The Moon’s diameter is 3,474 kilometres (2,159 mi),[4] a little more than a quarter of that of the Earth. Thus, the Moon’s surface area is less than a tenth that of the Earth (about a quarter the Earth’s land area, approximately as large as Russia, Canada, and the United States combined), and its volume is about 2 percent that of Earth. The pull of gravity at its surface is about 17 percent of that at the Earth’s surface.

The Moon is the only celestial body on which human beings have made a manned landing. While the Soviet Union’s Luna programme was the first to reach the Moon with unmanned spacecraft, the NASA Apollo program achieved the only manned missions to date, beginning with the first manned lunar mission by Apollo 8 in 1968, and six manned lunar landings between 1969 and 1972 – the first being Apollo 11 in 1969. Human exploration of the Moon temporarily ceased with the conclusion of the Apollo program, although a few robotic landers and orbiters have been sent to the Moon since that time. The U.S. has committed to return to the Moon by 2018.[5][6][7]

Name and etymology
The proper English name for Earth’s natural satellite is, simply, the Moon (capitalized).[8][9] Moon is a Germanic word, related to the Latin mensis (month). It is ultimately a derivative of the Proto-Indo-European root me-, also represented in measure[10] (time), with reminders of its importance in measuring time in words derived from it like Monday, month and menstrual. The related adjective is lunar, as well as an adjectival prefix seleno- and suffix -selene (from selēnē, σελήνη, the Ancient Greek word for the Moon). In English, the word moon exclusively meant “the Moon” until 1665, when it was extended to refer to the recently discovered natural satellites of other planets.[10] Subsequently, these objects were given distinct names in order to avoid confusion.[9] The Moon is occasionally referred to by its Latin name Luna, primarily in science fiction.

Lunar surface

Two sides of the Moon
The Moon is in synchronous rotation, which means it rotates about its axis in about the same time it takes to orbit the Earth. This results in it keeping nearly the same face turned towards the Earth at all times. The Moon used to rotate at a faster rate, but early in its history, its rotation slowed and became locked in this orientation as a result of frictional effects associated with tidal deformations caused by the Earth.[11]

Small variations (libration) in the angle from which the Moon is seen allow about 59% of its surface to be seen from the Earth (but only half at any instant).[4]

The side of the Moon that faces Earth is called the near side, and the opposite side the far side. The far side is often inaccurately called the “dark side,” but in fact, it is illuminated exactly as often as the near side: once per lunar day, during the new Moon phase we observe on Earth when the near side is dark. The far side of the Moon was first photographed by the Soviet probe Luna 3 in 1959. One distinguishing feature of the far side is its almost complete lack of maria.

Maria
The dark and relatively featureless lunar plains which can clearly be seen with the naked eye are called maria (singular mare), Latin for seas, since they were believed by ancient astronomers to be filled with water. These are now known to be vast solidified pools of ancient basaltic lava. The majority of these lavas erupted or flowed into the depressions associated with impact basins that formed by the collisions of meteors and comets with the lunar surface. (Oceanus Procellarum is a major exception in that it does not correspond to a known impact basin). Maria are found almost exclusively on the near side of the Moon, with the far side having only a few scattered patches covering about 2% of its surface,[12] compared with about 31% on the near side.[4] The most likely explanation for this difference is related to a higher concentration of heat-producing elements on the near-side hemisphere, as has been demonstrated by geochemical maps obtained from the Lunar Prospector gamma-ray spectrometer.[13][14] Several provinces containing shield volcanoes and volcanic domes are found within the near side maria.[15]

Terrae
The lighter-colored regions of the Moon are called terrae, or more commonly just highlands, since they are higher than most maria. Several prominent mountain ranges on the near side are found along the periphery of the giant impact basins, many of which have been filled by mare basalt. These are hypothesized to be the surviving remnants of the impact basin’s outer rims.[16] In contrast to the Earth, no major lunar mountains are believed to have formed as a result of tectonic events.[17]

From images taken by the Clementine mission in 1994, it appears that four mountainous regions on the rim of the 73 km-wide Peary crater at the Moon’s north pole remain illuminated for the entire lunar day. These peaks of eternal light are possible because of the Moon’s extremely small axial tilt to the ecliptic plane. No similar regions of eternal light were found at the south pole, although the rim of Shackleton crater is illuminated for about 80% of the lunar day. Other consequences of the Moon’s small axial tilt are regions that remain in permanent shadow at the bottoms of many polar craters.[18]

Impact craters
The surface of Earth’s Moon is marked by impact craters[19] which form when asteroids and comets collide with the lunar surface. There are about half a million craters with diameters greater than 1 km on the Moon.[citation needed] Since impact craters accumulate at a nearly constant rate, the number of craters per unit area superposed on a geologic unit can be used to estimate the age of the surface (see crater counting). The lack of an atmosphere, weather and recent geological processes ensures that many of these craters have remained relatively well preserved in comparison to those on Earth.

The largest crater on the Moon, which also has the distinction of being one of the largest known craters in the Solar System,[20] is the South Pole-Aitken basin. It is on the far side, between the South Pole and equator, and is some 2,240 km in diameter and 13 km in depth.[21] Prominent impact basins on the near side include Imbrium, Serenitatis, Crisium, and Nectaris.

Regolith
Blanketed atop the Moon’s crust is a highly comminuted (broken into ever smaller particles) and “impact gardened” surface layer called regolith. Since the regolith forms by impact processes, the regolith of older surfaces is generally thicker than for younger surfaces. In particular, it has been estimated that the regolith varies in thickness from about 3–5 m in the maria, and by about 10–20 m in the highlands.[22] Beneath the finely comminuted regolith layer is what is generally referred to as the megaregolith. This layer is much thicker (on the order of tens of kilometres) and comprises highly fractured bedrock.[23]

Astronauts have reported that the dust from the surface felt like snow and smelled like spent gunpowder.[24] The dust is mostly made of silicon dioxide glass (SiO2), most likely created from the meteors that have crashed into the Moon’s surface. It also contains calcium and magnesium.

Presence of water
The continuous bombardment of the Moon by comets and meteoroids has most likely added small amounts of water to the lunar surface. If so, sunlight would split much of this water into its constituent elements of hydrogen and oxygen, both of which would ordinarily escape into space over time, because of the Moon’s weak gravity. However, because of the slightness of the axial tilt of the Moon’s spin axis to the ecliptic plane—only 1.5°—some deep craters near the poles never receive direct light from the Sun and are thus in permanent shadow (see Shackleton crater). Water molecules that ended up in these craters could be stable for long periods of time.

Clementine has mapped craters at the lunar south pole[25] that are shadowed in this way, and computer simulations suggest that up to 14,000 km² might be in permanent shadow.[18] Results from the Clementine mission bistatic radar experiment are consistent with small, frozen pockets of water close to the surface, and data from the Lunar Prospector neutron spectrometer indicate that anomalously high concentrations of hydrogen are present in the upper metre of the regolith near the polar regions.[26] Estimate for the quantity of water on the Moon is 32 ounces per one ton of top layer of Moon’s surface.

Water ice can be mined and then split into its constituent hydrogen and oxygen atoms by means of nuclear generators or electric power stations equipped with solar panels. The presence of usable quantities of water on the Moon is an important factor in rendering lunar habitation cost-effective, since transporting water from Earth would be prohibitively expensive. However, recent observations made with the Arecibo planetary radar suggest that some of the near-polar Clementine radar data that were previously interpreted as being indicative of water ice might instead be a result of rocks ejected from young impact craters.[27] The question of how much water there is on the Moon has not been resolved.

In July 2008, small amounts of water were found in the interior of volcanic pearls from the Moon (brought to Earth by Apollo 15).[28]

On September 24, 2009, the Indian Space Research Organisation (ISRO) reported that their first lunar mission, Chandrayaan-1 using NASA’s Moon Mineralogy Mapper, found evidence of large quantities of water on the Moon’s surface, and that water is still presently being formed.[29][30] The instrument observed an absorption line in the spectrum of sunlight reflected from the Moon, indicating that light of a particular wavelength (around 2.8 microns) is being absorbed more readily than other nearby wavelengths. The position and shape of the line indicate the absorption is due to water. A nearby line also revealed the presence of the closely-related molecule hydroxyl, which consists of an oxygen atom with a single hydrogen atom. The exact abundance of water was not determined, but the team believed it could be as high as 1,000 parts per million in the top layer of Lunar soil.

Orbit and relationship to Earth
The Moon makes a complete orbit around the Earth with respect to the fixed stars about once every 27.3 days[nb 3](its sidereal period). However, since the Earth is moving in its orbit about the Sun at the same time, it takes slightly longer for the Moon to show its same phase to Earth, which is about 29.5 days[nb 4] (its synodic period).[4] Unlike most satellites of other planets, the Moon orbits near the ecliptic and not the Earth’s equatorial plane. It is the largest moon in the solar system relative to the size of its planet. (Charon is larger relative to the dwarf planet Pluto.) The natural satellites orbiting other planets are called “moons”, after Earth’s Moon.

Most of the tidal effects seen on the Earth are caused by the Moon’s gravitational pull, with the Sun making a somewhat smaller contribution. Tidal drag slows the Earth’s rotation by about 0.002 seconds per day per century.[60] As a result of the conservation of angular momentum, the slowing of Earth’s rotation is accompanied by an increase of the mean Earth-Moon distance of about 3.8 m per century, or 3.8 cm per year.[61] The Moon is exceptionally large relative to the Earth, being a quarter the diameter of the planet and 1/81 its mass. However, the Earth and Moon are still commonly considered a planet-satellite system, rather than a double-planet system, since the common centre of mass of the system (the barycentre) is located about 1,700 km beneath the surface of the Earth, or about a quarter of the Earth’s radius. The surface of the Moon is less than one-tenth that of the Earth, and only about a quarter the size of the Earth’s land area (or about as large as Russia, Canada, and the U.S. combined).

The current obliquity of the Moon means that the Sun never rises above 1.85° at the poles. The axial tilt of the Moon has remained at its present orientation for the past two billion years, allowing the craters at the poles to remain in permanent shadow for that length of time.[62] Prior to that point, the Moon had much larger values for its obliquity, possibly reaching angles as high as 77° for periods of several hundred thousand years.[63]

In 1997, the asteroid 3753 Cruithne was found to have an unusual Earth-associated horseshoe orbit. However, astronomers do not consider it to be a second moon of Earth, and its orbit is not stable in the long term.[64] Three other near-Earth asteroids, 54509 YORP, (85770) 1998 UP1 and 2002 AA29, which exist in orbits similar to Cruithne’s, have since been discovered.[65]

Exploration
The first leap in lunar observation was prompted by the invention of the telescope. Galileo Galilei made good use of this new instrument and observed mountains and craters on the Moon’s surface.

