trillion.[5][6] Roughly one comet per year is visible to the naked eye, though many of these are faint and unspectacular.[7] Particularly bright examples are called "Great Comets". Comets have been visited by unmanned probes such as the European Space Agency's Rosetta, which became the first ever to land a robotic spacecraft on a comet,[8] and NASA's Deep Impact, which blasted a crater on Comet Tempel 1 to study its interior.

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1 Etymology

Etymology[edit]

The word comet derives from the Old English cometa from the Latin comēta or comētēs. That, in turn, is a latinisation of the Greek κομήτης ("wearing long hair"), and the Oxford English Dictionary notes that the term (ἀστὴρ) κομήτης already meant "long-haired star, comet" in Greek. Κομήτης was derived from κομᾶν ("to wear the hair long"), which was itself derived from κόμη ("the hair of the head") and was used to mean "the tail of a comet".[9][10]

The astronomical symbol for comets is (☄), consisting of a small disc with three hairlike extensions.[11]

Physical characteristics[edit]

Nucleus[edit]

Main article: Comet nucleus

Nucleus of Comet 103P/Hartley as imaged during a spacecraft flyby. The nucleus is about 2 km in length.

Comet Borrelly exhibits jets, but has no surface ice.

Comet Wild 2 exhibits jets on light side and dark side, stark relief, and is dry.

The solid, core structure of a comet is known as the nucleus. Cometary nuclei are composed of an amalgamation of rock, dust, water ice, and frozen gases such as carbon dioxide, carbon monoxide, methane, and ammonia.[12] As such, they are popularly described as "dirty snowballs" after Fred Whipple's model.[13] However, some comets may have a higher dust content, leading them to be called "icy dirtballs".[14] Research conducted in 2014 suggests that comets are like "deep fried ice cream", in that their surfaces are formed of dense crystalline ice mixed with organic compounds, while the interior ice is colder and less dense.[15]

The surface of the nucleus is generally dry, dusty or rocky, suggesting that the ices are hidden beneath a surface crust several metres thick. In addition to the gases already mentioned, the nuclei contain a variety of organic compounds, which may include methanol, hydrogen cyanide, formaldehyde, ethanol, and ethane and perhaps more complex molecules such as long-chain hydrocarbons and amino acids.[16][17] In 2009, it was confirmed that the amino acid glycine had been found in the comet dust recovered by NASA's Stardust mission.[18] In August 2011, a report, based on NASA studies of 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.[19][20]

The outer surfaces of cometary nuclei have a very low albedo, making them among the least reflective objects found in the Solar System. The Giotto space probe found that the nucleus of Halley's Comet reflects about four percent of the light that falls on it,[21] and Deep Space 1 discovered that Comet Borrelly's surface reflects less than 3.0% of the light that falls on it;[21] by comparison, asphalt reflects seven percent of the light that falls on it. The dark surface material of the nucleus may consist of complex organic compounds. Solar heating drives off lighter volatile compounds, leaving behind larger organic compounds that tend to be very dark, like tar or crude oil. The low reflectivity of cometary surfaces enables them to absorb the heat necessary to drive their outgassing processes.[22]

Comet nuclei with radii of up to 30 kilometres (19 mi) have been observed,[23] but ascertaining their exact size is difficult.[24] The nucleus of P/2007 R5 is probably only 100–200 metres in diameter.[25] A lack of smaller comets being detected despite the increased sensitivity of instruments has led some to suggest that there is a real lack of comets smaller than 100 metres (330 ft) across.[26] Known comets have been estimated to have an average density of 0.6 g/cm3.[27] Because of their low mass, comet nuclei do not become spherical under their own gravity and therefore have irregular shapes.[28]

Roughly six percent of the near-Earth asteroids are thought to be extinct nuclei of comets that no longer experience outgassing,[29] including 14827 Hypnos and 3552 Don Quixote.

Results from the Rosetta and Philae spacecraft show that the nucleus of 67P/Churyumov–Gerasimenko has no magnetic field, which suggests that magnetism may not have played a role in the early formation of planetesimals.[30][31] Further, the ALICE spectrograph on Rosetta determined that electrons (within 1 km (0.62 mi) above the comet nucleus) produced from photoionization of water molecules by solar radiation, and not photons from the Sun as thought earlier, are responsible for the degradation of water and carbon dioxide molecules released from the comet nucleus into its coma.[32][33] Instruments on the Philae lander found at least sixteen organic compounds at the comet's surface, four of which (acetamide, acetone, methyl isocyanate and propionaldehyde) have been detected for the first time on a comet.[34][35][36]

Properties of some comets

NameDimensions

(km)Density(g/cm3)

Mass(kg)[37]

Refs

Halley's Comet 15 × 8 × 8 0.6 3×1014 [38][39]

Tempel 1 7.6 × 4.9 0.62 7.9×1013 [27][40]

19P/Borrelly 8 × 4 × 4 0.3 2.0×1013 [27]

81P/Wild 5.5 × 4.0 × 3.3 0.6 2.3×1013 [27][41]

67P/Churyumov–Gerasimenko

4.1 × 3.3 × 1.8 0.47

Coma[edit]

Main article: Coma (cometary)

Hubble image of Comet ISON shortly before perihelion.[44]

The streams of dust and gas thus released form a huge and extremely thin atmosphere around the comet called the "coma", and the force exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous "tail" to form pointing away from the Sun.[45]

The coma is generally made of  H2O and dust, with water making up to 90% of the volatiles that outflow from the nucleus when the comet is within 3 to 4 astronomical units (450,000,000 to 600,000,000 km; 280,000,000 to 370,000,000 mi) of the Sun.[46] The  H2O parent molecule is destroyed primarily through photodissociation and to a much smaller extent photoionization, with the solar wind playing a minor role in the destruction of water compared to photochemistry.[46] Larger dust particles are left along the comet's orbital path whereas smaller particles are pushed away from the Sun into the comet's tail by light pressure.[47]

Although the solid nucleus of comets is generally less than 60 kilometres (37 mi) across, the coma may be thousands or millions of kilometres across, sometimes becoming larger than the Sun.[48] For example, about a month after an outburst in October 2007, comet 17P/Holmes briefly had a tenuous dust atmosphere larger than the Sun.[49] The Great Comet of 1811 also had a coma roughly the diameter of the Sun.[50] Even though the coma can become quite large, its size can decrease about the time it crosses the orbit of Mars around 1.5 astronomical units (220,000,000 km; 140,000,000 mi) from the Sun.[50] At this distance the solar wind becomes strong enough to blow the gas and dust away from the coma, enlarging the tail.[50] Ion tails have been observed to extend one astronomical unit (150 million km) or more.[49]

Both the coma and tail are illuminated by the Sun and may become visible when a comet passes through the inner Solar System, the dust reflecting Sunlight directly and the gases glowing from ionisation.[51] Most comets are too faint to be visible without the aid of a telescope, but a few each decade become bright enough to be visible to the naked eye.[52] Occasionally a comet may experience a huge and sudden outburst of gas and dust, during which the size of the coma greatly increases for a period of time. This happened in 2007 to Comet Holmes.[49]

In 1996, comets were found to emit X-rays.[53] This greatly surprised astronomers because X-ray emission is usually associated with very high-temperature bodies. The X-rays are generated by the interaction between comets and the solar wind: when highly charged solar wind ions fly

through a cometary atmosphere, they collide with cometary atoms and molecules, "stealing" one or more electrons from the atom in a process called "charge exchange". This exchange or transfer of an electron to the solar wind ion is followed by its de-excitation into the ground state of the ion, leading to the emission of X-rays and far ultraviolet photons.[54]

Tails[edit]

Main article: Comet tail

Diagram of a comet showing the dust trail, the dust tail (or antitail) and the ion gas tail, which is formed by the solar wind flow.

Gas and snow jets on Comet Hartley 2

In the outer Solar System, comets remain frozen and inactive and are extremely difficult or impossible to detect from Earth due to their small size. Statistical detections of inactive comet nuclei in the Kuiper belt have been reported from observations by the Hubble Space Telescope [55] [56] but these detections have been questioned.[57][58] As a comet approaches the inner Solar System,

solar radiation causes the volatile materials within the comet to vaporize and stream out of the nucleus, carrying dust away with them.

