Asteroid

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

There are millions of asteroids, many thought to be the shattered remnants of planetesimals, bodies within the young Sun's solar nebula that never grew large enough to become planets.^[2] The large majority of known asteroids orbit in the asteroid belt between the orbits of Mars and Jupiter, or are co-orbital with Jupiter (the Jupiter trojans). However, other orbital families exist with significant populations, including the near-Earth asteroids. Individual asteroids are classified by their characteristic spectra, with the majority falling into three main groups: C-type, M-type, and S-type. These were named after and are generally identified with carbon-rich, metallic, and silicate (stony) compositions, respectively.

Only one asteroid, 4 Vesta, which has a relatively reflective surface, is normally visible to the naked eye, and this only in very dark skies when it is favorably positioned. Rarely, small asteroids passing close to Earth may be visible to the naked eye for a short time.^[3] As of November 2015, the Minor Planet Center had data on more than 1.25 million objects in the inner and outer Solar System, of which 750,000 had enough information to be given numbered designations.^[4]

On 22 January 2014, European Space Agency (ESA) scientists reported the detection, for the first definitive time, of water vapor on Ceres, the largest object in the asteroid belt.^[5] The detection was made by using the far-infrared abilities of the Herschel Space Observatory.^[6] The finding is unexpected because comets, not asteroids, are typically considered to "sprout jets and plumes". According to one of the scientists, "The lines are becoming more and more blurred between comets and asteroids."^[6]

Discovery

The first asteroid to be discovered, Ceres, was found in 1801 by Giuseppe Piazzi, and was originally considered to be a new planet.^{[note 1]} This was followed by the discovery of other similar bodies, which, with the equipment of the time, appeared to be points of light, like stars, showing little or no planetary disc, though readily distinguishable from stars due to their apparent motions. This prompted the astronomer Sir William Herschel to propose the term "asteroid",^[7] coined in Greek as ἀστεροειδής asteroeidēs 'star-like, star-shaped', from Ancient Greek ἀστήρ astēr 'star, planet'. In the early second half of the nineteenth century, the terms "asteroid" and "planet" (not always qualified as "minor") were still used interchangeably; for example, the Annual of Scientific Discovery for 1871, page 316, reads "Professor J. Watson has been awarded by the Paris Academy of Sciences, the astronomical prize, Lalande foundation, for the discovery of eight new asteroids in one year. The planet Lydia (No. 110), discovered by M. Borelly at the Marseilles Observatory [...] M. Borelly had previously discovered two planets bearing the numbers 91 and 99 in the system of asteroids revolving between Mars and Jupiter".

Historical methods

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

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

The first object, Ceres, was not discovered by a member of the group, but rather by accident in 1801 by Giuseppe Piazzi, director of the observatory of Palermo in Sicily. He discovered a new star-like object in Taurus and followed the displacement of this object during several nights. Later that year, Carl Friedrich Gauss used these observations to calculate the orbit of this unknown object, which was found to be between the planets Mars and Jupiter. Piazzi named it after Ceres, the Roman goddess of agriculture.

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

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

In 1891, Max Wolf pioneered the use of astrophotography to detect asteroids, which appeared as short streaks on long-exposure photographic plates. This dramatically increased the rate of detection compared with earlier visual methods: Wolf alone discovered 248 asteroids, beginning with 323 Brucia, whereas only slightly more than 300 had been discovered up to that point. It was known that there were many more, but most astronomers did not bother with them, calling them "vermin of the skies",^[8] a phrase variously attributed to Eduard Suess^[9] and Edmund Weiss.^[10] Even a century later, only a few thousand asteroids were identified, numbered and named.

Manual methods of the 1900s and modern reporting

Until 1998, asteroids were discovered by a four-step process. First, a region of the sky was photographed by a wide-field telescope, or astrograph. Pairs of photographs were taken, typically one hour apart. Multiple pairs could be taken over a series of days. Second, the two films or plates of the same region were viewed under a stereoscope. Any body in orbit around the Sun would move slightly between the pair of films. Under the stereoscope, the image of the body would seem to float slightly above the background of stars. Third, once a moving body was identified, its location would be measured precisely using a digitizing microscope. The location would be measured relative to known star locations.^[11]

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

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

Computerized methods

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

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

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

All these considerations helped spur the launch of highly efficient surveys that consist of Charge-Coupled Device (CCD) cameras and computers directly connected to telescopes. As of spring 2011, it was estimated that 89% to 96% of near-Earth asteroids one kilometer or larger in diameter had been discovered.^[12] A list of teams using such systems includes:^[13]

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

The LINEAR system alone has discovered 138,393 asteroids, as of 20 September 2013.^[14] Among all the surveys, 4711 near-Earth asteroids have been discovered^[15] including over 600 more than 1 km (0.6 mi) in diameter.

Terminology

Traditionally, small bodies orbiting the Sun were classified as asteroids, comets or meteoroids, with anything smaller than ten metres across being called a meteoroid.^[16][17] The term "asteroid" is ill-defined. It never had a formal definition, with the broader term minor planet being preferred by the International Astronomical Union. In 2006, the term "small Solar System body" was introduced to cover both most minor planets and comets.^[18][19] Other languages prefer "planetoid" (Greek for "planet-like"), and this term is occasionally used in English for larger minor planets such as the dwarf planets. The word "planetesimal" has a similar meaning, but refers specifically to the small building blocks of the planets that existed when the Solar System was forming. The term "planetule" was coined by the geologist William Daniel Conybeare to describe minor planets,^[20] but is not in common use. The three largest objects in the asteroid belt, 1 Ceres, 2 Pallas, and 4 Vesta, grew to the stage of protoplanets. Ceres is a dwarf planet, the only one in the inner Solar System.