The Cold War-inspired space race between the Soviet Union and the U.S. led to an acceleration of interest in the Moon. Unmanned probes, both flyby and impact/lander missions, were sent almost as soon as launcher capabilities would allow. The Soviet Union’s Luna program was the first to reach the Moon with unmanned spacecraft. The first man-made object to escape Earth’s gravity and pass near the Moon was Luna 1, the first man-made object to impact the lunar surface was Luna 2, and the first photographs of the normally occluded far side of the Moon were made by Luna 3, all in 1959. The first spacecraft to perform a successful lunar soft landing was Luna 9 and the first unmanned vehicle to orbit the Moon was Luna 10, both in 1966.[4] Moon samples have been brought back to Earth by three Luna missions (Luna 16, 20, and 24) and the Apollo missions 11 to 17 (except Apollo 13, which aborted its planned lunar landing).

The landing of the first humans on the Moon in 1969 is seen by many as the culmination of the space race.[74] Neil Armstrong became the first person to walk on the Moon as the commander of the American mission Apollo 11 by first setting foot on the Moon at 02:56 UTC on July 21, 1969. The American Moon landing and return was enabled by considerable technological advances, in domains such as ablation chemistry and atmospheric re-entry technology, in the early 1960s.

Scientific instrument packages were installed on the lunar surface during all of the Apollo missions. Long-lived ALSEP stations (Apollo lunar surface experiment package) were installed at the Apollo 12, 14, 15, 16, and 17 landing sites, whereas a temporary station referred to as EASEP (Early Apollo Scientific Experiments Package) was installed during the Apollo 11 mission. The ALSEP stations contained, among others, heat flow probes, seismometers, magnetometers, and corner-cube retroreflectors. Transmission of data to Earth was terminated on September 30, 1977 because of budgetary considerations.[75][76] Since the lunar laser ranging (LLR) corner-cube arrays are passive instruments, they are still being used. Ranging to the LLR stations is routinely performed from earth-based stations with an accuracy of a few centimetres, and data from this experiment are being used to place constraints on the size of the lunar core.[77]

&0000000000000036.00000036 years, &0000000000000294.000000294 days have now passed since Eugene Cernan and Harrison Schmitt, as part of the mission Apollo 17, left the surface of the Moon on December 14, 1972 (Cernan being the last to enter the LM) and no one has set foot on it since.
Astronaut Buzz Aldrin photographed by Neil Armstrong during the first Moon landing on July 20, 1969.

From the mid-1960s to the mid-1970s, there were 65 instances of artificial objects reaching the Moon (both manned and robotic, with ten in 1971 alone), with the last being Luna 24 in 1976. Only 18 of these were controlled Moon landings, with nine completing a round trip from Earth and returning samples of Moon rocks. The Soviet Union then turned its primary attention to Venus and space stations, and the U.S. to Mars and beyond. In 1990, Japan orbited the Moon with the Hiten spacecraft, becoming the third country to place a spacecraft into lunar orbit. The spacecraft released a smaller probe, Hagormo, in lunar orbit, but the transmitter failed, thereby preventing further scientific use of the mission.

In 1994, the U.S. finally returned to the Moon, robotically at least, sending the Joint Defense Department/NASA spacecraft Clementine. This mission obtained the first near-global topographic map of the Moon, and the first global multispectral images of the lunar surface. This was followed by the Lunar Prospector mission in 1998. The neutron spectrometer on Lunar Prospector indicated the presence of excess hydrogen at the lunar poles, which is likely to have been caused by the presence of water ice in the upper few meters of the regolith within permanently shadowed craters. The European spacecraft Smart 1 was launched September 27, 2003 and was in lunar orbit from November 15, 2004 to September 3, 2006.

On January 14, 2004, U.S. President George W. Bush called for a plan to resume manned missions to the Moon by 2020 (see Vision for Space Exploration).[78] NASA is now planning for the construction of a permanent outpost at one of the lunar poles.[79] The People’s Republic of China has expressed ambitious plans for exploring the Moon and has started the Chang’e program for lunar exploration, successfully launching its first spacecraft, Chang’e-1, on October 24, 2007. Like NASA, China hopes to land people on the Moon by 2020.[80] The U.S. launched the Lunar Reconnaissance Orbiter and the Lunar Crater Observation and Sensing Satellite on June 18, 2009 (the two missions were co-manifested). Russia also announced to resume its previously frozen project Luna-Glob, consisting of an unmanned lander and orbiter, which is slated to land in 2012.[81]

The Google Lunar X Prize, announced September 13, 2007, hopes to boost and encourage privately funded lunar exploration. The X Prize Foundation is offering anyone US$20 million who can land a robotic rover on the Moon and meet other specified criteria.

On September 14, 2007 the Japan Aerospace Exploration Agency launched SELENE, also known as Kaguya, a lunar orbiter which is fitted with a high-definition camera and two small satellites. The mission is expected to last one year.[82]

On October 22, 2008 India successfully launched the Chandrayaan I (a Sanskrit word literally meaning the ‘Moon-craft’) unmanned mission to the Moon and intends to launch several further unmanned missions. The country plans to launch Chandrayaan II in 2010 or 2011, which is slated to include a robotic lunar rover. India also has expressed its hope for a manned mission to the Moon by 2020.[83]

Terminology
Traditionally, small bodies orbiting the Sun were classified as asteroids, comets or meteoroids, with anything smaller than ten metres across being called a meteoroid.[1] The term “asteroid” is somewhat ill-defined. It never had a formal definition, with the broader term minor planet being preferred by the International Astronomical Union until 2006, when the term “small Solar System body” (SSSB) was introduced to cover both minor planets and comets. The 2006 definition of SSSB says that they “include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies”.[2] Other languages prefer “planetoid” (Greek for “planet-like”), and this term is occasionally used in English for the larger asteroids. The word “planetesimal” has a similar meaning, but refers specifically to the small building blocks of the planets that existed at the time the Solar System was forming. The term “planetule” was coined by the geologist William Daniel Conybeare to describe minor planets,[3] but is not in common use.

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.[citation needed]

For almost two centuries, from the discovery of the first asteroid, 1 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 additional small bodies that permanently resided further out than Jupiter, now called centaurs, they numbered them among the traditional asteroids, though there was debate over whether they should be classified as asteroids or as a new type of object. Then, when the first trans-Neptunian object, 1992 QB1, was discovered in 1992, and especially when large numbers of similar objects started turning up, new terms were invented to sidestep the issue: Kuiper Belt object (KBO), trans-Neptunian object (TNO), scattered-disc object (SDO), 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 TNOs were to venture close to the Sun, their volatile ices would sublimate, and traditional approaches would classify them as comets rather than asteroids.

The innermost of these are the Kuiper Belt Objects (KBOs), called “objects” partly to avoid the need to classify them as asteroids or comets.[4] KBOs are believed to be predominantly comet-like in composition, though some may be more akin to asteroids.[5] Furthermore, most do not have the highly eccentric orbits associated with comets, and the ones so far discovered are very much 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,[6] suggesting “a continuum between asteroids and comets” rather than a sharp dividing line.[7]

The minor planets beyond Jupiter’s orbit are rarely directly referred to as “asteroids”, but all are commonly lumped together under the term “asteroid” in popular presentations. For instance, a joint NASA-JPL public-outreach website states,

We include Trojans (bodies captured in Jupiter’s 4th and 5th Lagrange points), Centaurs (bodies in orbit between Jupiter and Neptune), and trans-Neptunian objects (orbiting beyond Neptune) in our definition of “asteroid” as used on this site, even though they may more correctly be called “minor planets” instead of asteroids.[8]

It is, however, becoming increasingly common for the term “asteroid” to be restricted to minor planets of the inner Solar System,[9] and therefore this article will restrict itself for the most part to the classical asteroids: objects of the main 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 which have sufficient 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.”[10] Currently only the largest object in the asteroid belt, Ceres, at about 950 km (590 mi) across, has been placed in the dwarf planet category, although there are several large asteroids (Vesta, Pallas, and Hygiea) that may be classified as dwarf planets when their shapes are better known.[11]

Formation
It is believed that planetesimals in the main 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. Both 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.[12] At least two asteroids, Ceres and Vesta, grew large enough to melt and differentiate, with heavy metallic elements sinking to the core, leaving rocky minerals in the crust.[13]

In the Nice model, a large number of Kuiper Belt objects are captured in the outer Main Belt, at distances greater than 2.6 AU. Most were subsequently ejected by Jupiter, but those that remained may be the D-type asteroids, and possibly include Ceres.[14]

Characteristics
Objects in the main asteroid belt vary greatly in size, from a diameter of 950 kilometres for the dwarf planet Ceres and over 500 kilometres for the asteroids 2 Pallas and 4 Vesta down to rocks just tens of metres across.[note 1] A few of the largest are roughly spherical and are very much like miniature planets. The vast majority, however, are much smaller and are irregularly shaped.

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, whereas Vesta is thought to have a nickel-iron core, olivine mantle, and basaltic crust,[15] and 10 Hygiea appears to have a primitive composition of undifferentiated carbonaceous chondrite. Many, perhaps most, of the smaller asteroids are piles of rubble held together loosely by gravity. Some have moons or are co-orbiting pairs of binary asteroids. All three conditions, as well as scattered asteroid families, may be the result of collisions which disrupted a parent asteroid.

Asteroids are believed to contain traces of amino-acids and other organic compounds, and some speculate that asteroid impacts may have seeded the early Earth with the chemicals necessary to initiate life, or may have even brought life itself to Earth (see also Panspermia).[16]

Only one asteroid, 4 Vesta (which has a particularly reflective surface), is normally visible to the naked eye, and this only in very dark skies when it is favorably positioned. Very rarely, small asteroids passing close to Earth may be naked-eye visible for a short period of time.[17]

The orbits of asteroids are often influenced by the gravity of other bodies in the solar system or the Yarkovsky effect.

Distribution within the Solar System
The vast majority of known asteroids orbit within the main asteroid belt between the orbits of Mars and Jupiter, generally in relatively low-eccentricity (i.e., not very elongated) orbits. This belt is currently estimated to contain between 1.1 and 1.9 million asteroids larger than 1 km (1 mi) in diameter,[20] and millions of smaller ones.[21] It is thought that these asteroids are 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. Although fewer Trojan asteroids sharing Jupiter’s orbit are currently known, it is thought that there are as many as there are asteroids in the main belt.