The streams of dust and gas each form their own distinct tail, pointing in slightly different directions. The tail of dust is left behind in the comet's orbit in such a manner that it often forms a curved tail called the type II or dust tail.[51] At the same time, the ion or type I tail, made of gases, always points directly away from the Sun because this gas is more strongly affected by the solar wind than is dust, following magnetic field lines rather than an orbital trajectory.[59] On occasions - such as when the Earth passes through a comet's orbital plane, and we see the track of the comet edge-on, a tail pointing in the opposite direction to the ion and dust tails may be seen – the antitail.[60] (The dust tail of the comet prior to its rounding of the Sun is collinear with the dust tail post the rounding).[clarification needed]

The observation of antitails contributed significantly to the discovery of solar wind.[61] The ion tail is formed as a result of the ionisation by solar ultra-violet radiation of particles in the coma. Once the particles have been ionized, they attain a net positive electrical charge, which in turn gives rise to an "induced magnetosphere" around the comet. The comet and its induced magnetic field form an obstacle to outward flowing solar wind particles. Because the relative orbital speed of the comet and the solar wind is supersonic, a bow shock is formed upstream of the comet in the flow direction of the solar wind. In this bow shock, large concentrations of cometary ions (called "pick-up ions") congregate and act to "load" the solar magnetic field with plasma, such that the field lines "drape" around the comet forming the ion tail.[62]

If the ion tail loading is sufficient, then the magnetic field lines are squeezed together to the point where, at some distance along the ion tail, magnetic reconnection occurs. This leads to a "tail disconnection event".[62] This has been observed on a number of occasions, one notable event being recorded on April 20, 2007, when the ion tail of Encke's Comet was completely severed while the comet passed through a coronal mass ejection. This event was observed by the STEREO space probe.[63]

In 2013 ESA scientists reported that the ionosphere of the planet Venus streams outwards in a manner similar to the ion tail seen streaming from a comet under similar conditions."[64][65]

Jets[edit]

Uneven heating can cause newly generated gases to break out of a weak spot on the surface of comet's nucleus, like a geyser.[66] These streams of gas and dust can cause the nucleus to spin, and even split apart.[66] In 2010 it was revealed dry ice (frozen carbon dioxide) can power jets of material flowing out of a comet nucleus.[67] This is known because a spacecraft got so close that it could see where the jets were coming out, and then measure the infrared spectrum at that point which shows what some of the materials are.[68]

Orbital characteristics[edit]

Most comets are small Solar System bodies with elongated elliptical orbits that take them close to the Sun for a part of their orbit and then out into the further reaches of the Solar System for the

remainder.[69] Comets are often classified according to the length of their orbital periods: The longer the period the more elongated the ellipse.

Short period[edit]

Main article: List of numbered comets

Periodic comets or short-period comets are generally defined as having orbital periods of less than 200 years.[70] They usually orbit more-or-less in the ecliptic plane in the same direction as the planets.[71] Their orbits typically take them out to the region of the outer planets (Jupiter and beyond) at aphelion; for example, the aphelion of Halley's Comet is a little beyond the orbit of Neptune. Comets whose aphelia are near a major planet's orbit are called its "family".[72] Such families are thought to arise from the planet capturing formerly long-period comets into shorter orbits.[73]

At the shorter extreme, Encke's Comet has an orbit that does not reach the orbit of Jupiter, and is known as an Encke-type comet. Short-period comets with orbital periods shorter than 20 years and low inclinations (up to 30 degrees) are called Jupiter-family comets (JFCs).[74][75] Those like Halley, with orbital periods of between 20 and 200 years and inclinations extending from zero to more than 90 degrees, are called Halley-type comets (HTCs).[76][77] As of 2015, only 75 HTCs have been observed, compared with 511 identified JFCs.[78]

Recently discovered main-belt comets form a distinct class, orbiting in more circular orbits within the asteroid belt.[79]

Because their elliptical orbits frequently take them close to the giant planets, comets are subject to further gravitational perturbations.[80] Short-period comets have a tendency for their aphelia to coincide with a giant planet's semi-major axis, with the JFCs being the largest group.[75] It is clear that comets coming in from the Oort cloud often have their orbits strongly influenced by the gravity of giant planets as a result of a close encounter. Jupiter is the source of the greatest perturbations, being more than twice as massive as all the other planets combined. These perturbations can deflect long-period comets into shorter orbital periods.[81][82]

Based on their orbital characteristics, short-period comets are thought to originate from the centaurs and the Kuiper belt/scattered disc [83] —a disk of objects in the trans-Neptunian region—whereas the source of long-period comets is thought to be the far more distant spherical Oort cloud (after the Dutch astronomer Jan Hendrik Oort who hypothesised its existence).[84] Vast swarms of comet-like bodies are believed to orbit the Sun in these distant regions in roughly circular orbits. Occasionally the gravitational influence of the outer planets (in the case of Kuiper belt objects) or nearby stars (in the case of Oort cloud objects) may throw one of these bodies into an elliptical orbit that takes it inwards toward the Sun to form a visible comet. Unlike the return of periodic comets, whose orbits have been established by previous observations, the appearance of new comets by this mechanism is unpredictable.[85]

Long period[edit]

See also: List of hyperbolic comets

Orbits of the Kohoutek Comet (red) and the Earth (blue), illustrating the high eccentricity of its orbit and its rapid motion when close to the Sun

Comets C/2012 F6 (Lemmon) (top) and C/2011 L4 (PANSTARRS) (bottom)

Long-period comets have highly eccentric orbits and periods ranging from 200 years to thousands of years.[86] An eccentricity greater than 1 when near perihelion does not necessarily mean that a comet will leave the Solar System.[87] For example, Comet McNaught had a heliocentric osculating eccentricity of 1.000019 near its perihelion passage epoch in January 2007 but is bound to the Sun with roughly a 92,600-year orbit because the eccentricity drops below 1 as it moves further from the Sun. The future orbit of a long-period comet is properly obtained when the osculating orbit is computed at an epoch after leaving the planetary region and is calculated with respect to the center of mass of the Solar System. By definition long-period comets remain gravitationally bound to the Sun; those comets that are ejected from the Solar System due to close passes by major planets are no longer properly considered as having "periods". The orbits of long-period comets take them far beyond the outer planets at aphelia, and the plane of their orbits need not lie near the ecliptic. Long-period comets such as Comet West and C/1999 F1 can have apoapsis distances of nearly 70,000 AU with orbital periods estimated around 6 million years.

Single-apparition or non-periodic comets are similar to long-period comets because they also have parabolic or slightly hyperbolic trajectories[86] when near perihelion in the inner Solar System. However, gravitational perturbations from giant planets cause their orbits to change. Single-apparition comets have a hyperbolic or parabolic osculating orbit which allows them to permanently exit the Solar System after a single pass of the Sun.[88] The Sun's Hill sphere has an unstable maximum boundary of 230,000 AU (1.1 parsecs (3.6 light-years)).[89] Only a few

hundred comets have been seen to reach a hyperbolic orbit (e > 1) when near perihelion[90] that using a heliocentric unperturbed two-body best-fit suggests they may escape the Solar System.

No comets with an eccentricity significantly greater than one have been observed,[90] so there are no confirmed observations of comets that are likely to have originated outside the Solar System. Comet C/1980 E1 had an orbital period of roughly 7.1 million years before the 1982 perihelion passage, but a 1980 encounter with Jupiter accelerated the comet giving it the largest eccentricity (1.057) of any known hyperbolic comet.[91] Comets not expected to return to the inner Solar System include C/1980 E1, C/2000 U5, C/2001 Q4 (NEAT), C/2009 R1, C/1956 R1, and C/2007 F1 (LONEOS).

Some authorities use the term "periodic comet" to refer to any comet with a periodic orbit (that is, all short-period comets plus all long-period comets),[92] whereas others use it to mean exclusively short-period comets.[86] Similarly, although the literal meaning of "non-periodic comet" is the same as "single-apparition comet", some use it to mean all comets that are not "periodic" in the second sense (that is, to also include all comets with a period greater than 200 years).

Early observations have revealed a few genuinely hyperbolic (i.e. non-periodic) trajectories, but no more than could be accounted for by perturbations from Jupiter. If comets pervaded interstellar space, they would be moving with velocities of the same order as the relative velocities of stars near the Sun (a few tens of km per second). If such objects entered the Solar System, they would have positive specific orbital energy and would be observed to have genuinely hyperbolic trajectories. A rough calculation shows that there might be four hyperbolic comets per century within Jupiter's orbit, give or take one and perhaps two orders of magnitude.[93]

Hyperbolic comet discoveries[94]

Year 2007 2008 2009 2010 2011 2012 2013 2014

Number 12 7 8 4 12 10 8 9

Oort cloud and Hills cloud[edit]

Main article: Oort cloud

The Oort cloud thought to surround the Solar System

The Oort cloud is thought to occupy a vast space from somewhere between 2,000 and 5,000 AU (0.03 and 0.08 ly)[95] to as far as 50,000 AU (0.79 ly)[76] from the Sun. Some estimates place the outer edge at between 100,000 and 200,000 AU (1.58 and 3.16 ly).[95] The region can be subdivided into a spherical outer Oort cloud of 20,000–50,000 AU (0.32–0.79 ly), and a doughnut-shaped inner Oort cloud of 2,000–20,000 AU (0.03–0.32 ly). The outer cloud is only weakly bound to the Sun and supplies the long-period (and possibly Halley-type) comets to inside the orbit of Neptune.[76] The inner Oort cloud is also known as the Hills cloud, named after J. G. Hills, who proposed its existence in 1981.[96] Models predict that the inner cloud should have tens or hundreds of times as many cometary nuclei as the outer halo;[96][97][98] it is seen as a possible source of new comets to resupply the relatively tenuous outer cloud as the latter's numbers are gradually depleted. The Hills cloud explains the continued existence of the Oort cloud after billions of years.[99]

Exocomets[edit]