When found, asteroids were seen as a class of objects distinct from comets, and there was no unified term for the two until "small Solar System body" was coined in 2006. The main difference between an asteroid and a comet is that a comet shows a coma due to sublimation of near surface ices by solar radiation. A few objects have ended up being dual-listed because they were first classified as minor planets but later showed evidence of cometary activity. Conversely, some (perhaps all) comets are eventually depleted of their surface volatile ices and become asteroids. A further distinction is that comets typically have more eccentric orbits than most asteroids; most "asteroids" with notably eccentric orbits are probably dormant or extinct comets.^[21]

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

The innermost of these are the Kuiper-belt objects, called "objects" partly to avoid the need to classify them as asteroids or comets.^[22] They are thought to be predominantly comet-like in composition, though some may be more akin to asteroids.^[23] Furthermore, most do not have the highly eccentric orbits associated with comets, and the ones so far discovered are larger than traditional comet nuclei. (The much more distant Oort cloud is hypothesized to be the main reservoir of dormant comets.) Other recent observations, such as the analysis of the cometary dust collected by the Stardust probe, are increasingly blurring the distinction between comets and asteroids,^[24] suggesting "a continuum between asteroids and comets" rather than a sharp dividing line.^[25]

The minor planets beyond Jupiter's orbit are sometimes also called "asteroids", especially in popular presentations.^[26] However, it is becoming increasingly common for the term "asteroid" to be restricted to minor planets of the inner Solar System.^[22] Therefore, this article will restrict itself for the most part to the classical asteroids: objects of the asteroid belt, Jupiter trojans, and near-Earth objects.

When the IAU introduced the class small Solar System bodies in 2006 to include most objects previously classified as minor planets and comets, they created the class of dwarf planets for the largest minor planets—those that have enough mass to have become ellipsoidal under their own gravity. According to the IAU, "the term 'minor planet' may still be used, but generally the term 'Small Solar System Body' will be preferred."^[27] Currently only the largest object in the asteroid belt, Ceres, at about 950 km (590 mi) across, has been placed in the dwarf planet category.

Formation

It is thought that planetesimals in the asteroid belt evolved much like the rest of the solar nebula until Jupiter neared its current mass, at which point excitation from orbital resonances with Jupiter ejected over 99% of planetesimals in the belt. Simulations and a discontinuity in spin rate and spectral properties suggest that asteroids larger than approximately 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.^[29] Ceres and Vesta grew large enough to melt and differentiate, with heavy metallic elements sinking to the core, leaving rocky minerals in the crust.^[30]

In the Nice model, many Kuiper-belt objects are captured in the outer asteroid belt, at distances greater than 2.6 AU. Most were later ejected by Jupiter, but those that remained may be the D-type asteroids, and possibly include Ceres.^[31]

Distribution within the Solar System

Various dynamical groups of asteroids have been discovered orbiting in the inner Solar System. Their orbits are perturbed by the gravity of other bodies in the Solar System and by the Yarkovsky effect. Significant populations include:

Asteroid belt

The majority of known asteroids orbit within the asteroid belt between the orbits of Mars and Jupiter, generally in relatively low-eccentricity (i.e. not very elongated) orbits. This belt is now estimated to contain between 1.1 and 1.9 million asteroids larger than 1 km (0.6 mi) in diameter,^[32] and millions of smaller ones. These asteroids may be remnants of the protoplanetary disk, and in this region the accretion of planetesimals into planets during the formative period of the Solar System was prevented by large gravitational perturbations by Jupiter.

Trojans

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

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

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

Near-Earth asteroids

Near-Earth asteroids, or NEAs, are asteroids that have orbits that pass close to that of Earth. Asteroids that actually cross Earth's orbital path are known as Earth-crossers. As of November 2014, 11,600 near-Earth asteroids are known^[12] and the number over one kilometre in diameter is estimated to be 900–1,000.

Characteristics

Size distribution

Asteroids vary greatly in size, from almost 7006100000000000000♠1000 km for the largest down to rocks just tens of metres across.^{[note 3]} The three largest are very much like miniature planets: they are roughly spherical, have at least partly differentiated interiors,^[33] and are thought to be surviving protoplanets. The vast majority, however, are much smaller and are irregularly shaped; they are thought to be either surviving planetesimals or fragments of larger bodies.

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

The mass of all the objects of the asteroid belt, lying between the orbits of Mars and Jupiter, is estimated to be about 2.8–7021320000000000000♠3.2×10²¹ kg, or about 4% of the mass of the Moon. Of this, Ceres comprises 7020950000000000000♠0.95×10²¹ kg, a third of the total.^[34] Adding in the next three most massive objects, Vesta (9%), Pallas (7%), and Hygiea (3%), brings this figure up to 51%; whereas the three after that, 511 Davida (1.2%), 704 Interamnia (1.0%), and 52 Europa (0.9%), only add another 3% to the total mass. The number of asteroids then increases rapidly as their individual masses decrease.

The number of asteroids decreases markedly with size. Although this generally follows a power law, there are 'bumps' at 7003500000000000000♠5 km and 7005100000000000000♠100 km, where more asteroids than expected from a logarithmic distribution are found.^[35]

The asteroids of the Solar System, categorized by size and number

Approximate number of asteroids (N) larger than a certain diameter (D)

D	100 m	300 m	500 m	1 km	3 km	5 km	10 km	30 km	50 km	100 km	200 km	300 km	500 km	900 km
N	~7007250000000000000♠25000000	7006400000000000000♠4000000	7006200000000000000♠2000000	7005750000000000000♠750000	7005200000000000000♠200000	7004900000000000000♠90000	7004100000000000000♠10000	7003110000000000000♠1100	600	200	30	5	3	1

Largest asteroids

Although their location in the asteroid belt excludes them from planet status, the three largest objects, Ceres, Vesta, and Pallas, are intact protoplanets that share many characteristics common to planets, and are atypical compared to the majority of "potato"-shaped asteroids.