The dwarf planet Ceres is the largest object in the asteroid belt, with a diameter of over 975 km (606 mi). The next largest are the asteroids 2 Pallas and 4 Vesta, both with diameters of over 500 km (311 mi). Normally Vesta is the only main belt asteroid that can, on occasion, become visible to the naked eye. However, on some very rare occasions, a near-Earth asteroid may briefly become visible without technical aid; see 99942 Apophis.
File:4 Vesta 1 Ceres Moon at 20 km (12 mi) per px.png
Left to right: 4 Vesta, 1 Ceres, Earth’s Moon

The mass of all the objects of the Main asteroid belt, lying between the orbits of Mars and Jupiter, is estimated to be about 3.0-3.6 × 1021 kg, or about 4 percent of the mass of the Moon. Of this, Ceres comprises 0.95 × 1021 kg, some 32 percent of the total.[22][23] Adding in the next three most massive asteroids, 4 Vesta (9%), 2 Pallas (7%), and 10 Hygiea (3%), brings this figure up to 51%; while the three after that, 511 Davida (1.2%), 704 Interamnia (1.0%), and 52 Europa (0.9%), only add another 3% to the total mass. The number of asteroids then increases rapidly as their individual masses decrease.

Various classes of asteroid have been discovered outside the main asteroid belt. Near-Earth asteroids have orbits in the vicinity of Earth’s orbit. Trojan asteroids are gravitationally locked into synchronisation with Jupiter, either leading or trailing the planet in its orbit. A couple trojans have been found orbiting with Mars.[note 2] A group of asteroids called Vulcanoids are hypothesised by some to lie very close to the Sun, within the orbit of Mercury, but none has so far been found.

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

Orbit groups and families
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 much tighter and result from the catastrophic break-up of a large parent asteroid sometime in the past.[24] Families have only been recognized within the main asteroid belt. They were first recognised by Kiyotsugu Hirayama in 1918 and are often called Hirayama families in his honor.

About 30% to 35% of the bodies in the main 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
Some asteroids have unusual horseshoe orbits that are co-orbital with the Earth or some other planet. Examples are 3753 Cruithne and 2002 AA29. The first instance of this type of orbital arrangement was discovered between Saturn’s moons Epimetheus and Janus.

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

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

Spectral classification
In 1975, an asteroid taxonomic system based on colour, albedo, and spectral shape was developed by Clark R. Chapman, David Morrison, and Ben Zellner.[25] 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 a number of other asteroid types. The number of types continues to grow as more asteroids are studied.

The two most widely used taxonomies currently 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.[26] 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 a number of smaller classes.[27]

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

Problems with spectral classification
Originally, spectral designations were based on inferences of an asteroid’s composition.[28] However, the correspondence between spectral class and composition is not always very good, and there are a variety of classifications in use. This has led to significant confusion. While asteroids of different spectral classifications are likely to be composed of different materials, there are no assurances that asteroids within the same taxonomic class are composed of similar materials.

At present, the spectral classification based on several coarse resolution spectroscopic surveys in the 1990s is still the standard. Scientists have been unable to agree on a better taxonomic system,[citation needed] largely due to the difficulty of obtaining detailed measurements consistently for a large sample of asteroids (e.g. finer resolution spectra, or non-spectral data such as densities would be very useful).

Discovery
The first named minor planet, 1 Ceres, was discovered in 1801 by Giuseppe Piazzi, and was originally considered a new planet.[note 3] This was followed by the discovery of other similar bodies, which with the equipment of the time appeared to be points of light, like stars, showing little or no planetary disc (though readily distinguishable from stars due to their apparent motions). This prompted the astronomer Sir William Herschel to propose the term “asteroid”, from Greek αστεροειδής, asteroeidēs = star-like, star-shaped, from ancient Greek Aστήρ, astēr = star. 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 8 new asteroids in one year. The planet Lydia (No. 110), discovered by M. Borelly at the Marseilles Observatory [...] M. Borelly had previously discovered 2 planets bearing the numbers 91 and 99 in the system of asteroids revolving between Mars and Jupiter” (emphasis added).

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

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

The first asteroid, 1 Ceres, was not discovered by a member of the group, but rather by accident in 1801 by Giuseppe Piazzi, director of the observatory of Palermo in Sicily. He discovered a new star-like object in Taurus and followed the displacement of this object during several nights. His colleague, Carl Friedrich Gauss, used these observations to determine the exact distance from this unknown object to the Earth. Gauss’ calculations placed the object between the planets Mars and Jupiter. Piazzi named it after Ceres, the Roman goddess of agriculture.

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

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

In 1891, however, Max Wolf pioneered the use of astrophotography to detect asteroids, which appeared as short streaks on long-exposure photographic plates. This dramatically increased the rate of detection compared with previous visual methods: Wolf alone discovered 248 asteroids, beginning with 323 Brucia, whereas only slightly more than 300 had been discovered up to that point. Still, a century later, only a few thousand asteroids were identified, numbered and named. It was known that there were many more, but most astronomers did not bother with them, calling them “vermin of the skies”.

Manual methods of the 1900s and modern reporting
Until 1998, asteroids were discovered by a four-step process. First, a region of the sky was photographed by a wide-field telescope, or Astrograph. Pairs of photographs were taken, typically one hour apart. Multiple pairs could be taken over a series of days. Second, the two films of the same region were viewed under a stereoscope. Any body in orbit around the Sun would move slightly between the pair of films. Under the stereoscope, the image of the body would appear to float slightly above the background of stars. Third, once a moving body was identified, its location would be measured precisely using a digitizing microscope. The location would be measured relative to known star locations.[29]

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 final 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 previous apparitions into a single orbit. If so, the object receives a catalogue number and the observer of the first apparition with a calculated orbit is declared the discoverer, and granted the honor of naming the object subject to the approval of the International Astronomical Union.

Computerized methods
There is increasing interest in identifying asteroids whose orbits cross Earth’s, and that could, given enough time, collide with Earth (see Earth-crosser asteroids). The three most important groups of near-Earth asteroids are the Apollos, Amors, and Atens. Various asteroid deflection strategies have been proposed, as early as the 1960s.

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

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

All of these considerations helped spur the launch of highly efficient automated systems that consist of Charge-Coupled Device (CCD) cameras and computers directly connected to telescopes. Since 1998, a large majority of the asteroids have been discovered by such automated systems. A list of teams using such automated systems includes:[30]

* The Lincoln Near-Earth Asteroid Research (LINEAR) team
* The Near-Earth Asteroid Tracking (NEAT) team
* Spacewatch
* The Lowell Observatory Near-Earth-Object Search (LONEOS) team
* The Catalina Sky Survey (CSS)
* The Campo Imperatore Near-Earth Objects Survey (CINEOS) team
* The Japanese Spaceguard Association
* The Asiago-DLR Asteroid Survey (ADAS)

The LINEAR system alone has discovered 97,470 asteroids, as of September 18, 2008.[31] Between all of the automated systems, 4711 near-Earth asteroids have been discovered[32] including over 600 more than 1 km (1 mi) in diameter. The rate of discovery peaked in 2000, when 38,679 minor planets were numbered, and has been going down steadily since then (719 minor planets were numbered in 2007).[33]

Exploration
Until the age of space travel, objects in the asteroid belt were merely pinpricks of light in even the largest telescopes and their shapes and terrain remained a mystery. The best modern ground-based telescopes, as well as the Earth-orbiting Hubble Space Telescope, can resolve a small amount of detail on the surfaces of the very 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.

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

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

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

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

In September 2005, the Japanese Hayabusa probe started studying 25143 Itokawa in detail and may return samples of its surface to earth. The Hayabusa mission has been plagued with difficulties, including the failure of two of its three control wheels, rendering it difficult to maintain its orientation to the sun to collect solar energy. Following that, the next asteroid encounters will involve the European Rosetta probe (launched in 2004), which flew by 2867 Šteins in 2008 and will buzz 21 Lutetia in 2010.

In September 2007, NASA launched the Dawn Mission, which will orbit the dwarf planet Ceres and the asteroid 4 Vesta in 2011-2015, with its mission possibly then extended to 2 Pallas.

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

The Sun’s hot corona continuously expands in space creating the solar wind, a hypersonic stream of charged particles that extends to the heliopause at roughly 100 AU. The bubble in the interstellar medium formed by the solar wind, the heliosphere, is the largest continuous structure in the Solar System.[17] [18]

The Sun is currently traveling through the Local Interstellar Cloud in the low-density Local Bubble zone of diffuse high-temperature gas, in the inner rim of the Orion Arm of the Milky Way Galaxy, between the larger Perseus and Sagittarius arms of the galaxy. Of the 50 nearest stellar systems within 17 light-years (1.6×1014 km) from the Earth, the Sun ranks 4th [19] in mass. Slightly different values for the magnitude have been published, for example 4.85[20] and 4.81.[21] The Sun orbits the center of the Milky Way galaxy at a distance of approximately 24,000–26,000 light years from the galactic center, moving generally in the direction of Cygnus and completing one revolution in about 225–250 million years (one Galactic year). Its orbital speed was thought to be 220 ± 20, km/s but a new estimate gives 251 km/s. [22][23] Since our galaxy is moving with respect to the cosmic microwave background radiation (CMB) in the direction of Hydra with a speed of 550 km/s, the Sun’s resultant velocity with respect to the CMB is about 370 km/s in the direction of Crater or Leo. [24]

Motion and location within the galaxy
The Sun lies close to the inner rim of the Milky Way Galaxy’s Orion Arm, in the Local Fluff or the Gould Belt, at a hypothesized distance of 7.5–8.5 kpc (25,000–28,000 lightyears) from the Galactic Center,[25][26][27][28] contained within the Local Bubble, a space of rarefied hot gas, possibly produced by the supernova remnant, Geminga.[29] The distance between the local arm and the next arm out, the Perseus Arm, is about 6,500 light-years.[30] The Sun, and thus the Solar System, is found in what scientists call the galactic habitable zone.

The Apex of the Sun’s Way, or the solar apex, is the direction that the Sun travels through space in the Milky Way. The general direction of the Sun’s galactic motion is towards the star Vega near the constellation of Hercules, at an angle of roughly 60 sky degrees to the direction of the Galactic Center. If one were to observe it from Alpha Centauri, the closest star system, the Sun would appear to be in the constellation Cassiopeia.[31]

The Sun’s orbit around the Galaxy is expected to be roughly elliptical with the addition of perturbations due to the galactic spiral arms and non-uniform mass distributions. In addition the Sun oscillates up and down relative to the galactic plane approximately 2.7 times per orbit. This is very similar to how a simple harmonic oscillator works with no drag force (damping) term. It has been argued that the Sun’s passage through the higher density spiral arms often coincides with mass extinctions on Earth, perhaps due to increased impact events.[32] It takes the Solar System about 225–250 million years to complete one orbit of the galaxy (a galactic year),[33] so it is thought to have completed 20–25 orbits during the lifetime of the Sun. The orbital speed of the Solar System about the center of the Galaxy is approximately 251 km/s.[22] At this speed, it takes around 1,400 years for the Solar System to travel a distance of 1 light-year, or 8 days to travel 1 AU.[34]
Motion of Barycenter of Solar System relative to the Sun.