Main article: Exocomet

Exocomets beyond our Solar System have also been detected and may be common in the Milky Way Galaxy.[100] The first exocomet system detected was around Beta Pictoris, a very young type A V star, in 1987.[101][102] A total of 10 such exocomet systems have been identified as of 2013, using the absorption spectrum caused by the large clouds of gas emitted by comets when passing close to their star.[100][101]

Effects of comets[edit]

Connection to meteor showers[edit]

Diagram of Perseids meteors

As a result of outgassing, comets leave in their wake a trail of solid debris too large to be swept away by radiation pressure and the solar wind.[103] If the comet's path crosses the path the Earth follows in orbit around the Sun, then at that point there are likely to be meteor showers as Earth passes through the trail of debris. The Perseid meteor shower, for example, occurs every year between August 9 and August 13, when Earth passes through the orbit of Comet Swift–Tuttle.[104] Halley's Comet is the source of the Orionid shower in October.[104]

Comets and impact on life[edit]

Many comets and asteroids collided into Earth in its early stages. Many scientists believe that comets bombarding the young Earth about 4 billion years ago brought the vast quantities of water that now fill the Earth's oceans, or at least a significant portion of it. Other researchers have cast doubt on this theory.[105] The detection of organic molecules, including polycyclic aromatic hydrocarbons,[15] in significant quantities in comets has led some to speculate that comets or meteorites may have brought the precursors of life—or even life itself—to Earth.[106] In 2013 it was suggested that impacts between rocky and icy surfaces, such as comets, had the potential to create the amino acids that make up proteins through shock synthesis.[107]

It is suspected that comet impacts have, over long timescales, also delivered significant quantities of water to the Earth's Moon, some of which may have survived as lunar ice.[108] Comet and meteoroid impacts are also believed responsible for the existence of tektites and australites.[109]

Fate of comets[edit]

Departure (ejection) from Solar System[edit]

If a comet is traveling fast enough, it may leave the Solar System; such is the case for hyperbolic comets. To date, comets are only known to be ejected by interacting with another object in the Solar System, such as Jupiter.[110] An example of this is thought to be Comet C/1980 E1, which was shifted from a predicted orbit of 7.1 million years around the Sun, to a hyperbolic trajectory, after a 1980 encounter with the planet Jupiter.[111]

Volatiles exhausted[edit]

Main article: Extinct comet

Jupiter-family comets and long-period comets appear to follow very different fading laws. The JFCs are active over a lifetime of about 10,000 years or ~1,000 orbits whereas long-period comets fade much faster. Only 10% of the long-period comets survive more than 50 passages to small perihelion and only 1% of them survive more than 2,000 passages.[29] Eventually most of the volatile material contained in a comet nucleus evaporates away, and the comet becomes a small, dark, inert lump of rock or rubble that can resemble an asteroid.[112] Some asteroids in elliptical orbits are now identified as extinct comets.[113] Roughly six percent of the near-Earth asteroids are thought to be extinct nuclei of comets that no longer emit gas.[29]

Breakup and collisions[edit]

The nucleus of some comets may be fragile, a conclusion supported by the observation of comets splitting apart.[114] A significant cometary disruption was that of Comet Shoemaker–Levy 9, which was discovered in 1993. A close encounter in July 1992 had broken it into pieces, and over a period of six days in July 1994, these pieces fell into Jupiter's atmosphere—the first time astronomers had observed a collision between two objects in the Solar System.[115][116] Other splitting comets include 3D/Biela in 1846 and 73P/Schwassmann–Wachmann from 1995 to 2006.[117] Greek historian Ephorus reported that a comet split apart as far back as the winter of 372–373 BC.[118] Comets are suspected of splitting due to thermal stress, internal gas pressure, or impact.[119]

Comets 42P/Neujmin and 53P/Van Biesbroeck appear to be fragments of a parent comet. Numerical integrations have shown that both comets had a rather close approach to Jupiter in January 1850, and that, before 1850, the two orbits were nearly identical.[120]

Some comets have been observed to break up during their perihelion passage, including great comets West and Ikeya–Seki. Biela's Comet was one significant example, when it broke into two pieces during its passage through the perihelion in 1846. These two comets were seen separately in 1852, but never again afterward. Instead, spectacular meteor showers were seen in 1872 and 1885 when the comet should have been visible. A minor meteor shower, the Andromedids, occurs annually in November, and it is caused when the Earth crosses the orbit of Biela's Comet.[121]

Some comets meet a more spectacular end – either falling into the Sun[122] or smashing into a planet or other body. Collisions between comets and planets or moons were common in the early Solar System: some of the many craters on the Moon, for example, may have been caused by comets. A recent collision of a comet with a planet occurred in July 1994 when Comet Shoemaker–Levy 9 broke up into pieces and collided with Jupiter.[123]

Nomenclature[edit]

Main article: Naming of comets

Halley's Comet (photo, 1910)

The names given to comets have followed several different conventions over the past two centuries. Prior to the early 20th century, most comets were simply referred to by the year when they appeared, sometimes with additional adjectives for particularly bright comets; thus, the "Great Comet of 1680", the "Great Comet of 1882", and the "Great January comet of 1910".

After Edmund Halley demonstrated that the comets of 1531, 1607, and 1682 were the same body and successfully predicted its return in 1759 by calculating its orbit, that comet became known as Halley's Comet.[125] Similarly, the second and third known periodic comets, Encke's Comet [126] and Biela's Comet,[127] were named after the astronomers who calculated their orbits rather than their original discoverers. Later, periodic comets were usually named after their discoverers, but comets that had appeared only once continued to be referred to by the year of their apparition.[128]

In the early 20th century, the convention of naming comets after their discoverers became common, and this remains so today. A comet can be named after its discoverers, or an instrument or program that helped to find it.[128]

History of study[edit]

Main article: Observational history of comets

Early observations and thought[edit]

Halley's Comet appeared in 1066, prior to the Battle of Hastings (Bayeux Tapestry).

From ancient sources, such as Chinese oracle bones, it is known that their appearances have been noticed by humans for millennia.[129] Until the sixteenth century, comets were usually considered

bad omens of deaths of kings or noble men, or coming catastrophes, or even interpreted as attacks by heavenly beings against terrestrial inhabitants.[130][131]

Aristotle believed that comets were atmospheric phenomena, due to the fact that they could appear outside of the Zodiac and vary in brightness over the course of a few days.[132] Pliny the Elder believed that comets were connected with political unrest and death.[133]

In the 16th century Tycho Brahe demonstrated that comets must exist outside the Earth's atmosphere by measuring the parallax of the Great Comet of 1577 from observations collected by geographically separated observers. Within the precision of the measurements, this implied the comet must be at least four times more distant than from the Earth to the Moon.[134][135]

Orbital studies[edit]

Further information: Observational history of comets § Orbital studies

The orbit of the comet of 1680, fitted to a parabola, as shown in Isaac Newton's Principia

Isaac Newton, in his Principia Mathematica of 1687, proved that an object moving under the influence of his inverse square law of universal gravitation must trace out an orbit shaped like one of the conic sections, and he demonstrated how to fit a comet's path through the sky to a parabolic orbit, using the comet of 1680 as an example.[136]

In 1705, Edmond Halley (1656–1742) applied Newton's method to twenty-three cometary apparitions that had occurred between 1337 and 1698. He noted that three of these, the comets of 1531, 1607, and 1682, had very similar orbital elements, and he was further able to account for the slight differences in their orbits in terms of gravitational perturbation by Jupiter and Saturn. Confident that these three apparitions had been three appearances of the same comet, he predicted that it would appear again in 1758–9.[137] Halley's predicted return date was later refined by a team of three French mathematicians: Alexis Clairaut, Joseph Lalande, and Nicole-Reine Lepaute, who predicted the date of the comet's 1759 perihelion to within one month's accuracy.[138] When the comet returned as predicted, it became known as Halley's Comet (with the latter-day designation of 1P/Halley). It will next appear in 2061.[139]

Studies of physical characteristics[edit]

Further information: Observational history of comets § Studies of physical characteristics

From his huge vapouring train perhaps to shakeReviving moisture on the numerous orbs,

Thro' which his long ellipsis winds; perhapsTo lend new fuel to declining suns,To light up worlds, and feed th' ethereal fire.