Ceres is the only asteroid with a fully ellipsoidal shape and hence the only one that is a dwarf planet.^[38] It has a much higher absolute magnitude than the other asteroids, of around 3.32,^[39] and may possess a surface layer of ice.^[40] Like the planets, Ceres is differentiated: it has a crust, a mantle and a core.^[40] No meteorites from Ceres have been found on Earth.

Vesta, too, has a differentiated interior, though it formed inside the Solar System's frost line, and so is devoid of water;^[41] its composition is mainly of basaltic rock such as olivine.^[42] Aside from the large crater at its southern pole, Rheasilvia, Vesta also has an ellipsoidal shape. Vesta is the parent body of the Vestian family and other V-type asteroids, and is the source of the HED meteorites, which constitute 5% of all meteorites on Earth.

Pallas is unusual in that, like Uranus, it rotates on its side, with its axis of rotation tilted at high angles to its orbital plane.^[43] Its composition is similar to that of Ceres: high in carbon and silicon, and perhaps partially differentiated.^[44] Pallas is the parent body of the Palladian family of asteroids,

The fourth-most-massive asteroid, Hygiea, is the largest carbonaceous asteroid and, unlike the other largest asteroids, lies relatively close to the plane of the ecliptic.^[45] It is the largest member and presumed parent body of the Hygiean family of asteroids. Between them, the four largest asteroids constitute half the mass of the asteroid belt.

Attributes of largest asteroids
Name	Orbital radius (AU)	Orbital period (years)	Inclination to ecliptic	Orbital eccentricity	Diameter (km)	Diameter (% of Moon)	Mass (×1018 kg)	Mass (% of Ceres)	Density[46] g/cm3	Rotation period (hr)	Axial tilt	Surface temperature
Vesta	2.36	3.63	7.1°	0.089	573×557×446 (mean 525)	15%	260	28%	3.44 ± 0.12	5.34	29°	85–270 K
Ceres	2.77	4.60	10.6°	0.079	975×975×909 (mean 952)	28%	940	100%	2.12 ± 0.04	9.07	≈ 3°	167 K
Pallas	2.77	4.62	34.8°	0.231	580×555×500 (mean 545)	16%	210	22%	2.71 ± 0.11	7.81	≈ 80°	164 K
Hygiea	3.14	5.56	3.8°	0.117	530×407×370 (mean 430)	12%	87	9%	2.76 ± 1.2	27.6	≈ 60°	164 K

Rotation

Measurements of the rotation rates of large asteroids in the asteroid belt show that there is an upper limit. No asteroid with a diameter larger than 100 meters has a rotation period smaller than 2.2 hours. For asteroids rotating faster than approximately this rate, the inertia at the surface is greater than the gravitational force, so any loose surface material would be flung out. However, a solid object should be able to rotate much more rapidly. This suggests that most asteroids with a diameter over 100 meters are rubble piles formed through accumulation of debris after collisions between asteroids.^[47]

Composition

The physical composition of asteroids is varied and in most cases poorly understood. Ceres appears to be composed of a rocky core covered by an icy mantle, where Vesta is thought to have a nickel-iron core, olivine mantle, and basaltic crust.^[48] 10 Hygiea, however, which appears to have a uniformly primitive composition of carbonaceous chondrite, is thought to be the largest undifferentiated asteroid. Most of the smaller asteroids are thought to be piles of rubble held together loosely by gravity, though the largest are probably solid. Some asteroids have moons or are co-orbiting binaries: Rubble piles, moons, binaries, and scattered asteroid families are thought to be the results of collisions that disrupted a parent asteroid.

Asteroids contain traces of amino acids and other organic compounds, and some speculate that asteroid impacts may have seeded the early Earth with the chemicals necessary to initiate life, or may have even brought life itself to Earth (also see panspermia).^[49] In August 2011, a report, based on NASA studies with meteorites found on Earth, was published suggesting DNA and RNA components (adenine, guanine and related organic molecules) may have been formed on asteroids and comets in outer space.^[50][51][52]

Composition is calculated from three primary sources: albedo, surface spectrum, and density. The last can only be determined accurately by observing the orbits of moons the asteroid might have. So far, every asteroid with moons has turned out to be a rubble pile, a loose conglomeration of rock and metal that may be half empty space by volume. The investigated asteroids are as large as 280 km in diameter, and include 121 Hermione (268×186×183 km), and 87 Sylvia (384×262×232 km). Only half a dozen asteroids are larger than 87 Sylvia, though none of them have moons; however, some smaller asteroids are thought to be more massive, suggesting they may not have been disrupted, and indeed 511 Davida, the same size as Sylvia to within measurement error, is estimated to be two and a half times as massive, though this is highly uncertain. The fact that such large asteroids as Sylvia can be rubble piles, presumably due to disruptive impacts, has important consequences for the formation of the Solar System: Computer simulations of collisions involving solid bodies show them destroying each other as often as merging, but colliding rubble piles are more likely to merge. This means that the cores of the planets could have formed relatively quickly.^[53]

On 7 October 2009, the presence of water ice was confirmed on the surface of 24 Themis using NASA’s Infrared Telescope Facility. The surface of the asteroid appears completely covered in ice. As this ice layer is sublimated, it may be getting replenished by a reservoir of ice under the surface. Organic compounds were also detected on the surface.^{[54][55][56][57]} Scientists hypothesize that some of the first water brought to Earth was delivered by asteroid impacts after the collision that produced the Moon. The presence of ice on 24 Themis supports this theory.^[56]

In October 2013, water was detected on an extrasolar body for the first time, on an asteroid orbiting the white dwarf GD 61.^[58]