The Sun’s motion about the centre of mass of the Solar System is complicated by perturbations from the planets. Every few hundred years this motion switches between prograde and retrograde[35].

Characteristics
The Sun is a yellow main sequence star comprising about 99.86% of the total mass of the Solar System. It is a near-perfect sphere, with an oblateness estimated at about 9 millionths,[36] which means that its polar diameter differs from its equatorial diameter by only 10 km (6 mi). As the Sun exists in a plasmatic state and is not solid, it rotates faster at its equator than at its poles. This behavior is known as differential rotation. The period of this actual rotation is approximately 25.6 days at the equator and 33.5 days at the poles. However, due to our constantly changing vantage point from the Earth as it orbits the Sun, the apparent rotation of the star at its equator is about 28 days.[37] The centrifugal effect of this slow rotation is 18 million times weaker than the surface gravity at the Sun’s equator. The tidal effect of the planets is even weaker, and does not significantly affect the shape of the Sun.[38]

The Sun is a Population I, or heavy element-rich,[note 1] star.[39] The formation of the Sun may have been triggered by shockwaves from one or more nearby supernovae.[40] This is suggested by a high abundance of heavy elements in the Solar System, such as gold and uranium, relative to the abundances of these elements in so-called Population II (heavy element-poor) stars. These elements could most plausibly have been produced by endergonic nuclear reactions during a supernova, or by transmutation via neutron absorption inside a massive second-generation star.[39]

The Sun does not have a definite boundary as rocky planets do, and in its outer parts the density of its gases drops approximately exponentially with increasing distance from its center.[41] Nevertheless, it has a well-defined interior structure, described below. The Sun’s radius is measured from its center to the edge of the photosphere. This is simply the layer above which the gases are too cool or too thin to radiate a significant amount of light, and is therefore the surface most readily visible to the naked eye.[42]

The solar interior is not directly observable, and the Sun itself is opaque to electromagnetic radiation. However, just as seismology uses waves generated by earthquakes to reveal the interior structure of the Earth, the discipline of helioseismology makes use of pressure waves (infrasound) traversing the Sun’s interior to measure and visualize the star’s inner structure.[43] Computer modeling of the Sun is also used as a theoretical tool to investigate its deeper layers.

Core
The core of the Sun is considered to extend from the center to about 0.2 solar radius.[1] It is the hottest part of the Solar System. It has a density of up to 150,000 kg/m³ (150 times the density of liquid water) and a temperature of close to 15,000,000 kelvins (by contrast, the surface of the Sun is close to 6,000 kelvins). The core is made of hot, dense gas in the plasmic state.

Energy production
Energy is produced by exothermic thermonuclear reactions (nuclear fusion) that mainly convert hydrogen into helium. The core is the only location in the Sun that produces an appreciable amount of heat via fusion: the rest of the star is heated by energy that is transferred outward from the core. All of the energy produced by fusion in the core must travel through many successive layers to the solar photosphere before it escapes into space as sunlight or kinetic energy of particles.

Statistics
About 3.6 × 1038 protons (hydrogen nuclei) are converted into helium nuclei every second, releasing energy at the matter-energy conversion rate of 4.3 million tonnes per second, 380 yottawatts (3.8 × 1026 watts), equivalent to 9.1 × 1010 megatons of TNT per second. The rate of nuclear fusion depends strongly on density, so the fusion rate in the core is in a self-correcting equilibrium: a slightly higher rate of fusion would cause the core to heat up more and expand slightly against the weight of the outer layers, reducing the fusion rate and correcting the perturbation; and a slightly lower rate would cause the core to cool and shrink slightly, increasing the fusion rate and again reverting it to its present level.

Energy transfer
The high-energy photons (gamma rays and x-rays) released in fusion reactions take a long time to reach the Sun’s surface, slowed down by the indirect path taken, as well as by constant absorption and reemission at lower energies in the solar mantle. Estimates of the “photon travel time” range from as much as 50 million years[2] to as little as 17,000 years.[3] After a final trip through the convective outer layer to the transparent “surface” of the photosphere, the photons escape as visible light. Each gamma ray in the Sun’s core is converted into several million visible light photons before escaping into space. Neutrinos are also released by the fusion reactions in the core, but unlike photons they very rarely interact with matter, so almost all are able to escape the Sun immediately. For many years measurements of the number of neutrinos produced in the Sun were much lower than theories predicted, a problem which was recently resolved through a better understanding of the effects of neutrino oscillation.

Name and symbol
The word comet came to the English language through the Latin cometes from the Greek word komē, meaning “hair of the head”; Aristotle first used the derivation komētēs to depict comets as “stars with hair.” The astronomical symbol for comets (☄) accordingly consists of a disc with a hairlike tail.

Orbits and origin
Comets have a variety of different orbital periods, ranging from a few years, to hundreds of thousands of years, while some are believed to pass only once through the inner Solar System before being thrown out into interstellar space. Short-period comets are thought to originate in the Kuiper Belt, or associated scattered disc,[1] which lie beyond the orbit of Neptune. Long-period comets are believed to originate in the Oort cloud, consisting of debris left over from the condensation of the solar nebula, located well-beyond the Kuiper Belt. Comets are thrown from these outer reaches of the Solar System towards the Sun by gravitational perturbations from the outer planets (in the case of Kuiper Belt objects) or nearby stars (in the case of Oort Cloud objects), or as a result of collisions between objects within these regions.

Comets are distinguished from asteroids by the presence of a coma or tail, though very old comets that have lost all their volatile materials may come to resemble asteroids (see extinct comets).[2] Asteroids are also believed to have a different origin from comets, having formed in the inner Solar System rather than the outer Solar System,[3] but recent findings[4] have somewhat blurred the distinction between asteroids and comets (see centaurs and asteroid terminology).

As of May 2009[update] there are a reported 3,648 known comets[5] of which about 1500 are Kreutz Sungrazers and about 400 are short-period.[6] This number is steadily increasing. However, this represents only a tiny fraction of the total potential comet population: the reservoir of comet-like bodies in the outer solar system may number one trillion.[7] The number of comets visible to the naked-eye averages to roughly one per year, though many of these are faint and unspectacular.[8] When a historically bright or notable naked-eye comet is witnessed by many people, it may be termed a Great Comet.

Physical characteristics

Nucleus
Comet nuclei are known to range from about 100 meters to more than 40 kilometers across. They are composed of rock, dust, water ice, and frozen gases such as carbon monoxide, carbon dioxide, methane and ammonia.[9] Because of their low mass, comet nuclei do not become spherical under their own gravity, and thus have irregular shapes.

They are often popularly described as “dirty snowballs”, though recent observations have revealed dry dusty or rocky surfaces, suggesting that the ices are hidden beneath the crust (see Debate over comet composition). Comets also contain a variety of organic compounds; in addition to the gases already mentioned, these may include methanol, hydrogen cyanide, formaldehyde, ethanol and ethane, and perhaps more complex molecules such as long-chain hydrocarbons and amino acids.[10][11][12] In 2009, it was confirmed that the amino acid glycine had been found in the comet dust recovered by NASA’s Stardust mission.[13]

Surprisingly, cometary nuclei are among the darkest objects known to exist in the solar system. The Giotto probe found that Comet Halley’s nucleus reflects approximately 4% of the light that falls on it,[14] and Deep Space 1 discovered that Comet Borrelly’s surface reflects 2.4–3.0% of the light that falls on it;[14] by comparison, asphalt reflects 7% of the light that falls on it. It is thought that complex organic compounds are the dark surface material. Solar heating drives off volatile compounds leaving behind heavy long-chain organics[clarification needed] that tend to be very dark, like tar or crude oil. The very darkness of cometary surfaces allows them to absorb the heat necessary to drive their outgassing.

Coma and tail
In the outer solar system, comets remain frozen 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 the Hubble Space Telescope observations,[15][16] but these detections have been questioned,[17][18] and have not yet been independently confirmed. 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 thus released form a huge, extremely tenuous 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, which points away from the sun.

Both the coma and tail are illuminated by the Sun and may become visible from Earth when a comet passes through the inner solar system, the dust reflecting sunlight directly and the gases glowing from ionisation. 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. Occasionally a comet may experience a huge and sudden outburst of gas and dust, during which the size of the coma temporarily greatly increases in size. This happened in 2007 to Comet Holmes.[citation needed]

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[citation needed] tail called the antitail. At the same time, the ion tail, made of gases, always points directly away from the Sun, as this gas is more strongly affected by the solar wind than is dust, following magnetic field lines rather than an orbital trajectory. Parallax viewing from the Earth may sometimes mean the tails appear to point in opposite directions.[19]

While the solid nucleus of comets is generally less than 50 km across, the coma may be larger than the Sun, and ion tails have been observed to extend 1 astronomical unit (150 million km) or more.[9] The observation of antitails contributed significantly to the discovery of solar wind.[20] The ion tail is formed as a result of the photoelectric effect[dubious – discuss] of solar ultra-violet radiation acting on particles in the coma. Once the particles have been ionised, 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. As 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.[21]
Comet Encke loses its tail

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”.[21] This has been observed on a number of occasions, notable among which was on the 20th. April 2007 when the ion tail of comet Encke was completely severed as the comet passed through a coronal mass ejection. This event was observed by the STEREO spacecraft.[22]

Comets were found to emit X-rays in 1996.[23] This surprised researchers, because X-ray emission is usually associated with very high-temperature bodies. The X-rays are thought to be generated by the interaction between comets and the solar wind: when highly charged ions fly through a cometary atmosphere, they collide with cometary atoms and molecules, “ripping off” one or more electrons from the comet. This ripping off leads to the emission of X-rays and far ultraviolet photons.[24]

Orbital characteristics
Most comets have elongated elliptical orbits (oval shaped) 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. Comets are often classified according to the length of their orbital period; the longer the period the more elongated the ellipse.