James Thomson The Seasons (1730; 1748)[140]

Isaac Newton described comets as compact and durable solid bodies moving in oblique orbit and their tails as thin streams of vapor emitted by their nuclei, ignited or heated by the Sun. Newton suspected that comets were the origin of the life-supporting component of air.[141]

As early as the 18th century, some scientists had made correct hypotheses as to comets' physical composition. In 1755, Immanuel Kant hypothesized that comets are composed of some volatile substance, whose vaporization gives rise to their brilliant displays near perihelion.[142] In 1836, the German mathematician Friedrich Wilhelm Bessel, after observing streams of vapor during the appearance of Halley's Comet in 1835, proposed that the jet forces of evaporating material could be great enough to significantly alter a comet's orbit, and he argued that the non-gravitational movements of Encke's Comet resulted from this phenomenon.[143]

In 1950, Fred Lawrence Whipple proposed that rather than being rocky objects containing some ice, comets were icy objects containing some dust and rock.[144] This "dirty snowball" model soon became accepted and appeared to be supported by the observations of an armada of spacecraft (including the European Space Agency's Giotto probe and the Soviet Union's Vega 1 and Vega 2) that flew through the coma of Halley's Comet in 1986, photographed the nucleus, and observed jets of evaporating material.[145]

On 22 January 2014, ESA scientists reported the detection, for the first definitive time, of water vapor on the dwarf planet Ceres, the largest object in the asteroid belt.[146] The detection was made by using the far-infrared abilities of the Herschel Space Observatory.[147] 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."[147] On 11 August 2014, astronomers released studies, using the Atacama Large Millimeter/Submillimeter Array (ALMA) for the first time, that detailed the distribution of HCN, HNC, H2CO, and dust inside the comae of comets C/2012 F6 (Lemmon) and C/2012 S1 (ISON).[148][149]

Spacecraft missions[edit]

See also: List of comets visited by spacecraft

Deep Impact. Debate continues about how much ice is in a comet. In 2001, the Deep Space 1 spacecraft obtained high-resolution images of the surface of Comet Borrelly. It was found that the surface of comet Borrelly is hot and dry, with a temperature of between 26 to 71 °C (79 to 160 °F), and extremely dark, suggesting that the ice has been removed by solar heating and maturation, or is hidden by the soot-like material that covers Borrelly's.[150] In July 2005, the Deep Impact probe blasted a crater on Comet Tempel 1 to study its interior. The mission yielded results suggesting that the majority of a comet's water ice is below the surface and that these reservoirs feed the jets of

vaporised water that form the coma of Tempel 1.[151] Renamed EPOXI, it made a flyby of Comet Hartley 2 on November 4, 2010.

Stardust. Data from the Stardust mission show that materials retrieved from the tail of Wild 2 were crystalline and could only have been "born in fire," at extremely high temperatures of over 1,000 °C (1,830 °F).[152][153] Although comets formed in the outer Solar System, radial mixing of material during the early formation of the Solar System is thought to have redistributed material throughout the proto-planetary disk,[154] so comets also contain crystalline grains that formed in the hot inner Solar System. This is seen in comet spectra as well as in sample return missions. More recent still, the materials retrieved demonstrate that the "comet dust resembles asteroid materials".[155] These new results have forced scientists to rethink the nature of comets and their distinction from asteroids.[156]

Rosetta. The Rosetta probe is presently in erratic orbit around Comet Churyumov–Gerasimenko. On November 12, 2014, its lander Philae successfully landed on the comet's surface, the first time a spacecraft has ever landed on such an object in history.[157]

Great comets[edit]

Main article: Great Comet

Woodcut of the Great Comet of 1577

Approximately once a decade, a comet becomes bright enough to be noticed by a casual observer, leading such comets to be designated as Great Comets.[118] Predicting whether a comet will become a great comet is notoriously difficult, as many factors may cause a comet's brightness to depart drastically from predictions.[158] Broadly speaking, if a comet has a large and active nucleus, will pass close to the Sun, and is not obscured by the Sun as seen from the Earth when at its brightest, it has a chance of becoming a great comet. However, Comet Kohoutek in 1973 fulfilled all the criteria and was expected to become spectacular but failed to do so.[159] Comet West, which appeared three years later, had much lower expectations but became an extremely impressive comet.[160]

The late 20th century saw a lengthy gap without the appearance of any great comets, followed by the arrival of two in quick succession—Comet Hyakutake in 1996, followed by Hale–Bopp, which reached maximum brightness in 1997 having been discovered two years earlier. The first great comet of the 21st century was C/2006 P1 (McNaught), which became visible to naked eye observers in January 2007. It was the brightest in over 40 years.[161]

Sungrazing comets[edit]

Main article: Sungrazing comet

A sungrazing comet is a comet that passes extremely close to the Sun at perihelion, generally within a few million kilometres.[162] Although small sungrazers can be completely evaporated during such a close approach to the Sun, larger sungrazers can survive many perihelion passages. However, the strong tidal forces they experience often lead to their fragmentation.[163]

About 90% of the sungrazers observed with SOHO are members of the Kreutz group, which all originate from one giant comet that broke up into many smaller comets during its first passage through the inner Solar System.[164] The remainder contains some sporadic sungrazers, but four other related groups of comets have been identified among them: the Kracht, Kracht 2a, Marsden, and Meyer groups. The Marsden and Kracht groups both appear to be related to Comet 96P/Machholz, which is also the parent of two meteor streams, the Quadrantids and the Arietids.[165]

Unusual comets[edit]

Of the thousands of known comets, some exhibit unusual properties. Encke's Comet (2P/Encke) orbits from outside the asteroid belt to just inside the orbit of the planet Mercury whereas the Comet 29P/Schwassmann–Wachmann currently travels in a nearly circular orbit entirely between the orbits of Jupiter and Saturn.[166] 2060 Chiron, whose unstable orbit is between Saturn and Uranus, was originally classified as an asteroid until a faint coma was noticed.[167] Similarly, Comet Shoemaker–Levy 2 was originally designated asteroid 1990 UL3.[168] (See also Fate of comets.)

Centaurs[edit]

Main article: Centaur (minor planet)

Centaurs typically behave with characteristics of both asteroids and comets.[169] Centaurs can be classified as comets such as 60558 Echeclus, and 166P/NEAT. 166P/NEAT was discovered while it exhibited a coma, and so is classified as a comet despite its orbit, and 60558 Echeclus was discovered without a coma but later became active,[170] and was then classified as both a comet and an asteroid (174P/Echeclus). One plan for Cassini involved sending it to a centaur, but NASA decided to destroy it instead.[171]

Observation[edit]

A comet may be discovered photographically using a wide-field telescope or visually with binoculars. However, even without access to optical equipment, it is still possible for the amateur astronomer to discover a sungrazing comet online by downloading images accumulated by some satellite observatories such as SOHO.[25] SOHO's 2000th comet was discovered by Polish amateur astronomer Michał Kusiak on 26 December 2010[172] and both discoverers of Hale-Bopp used amateur equipment (although Hale was not an amateur).

Lost[edit]

Main article: Lost comet

A number of periodic comets discovered in earlier decades or previous centuries are now lost comets. Their orbits were never known well enough to predict future appearances or the comets have disintegrated. However, occasionally a "new" comet is discovered, and calculation of its orbit shows it to be an old "lost" comet. An example is Comet 11P/Tempel–Swift–LINEAR, discovered in 1869 but unobservable after 1908 because of perturbations by Jupiter. It was not found again until accidentally rediscovered by LINEAR in 2001.[173] There are at least 18 comets that fit this category.[174]

AsteroidFrom Wikipedia, the free encyclopediaJump to: navigation, search For other uses, see Asteroid (disambiguation).

253 Mathilde, a C-type asteroid measuring about 50 kilometres (30 mi) across, covered in craters half that size. Photograph taken in 1997 by the NEAR Shoemaker probe.

Asteroids are minor planets, especially those of the inner Solar System. The larger ones have also been called planetoids. These terms have historically been applied to any astronomical object orbiting the Sun that did not show the disc of a planet and was not observed to have the characteristics of an active comet. As minor planets in the outer Solar System were discovered and found to have volatile-based surfaces that resemble those of comets, they were often distinguished from asteroids of the asteroid belt.[1] In this article, the term "asteroid" is restricted to the minor planets of the inner Solar System or co-orbital with Jupiter.

There are millions of asteroids, many thought to be the shattered remnants of planetesimals, bodies within the young Sun's solar nebula that never grew large enough to become planets.[2] The large majority of known asteroids orbit in the asteroid belt between the orbits of Mars and Jupiter, or are 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.

Only one asteroid, 4 Vesta, which has a relatively 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 visible to the naked eye for a short time.[3] As of September 2013, the Minor Planet Center had data on more than one million objects in the inner and outer Solar System, of which 625,000 had enough information to be given numbered designations.[4]

On 22 January 2014, ESA scientists reported the detection, for the first definitive time, of water vapor on Ceres, the largest object in the asteroid belt.[5] The detection was made by using the far-infrared abilities of the Herschel Space Observatory.[6] 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."[6]

Naming[edit]

Main article: Minor planet § Naming

2013 EC, shown here in radar images, has a provisional designation

A newly discovered asteroid is given a provisional designation (such as 2002 AT 4 ) 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[edit]

Main article: Astronomical 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 multiple variants.[7]

Asteroid Symbol Year

1 Ceres ⚳ Ceres' scythe, reversed to double as the letter C 1801

2 Pallas ⚴ Athena's (Pallas') spear 1801

3 Juno ⚵ A star mounted on a scepter,for Juno, the Queen of Heaven

1804

4 Vesta ⚶ The altar and sacred fire of Vesta 1807

5 Astraea A scale, or an inverted anchor, symbols of justice 18456 Hebe Hebe's cup 18477 Iris A rainbow (iris) and a star 18478 Flora A flower (flora) (specifically the Rose of England) 18479 Metis The eye of wisdom and a star 184810 Hygiea Hygiea's serpent and a star, or the Rod of Asclepius 184911 Parthenope A harp, or a fish and a star; symbols of the sirens 185012 Victoria The laurels of victory and a star 185013 Egeria A shield, symbol of Egeria's protection, and a star 1850