Surface features

Most asteroids outside the "big four" (Ceres, Pallas, Vesta, and Hygiea) are likely to be broadly similar in appearance, if irregular in shape. 50-km (31-mi) 253 Mathilde is a rubble pile saturated with craters with diameters the size of the asteroid's radius, and Earth-based observations of 300-km (186-mi) 511 Davida, one of the largest asteroids after the big four, reveal a similarly angular profile, suggesting it is also saturated with radius-size craters.^[59] Medium-sized asteroids such as Mathilde and 243 Ida that have been observed up close also reveal a deep regolith covering the surface. Of the big four, Pallas and Hygiea are practically unknown. Vesta has compression fractures encircling a radius-size crater at its south pole but is otherwise a spheroid. Ceres seems quite different in the glimpses Hubble has provided, with surface features that are unlikely to be due to simple craters and impact basins, but details will be expanded with the Dawn spacecraft, which entered Ceres orbit on 6 March 2015.^[60]

Color

Asteroids become darker and redder with age due to space weathering.^[61] However evidence suggests most of the color change occurs rapidly, in the first hundred thousands years, limiting the usefulness of spectral measurement for determining the age of asteroids.^[62]

Classification

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

Orbital classification

Many asteroids have been placed in groups and families based on their orbital characteristics. Apart from the broadest divisions, it is customary to name a group of asteroids after the first member of that group to be discovered. Groups are relatively loose dynamical associations, whereas families are tighter and result from the catastrophic break-up of a large parent asteroid sometime in the past.^[63] Families have only been recognized within the asteroid belt. They were first recognized by Kiyotsugu Hirayama in 1918 and are often called Hirayama families in his honor.

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

Quasi-satellites and horseshoe objects

Some asteroids have unusual horseshoe orbits that are co-orbital with 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 earlier status. Both Earth and Venus are known to have quasi-satellites.

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

Spectral classification

In 1975, an asteroid taxonomic system based on color, albedo, and spectral shape was developed by Clark R. Chapman, David Morrison, and Ben Zellner.^[64] These properties are thought to correspond to the composition of the asteroid's surface material. The original classification system had three categories: C-types for dark carbonaceous objects (75% of known asteroids), S-types for stony (silicaceous) objects (17% of known asteroids) and U for those that did not fit into either C or S. This classification has since been expanded to include many other asteroid types. The number of types continues to grow as more asteroids are studied.

The two most widely used taxonomies now used are the Tholen classification and SMASS classification. The former was proposed in 1984 by David J. Tholen, and was based on data collected from an eight-color asteroid survey performed in the 1980s. This resulted in 14 asteroid categories.^[65] In 2002, the Small Main-Belt Asteroid Spectroscopic Survey resulted in a modified version of the Tholen taxonomy with 24 different types. Both systems have three broad categories of C, S, and X asteroids, where X consists of mostly metallic asteroids, such as the M-type. There are also several smaller classes.^[66]

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

Problems

Originally, spectral designations were based on inferences of an asteroid's composition.^[67] However, the correspondence between spectral class and composition is not always very good, and a variety of classifications are in use. This has led to significant confusion. Although asteroids of different spectral classifications are likely to be composed of different materials, there are no assurances that asteroids within the same taxonomic class are composed of similar materials.

Naming

A newly discovered asteroid is given a provisional designation (such as 2002 AT4) consisting of the year of discovery and an alphanumeric code indicating the half-month of discovery and the sequence within that half-month. Once an asteroid's orbit has been confirmed, it is given a number, and later may also be given a name (e.g. 433 Eros). The formal naming convention uses parentheses around the number (e.g. (433) Eros), but dropping the parentheses is quite common. Informally, it is common to drop the number altogether, or to drop it after the first mention when a name is repeated in running text.

Symbols

The first asteroids to be discovered were assigned iconic symbols like the ones traditionally used to designate the planets. By 1855 there were two dozen asteroid symbols, which often occurred in multiple variants.^[68]

Asteroid	Symbol		Year
1 Ceres	⚳	Ceres' scythe, reversed to double as the letter C	1801
2 Pallas	⚴	Athena's (Pallas') spear	1801
3 Juno	⚵	A star mounted on a scepter, for Juno, the Queen of Heaven	1804
4 Vesta	⚶	The altar and sacred fire of Vesta	1807
5 Astraea		A scale, or an inverted anchor, symbols of justice	1845
6 Hebe		Hebe's cup	1847
7 Iris		A rainbow (iris) and a star	1847
8 Flora		A flower (flora), specifically the Rose of England	1847
9 Metis		The eye of wisdom and a star	1848
10 Hygiea		Hygiea's serpent and a star, or the Rod of Asclepius	1849
11 Parthenope		A harp, or a fish and a star; symbols of the sirens	1850
12 Victoria		The laurels of victory and a star	1850
13 Egeria		A shield, symbol of Egeria's protection, and a star	1850
14 Irene		A dove carrying an olive branch (symbol of irene 'peace') with a star on its head,[69] or an olive branch, a flag of truce, and a star	1851
15 Eunomia		A heart, symbol of good order (eunomia), and a star	1851
16 Psyche		A butterfly's wing, symbol of the soul (psyche), and a star	1852
17 Thetis		A dolphin, symbol of Thetis, and a star	1852
18 Melpomene		The dagger of Melpomene, and a star	1852
19 Fortuna		The wheel of fortune and a star	1852
26 Proserpina		Proserpina's pomegranate	1853
28 Bellona		Bellona's whip and lance[70]	1854
29 Amphitrite		The shell of Amphitrite and a star	1854
35 Leukothea		A lighthouse beacon, symbol of Leucothea[71]	1855
37 Fides		The cross of faith (fides)[72]	1855