* Short-period comets are generally defined as having orbital periods of less than 200 years. They usually orbit more-or-less in the ecliptic plane in the same direction as the planets. Their orbits typically take them out to the region of the outer planets (Jupiter and beyond) at aphelion; for example, Comet Halley’s aphelion is a little way beyond the orbit of Neptune. At the shorter extreme, Comet Encke has an orbit which never places it farther from the Sun than Jupiter. Short-period comets are further divided into the Jupiter family (periods less than 20 years) and Halley family (periods between 20 and 200 years).
* Long-period comets have highly eccentric (elongated) orbits and periods ranging from 200 years to thousands or even millions of years. (However, by definition they 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”.) Their orbits take them far beyond the outer planets at aphelia, and the plane of their orbits need not lie near the ecliptic.
* Single-apparition comets are similar to long-period comets, but have parabolic or hyperbolic trajectories which will cause them to permanently exit the solar system after passing the Sun once.[31]
* 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),[32] while others use it to mean exclusively short-period comets.[31] 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).
* Recently discovered main-belt comets form a distinct class, orbiting in more circular orbits within the asteroid belt.[33][34]

Based on their orbital characteristics, short-period comets are thought to originate from the centaurs and the Kuiper belt/scattered disk[1]—a disk of objects in the transneptunian 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).[35] 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 towards 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.

Since their elliptical orbits frequently take them close to the giant planets, comets are subject to further gravitational perturbations. Short period comets display a tendency for their aphelia to coincide with a giant planet’s orbital radius, with the Jupiter family of comets being the largest, as the histogram shows. 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, in addition to being the swiftest of the giant planets. These perturbations may sometimes deflect long-period comets into shorter orbital periods (Halley’s Comet being a possible example).

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 kilometres per second). If such objects entered the solar system, they would have positive total energies, and would be observed to have genuinely hyperbolic trajectories. A rough calculation shows that there might be four hyperbolic comets per century,[36] within Jupiter’s orbit, give or take one and perhaps two orders of magnitude.[citation needed]

A number of periodic comets discovered in earlier decades or previous centuries are now “lost.” Their orbits were never known well enough to predict future appearances. However, occasionally a “new” comet will be discovered and upon calculation of its orbit it turns out 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.[37]

Comet nomenclature
The names given to comets have followed several different conventions over the past two centuries. Before any systematic naming convention was adopted, comets were named in a variety of ways. Prior to the early 20th century, most comets were simply referred to by the year in which they appeared, sometimes with additional adjectives for particularly bright comets; thus, the “Great Comet of 1680″ (Kirch’s Comet), the “Great September Comet of 1882,” and the “Daylight Comet of 1910″ (“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, that comet became known as Comet Halley.[38] Similarly, the second and third known periodic comets, Comet Encke[39] and Comet Biela,[40] 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.

In the early 20th century, the convention of naming comets after their discoverers became common, and this remains so today. A comet is named after up to three independent discoverers. In recent years, many comets have been discovered by instruments operated by large teams of astronomers, and in this case, comets may be named for the instrument. For example, Comet IRAS-Araki-Alcock was discovered independently by the IRAS satellite and amateur astronomers Genichi Araki and George Alcock. In the past, when multiple comets were discovered by the same individual, group of individuals, or team, the comets’ names were distinguished by adding a numeral to the discoverers’ names (but only for periodic comets); thus Comets Shoemaker-Levy 1–9. Today, the large numbers of comets discovered by some instruments has rendered this system impractical, and no attempt is made to ensure that each comet has a unique name. Instead, the comets’ systematic designations are used to avoid confusion.

Until 1994, comets were first given a provisional designation consisting of the year of their discovery followed by a lowercase letter indicating its order of discovery in that year (for example, Comet 1969i (Bennett) was the 9th comet discovered in 1969). Once the comet had been observed through perihelion and its orbit had been established, the comet was given a permanent designation of the year of its perihelion, followed by a Roman numeral indicating its order of perihelion passage in that year, so that Comet 1969i became Comet 1970 II (it was the second comet to pass perihelion in 1970)[41]

Increasing numbers of comet discoveries made this procedure awkward, and in 1994 the International Astronomical Union approved a new naming system. Comets are now designated by the year of their discovery followed by a letter indicating the half-month of the discovery and a number indicating the order of discovery (a system similar to that already used for asteroids), so that the fourth comet discovered in the second half of February 2006 would be designated 2006 D4. Prefixes are also added to indicate the nature of the comet:

* P/ indicates a periodic comet (defined for these purposes as any comet with an orbital period of less than 200 years or confirmed observations at more than one perihelion passage);
* C/ indicates a non-periodic comet (defined as any comet that is not periodic according to the preceding definition);
* X/ indicates a comet for which no reliable orbit could be calculated (generally, historical comets);
* D/ indicates a comet which has broken up or been lost, referred to as dark comet;[42]
* A/ indicates an object that was mistakenly identified as a comet, but is actually a minor planet.

After their second observed perihelion passage, periodic comets are also assigned a number indicating the order of their discovery.[43] So Halley’s Comet, the first comet to be identified as periodic, has the systematic designation 1P/1682 Q1. Comet Hale-Bopp’s designation is C/1995 O1. Comets which first received a minor planet designation keep the latter, which leads to some odd names such as P/2004 EW38 (Catalina-LINEAR).

There are only five objects that are cross-listed as both comets and asteroids: 2060 Chiron (95P/Chiron), 4015 Wilson-Harrington (107P/Wilson-Harrington), 7968 Elst-Pizarro (133P/Elst-Pizarro), 60558 Echeclus (174P/Echeclus), and 118401 LINEAR (176P/LINEAR).

History of comet study

Early observations and thought
Before the invention of the telescope, comets seemed to appear out of nowhere in the sky and gradually vanish out of sight. They 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.[44] From ancient sources, such as Chinese oracle bones, it is known that their appearances have been noticed by humans for millennia. Some authorities interpret references to “falling stars” in Gilgamesh, the Book of Revelation and the Book of Enoch as references to comets, or possibly bolides.

In the first book of his Meteorology, Aristotle propounded the view of comets that would hold sway in Western thought for nearly two thousand years. He rejected the ideas of several earlier philosophers that comets were planets, or at least a phenomenon related to the planets, on the grounds that while the planets confined their motion to the circle of the Zodiac, comets could appear in any part of the sky.[45] Instead, he described comets as a phenomenon of the upper atmosphere, where hot, dry exhalations gathered and occasionally burst into flame. Aristotle held this mechanism responsible for not only comets, but also meteors, the aurora borealis, and even the Milky Way.[46]

A few later classical philosophers did dispute this view of comets. Seneca the Younger, in his Natural Questions, observed that comets moved regularly through the sky and were undisturbed by the wind, behavior more typical of celestial than atmospheric phenomena. While he conceded that the other planets do not appear outside the Zodiac, he saw no reason that a planet-like object could not move through any part of the sky, humanity’s knowledge of celestial things being very limited.[47] However, the Aristotelian viewpoint proved more influential, and it was not until the 16th century that it was demonstrated that comets must exist outside the Earth’s atmosphere.

In 1577, a bright comet was visible for several months. The Danish astronomer Tycho Brahe used measurements of the comet’s position taken by himself and other, geographically separated, observers to determine that the comet had no measurable parallax. Within the precision of the measurements, this implied the comet must be at least four times more distant from the earth than the moon.[48]

One very famous old recording of a comet is the appearance of Halley’s Comet on the Bayeux Tapestry, which records the Norman conquest of England in AD 1066.[49]

In popular culture
The depiction of comets in popular culture is firmly rooted in the long Western tradition of seeing comets as harbingers of doom and as omens of world-altering change.[78] Halley’s Comet alone has caused a slew of frightful or excited publications of all sorts at each of its reappearances. It was especially noted that the birth and death of some notable persons coincided with separate appearances of the comet, such as with writers Mark Twain (who correctly speculated that he’d “go out with the comet” in 1910)[78] and Eudora Welty, to whose life Mary Chapin Carpenter dedicated the song Halley Came to Jackson.[78]

In science fiction, the impact of comets has been depicted as a threat overcome by technology and heroism (Deep Impact, 1998), or as a trigger of global apocalypse (Lucifer’s Hammer, 1979) or of waves of zombies (Night of the Comet, 1984).[78] Near impacts have been depicted in Jules Verne’s Off on a Comet and Tove Jansson’s Comet in Moominland, while a human expedition visits Halley’s Comet in Arthur C. Clarke’s 2061: Odyssey Three.

Overview

Meteoroid
The current official definition of a meteoroid from the International Astronomical Union is “a solid object moving in interplanetary space, of a size considerably smaller than an asteroid and considerably larger than an atom.”[1] The Royal Astronomical Society has proposed a new definition where a meteoroid is between 100 µm and 10 m across.[2] The NEO definition includes larger objects, up to 50 m in diameter, in this category. Very small meteoroids are known as micrometeoroids (see also interplanetary dust).

The composition of meteoroids can be determined as they pass through Earth’s atmosphere from their trajectories and the light spectra of the resulting meteor. Their effects on radio signals also yield 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,[3] to nickel-iron rich dense rocks.

Meteoroids travel around the sun in a variety of orbits and at various velocities. The fastest ones move at about 26 miles per second (42 kilometers per second) through space in the vicinity of Earth’s orbit. The earth travels at about 18 miles per second (29 kilometers per second). Thus, when meteoroids meet the Earth’s atmosphere head-on (which would only occur if the meteor were in a retrograde orbit), the combined speed may reach about 44 miles per second (71 kilometers per second).

Meteor
A meteor is the visible streak of light that occurs when a meteoroid enters the Earth’s atmosphere. Meteors typically occur in the mesosphere, and most range in altitude from 75 km to 100 km.[4] Millions of meteors occur in the Earth’s atmosphere every day. Most meteoroids that cause meteors are about the size of a pebble. They become visible between about 40 and 75 miles (65 and 120 kilometers) above the earth. They disintegrate at altitudes of 30 to 60 miles (50 to 95 kilometers). Meteors have roughly a fifty percent chance of a daylight (or near daylight) collision with the Earth as the Earth orbits in the direction of roughly west at noon. Most meteors are, however, observed at night as low light conditions allow fainter meteors to be observed.

For bodies with a size scale larger than the atmospheric mean free path (10 cm to several metres)[clarification needed] the visibility is due to the air friction that heats the meteoroid so that it glows and creates a shining trail of gases and melted meteoroid particles. The gases include vaporized meteoroid material and atmospheric gases that heat up when the meteoroid passes through the atmosphere. Most meteors glow for about a second. A relatively small percentage of meteoroids hit the Earth’s atmosphere and then pass out again: these are termed Earth-grazing fireballs.