14 IreneA dove carrying an olive branch (symbol ofirene 'peace') with a star on its head,[8] oran olive branch, a flag of truce, and a star

1851

15 EunomiaA heart, symbol of good order(eunomia), and a star

1851

16 PsycheA butterfly's wing, symbol ofthe soul (psyche), and a star

1852

17 Thetis A dolphin, symbol of Thetis, and a star 185218 Melpomene The dagger of Melpomene, and a star 185219 Fortuna The wheel of fortune and a star 185226 Proserpina Proserpina's pomegranate 185328 Bellona Bellona's whip and lance[9] 185429 Amphitrite The shell of Amphitrite and a star 185435 Leukothea A lighthouse beacon, symbol of Leucothea [10] 185537 Fides The cross of faith (fides)[11] 1855

In 1851,[12] 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 ① to the fifth, Astraea, while continuing to designate the first four only with their existing iconic symbols). The numbered-circle 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.[13] That year Astraea's number

was increased to ⑤, 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.[8]

Discovery[edit]

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

The first asteroid to be discovered, Ceres, was found in 1801 by Giuseppe Piazzi, and was originally considered to be a new planet.[note 1] This was followed by the discovery of other similar bodies, which, with the equipment of the time, appeared to be points of light, like stars, showing little or no planetary disc, though readily distinguishable from stars due to their apparent motions. This prompted the astronomer Sir William Herschel to propose the term "asteroid",[14] coined in Greek as ἀστεροειδής asteroeidēs 'star-like, star-shaped', from Ancient Greek ἀστήρ 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[edit]

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.

First asteroid image (Ceres and Vesta) from Mars – viewed by Curiosity (20 April 2014).

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. 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 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, 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,[15] calling them "vermin of the skies", a phrase variously attributed to Eduard Suess [16] and Edmund Weiss.[17] Even a century later, only a few thousand asteroids were identified, numbered and named.

Manual methods of the 1900s and modern reporting[edit]

Until 1998, asteroids were discovered by a four-step process. First, a region of the sky was photographed by a wide-field telescope, or astrograph. Pairs of photographs were taken, typically one hour apart. Multiple pairs could be taken over a series of days. Second, the two films or plates of the same region were viewed under a stereoscope. Any body in orbit around the Sun would move slightly between the pair of films. Under the stereoscope, the image of the body would seem to float slightly above the background of stars. Third, once a moving body was

identified, its location would be measured precisely using a digitizing microscope. The location would be measured relative to known star locations.[18]

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

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

Computerized methods[edit]

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 Earth in 1937. Astronomers began to realize the possibilities of Earth impact.

Two events in later decades increased the alarm: the increasing acceptance of Walter Alvarez' hypothesis that an impact event resulted in the Cretaceous–Paleogene extinction, and the 1994 observation of Comet Shoemaker-Levy 9 crashing into Jupiter. The U.S. military also

declassified the information that its military satellites, built to detect nuclear explosions, had detected hundreds of upper-atmosphere impacts by objects ranging from one to 10 metres across.

All these considerations helped spur the launch of highly efficient surveys that consist of Charge-Coupled Device (CCD) cameras and computers directly connected to telescopes. As of spring 2011, it was estimated that 89% to 96% of near-Earth asteroids one kilometer or larger in diameter had been discovered.[19] A list of teams using such systems includes:[20]

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 Object Survey (CINEOS) team The Japanese Spaceguard Association The Asiago-DLR Asteroid Survey (ADAS)

The LINEAR system alone has discovered 138,393 asteroids, as of 20 September 2013.[21] Among all the surveys, 4711 near-Earth asteroids have been discovered[22] including over 600 more than 1 km (0.6 mi) in diameter.

Terminology[edit]

A composite image, to scale, of the asteroids which have been imaged at high resolution. As of 2011 they are, from largest to smallest: 4 Vesta, 21 Lutetia, 253 Mathilde, 243 Ida and its moon Dactyl, 433 Eros, 951 Gaspra, 2867 Šteins, 25143 Itokawa.

The largest asteroid in the previous image, Vesta (left), with Ceres (center) and the Moon (right) shown to scale.

Traditionally, small bodies orbiting the Sun were classified as asteroids, comets or meteoroids, with anything smaller than ten metres across being called a meteoroid.[23][24] 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. In 2006, the term "small Solar System body" was introduced to cover both most minor planets and comets.[25] Other languages prefer "planetoid" (Greek for "planet-like"), and this term is occasionally used in English for larger minor planets such as the dwarf planets. 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,[26] 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 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 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.[27]

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 considered as asteroids or as a new type of object. Then, when the first trans-Neptunian object (other than Pluto), 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.[28] They are believed to be predominantly comet-like in composition, though some may be more akin to asteroids.[29] 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,[30] suggesting "a continuum between asteroids and comets" rather than a sharp dividing line.[31]

The minor planets beyond Jupiter's orbit are sometimes also called "asteroids", especially in popular presentations.[32] However, it is becoming increasingly common for the term "asteroid" to be restricted to minor planets of the inner Solar System.[28] 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."[33] Currently only the largest object in the asteroid belt, Ceres, at about 950 km (590 mi) across, has been placed in the dwarf planet category.

Formation[edit]

It is believed that planetesimals in the asteroid belt evolved much like the rest of the solar nebula until Jupiter neared its current mass, at which point excitation from orbital resonances with Jupiter ejected over 99% of planetesimals in the belt. Simulations and a discontinuity in spin rate and spectral properties suggest that asteroids larger than approximately 120 km (75 mi) in diameter accreted during that early era, whereas smaller bodies are fragments from collisions between asteroids during or after the Jovian disruption.[34] Ceres and Vesta grew large enough to melt and differentiate, with heavy metallic elements sinking to the core, leaving rocky minerals in the crust.[35]

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.[36]

Distribution within the Solar System[edit]

See also: list of minor-planet groups, List of notable asteroids and List of minor planets

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

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[edit]

Main article: 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 1 km (0.6 mi) in diameter,[37] 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[edit]

Main article: Trojan (astronomy)

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

A couple of trojans have also been found orbiting with Mars.[note 2]

Near-Earth asteroids[edit]

Main article: Near-Earth asteroids

Near-Earth 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 Earth-crossers. As of November 2014, 11,600 near-Earth asteroids are known[19] and the number over one kilometre in diameter is estimated to be 900–1,000.

Frequency of bolides, small asteroids roughly 1 to 20 meters in diameter impacting Earth's atmosphere.

Characteristics[edit]

Size distribution[edit]

Sizes of the first ten asteroids to be discovered, compared to the Moon

Ceres as imaged by Dawn on 4 February 2015

Asteroids vary greatly in size, from almost 1000 km for the largest down to rocks just tens of metres across.[note 3] The three largest are very much like miniature planets: they are roughly spherical, have at least partly differentiated interiors,[38] and are thought to be surviving protoplanets. The vast majority, however, are much smaller and are irregularly shaped; they are thought to be either surviving planetesimals or fragments of larger bodies.

The dwarf planet Ceres is by far the largest asteroid, with a diameter of 975 km (610 mi). The next largest are 2 Pallas and 4 Vesta, both with diameters of just over 500 km (300 mi). Vesta is the only main-belt asteroid that can, on occasion, be visible to the naked eye. On some rare occasions, a near-Earth asteroid may briefly become visible without technical aid; see 99942 Apophis.

The mass of all the objects of the asteroid belt, lying between the orbits of Mars and Jupiter, is estimated to be about 2.8–3.2×1021 kg, or about 4% of the mass of the Moon. Of this, Ceres comprises 0.95×1021 kg, a third of the total.[39] Adding in the next three most massive objects, Vesta (9%), Pallas (7%), and Hygiea (3%), brings this figure up to 51%; whereas 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.[40]

The asteroids of the Solar System, categorized by size and numberApproximate number of asteroids (N) larger than a certain diameter (D)

D 100 m 300 m 500 m 1 km 3 km 5 km10 km

30 km

50 km

100 km

200 km

300 km

500 km

900 km

N~25000000

4000000

2000000

750000

200000

90000

10000

1100 600 200 30 5 3 1

Largest asteroids[edit]

See also: Largest asteroids

The relative masses of the twelve largest asteroids known,[41] compared to the remaining mass of the asteroid belt.[42]       1 Ceres      4 Vesta      2 Pallas      10 Hygiea      31 Euphrosyne

      704 Interamnia      511 Davida      532 Herculina      15 Eunomia      3 Juno

      16 Psyche      52 Europa      all others

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 "potato"-shaped asteroids.

Ceres is the only asteroid with a fully ellipsoidal shape and hence the only one that is a dwarf planet.[43] It has a much higher absolute magnitude than the other asteroids, of around 3.32,[44] and may possess a surface layer of ice.[45] Like the planets, Ceres is differentiated: it has a crust, a mantle and a core.[45] No meteorites from Ceres have been found on Earth.

Vesta, too, has a differentiated interior, though it formed inside the Solar System's frost line, and so is devoid of water;[46] its composition is mainly of basaltic rock such as olivine.[47] 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 V-type 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.[48] Its composition is similar to that of Ceres: high in carbon and silicon, and perhaps partially differentiated.[49] Pallas is the parent body of the Palladian family of asteroids,

The fourth-most-massive asteroid, Hygiea, is the largest carbonaceous asteroid and, unlike the other largest asteroids, lies relatively close to the plane of the ecliptic.[50] It is the largest member and presumed parent body of the Hygiean family of asteroids. Between them, the four largest asteroids constitute half the mass of the asteroid belt.