In 1851,^[73] after the fifteenth asteroid (Eunomia) had been discovered, Johann Franz Encke made a major change in the upcoming 1854 edition of the Berliner Astronomisches Jahrbuch (BAJ, Berlin Astronomical Yearbook). He introduced a disk (circle), a traditional symbol for a star, as the generic symbol for an asteroid. The circle was then numbered in order of discovery to indicate a specific asteroid (although he assigned ① to the fifth, Astraea, while continuing to designate the first four only with their existing iconic symbols). The numbered-circle convention was quickly adopted by astronomers, and the next asteroid to be discovered (16 Psyche, in 1852) was the first to be designated in that way at the time of its discovery. However, Psyche was given an iconic symbol as well, as were a few other asteroids discovered over the next few years (see chart above). 20 Massalia was the first asteroid that was not assigned an iconic symbol, and no iconic symbols were created after the 1855 discovery of 37 Fides.^[74] That year Astraea's number was increased to ⑤, but the first four asteroids, Ceres to Vesta, were not listed by their numbers until the 1867 edition. The circle was soon abbreviated to a pair of parentheses, which were easier to typeset and sometimes omitted altogether over the next few decades, leading to the modern convention.^[69]

Exploration

Until the age of space travel, objects in the asteroid belt were merely pinpricks of light in even the largest telescopes and their shapes and terrain remained a mystery. The best modern ground-based telescopes and the Earth-orbiting Hubble Space Telescope can resolve a small amount of detail on the surfaces of the largest asteroids, but even these mostly remain little more than fuzzy blobs. Limited information about the shapes and compositions of asteroids can be inferred from their light curves (their variation in brightness as they rotate) and their spectral properties, and asteroid sizes can be estimated by timing the lengths of star occulations (when an asteroid passes directly in front of a star). Radar imaging can yield good information about asteroid shapes and orbital and rotational parameters, especially for near-Earth asteroids. In terms of delta-v and propellant requirements, NEOs are more easily accessible than the Moon.^[75]

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

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

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

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

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

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

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

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

Planned and future missions

The Japan Aerospace Exploration Agency (JAXA) launched the Hayabusa 2 space probe in December 2014, and plans to return samples from 162173 Ryugu in December 2020.

In May 2011, NASA selected the OSIRIS-REx sample return mission to asteroid 101955 Bennu; it is expected to launch in September 2016.

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

In early 2013, NASA announced the planning stages of a mission to capture a near-Earth asteroid and move it into lunar orbit where it could possibly be visited by astronauts and later impacted into the Moon.^[76] On 19 June 2014, NASA reported that asteroid 2011 MD was a prime candidate for capture by a robotic mission, perhaps in the early 2020s.^[77]

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

In the U.S. Discovery program the Psyche (spacecraft) proposal to 16 Psyche and Lucy (spacecraft) to Jupiter trojans made it to the semifinalist stage of mission selection.

Fiction

Asteroids and the asteroid belt are a staple of science fiction stories. Asteroids play several potential roles in science fiction: as places human beings might colonize, resources for extracting minerals, hazards encountered by spaceships traveling between two other points, and as a threat to life on Earth by potential impact.

See also

Notes

1. ↑ Ceres is the largest asteroid and is now classified as a dwarf planet. All other asteroids are now classified as small Solar System bodies along with comets, centaurs, and the smaller trans-Neptunian objects.
2. ↑ Neptune also has a few known trojans, and these are thought to actually be much more numerous than the Jovian trojans. However, they are often included in the trans-Neptunian population rather than counted with the asteroids.
3. ↑ Below 7001100000000000000♠10 m, these rocks are by convention considered to be meteoroids.

References

External links

• Alphabetical list of minor planet names (ASCII) (Minor Planet Center)
• Asteroid articles in Planetary Science Research Discoveries
• IAU Committee on Small Body Nomenclature
• JPL Asteroid Watch Site
• NASA Asteroid and Comet Watch Site
• Near Earth Asteroid Tracking (NEAT)
• Near Earth Objects Dynamic Site
• NEO MAP (Armagh Observatory)
• Spaceguard Centre
• TECA Table of next close approaches to the Earth
• When Did the Asteroids Become Minor Planets?

Asteroids Main Notable asteroids Physical characteristics dynamic method Impact avoidance Tracking Capture Gravitational keyhole Earth-crosser Close approaches Binary moon contact P–L Distribution Interior to Earth vulcanoids Mercury-crossers Venus-crossers Main-belt Kirkwood gap comets Trojans Venus trojan Earth Mars Jupiter Uranus trojan Neptune trojan Near-Earth Aten Amor Apollo Apohele Earth-crossers Between Earth and the main belt Mars crossers Mars trojans Outer Solar System, does not include distant minor planets Jupiter-crossers Jupiter trojan Classification Orbital Group Family (list) Spectral Tholen C-group B-type F-type G-type C-type S-type X-group M-type E-type P-type small classes A-type D-type J-type T-type Q-type R-type V-type SMASS C-group B-type C-type Cg Ch Cgh Cb S-group A-type Q-type R-type K-type L-type S-type Sa Sq Sr Sk Sl X-group X-type Xe Xc Xk small types T-type D-type Ld-type O-type V-type Exploration Asteroid Redirect Mission Asteroids visited by spacecraft Human mission Mining Ceres Colonization Lists Near-Earth Minor planets Lost Discovering observatories Space missions See also: Asteroids in fiction Asteroids in astrology occultation Pronunciation asteroids Jupiter Trojans Comet