Meteors may occur in showers, which arise when the Earth passes through a trail of debris left by a comet, or as “random” or “sporadic” meteors, not associated with a specific single cause. 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 incoming meteors or meteorites have been calculated. All of them came from orbits from the vicinity of the asteroid belt.[5]

Fireball
A fireball is a brighter-than-usual meteor. The International Astronomical Union defines a fireball as “a meteor brighter than any of the planets” (magnitude -4 or greater).[6] 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.[7]

Bolide
The word bolide comes from the Greek βολις, (bolis) which can mean a missile or to flash. The IAU has no official definition of bolide and generally considers the term synonymous with fireball. The term is more often used among geologists than astronomers where it means a very large impactor. For example, the USGS uses the term to mean a generic large crater-forming projectile “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”.[8] Astronomers tend to use the term to mean an exceptionally bright fireball, particularly one that explodes (sometimes called a detonating fireball).

Meteorite
A meteorite is a portion of a meteoroid or asteroid that survives its passage through the atmosphere and impact with the ground without being destroyed.[9] 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.

Sound
There are anecdotal reports of sounds being heard from meteors entering the Earth’s atmosphere.[11] This would seem impossible, given the relatively slow speed of sound. Any sound generated by a meteor in the upper atmosphere, such as a sonic boom, should not be heard until many seconds after the meteor disappeared. However, in certain instances, for example during the Leonid meteor shower of 2001, several people reported sounds described as “crackling”, “swishing”, or “hissing”[12] occurring at the same instant as a meteor flare. Similar sounds have also been reported during intense displays of Earth’s auroras[citation needed].

Sound recordings made under controlled conditions in Mongolia in 1998 by a team led by Slaven Garaj, a physicist at the Swiss Federal Institute of Technology at Lausanne, support the contention that the sounds are real.[13]

How these sounds could be generated, assuming they are in fact real, remains something of a mystery. It has been hypothesized by some scientists at NASA as that the turbulent ionized wake of a meteor interacts with the magnetic field of the Earth, generating pulses of radio waves. As the trail dissipates, megawatts of electromagnetic energy 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.[14][15][16][17] This proposed mechanism, although proven to be plausible by laboratory work, remains unsupported by corresponding measurements in the field.

Frequency of large meteors
The biggest asteroid to hit Earth on any given day is likely to be about 40 centimeters, 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 inches) to roughly 300 meters (1,000 feet), the rate at which Earth receives meteors obeys a power-law distribution (meaning there is no typical size in the conventional sense) as follows :

N( > D) = 37D − 2.7

where N(>D) is the expected number of objects larger than a diameter of D meters to hit Earth in a year.[18] 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.

History
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.”[22] He was referring to Yale chemistry professor Benjamin Silliman’ investigation of an 1807 meteorite that fell in Weston, Connecticut.[22] 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.[23] People all across the Eastern US 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.[24]

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 its return in 1867, which drew the attention of other astronomers. Hubert A. Newton’s more thorough historical work led to a refined prediction of 1866, which proved to be correct.[23] 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.”[25]

Historically, galaxies have been categorized according to their apparent shape (usually referred to as their visual morphology). A common form is the elliptical galaxy,[5] which has an ellipse-shaped light profile. Spiral galaxies are disk-shaped assemblages with curving, dusty arms. Galaxies with irregular or unusual shapes are known as peculiar galaxies, and typically result from disruption by the gravitational pull of neighboring galaxies. Such interactions between nearby galaxies, which may ultimately result in galaxies merging, may induce episodes of significantly increased star formation, producing what is called a starburst galaxy. Small galaxies that lack a coherent structure could also be referred to as irregular galaxies.[6]

There are probably more than 100 billion (1011) galaxies in the observable universe.[7] Most galaxies are 1,000 to 100,000[4] parsecs in diameter and are usually separated by distances on the order of millions of parsecs (or megaparsecs).[8] Intergalactic space (the space between galaxies) is filled with a tenuous gas of an average density less than one atom per cubic meter. The majority of galaxies are organized into a hierarchy of associations called clusters, which, in turn, can form larger groups called superclusters. These larger structures are generally arranged into sheets and filaments, which surround immense voids in the universe.[9]

Although it is not yet well understood, dark matter appears to account for around 90% of the mass of most galaxies. Observational data suggests that supermassive black holes may exist at the center of many, if not all, galaxies. They are proposed to be the primary cause of active galactic nuclei found at the core of some galaxies. The Milky Way galaxy appears to harbor at least one such object within its nucleus.[10]

Observation history
The realization that we live in a galaxy, and that there were, in fact, many other galaxies, parallels discoveries that were made about the Milky Way and other nebulae in the night sky.

The Milky Way
The Greek philosopher Democritus (450–370 B.C.) proposed that the bright band on the night sky known as the Milky Way might consist of distant stars.[14] Aristotle (384–322 B.C.), however, believed the Milky Way to be caused by “the ignition of the fiery exhalation of some stars which were large, numerous and close together” and that the “ignition takes place in the upper part of the atmosphere, in the region of the world which is continuous with the heavenly motions.”[15] The Arabian astronomer, Alhazen (965–1037 A.D.), refuted this by making the first attempt at observing and measuring the Milky Way’s parallax,[16] and he thus “determined that because the Milky Way had no parallax, it was very remote from the earth and did not belong to the atmosphere.”[17]

The Persian astronomer, Abū Rayhān al-Bīrūnī (973–1048), proposed the Milky Way galaxy to be a collection of countless nebulous stars.[18] Ibn Bajjah (“Avempace”, d. 1138) proposed the Milky Way to be made up of many stars but appears to be a continuous image due to the effect of refraction in the Earth’s atmosphere.[15] Ibn Qayyim Al-Jawziyya (1292–1350) proposed the Milky Way galaxy to be “a myriad of tiny stars packed together in the sphere of the fixed stars” and that these stars are larger than planets.[19]

Actual proof of the Milky Way consisting of many stars came in 1610 when Galileo Galilei used a telescope to study the Milky Way and discovered that it is composed of a huge number of faint stars.[20] In 1750 Thomas Wright, in his An original theory or new hypothesis of the universe, speculated (correctly) that the Galaxy might be a rotating body of a huge number of stars held together by gravitational forces, akin to the solar system but on a much larger scale. The resulting disk of stars can be seen as a band on the sky from our perspective inside the disk.[21] In a treatise in 1755, Immanuel Kant elaborated on Wright’s idea about the structure of the Milky Way.
The shape of the Milky Way as deduced from star counts by William Herschel in 1785; the solar system was assumed to be near the center.

The first attempt to describe the shape of the Milky Way and the position of the Sun in it was carried out by William Herschel in 1785 by carefully counting the number of stars in different regions of the sky. He produced a diagram of the shape of the galaxy with the solar system close to the center.[22][23] Using a refined approach, Kapteyn in 1920 arrived at the picture of a small (diameter about 15 kiloparsecs) ellipsoid galaxy with the Sun close to the center. A different method by Harlow Shapley based on the cataloguing of globular clusters led to a radically different picture: a flat disk with diameter approximately 70 kiloparsecs and the Sun far from the center.[21] Both analyses failed to take into account the absorption of light by interstellar dust present in the galactic plane, but after Robert Julius Trumpler quantified this effect in 1930 by studying open clusters, the present picture of our galaxy, the Milky Way, emerged.[24]

Types and morphology
Galaxies come in three main types: ellipticals, spirals, and irregulars. A slightly more extensive description of galaxy types based on their appearance is given by the Hubble sequence. Since the Hubble sequence is entirely based upon visual morphological type, it may miss certain important characteristics of galaxies such as star formation rate (in starburst galaxies) and activity in the core (in active galaxies).[6]

Ellipticals
The Hubble classification system rates elliptical galaxies on the basis of their ellipticity, ranging from E0, being nearly spherical, up to E7, which is highly elongated. These galaxies have an ellipsoidal profile, giving them an elliptical appearance regardless of the viewing angle. Their appearance shows little structure and they typically have relatively little interstellar matter. Consequently these galaxies also have a low portion of open clusters and a reduced rate of new star formation. Instead the galaxy is dominated by generally older, more evolved stars that are orbiting the common center of gravity in random directions. In this sense they have some similarity to the much smaller globular clusters.[42]

The largest galaxies are giant ellipticals. Many elliptical galaxies are believed to form due to the interaction of galaxies, resulting in a collision and merger. They can grow to enormous sizes (compared to spiral galaxies, for example), and giant elliptical galaxies are often found near the core of large galaxy clusters.[43] Starburst galaxies are the result of such a galactic collision that can result in the formation of an elliptical galaxy.[42]

Spirals
Spiral galaxies consist of a rotating disk of stars and interstellar medium, along with a central bulge of generally older stars. Extending outward from the bulge are relatively bright arms. In the Hubble classification scheme, spiral galaxies are listed as type S, followed by a letter (a, b, or c) that indicates the degree of tightness of the spiral arms and the size of the central bulge. An Sa galaxy has tightly wound, poorly defined arms and possesses a relatively large core region. At the other extreme, an Sc galaxy has open, well-defined arms and a small core region.[44]

In spiral galaxies, the spiral arms do have the shape of approximate logarithmic spirals, a pattern that can be theoretically shown to result from a disturbance in a uniformly rotating mass of stars. Like the stars, the spiral arms also rotate around the center, but they do so with constant angular velocity. That means that stars pass in and out of spiral arms, with stars near the galactic core orbiting faster than the arms are moving while stars near the outer parts of the galaxy typically orbit more slowly than the arms. The spiral arms are thought to be areas of high density matter, or “density waves”. As stars move through an arm, the space velocity of each stellar system is modified by the gravitational force of the higher density. (The velocity returns to normal after the stars depart on the other side of the arm.) This effect is akin to a “wave” of slowdowns moving along a highway full of moving cars. The arms are visible because the high density facilitates star formation, and therefore they harbor many bright and young stars.
NGC 1300, an example of a barred spiral galaxy.