Attributes of largest asteroids

Name

Orbital

radius (AU)

Orbital

period(years

)

Inclinationto

ecliptic

Orbitaleccentrici

ty

Diameter(km)

Diameter

(% of Moon)

Mass

(×1018

kg)

Mass(% of Ceres

)

Density[51]

g/cm3

Rotation

period(hr)

Axial tilt

Surfacetemperatu

re

Vesta 2.36 3.63 7.1° 0.089573×557×4

46(mean 525)

15% 260 28%3.44 ± 0.12

5.34 29°85–270

K

Ceres 2.77 4.60 10.6° 0.079975×975×9

09(mean 952)

28% 940100%

2.12 ± 0.04

9.07 ≈ 3° 167 K

Pallas 2.77 4.62 34.8° 0.231580×555×5

00(mean 545)

16% 210 22%2.71 ± 0.11

7.81≈

80°164 K

Hygiea

3.14 5.56 3.8° 0.117530×407×3

70(mean 430)

12% 87 9%2.76 ±

1.227.6

≈ 60°

164 K

Rotation[edit]

Measurements of the rotation rates of large asteroids in the asteroid belt show that there is an upper limit. No asteroid with a diameter larger than 100 meters has a rotation period smaller than 2.2 hours. For asteroids rotating faster than approximately this rate, the inertia at the surface is greater than the gravitational force, so any loose surface material would be flung out. However, a solid object should be able to rotate much more rapidly. This suggests that most asteroids with a diameter over 100 meters are rubble piles formed through accumulation of debris after collisions between asteroids.[52]

Composition[edit]

Cratered terrain on 4 Vesta

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.[53] 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 (also see panspermia).[54] 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.[55][56][57]

Asteroid collision – building planets (artist concept).

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.[58]

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 sublimated, it may be getting replenished by a reservoir of ice under the surface. Organic compounds were also detected on the surface.[59][60][61][62] 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.[61]

In October 2013, water was detected on an extrasolar body for the first time, on an asteroid orbiting the white dwarf GD 61.[63]

Surface features[edit]

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 Earth-based 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 radius-size craters.[64] 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 be expanded with the Dawn spacecraft, which entered Ceres orbit on 6 March 2015.[65]

Color[edit]

Asteroids become darker and redder with age due to space weathering.[66] However evidence suggests most of the color change occurs rapidly, in the first hundred thousands years, limiting the usefulness of spectral measurement for determining the age of asteroids.[67]

Classification[edit]

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

Orbital classification[edit]

Main articles: Asteroid group and Asteroid family

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.[68] Families have only been recognized within the asteroid belt. They were first recognized by Kiyotsugu Hirayama in 1918 and are often called Hirayama families in his honor.

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

Quasi-satellites and horseshoe objects[edit]

Some asteroids have unusual horseshoe orbits that are co-orbital with Earth or some other planet. Examples are 3753 Cruithne and 2002 AA 29 . 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[edit]

Main article: Asteroid spectral types

This picture of 433 Eros shows the view looking from one end of the asteroid across the gouge on its underside and toward the opposite end. Features as small as 35 m (115 ft) across can be seen.

In 1975, an asteroid taxonomic system based on color, albedo, and spectral shape was developed by Clark R. Chapman, David Morrison, and Ben Zellner.[69] 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.[70] 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.[71]

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[edit]

Originally, spectral designations were based on inferences of an asteroid's composition.[72] 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 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.

Exploration[edit]

See also: Sample return mission, Asteroid mining and Colonization of the asteroids

951 Gaspra is the first asteroid to be imaged in close-up (enhanced color).

Vesta imaged by the Dawn spacecraft

Several views of 433 Eros in natural colour

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.[73]

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 13 June 2010.

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

In September 2007, NASA launched the Dawn Mission, which orbited 4 Vesta from July 2011 to September 2012, and is planned to orbit the dwarf planet 1 Ceres in 2015. 4 Vesta is the second-largest asteroid visited to date.

On 13 December 2012, China's lunar orbiter Chang'e 2 flew within 2 miles (3.2 km) of the asteroid 4179 Toutatis on an extended mission.

Planned and future missions[edit]

The Japan Aerospace Exploration Agency (JAXA) plans to launch around 2015 the improved Hayabusa 2 space probe and to return asteroid samples by 2020. Current target for the mission is the C-type asteroid (162173) 1999 JU 3 .

In May 2011, NASA announced the OSIRIS-REx sample return mission to asteroid 1999 RQ36, and is expected to launch in 2016.

On 15 February 2013, an asteroid measuring approximately 18 metres (59 feet) with a mass of about 9,100 tonnes (10,000 short tons) exploded over Chelyabinsk, Russia causing 1,500 injuries and damaging 7,000 buildings. Small samples of the rocky Chelyabinsk meteorite were quickly recovered and analyzed with a larger fragment found several months later.

In early 2013, NASA announced the planning stages of a mission to capture a near-Earth asteroid and move it into lunar orbit where it could possibly be visited by astronauts and later impacted into the Moon.[74] 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.[75]

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[edit]

Main article: Asteroids in 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.

MeteoroidFrom Wikipedia, the free encyclopediaJump to: navigation, search See also: Meteor and Meteorite

From a meteoroid to a meteor and meteorite.Animated illustration of different phases as a meteoroid enters the atmosphere to become visible as a meteor and impact the Earth's surface as a meteorite.

A meteoroid is a small rocky or metallic body travelling through space. Meteoroids are significantly smaller than asteroids, and range in size from small grains to 1 meter-wide objects. Objects smaller than this are classified as micrometeoroids or space dust.[1][2][3][4] Most are fragments from comets or asteroids, whereas others are collision impact debris ejected from bodies such as the Moon or Mars.[5][6][7][8]

When a meteoroid, comet or asteroid enters the Earth's atmosphere at a speed typically in excess of 20 km/s (72,000 km/h; 45,000 mph), aerodynamic heating produces a streak of light, both from the glowing object and the trail of glowing particles that it leaves in its wake. This phenomenon is called a meteor or "shooting star". A series of many meteors appearing seconds or minutes apart, and appearing to originate from the same fixed point in the sky, is called a meteor shower. If a meteor withstands ablation from its atmospheric entry and impacts with the ground, then it is called a meteorite.

Around 15,000 tonnes of meteoroids, micrometeoroids and different forms of space dust enter Earth's atmosphere each year.[9]

Meteoroids[edit]

See also: Micrometeoroid

Meteoroid embedded in aerogel. The meteoroid is 10 µm in diameter and its track is 1.5 mm long.

2008 TC3 meteorite fragments found on February 28, 2009 in the Nubian Desert, Sudan

In 1961, the International Astronomical Union defined a meteoroid as "a solid object moving in interplanetary space, of a size considerably smaller than an asteroid and considerably larger than an atom".[10][11] In 1995, Beech and Steel, writing in Quarterly Journal of the Royal Astronomical Society, proposed a new definition where a meteoroid would be between 100 µm and 10 meters across.[12] Following the discovery of asteroids below 10 m in size,[clarification needed] Rubin and Grossman refined the Beech and Steel definition of meteoroid to objects between 10 µm and 1 m in diameter.[2] According to Rubin and Grossman the minimum size of an asteroid is given by what can be discovered from Earth-bound telescopes, so the distinction between meteoroid and asteroid is fuzzy. The smallest asteroid ever discovered (based on absolute magnitude) is 2008 TS26 with an absolute magnitude of 33.2,[13] and an estimated size of 1-meter.[14] Objects smaller than meteoroids are classified as micrometeoroids and cosmic dust. The Minor Planet Center does not use the term "meteoroid".

Composition[edit]

Almost all meteoroids contain extraterrestrial nickel and irons. They have three main classifications, irons, stones and stony-irons. Some stone meteoroids contain grain-like inclusions known as chondrules and are called chondrites. Stoney meteoroids without these features are called "achondrites", which are typically formed from extraterrestrial igneous activity; they contain little or no extraterrestrial iron.[15] The composition of meteoroids can be inferred as they pass through the Earth's atmosphere from their trajectories and the light spectra of the resulting meteor. Their effects on radio signals also give information, especially useful for daytime meteors which are otherwise very difficult to observe. From these trajectory measurements, meteoroids have been found to have many different orbits, some clustering in streams (see meteor showers) often associated with a parent comet, others apparently sporadic. Debris from meteoroid streams may eventually be scattered into other orbits. The light spectra, combined with trajectory and light curve measurements, have yielded various compositions and densities, ranging from fragile snowball-like objects with density about a quarter that of ice,[16] to nickel-iron rich dense rocks. The study of meteorites also gives insights into the composition of non-ephemeral meteoroids.