Ceres Geography Craters Abellio Achita Annona Anura Asari Attis Azacca Belun Besua Bonsu Chaminuka Coniraya Cozobi Dada Dantu Doliku Darzamat Datan Ernutet Ezinu Fejokoo Fluusa Gaue Geshtin Ghanan Hamori Haulani Heneb Homshuk Ikapati Inamahari Insitor Jaja Jarimba Jarovit Juling Kaikara Kait Kerwan Kirnis Kondos Kumitoga Kupalo Liber Lono Meanderi Messor Mondamin Nawish Ninsar Occator Oltagon Omonga Oxo Piuku Rao Rongo Roskva Sekhet Shakaema Sintana Tafakula Tahu Takel Tibong Toharu Tupo Urvara Victa Vinotonus Yalode Zadeni Montes Ahuna Mons Liberalia Mons Ysolo Mons Catenae Gerber Catena Samhain Catena Rupēs Niman Rupes Plana Erntedank Planum Tholi Dalien Tholus Wangala Tholus Other features Bright spots Astronomy Classification Asteroid Definition of planet Dwarf planet Planet 2006 definition of planet Discovery Barnaba Oriani Carl Friedrich Gauss Francis Wollaston Franz Xaver von Zach Giuseppe Piazzi Heinrich Olbers Jérôme Lalande Johann Elert Bode Tobias Mayer Possible trojans 2006 SV23 1372 Haremari Exploration Dawn mission Related Ceres in fiction Colonization In astrology Solar System bodies formerly considered to be planets