A majority of spiral galaxies have a linear, bar-shaped band of stars that extends outward to either side of the core, then merges into the spiral arm structure.[45] In the Hubble classification scheme, these are designated by an SB, followed by a lower-case letter (a, b or c) that indicates the form of the spiral arms (in the same manner as the categorization of normal spiral galaxies). Bars are thought to be temporary structures that can occur as a result of a density wave radiating outward from the core, or else due to a tidal interaction with another galaxy.[46] Many barred spiral galaxies are active, possibly as a result of gas being channeled into the core along the arms.[47]

Our own galaxy is a large disk-shaped barred-spiral galaxy[48] about 30 kiloparsecs in diameter and a kiloparsec in thickness. It contains about two hundred billion (2×1011)[49] stars and has a total mass of about six hundred billion (6×1011) times the mass of the Sun.[50]

Other morphologies
Peculiar galaxies are galactic formations that develop unusual properties due to tidal interactions with other galaxies. An example of this is the ring galaxy, which possesses a ring-like structure of stars and interstellar medium surrounding a bare core. A ring galaxy is thought to occur when a smaller galaxy passes through the core of a spiral galaxy.[51] Such an event may have affected the Andromeda Galaxy, as it displays a multi-ring-like structure when viewed in infrared radiation.[52]

A lenticular galaxy is an intermediate form that has properties of both elliptical and spiral galaxies. These are categorized as Hubble type S0, and they possess ill-defined spiral arms with an elliptical halo of stars.[53] (Barred lenticular galaxies receive Hubble classification SB0.)
NGC 5866, an example of a lenticular galaxy. Credit: NASA/ESA.

In addition to the classifications mentioned above, there are a number of galaxies that can not be readily classified into an elliptical or spiral morphology. These are categorized as irregular galaxies. An Irr-I galaxy has some structure but does not align cleanly with the Hubble classification scheme. Irr-II galaxies do not possess any structure that resembles a Hubble classification, and may have been disrupted.[54] Nearby examples of (dwarf) irregular galaxies include the Magellanic Clouds.

Dwarfs
Despite the prominence of large elliptical and spiral galaxies, most galaxies in the universe appear to be dwarf galaxies. These tiny galaxies are about one hundredth the size of the Milky Way, containing only a few billion stars. Ultra-compact dwarf galaxies have recently been discovered that are only 100 parsecs across.[55]

Many dwarf galaxies may orbit a single larger galaxy; the Milky Way has at least a dozen such satellites, with an estimated 300–500 yet to be discovered.[56] Dwarf galaxies may also be classified as elliptical, spiral, or irregular. Since small dwarf ellipticals bear little resemblance to large ellipticals, they are often called dwarf spheroidal galaxies instead.

A study of 27 Milky Way neighbors found that dwarf galaxies were all approximately 10 million solar masses, regardless of whether they have thousands or millions of stars. This has led to the suggestion that galaxies are largely formed by dark matter, and that the minimum size may indicate a form of warm dark matter incapable of gravitational coalescence on a smaller scale.[57]

Evolution
Within a billion years of a galaxy’s formation, key structures begin to appear. Globular clusters, the central supermassive black hole, and a galactic bulge of metal-poor Population II stars form. The creation of a supermassive black hole appears to play a key role in actively regulating the growth of galaxies by limiting the total amount of additional matter added.[77] During this early epoch, galaxies undergo a major burst of star formation.[78]

During the following two billion years, the accumulated matter settles into a galactic disc.[79] A galaxy will continue to absorb infalling material from high velocity clouds and dwarf galaxies throughout its life.[80] This matter is mostly hydrogen and helium. The cycle of stellar birth and death slowly increases the abundance of heavy elements, eventually allowing the formation of planets.[81]

The evolution of galaxies can be significantly affected by interactions and collisions. Mergers of galaxies were common during the early epoch, and the majority of galaxies were peculiar in morphology.[82] Given the distances between the stars, the great majority of stellar systems in colliding galaxies will be unaffected. However, gravitational stripping of the interstellar gas and dust that makes up the spiral arms produces a long train of stars known as tidal tails. Examples of these formations can be seen in NGC 4676[83] or the Antennae Galaxies.[84]

As an example of such an interaction, the Milky Way galaxy and the nearby Andromeda Galaxy are moving toward each other at about 130 km/s, and—depending upon the lateral movements—the two may collide in about five to six billion years. Although the Milky Way has never collided with a galaxy as large as Andromeda before, evidence of past collisions of the Milky Way with smaller dwarf galaxies is increasing.[85]

Such large-scale interactions are rare. As time passes, mergers of two systems of equal size become less common. Most bright galaxies have remained fundamentally unchanged for the last few billion years, and the net rate of star formation probably also peaked approximately ten billion years ago.[86]
[edit] Future trends

At present, most star formation occurs in smaller galaxies where cool gas is not so depleted.[82] Spiral galaxies, like the Milky Way, only produce new generations of stars as long as they have dense molecular clouds of interstellar hydrogen in their spiral arms.[87] Elliptical galaxies are already largely devoid of this gas, and so form no new stars.[88] The supply of star-forming material is finite; once stars have converted the available supply of hydrogen into heavier elements, new star formation will come to an end.[89]

The current era of star formation is expected to continue for up to one hundred billion years, and then the “stellar age” will wind down after about ten trillion to one hundred trillion years (1013–1014 years), as the smallest, longest-lived stars in our astrosphere, tiny red dwarfs, begin to fade. At the end of the stellar age, galaxies will be composed of compact objects: brown dwarfs, white dwarfs that are cooling or cold (“black dwarfs”), neutron stars, and black holes. Eventually, as a result of gravitational relaxation, all stars will either fall into central supermassive black holes or be flung into intergalactic space as a result of collisions.[89][90]

The term planet is ancient, with ties to history, science, mythology, and religion. The planets were originally seen by many early cultures as divine, or as emissaries of the gods. As scientific knowledge advanced, human perception of the planets changed, incorporating a number of disparate objects. In 2006, the International Astronomical Union officially adopted a resolution defining planets within the Solar System. This definition has been both praised and criticized, and remains disputed by some scientists.

The planets were thought by Ptolemy to orbit the Earth in deferent and epicycle motions. Though the idea that the planets orbited the Sun had been suggested many times, it was not until the 17th century that this view was supported by evidence from the first telescopic astronomical observations, performed by Galileo Galilei. By careful analysis of the observation data, Johannes Kepler found the planets’ orbits to be not circular, but elliptical. As observational tools improved, astronomers saw that, like Earth, the planets rotated around tilted axes, and some share such features as ice-caps and seasons. Since the dawn of the Space Age, close observation by probes has found that Earth and the other planets share characteristics such as volcanism, hurricanes, tectonics, and even hydrology. Since 1992, through the discovery of hundreds of planets around other stars, called extrasolar planets, scientists are beginning to understand that planets throughout the Milky Way Galaxy share characteristics in common with our own. As of September 2009, there are 374 known extrasolar planets, ranging from the size of gas giants to that of terrestrial planets.[3]

Planets are generally divided into two main types: large, low-density gas giants, and smaller, rocky terrestrials. Under IAU definitions, there are eight planets in the Solar System. In order from the Sun, they are the four terrestrials, Mercury, Venus, Earth, and Mars, then the four gas giants, Jupiter, Saturn, Uranus, and Neptune. The Solar System also contains at least five dwarf planets: Ceres, Pluto (originally classified as the Solar System’s ninth planet), Makemake, Haumea and Eris. With the exception of Mercury, Venus, Ceres and Makemake, all of these are orbited by one or more natural satellites.

History
The idea of planets has evolved over its history, from the divine wandering stars of antiquity to the earthly objects of the scientific age. The concept has also now expanded to include worlds not only in the Solar System, but in hundreds of other extrasolar systems. The ambiguities inherent in defining planets have led to much scientific controversy.

In ancient times, astronomers noted how certain lights moved across the sky in relation to the other stars. Ancient Greeks called these lights “πλάνητες ἀστέρες” (planetes asteres: wandering stars) or simply “πλανήτοι” (planētoi: wanderers),[4] from which today’s word “planet” was derived.[5][6] In ancient Greece, China, Babylon and indeed all pre-modern civilisations,[7][8] it was almost universally believed that Earth was in the centre of the Universe and that all the “planets” circled the Earth. The reasons for this perception were that stars and planets appeared to revolve around the Earth each day,[9] and the apparently common sense perception that the Earth was solid and stable, and that it is not moving but at rest.

Babylon
The first Western civilisation known to possess a functional theory of the planets were the Babylonians, who lived in Mesopotamia in the first and second millennia BC. The oldest surviving planetary astronomical text is the Babylonian Venus tablet of Ammisaduqa, a 7th century BC copy of a list of observations of the motions of the planet Venus that probably dates as early as the second millennium BC.[10] The Babylonians also laid the foundations of what would eventually become Western astrology.[11] The Enuma anu enlil, written during the Neo-Assyrian period in the 7th century BC,[12] comprises a list of omens and their relationships with various celestial phenomena including the motions of the planets.[13] The Sumerians, predecessors of the Babylonians who are considered as one of the first civilizations and are credited with the invention of writing, had identified at least Venus by 1500 BC.

Ancient Greece to Medieval Europe
The ancient Greek cosmological system was taken from that of the Babylonians,[14] from whom they began to acquire astronomical learning from around 600 BC, including the constellations and the zodiac.[16] In the 6th century BC, the Babylonians’ astronomical knowledge at the time was far in advance of the Greeks. The earliest known Greek sources, such as the Iliad and the Odyssey, do not mention the planets.[11]

By the first century BC, the Greeks had begun to develop their own mathematical schemes for predicting the positions of the planets. These schemes, which were based on geometry rather than the arithmetic of the Babylonians, would eventually eclipse the Babylonians’ theories in complexity and comprehensiveness, and account for most of the astronomical movements observed from Earth with the naked eye. These theories would reach their fullest expression in the Almagest written by Ptolemy in the 2nd century AD. So complete was the domination of Ptolemy’s model that it superseded all previous works on astronomy and remained the definitive astronomical text in the Western world for 13 centuries.[10][17]

To the Greeks and Romans there were seven known planets, each presumed to be circling the Earth according to the complex laws laid out by Ptolemy. They were, in increasing order from Earth (in Ptolemy’s order): the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn.[6][17][18]

European Renaissance
The five naked-eye planets may have been known since ancient times, and have had a significant impact on mythology, religious cosmology, and ancient astronomy. As scientific knowledge progressed, however, understanding of the term “planet” changed from something that moved across the sky (in relation to the star field); to a body that orbited the Earth (or that were believed to do so at the time); and in the 16th century to something that directly orbited the Sun when the heliocentric model of Copernicus, Galileo and Kepler gained sway.

Thus the Earth became included in the list of planets,[19] while the Sun and Moon were excluded. At first, when the first satellites of Jupiter and Saturn were discovered in the 17th century, the terms “planet” and “satellite” were used interchangeably – although the latter would gradually become more prevalent in the following century.[20] Until the mid-19th century, the number of “planets” rose rapidly since any newly discovered object directly orbiting the Sun was listed as a planet by the scientific community.