In the Solar System[edit]

Meteoroids travel around the Sun in a variety of orbits and at various velocities. The fastest ones move at about 42 kilometers per second through space in the vicinity of Earth's orbit.[citation needed] The Earth travels at about 29.6 kilometers per second. Thus, when meteoroids meet Earth's atmosphere head-on (which only occurs when meteors are in a retrograde orbit such as the Eta Aquarids, which are

associated with the retrograde Halley's Comet), the combined speed may reach about 71 kilometers per second. Meteoroids moving through Earth's orbital space average about 20 km/s.[17]

On January 17, 2013 at 05:21 PST, a 1 meter-sized comet from the Oort cloud entered Earth atmosphere over a wide area in California and Nevada.[18] The object had a retrograde orbit with perihelion at 0.98 ± 0.03 AU. It approached from the direction of the constellation Virgo, and collided head-on with Earth atmosphere at 72 ± 6 km/s[18] vapourising more than 100 km above ground over a period of several seconds.

Collision with Earth's atmosphere[edit]

When meteoroids intersect with the Earth's atmosphere at night, they are likely to become visible as meteors. If meteoroids survive the entry through the atmosphere and reach the Earth's surface, they are called meteorites. Meteorites are transformed in structure and chemistry by the heat of entry and force of impact. A noted 4-meter asteroid, 2008 TC3, was observed in space on a collision course with Earth on 6 October 2008 and entered the Earth's atmosphere the next day, striking a remote area of northern Sudan. It was the first time that a meteoroid had been observed in space and tracked prior to impacting Earth.[10] NASA has produced a map showing the most notable asteroid collisions with Earth and its atmosphere from 1994 to 2013 from data gathered by U.S. government sensors (see below).

Meteors[edit]

"Meteor" redirects here. For other uses, see Meteor (disambiguation).

Meteor seen from the site of the Atacama Large Millimeter Array[19]

World map of large meteoric events (also see Fireball below) [20]

A meteor, known colloquially as a "shooting star" or "falling star", is the passage of a meteoroid, micrometeoroid, comet or asteroid into the Earth's atmosphere, heated from collisions with air particles in the upper atmosphere[21] and shedding glowing material in its wake sufficiently to create a visible

streak of light.[10][22] Meteors typically occur in the mesosphere at altitudes from 76 to 100 km (47 to 62 mi).[23] The root word meteor comes from the Greek meteōros, meaning "high in the air."[22]

Millions of meteors occur in the Earth's atmosphere daily. Most meteoroids that cause meteors are about the size of a grain of sand. Meteors may occur in showers, which arise when the Earth passes through a stream of debris left by a comet, or as "random" or "sporadic" meteors, not associated with a specific stream of space debris. A number of specific meteors have been observed, largely by members of the public and largely by accident, but with enough detail that orbits of the meteoroids producing the meteors have been calculated. All of the orbits passed through the asteroid belt.[24] The atmospheric velocities of meteors result from the movement of Earth around the Sun at about 30 km/s (18 miles/second),[25] the orbital speeds of meteoroids, and the gravity well of Earth.

Meteors become visible between about 75 to 120 km (47 to 75 mi) above the Earth. They usually disintegrate at altitudes of 50 to 95 km (31 to 59 mi).[26] Meteors have roughly a fifty percent chance of a daylight (or near daylight) collision with the Earth. Most meteors are, however, observed at night, when darkness allows fainter objects to be recognized. For bodies with a size scale larger than (10 cm to several meters) meteor visibility is due to the atmospheric ram pressure (not friction) that heats the meteoroid so that it glows and creates a shining trail of gases and melted meteoroid particles. The gases include vaporised meteoroid material and atmospheric gases that heat up when the meteoroid passes through the atmosphere. Most meteors glow for about a second.

History[edit]

Although meteors have been known since ancient times, they were not known to be an astronomical phenomenon until early in the 19th century. Prior to that, they were seen in the West as an atmospheric phenomenon, like lightning, and were not connected with strange stories of rocks falling from the sky. Thomas Jefferson wrote "I would more easily believe that (a) Yankee professor would lie than that stones would fall from heaven."[27] He was referring to Yale chemistry professor Benjamin Silliman's investigation of an 1807 meteorite that fell in Weston, Connecticut.[27] Silliman believed the meteor had a cosmic origin, but meteors did not attract much attention from astronomers until the spectacular meteor storm of November 1833.[28] People all across the eastern United States saw thousands of meteors, radiating from a single point in the sky. Astute observers noticed that the radiant, as the point is now called, moved with the stars, staying in the constellation Leo.[29]

The astronomer Denison Olmsted made an extensive study of this storm, and concluded it had a cosmic origin. After reviewing historical records, Heinrich Wilhelm Matthias Olbers predicted the storm's return in 1867, which drew the attention of other astronomers to the phenomenon. Hubert A. Newton's more thorough historical work led to a refined prediction of 1866, which proved to be correct.[28] With Giovanni Schiaparelli's success in connecting the Leonids (as they are now called) with comet Tempel-Tuttle, the cosmic origin of meteors was now firmly established. Still, they remain an atmospheric phenomenon, and retain their name "meteor" from the Greek word for "atmospheric".[30]

Fireball[edit]

Play media

Footage of a superbolide, a very bright fireball that exploded over Chelyabinsk Oblast, Russia in 2013.

A fireball is a brighter-than-usual meteor. The International Astronomical Union (IAU) defines a fireball as "a meteor brighter than any of the planets" (apparent magnitude −4 or greater).[31] The International Meteor Organization (an amateur organization that studies meteors) has a more rigid definition. It defines a fireball as a meteor that would have a magnitude of −3 or brighter if seen at zenith. This definition corrects for the greater distance between an observer and a meteor near the horizon. For example, a meteor of magnitude −1 at 5 degrees above the horizon would be classified as a fireball because if the observer had been directly below the meteor it would have appeared as magnitude −6.[32]

Fireballs reaching apparent magnitude −14 or brighter are called bolides.[33] The IAU has no official definition of "bolide", and generally considers the term synonymous with "fireball". Astronomers often use "bolide" to identify an exceptionally bright fireball, particularly one that explodes. They are sometimes called detonating fireballs (also see List of meteor air bursts). It may also be used to mean a fireball which creates audible sounds. In the late twentieth century, bolide has also come to mean any object that hits the Earth and explodes, with no regard to its composition (asteroid or comet).[34] The word bolide comes from the Greek βολίς (bolis) [35] which can mean a missile or to flash. If the magnitude of a bolide reaches −17 or brighter it is known as a superbolide.[33][36] A relatively small percentage of fireballs hit the Earth's atmosphere and then pass out again: these are termed Earth-grazing fireballs. Such an event happened in broad daylight over North America in 1972. Another rare phenomena are meteor processions, where the meteor breaks up into several fireballs traveling nearly parallel to the surface of the Earth.

For 2014 there were 3,751 fireballs recorded at the American Meteor Society.[37] There are probably more than 500,000 fireballs a year,[38] but most will go unnoticed because most will occur over the ocean and half will occur during daytime.

Fireball Sightings reported to the American Meteor Society [37]

Year 2008 2009 2010 2011 2012 2013 2014

Number 726 692 948 1,629 2,326 3,556 3,751

Effect on atmosphere[edit]

"Ionization trail" and "Dark flight (astronomy)" redirect here. For the movie, see Dark Flight.

Ionization trail of a Perseid meteor seen against the constellation Corona Borealis with its ring of stars

Entry of meteoroids into the Earth's atmosphere produces three main effects: ionization of atmospheric molecules, dust that the meteoroid sheds, and the sound of passage. During the entry of a meteoroid or asteroid into the upper atmosphere, an ionization trail is created, where the air molecules are ionized by the passage of the meteor. Such ionization trails can last up to 45 minutes at a time.

Small, sand-grain sized meteoroids are entering the atmosphere constantly, essentially every few seconds in any given region of the atmosphere, and thus ionization trails can be found in the upper atmosphere more or less continuously. When radio waves are bounced off these trails, it is called meteor burst communications. Meteor radars can measure atmospheric density and winds by measuring the decay rate and Doppler shift of a meteor trail. Most meteoroids burn up when they enter the atmosphere. The left-over debris is called meteoric dust or just meteor dust. Meteor dust particles can persist in the atmosphere for up to several months. These particles might affect climate, both by scattering electromagnetic radiation and by catalyzing chemical reactions in the upper atmosphere.[39] Larger meteors can enter dark flight after deceleration where the meteorite (or fragments) fall at terminal velocity.[40] Dark flight starts when the meteorite(s) decelerate to about 2–4 km/s (4,500–8,900 mph).[41] Larger fragments will fall further down the strewn field.

Colors[edit]

A meteor of the Leonid meteor shower. The photograph shows the meteor, afterglow, and wake as distinct components.