Comets Features Nucleus Coma Tails Antitail Comet dust Meteor shower Types Periodic Numbered Lost Halley-type Jupiter-family Encke-type Main-belt Non-periodic Hyperbolic Unknown-orbit Great Comet Sungrazing (Kreutz) Extinct Exocomet Interstellar Related Naming of comets Centaur Comet discoverers LINEAR Extraterrestrial atmospheres Oort cloud Small Solar System body Asteroid Space missions Comet Hopper CONTOUR CRAF Deep Impact/EPOXI Deep Space 1 Giotto Hayabusa Mk2 ICE Marco Polo Rosetta Philae Timeline Sakigake Stardust/NeXT Suisei Ulysses Vega program Vega 1 Vega 2 Vesta Latest C/2015 G2 (MASTER) C/2015 F5 (SWAN-XingMing) C/2015 F3 C/2014 Q2 (Lovejoy) C/2014 E2 (Jacques) C/2013 US10 (Catalina) C/2013 A1 (Siding Spring) C/2012 S4 (PANSTARRS) C/2012 K1 (PANSTARRS) Culture and speculation Antimatter comet Comets in fiction Fictional comets Comet vintages   Lists of comets (more) Periodic comets Numbered 1P/Halley 2P/Encke 3D/Biela 4P/Faye 5D/Brorsen 6P/d'Arrest 7P/Pons–Winnecke 8P/Tuttle 9P/Tempel 10P/Tempel 11P/Tempel–Swift–LINEAR 12P/Pons–Brooks 13P/Olbers 14P/Wolf 15P/Finlay 16P/Brooks 17P/Holmes 18D/Perrine–Mrkos 19P/Borrelly 20D/Westphal 21P/Giacobini–Zinner 22P/Kopff 23P/Brorsen–Metcalf 24P/Schaumasse 25D/Neujmin 26P/Grigg–Skjellerup 27P/Crommelin 28P/Neujmin 29P/Schwassmann–Wachmann 30P/Reinmuth 31P/Schwassmann–Wachmann 32P/Comas Solà 33P/Daniel 34D/Gale 35P/Herschel–Rigollet 36P/Whipple 37P/Forbes 38P/Stephan–Oterma 39P/Oterma 40P/Väisälä 41P/Tuttle–Giacobini–Kresák 42P/Neujmin 43P/Wolf–Harrington 44P/Reinmuth 45P/Honda–Mrkos–Pajdušáková 46P/Wirtanen 47P/Ashbrook–Jackson 48P/Johnson 49P/Arend–Rigaux 50P/Arend 51P/Harrington 52P/Harrington–Abell 53P/Van Biesbroeck 54P/de Vico–Swift–NEAT 55P/Tempel–Tuttle 56P/Slaughter–Burnham 57P/du Toit–Neujmin–Delporte 58P/Jackson–Neujmin 59P/Kearns–Kwee 60P/Tsuchinshan 61P/Shajn–Schaldach 62P/Tsuchinshan 63P/Wild 64P/Swift–Gehrels 65P/Gunn 66P/du Toit 67P/Churyumov–Gerasimenko 68P/Klemola 69P/Taylor 70P/Kojima 71P/Clark 73P/Schwassmann–Wachmann 74P/Smirnova–Chernykh 75D/Kohoutek 76P/West–Kohoutek–Ikemura 77P/Longmore 78P/Gehrels 79P/du Toit-Hartley 81P/Wild 82P/Gehrels 83D/Russell 84P/Giclas 85P/Boethin 87P/Bus 88P/Howell 91P/Russell 92P/Sanguin 93P/Lovas 94P/Russell 95P/Chiron 96P/Machholz 98P/Takamizawa 99P/Kowal 100P/Hartley 1 101P/Chernykh 102P/Shoemaker 103P/Hartley 105P/Singer Brewster 107P/Wilson–Harrington 109P/Swift–Tuttle 111P/Helin–Roman–Crockett 112P/Urata–Niijima 113P/Spitaler 114P/Wiseman–Skiff 115P/Maury 116P/Wild 117P/Helin–Roman–Alu 118P/Shoemaker–Levy 119P/Parker–Hartley 120P/Mueller 121P/Shoemaker–Holt 122P/de Vico 125P/Spacewatch 128P/Shoemaker–Holt 129P/Shoemaker–Levy 130P/McNaught–Hughes 132P/Helin–Roman–Alu 133P/Elst–Pizarro 136P/Mueller 138P/Shoemaker–Levy 139P/Väisälä–Oterma 143P/Kowal–Mrkos 144P/Kushida 147P/Kushida–Muramatsu 152P/Helin–Lawrence 153P/Ikeya–Zhang 157P/Tritton 158P/Kowal–LINEAR 159P/LONEOS 160P/LINEAR 161P/Hartley–IRAS 163P/NEAT 164P/Christensen 165P/LINEAR 166P/NEAT 167P/CINEOS 168P/Hergenrother 169P/NEAT 170P/Christensen 171P/Spahr 172P/Yeung 173P/Mueller 174P/Echeclus 176P/LINEAR 177P/Barnard 178P/Hug–Bell 206P/Barnard–Boattini 208P/McMillan 209P/LINEAR 238P/Read 246P/NEAT 255P/Levy 271P/van Houten–Lemmon 273P/Pons–Gambart 276P/Vorobjov 289P/Blanpain 311P/PANSTARRS (formerly P/2013 P5) 322P/SOHO (formerly P/2007 R5) Not numbered D/1993 F2 (Shoemaker–Levy 9) P/1997 B1 (Kobayashi) P/2010 A2 (LINEAR) P/2010 B2 (WISE) P/2010 V1 (Ikeya–Murakami) P/2011 NO1 (Elenin) Lost Recovered 11P/Tempel–Swift–LINEAR 15P/Finlay 17P/Holmes 27P/Crommelin 54P/de Vico–Swift–NEAT 69P/Taylor 73P/Schwassmann–Wachmann 113P/Spitaler 107P/Wilson–Harrington 177P/Barnard 206P/Barnard–Boattini Not recovered D/1770 L1 (Lexell) 3D/Biela 5D/Brorsen 18D/Perrine–Mrkos 20D/Westphal 25D/Neujmin 34D/Gale 75D/Kohoutek Asteroids with comet features 2060 Chiron (95P) 7968 Elst–Pizarro (133P) 4015 Wilson–Harrington (107P) 60558 Echeclus (174P) 118401 LINEAR (176P), formerly 1999 RE70 Non-periodic comets Until 1910 C/-43 K1 (Caesar's Comet) X/1106 C1 (Great Comet of 1106) C/1577 V1 (Great Comet of 1577) C/1652 Y1 C/1680 V1 (Great Comet of 1680, Kirsch's Comet, Newton's Comet)) C/1702 H1 (Comet of 1702) C/1729 P1 (Comet of 1729, Comet Sarabat) C/1743 X1 (Great Comet of 1744, Comet Klinkenberg-Chéseaux) C/1760 A1 (Great Comet of 1760) C/1769 P1 (Great Comet of 1769) C/1807 R1 (Great Comet of 1807) C/1811 F1 (Great Comet of 1811) C/1819 N1 (Great Comet of 1819) C/1823 Y1 (Great Comet of 1823) C/1843 D1 (Great March Comet of 1843) C/1847 T1 (Miss Mitchell's Comet) C/1858 L1 (Comet Donati) C/1861 G1 (Comet Thatcher) C/1861 J1 (Great Comet of 1861) X/1872 X1 (Pogson's Comet) C/1882 R1 (Great Comet of 1882) C/1887 B1 (Great Southern Comet of 1887) C/1910 A1 (Great January Comet of 1910) After 1910 C/1911 O1 (Brooks) C/1911 S3 (Beljawsky) C/1927 X1 (Skjellerup–Maristany) C/1931 P1 (Ryves) C/1948 V1 (Eclipse) C/1956 R1 (Arend–Roland) C/1961 R1 (Humason) C/1963 R1 (Pereyra) C/1965 S1 (Ikeya-Seki) C/1969 Y1 (Bennett) C/1970 K1 (White–Ortiz–Bolelli) C/1973 E1 (Kohoutek) C/1975 V1 (West) C/1980 E1 (Bowell) C/1983 H1 (IRAS–Araki–Alcock) C/1989 X1 (Austin) C/1989 Y1 (Skorichenko–George) C/1992 J1 (Spacewatch–Rabinowitz) C/1993 Y1 (McNaught–Russell) C/1995 O1 (Hale–Bopp) C/1996 B2 (Hyakutake) C/1997 L1 (Zhu–Balam) C/1998 H1 (Stonehouse) C/1999 F1 (Catalina) C/1999 S4 (LINEAR) C/2000 U5 (LINEAR) C/2000 W1 (Utsunomiya-Jones) C/2001 OG108 (LONEOS) C/2001 Q4 (NEAT) C/2002 T7 (LINEAR) C/2004 Q2 (Machholz) C/2006 A1 (Pojmański) C/2006 M4 (SWAN) C/2006 P1 (McNaught) C/2007 E2 (Lovejoy) C/2007 F1 (LONEOS) C/2007 K5 (Lovejoy) C/2007 N3 (Lulin) C/2007 Q3 (Siding Spring) C/2007 W1 (Boattini) C/2009 F6 (Yi–SWAN) C/2009 R1 (McNaught) C/2010 X1 (Elenin) C/2011 L4 (PANSTARRS) C/2011 W3 (Lovejoy) C/2012 E2 (SWAN) C/2012 F6 (Lemmon) C/2012 K1 (PANSTARRS) C/2012 S1 (ISON) C/2012 S4 (PANSTARRS) C/2013 A1 (Siding Spring) C/2013 R1 (Lovejoy) C/2013 US10 (Catalina) C/2013 V5 (Oukaimeden) C/2014 E2 (Jacques) C/2014 Q2 (Lovejoy) After 1910 (by name) Arend–Roland Austin Beljawsky Bennett Boattini Bowell Brooks Catalina C/1999 F1 C/2013 US10 Eclipse Elenin Hale-Bopp Humason Hyakutake Ikeya-Seki IRAS–Araki–Alcock ISON Jacques Kohoutek Lemmon LINEAR C/1999 S4 C/2000 U5 C/2002 T7 LONEOS C/2001 OG108 C/2007 F1 Lovejoy C/2007 E2 C/2007 K5 C/2011 W3 C/2013 R1 C/2014 Q2 Lulin Machholz McNaught C/2006 P1 C/2009 R1 McNaught–Russell NEAT Oukaimeden Pan-STARRS C/2011 L4 C/2012 K1 C/2012 S4 311P Pereyra Pojmański Ryves Spacewatch–Rabinowitz Siding Spring C/2007 Q3 C/2013 A1 Skjellerup–Maristany Skorichenko–George Stonehouse SWAN C/2006 M4 C/2012 E2 Utsunomiya–Jones West White–Ortiz–Bolelli Yi–SWAN Zhu–Balam Category Commons Wikinews