19th Century
In the 19th century astronomers began to realize that recently discovered bodies that had been classified as planets for almost half a century (such as Ceres, Pallas, and Vesta) were very different from the traditional ones. These bodies shared the same region of space between Mars and Jupiter (the Asteroid belt), and had a much smaller mass; as a result they were reclassified as “asteroids.” In the absence of any formal definition, a “planet” came to be understood as any “large” body that orbited the Sun. Since there was a dramatic size gap between the asteroids and the planets, and the spate of new discoveries seemed to have ended after the discovery of Neptune in 1846, there was no apparent need to have a formal definition.[21]

20th Century
However, in the 20th century, Pluto was discovered. After initial observations led to the belief it was larger than Earth,[22] the object was immediately accepted as the ninth planet. Further monitoring found the body was actually much smaller: in 1936, Raymond Lyttleton suggested that Pluto may be an escaped satellite of Neptune,[23] and Fred Whipple suggested in 1964 that Pluto may be a comet.[24] However, as it was still larger than all known asteroids and seemingly did not exist within a larger population,[25] it kept its status until 2006.
Planets 1930-2006
Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto

In 1992, astronomers Aleksander Wolszczan and Dale Frail announced the discovery of planets around a pulsar, PSR B1257+12.[26] This discovery is generally considered to be the first definitive detection of a planetary system around another star. Then, on October 6, 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting an ordinary main-sequence star (51 Pegasi).[27]

The discovery of extrasolar planets led to another ambiguity in defining a planet; the point at which a planet becomes a star. Many known extrasolar planets are many times the mass of Jupiter, approaching that of stellar objects known as “brown dwarfs”.[28] Brown dwarfs are generally considered stars due to their ability to fuse deuterium, a heavier isotope of hydrogen. While stars more massive than 75 times that of Jupiter fuse hydrogen, stars of only 13 Jupiter masses can fuse deuterium. However, deuterium is quite rare, and most brown dwarfs would have ceased fusing deuterium long before their discovery, making them effectively indistinguishable from supermassive planets.[29]

Mythology
The names for the planets in the Western world are derived from the naming practices of the Romans, which ultimately derive from those of the Greeks and the Babylonians. In ancient Greece, the two great luminaries the Sun and the Moon were called Helios and Selene; the farthest planet was called Phainon, the shiner; followed by Phaethon, “bright”; the red planet was known as Pyroeis, the “fiery”; the brightest was known as Phosphoros, the light bringer; and the fleeting final planet was called Stilbon, the gleamer. The Greeks also made each planet sacred to one of their pantheon of gods, the Olympians: Helios and Selene were the names of both planets and gods; Phainon was sacred to Kronos, the Titan who fathered the Olympians; Phaethon was sacred to Zeús, Kronos’s son who deposed him as king; Pyroeis was given to Ares, son of Zeus and god of war; Phosphorus was ruled by Aphrodite, the goddess of love; and Hermes, messenger of the gods and god of learning and wit, ruled over Stilbon.[10]

The Greek practice of grafting of their gods’ names onto the planets was almost certainly borrowed from the Babylonians. The Babylonians named Phosphorus after their goddess of love, Ishtar; Pyroeis after their god of war, Nergal, Stilbon after their god of wisdom Nabu, and Phaethon after their chief god, Marduk.[43] There are too many concordances between Greek and Babylonian naming conventions for them to have arisen separately.[10] The translation was not perfect. For instance, the Babylonian Nergal was a god of war, and thus the Greeks identified him with Ares. However, unlike Ares, Nergal was also god of pestilence and the underworld.[44]

Today, most people in the western world know the planets by names derived from the Olympian pantheon of gods. While modern Greeks still use their ancient names for the planets, other European languages, because of the influence of the Roman Empire and, later, the Catholic Church, use the Roman (or Latin) names rather than the Greek ones. The Romans, who, like the Greeks, were Indo-Europeans, shared with them a common pantheon under different names but lacked the rich narrative traditions that Greek poetic culture had given their gods. During the later period of the Roman Republic, Roman writers borrowed much of the Greek narratives and applied them to their own pantheon, to the point where they became virtually indistinguishable.[45] When the Romans studied Greek astronomy, they gave the planets their own gods’ names: Mercurius (for Hermes), Venus (Aphrodite), Mars (Ares), Iuppiter (Zeus) and Saturnus (Kronos). When subsequent planets were discovered in the 18th and 19th centuries, the naming practice was retained: Uranus (Ouranos) and Neptūnus (Poseidon).

Some Romans, following a belief possibly originating in Mesopotamia but developed in Hellenistic Egypt, believed that the seven gods after whom the planets were named took hourly shifts in looking after affairs on Earth. The order of shifts went Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon (from the farthest to the closest planet).[46] Therefore, the first day was started by Saturn (1st hour), second day by Sun (25th hour), followed by Moon (49th hour), Mars, Mercury, Jupiter and Venus. Since each day was named by the god that started it, this is also the order of the days of the week in the Roman calendar after the Nundinal cycle was rejected – and still preserved many modern languages.[47] Sunday, Monday, and Saturday are straightforward translations of these Roman names. In English the other days were renamed after Tiw, (Tuesday) Wóden (Wednesday), Thunor (Thursday), and Fríge (Friday), the Anglo-Saxon gods considered similar or equivalent to Mars, Mercury, Jupiter, and Venus respectively.

Since Earth was only generally accepted as a planet in the 17th century,[19] there is no tradition of naming it after a god (the same is true, in English at least, of the Sun and the Moon, though they are no longer considered planets). The name originates from the 8th century Anglo-Saxon word erda, which means ground or soil and was first used in writing as the name of the sphere of the Earth perhaps around 1300.[48][49] It is the only planet whose name in English is not derived from Greco-Roman mythology. Many of the Romance languages retain the old Roman word terra (or some variation of it) that was used with the meaning of “dry land” (as opposed to “sea”).[50] However, the non-Romance languages use their own respective native words. The Greeks retain their original name, Γή (Ge or Yi); the Germanic languages, including English, use a variation of an ancient Germanic word ertho, “ground,”[49] as can be seen in the English Earth, the German Erde, the Dutch Aarde, and the Scandinavian Jorde.

Non-European cultures use other planetary naming systems. India uses a naming system based on the Navagraha, which incorporates the seven traditional planets (Surya for the Sun, Chandra for the Moon, and Budha, Shukra, Mangala, Bṛhaspati and Shani for the traditional planets Mercury, Venus, Mars, Jupiter and Saturn) and the ascending and descending lunar nodes Rahu and Ketu. China and the countries of eastern Asia influenced by it (such as Japan, Korea and Vietnam) use a naming system based on the five Chinese elements: water (Mercury), metal (Venus), fire (Mars), wood (Jupiter) and earth (Saturn).[47]

Formation
It is not known with certainty how planets are formed. The prevailing theory is that they are formed during the collapse of a nebula into a thin disk of gas and dust. A protostar forms at the core, surrounded by a rotating protoplanetary disk. Through accretion (a process of sticky collision) dust particles in the disk steadily accumulate mass to form ever-larger bodies. Local concentrations of mass known as planetesimals form, and these accelerate the accretion process by drawing in additional material by their gravitational attraction. These concentrations become ever denser until they collapse inward under gravity to form protoplanets.[51] After a planet reaches a diameter larger than the Earth’s moon, it begins to accumulate an extended atmosphere, greatly increasing the capture rate of the planetesimals by means of atmospheric drag.[52]
An artist’s impression of protoplanetary disk

When the protostar has grown such that it ignites to form a star, the surviving disk is removed from the inside outward by photoevaporation, the solar wind, Poynting-Robertson drag and other effects.[53][54] Thereafter there still may be many protoplanets orbiting the star or each other, but over time many will collide, either to form a single larger planet or release material for other larger protoplanets or planets to absorb.[55] Those objects that have become massive enough will capture most matter in their orbital neighbourhoods to become planets. Meanwhile, protoplanets that have avoided collisions may become natural satellites of planets through a process of gravitational capture, or remain in belts of other objects to become either dwarf planets or small Solar System bodies.

The energetic impacts of the smaller planetesimals (as well as radioactive decay) will heat up the growing planet, causing it to at least partially melt. The interior of the planet begins to differentiate by mass, developing a denser core.[56] Smaller terrestrial planets lose most of their atmospheres because of this accretion, but the lost gases can be replaced by outgassing from the mantle and from the subsequent impact of comets.[57] (Smaller planets will lose any atmosphere they gain through various escape mechanisms.)

With the discovery and observation of planetary systems around stars other than our own, it is becoming possible to elaborate, revise or even replace this account. The level of metallicity – an astronomical term describing the abundance of chemical elements with an atomic number greater than 2 (helium) – is now believed to determine the likelihood that a star will have planets.[58] Hence it is thought less likely that a metal-poor, population II star will possess a more substantial planetary system than a metal-rich population I star.

Solar System
According to the IAU’s current definitions, there are eight planets and five dwarf planets in the Solar System. In increasing distance from the Sun, the planets are:

1. ☿ Mercury
2. ♀ Venus
3. ⊕ Earth
4. ♂ Mars
5. ♃ Jupiter
6. ♄ Saturn
7. ♅ Uranus
8. ♆ Neptune

Jupiter is the largest, at 318 Earth masses, while Mercury is smallest, at 0.055 Earth masses.

The planets of the Solar System can be divided into categories based on their composition:

* Terrestrials: Planets that are similar to Earth, with bodies largely composed of rock: Mercury, Venus, Earth and Mars.
* Gas giants (Jovians): Planets with a composition largely made up of gaseous material and are significantly more massive than terrestrials: Jupiter, Saturn, Uranus, Neptune. Ice giants, comprising Uranus and Neptune, are a sub-class of gas giants, distinguished from gas giants by their significantly lower mass, and by depletion in hydrogen and helium in their atmospheres together with a significantly higher proportion of rock and ice.
* Dwarf planets: Before the August 2006 decision, several objects were proposed by astronomers, including at one stage by the IAU, as planets. However in 2006 several of these objects were reclassified as dwarf planets, objects distinct from planets. Currently five dwarf planets in the Solar System are recognized by the IAU: Ceres, Pluto, Haumea, Makemake and Eris. Several other objects in both the Asteroid belt and the Kuiper belt are under consideration, with as many as 50 that could eventually qualify. There may be as many as 200 that could be discovered once the Kuiper belt has been fully explored. Dwarf planets share many of the same characteristics as planets, although notable differences remain – namely that they are not dominant in their orbits. By definition, all dwarf planets are members of larger populations. Ceres is the largest body in the asteroid belt, while Pluto, Haumea, and Makemake are members of the Kuiper belt and Eris is a member of the scattered disc. Scientists such as Mike Brown believe that there may soon be over forty trans-Neptunian objects that qualify as dwarf planets under the IAU’s recent definition.