The visible light produced by a meteor may take on various hues, depending on the chemical composition of the meteoroid, and the speed of its movement through the atmosphere. As layers of the meteoroid abrade and ionize, the color of the light emitted may change according to the layering of minerals. Colors of meteors depend on the relative influence of the metallic content of the meteoroid versus the superheated air plasma, which its passage engenders:[42]

Orange-yellow (sodium)

Yellow (iron)

Blue-green (magnesium)

Violet (calcium)

Red (atmospheric nitrogen and oxygen)

Acoustic manifestations[edit]

Sound generated by a meteor in the upper atmosphere, such as a sonic boom, typically arrives many seconds after the visual light from a meteor disappears. Occasionally, as with the Leonid meteor shower of 2001,"crackling", "swishing", or "hissing" sounds have been reported,[43] occurring at the same instant as a meteor flare. Similar sounds have also been reported during intense displays of Earth's auroras.[44][45][46][47]

Theories on the generation of these sounds may partially explain them. For example, scientists at NASA suggested that the turbulent ionized wake of a meteor interacts with the Earth's magnetic field, generating pulses of radio waves. As the trail dissipates, megawatts of electromagnetic power could be released, with a peak in the power spectrum at audio frequencies. Physical vibrations induced by the electromagnetic impulses would then be heard if they are powerful enough to make grasses, plants, eyeglass frames, and other conductive materials vibrate.[48][49][50][51] This proposed mechanism, although proven to be plausible by laboratory work, remains unsupported by corresponding measurements in the field. Sound recordings made under controlled conditions in Mongolia in 1998 support the contention that the sounds are real.[52] (Also see Bolide.)

Meteor shower[edit]

Main articles: Meteor shower and List of meteor showers

Multiple meteors photographed over an extended exposure time during a meteor shower

Meteor shower on chart

A meteor shower is the result of an interaction between a planet, such as Earth, and streams of debris from a comet or other source. The passage of the Earth through cosmic debris from comets and other sources is a recurring event in many cases. Comets can produce debris by water vapor drag, as demonstrated by Fred Whipple in 1951,[53] and by breakup. Each time a comet swings by the Sun in its orbit, some of its ice vaporizes and a certain amount of meteoroids will be shed. The meteoroids spread out along the entire orbit of the comet to form a meteoroid stream, also known as a "dust trail" (as opposed to a comet's "dust tail" caused by the very small particles that are quickly blown away by solar radiation pressure).

The frequency of fireball sightings increases by about 10-30% during the weeks of vernal equinox.[54] Even meteorite falls are more common during the northern hemisphere's spring season. Although this phenomenon has been known for quite some time, the reason behind the anomaly is not fully understood by scientists. Some researchers attribute this to an intrinsic variation in the meteoroid population along Earth's orbit, with a peak in big fireball-producing debris around spring and early summer. Others have pointed out that during this period the ecliptic is (in the northern hemisphere) high in the sky in the late afternoon and early evening. This means that fireball radiants with an asteroidal source are high in the sky (facilitating relatively high rates) at the moment the meteoroids "catch up" with earth, coming from behind going in the same direction as Earth. This causes relatively low relative speeds and from this low entry speeds, which facilitates survival of meteorites.[55] It also generates high fireball rates in the early evening, increasing chances of eye witness reports. This explains a part, but perhaps not all of the seasonal variation. Research is in progress for mapping the orbits of the meteors to gain a better understanding of the phenomenon.[56]

Notable meteors[edit]

See also: Near-Earth object § Notable objects

1992—Peekskill, New York

The Peekskill Meteorite was filmed on October 9, 1992 by at least 16 independent videographers.[57] Eyewitness accounts indicate the fireball entry of the Peekskill meteorite started over West Virginia at 23:48 UT (±1 min). The fireball, which traveled in a northeasterly direction, had a pronounced greenish colour, and attained an estimated peak visual magnitude of −13. During a luminous flight time that exceeded 40 seconds the fireball covered a ground path of some 700 to 800 km.[58] One meteorite recovered at Peekskill, New York, for which the event and object gained their name, had a mass of

12.4 kg (27 lb) and was subsequently identified as an H6 monomict breccia meteorite.[59] The video record suggests that the Peekskill meteorite had several companions over a wide area. The companions are unlikely to be recovered in the hilly, wooded terrain in the vicinity of Peekskill.

2009—Bone, Indonesia

A large fireball was observed in the skies near Bone, Indonesia on October 8, 2009. This was thought to be caused by an asteroid approximately 10 meters in diameter. The fireball contained an estimated energy of 50 kilotons of TNT, or about twice the Nagasaki atomic bomb. No injuries were reported.[60]

2009—Southwestern US

A large bolide was reported on 18 November 2009 over southeastern California, northern Arizona, Utah, Wyoming, Idaho and Colorado. At 00:07 local time a security camera at the high altitude W. L. Eccles Observatory (2930 m above sea level) recorded a movie of the passage of the object to the north.[61][62] Of particular note in this video is the spherical "ghost" image slightly trailing the main object (this is likely a lens reflection of the intense fireball), and the bright fireball explosion associated with the breakup of a substantial fraction of the object. An object trail can be seen to continue northward after the bright fireball event. The shock from the final breakup triggered seven seismological stations in northern Utah; a timing fit to the seismic data yielded a terminal location of the object at 40.286 N, -113.191 W, altitude 27 km.[63] This is above the Dugway Proving Grounds, a closed Army testing base.

2013—Chelyabinsk Oblast, Russia

The Chelyabinsk meteor was an extremely bright, exploding fireball, known as superbolide, measuring about 17 to 20 meters across, with an estimated initial mass of 11,000 tonnes, as the relatively small asteroid entered the earth's atmosphere.[64][65] It was the largest known natural object to have entered Earth's atmosphere since the Tunguska event in 1908. Over 1,500 people were injured mostly by glass from shattered windows caused by the air burst approximately 25 to 30 km above the environs of Chelyabinsk, Russia on 15 February 2013. An increasingly bright streak was observed during morning daylight with a large contrail lingering behind. At no less than 1 minute and up to at least 3 minutes after the object peaked in intensity (depending on distance from trail), a large concussive blast was heard that shattered windows and set-off car alarms, which was followed by a number of smaller explosions.[66]

Gallery of meteors[edit]

Orionid

Orionid

Two Orionids and Milky Way

Multi-colored Orionid

Orionid

The brightest meteor, a fireball, leaves a smoky, persistent trail drifting in high-altitude winds, which is seen at the right-hand side of the image left by Orionid.

A fireball seen over the desert of Central Australia. Although this occurred during the Lyrids, its North-East entry angle indicates it is sporadic.

Looking down from the International Space Station at a meteor as it passes through the atmosphere

Possible meteor photographed from Mars, March 7, 2004, by MER Spirit

Comet Shoemaker–Levy 9 colliding with Jupiter: The sequence shows fragment W turning into a fireball on the planets's dark side.

Meteorites[edit]

Main article: Meteorite

Murnpeowie meteorite, an iron meteorite with regmaglypts resembling thumbprints (Australia, 1910)

A meteorite is a portion of a meteoroid or asteroid that survives its passage through the atmosphere and hits the ground without being destroyed.[67] Meteorites are sometimes, but not always, found in association with hypervelocity impact craters; during energetic collisions, the entire impactor may be vaporized, leaving no meteorites. Geologists use the term, "bolide", in a different sense from astronomers to indicate a very large impactor. For example, the USGS uses the term to mean a generic large crater-forming projectile in a manner "to imply that we do not know the precise nature of the impacting body ... whether it is a rocky or metallic asteroid, or an icy comet for example".[68]

Meteoroids also hit other bodies in the solar system. On such stony bodies as the Moon or Mars with no or little atmosphere, they leave enduring craters.

Frequency of impacts[edit]

See also: Planet Earth collision probability with near-Earth objects

The diameter of the biggest impactor to hit Earth on any given day is likely to be about 40 centimeters (16 inches) in a given year about 4 meters, and in a given century about 20 meters. These statistics are obtained by the following:

Over at least the range from 5 centimeters (2.0 inches) to roughly 300 meters (980 feet), the rate at which Earth receives meteors obeys a power-law distribution as follows:

where N (>D) is the expected number of objects larger than a diameter of D meters to hit Earth in a year.[69] This is based on observations of bright meteors seen from the ground and space, combined with surveys of near-Earth asteroids. Above 300 meters in diameter, the predicted rate is somewhat higher, with a two-kilometer asteroid (one million-megaton TNT equivalent) every couple of million years — about 10 times as often as the power-law extrapolation would predict.

Impact craters[edit]

Main article: Impact crater

Meteoroid collisions with solid Solar System objects, including the Moon, Mercury, Callisto, Ganymede and most small moons and asteroids, create impact craters, which are the dominant geographic features of many of those objects. On other planets and moons with active surface geological processes, such as Earth, Venus, Mars, Europa, Io and Titan, visible impact craters may become eroded, buried or transformed by tectonics over time. In early literature, before the significance of impact cratering was widely recognised, the terms cryptoexplosion or cryptovolcanic structure were often used to describe what are now recognised as impact-related features on Earth.[70] Molten terrestrial material ejected from a meteorite impact crater can cool and solidify into an object known as a tektite. These are often mistaken for meteorites.

Gallery of meteorites[edit]

Two tektites, molten terrestrial ejecta from a meteorite impact

A partial slice of the Esquel pallasite

Willamette Meteorite, from Oregon, USA

Meteorite, which fell in Wisconsin in 1868.

Marília Meteorite, a chondrite H4, which fell in Marília, Brazil (1971)

Children posing behind the Tucson Meteorite at the Arizona Museum of Natural History

Meteorite with brecciation and carbon inclusions from Tindouf, Algeria[71]