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Minor planets navigator 1 Ceres 2 Pallas

Small Solar System bodies Minor planets Designation Groups Moons Meanings of names Pronunciation of names Asteroids Aten asteroid Families Jupiter Trojans Main-belt Near-Earth Spectral types Distant minor planets Centaurs Damocloids Neptune trojans Trans-Neptunian objects Detached Kuiper belt Oort cloud Scattered disc Comets Extinct Great Lost Main-belt Non-periodic Periodic Sungrazing Meteoroids Bolide Dust Fireball Meteor Meteorite Tektite Lists / categories Asteroid groups and families Asteroid moons Binary asteroids Minor planets

Spacecraft missions to minor planets including dwarf planets and asteroids Orbiters NEAR Shoemaker Hayabusa Dawn Hayabusa 2 (en route) Landers NEAR Shoemaker MASCOT (en route) MINERVA-II 1A, 1B, 2 (en route) Flybys Galileo NEAR Shoemaker Cassini–Huygens Deep Space 1 Stardust/NExT Rosetta New Horizons Chang'e 2 Failed Clementine MINERVA PROCYON Planned OSIRIS-REx (lander, 2016) Near-Earth Asteroid Scout (probe, 2018) Proposed AIDA (AIM/ DART) Asteroid Redirect Mission Psyche Orbiter Cancelled AGORA CRAF Don Quijote Hayabusa Mk2 MAOSEP Marco Polo MarcoPolo-R New Horizons 2 Vesta Related Asteroid belt Asteroid capture Asteroid mining Human mission to an asteroid Colonization of asteroids Ceres Colonization Pluto Exploration Vesta Small Solar System bodies Near-Earth object Trans-Neptunian object Colonization Trojan Spacecraft missions to comets List of artificial objects on extra-terrestrial surfaces Italics indicate active current missions.

Modern impact events On Earth Pre-2000 1490 Ch'ing-yang 1783 Great Meteor 1860 Great Meteor 1908 Tunguska 1913 Great Meteor Procession 1930 Curuçá River 1938 Chicora meteor 1947 Sikhote-Alin meteorite 1969 Murchison meteorite 1972 Great Daylight Fireball 1990 Earth-grazing meteoroid Post-2000 2002 Eastern Mediterranean 2002 Vitim 2007 Carancas 2008 TC3 impact 2009 Sulawesi superbolide 2012 Sutter's Mill meteorite 2012 UK meteoroid 2012 Novato meteorite 2013 Chelyabinsk meteor Chelyabinsk meteorite 2014 AA impact 2014 Ontario fireball 2015 Kerala meteorite 2015 Thailand bolide WT1190F impact On Jupiter 1994 Comet Shoemaker–Levy 9 2009 Jupiter impact 2010 Jupiter impact Lists Asteroid close approaches to Earth Comets Meteor air bursts Meteorite falls Minor planets crossing Earth's orbit See also Asteroid impact avoidance Bolide Earth-grazing fireball Meteor procession Meteor shower Meteorite Meteoroid Near-Earth object Potentially hazardous object

Planetary defense Main topics Asteroid Bolide Earth-grazing fireball Impact event List of meteor air bursts Meteor procession Meteor shower Meteorite Meteoroid Near-Earth object Potentially hazardous object Defense Asteroid impact avoidance Asteroid close approaches Earth-crossing minor planets Gravity tractor Ion Beam Shepherd Damage scales Palermo scale Torino scale Space probes AIDA Asteroid Redirect Mission Dawn Deep Impact Halley Armada Hayabusa Hayabusa 2 PROCYON NEAR Shoemaker New Horizons OSIRIS-REx Rosetta Philae Stardust NEO tracking ATLAS Catalina Sky Survey LINEAR LONEOS NEAT NEOSSat NEOCam NEODyS OGS Telescope Orbit@home Pan-STARRS SCAP Sentinel Space Telescope Sentry Spacewatch WISE Organizations B612 Foundation Japan Spaceguard Association Meteoritical Society NEOShield Spaceguard The Spaceguard Foundation Space Situational Awareness Programme Planetary Defense Coordination Office Potential threats 1950 DA 1994 WR12 101955 Bennu 2002 MN (410777) 2009 FD 2010 RF12 99942 Apophis Films/video (1916) The End of the World (1951) When Worlds Collide (1962) Gorath (1968) The Green Slime (1968) The Paradise Syndrome (1978) A Fire in the Sky (1979) Meteor (1995) NOVA: Doomsday Asteroid (1997) Asteroid (1998) Deep Impact (1998) Armageddon (1999) Judgment Day (2004) Post Impact (2006) Deadly Skies (2007) Super Comet: After the Impact (2009) Impact (2009) NOVA: Last Extinction (2010) Meteor Storm (2010) HORIZON: Asteroids–The Good, the Bad and the Ugly (2011) Melancholia (2013) NOVA: Meteor Strike (2013) NOVA: Asteroid: Doomsday or Payday? (2015) Impact Earth

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