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
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. In this article, the term
"asteroid" refers to the minor planets of the inner Solar System
including those co-orbital with Jupiter.
There are millions of asteroids, many thought to be the shattered
remnants of planetesimals, bodies within the young Sun's solar nebula
that never grew large enough to become planets. 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 objects. 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. The size of asteroids
varies greatly, the largest is almost 1,000 km (625 mi)
Asteroids are differentiated from comets and meteoroids. In the case
of comets, the difference is one of composition: while asteroids are
mainly composed of mineral and rock, comets are composed of dust and
ice. In addition, asteroids formed closer to the sun, preventing the
development of the aforementioned cometary ice. The difference
between asteroids and meteoroids is mainly one of size: meteoroids
have a diameter of less than one meter, whereas asteroids have a
diameter of greater than one meter. Finally, meteoroids can be
composed of either cometary or asteroidal materials.
Only one asteroid, 4 Vesta, which has a relatively reflective surface,
is normally visible to the naked eye, and this only in very dark skies
when it is favorably positioned. Rarely, small asteroids passing close
Earth may be visible to the naked eye for a short time. As of
October 2017[update], the Minor
Planet Center had data on almost
745,000 objects in the inner and outer Solar System, of which almost
504,000 had enough information to be given numbered designations.
United Nations declared June 30 as International
Asteroid Day to
educate the public about asteroids. The date of International Asteroid
Day commemorates the anniversary of the Tunguska asteroid impact over
Siberia, Russian Federation, on 30 June 1908.
1.1 Historical methods
1.2 Manual methods of the 1900s and modern reporting
1.3 Computerized methods
4 Distribution within the Solar System
5.1 Size distribution
5.1.1 Largest asteroids
5.4 Surface features
6.1 Orbital classification
6.1.1 Quasi-satellites and horseshoe objects
6.2 Spectral classification
8.1 Planned and future missions
11 See also
14 External links
Sizes of the first ten asteroids to be discovered, compared to the
243 Ida and its moon Dactyl. Dactyl is the first satellite of an
asteroid to be discovered.
The first asteroid to be discovered, Ceres, was 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",[note 2] coined in Greek as
ἀστεροειδής, or asteroeidēs, meaning 'star-like,
star-shaped', and derived from the 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. [note 3]
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Asteroid discovery methods have dramatically improved over the past
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
Uranus at the distance predicted by the law. This task
required that hand-drawn sky charts be prepared for all stars in the
zodiacal band down to an agreed-upon limit of faintness. On subsequent
nights, the sky would be charted again and any moving object would,
hopefully, be spotted. The expected motion of the missing planet was
about 30 seconds of arc per hour, readily discernible by observers.
First asteroid image (Ceres and Vesta) from
Mars – viewed by
Curiosity (20 April 2014).
The first object, Ceres, was not discovered by a member of the group,
but rather by accident in 1801 by Giuseppe Piazzi, director of the
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
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.
Karl Ludwig Hencke
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", a phrase variously attributed to Eduard
Suess and Edmund Weiss. Even a century later, only a few
thousand asteroids were identified, numbered and named.
Manual methods of the 1900s and modern reporting
Until 1998, asteroids were discovered by a four-step process. First, a
region of the sky was photographed by a wide-field telescope, or
astrograph. Pairs of photographs were taken, typically one hour apart.
Multiple pairs could be taken over a series of days. Second, the two
films 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.
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.
2004 FH is the center dot being followed by the sequence; the object
that flashes by during the clip is an artificial satellite.
There is increasing interest in identifying asteroids whose orbits
cross Earth's, and that could, given enough time, collide with Earth
(see Earth-crosser asteroids). The three most important groups of
Earth asteroids are the Apollos, Amors, and Atens. Various
asteroid deflection strategies have been proposed, as early as the
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
1937. Astronomers began to realize the possibilities of
Two events in later decades increased the alarm: the increasing
acceptance of the
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 ten meters 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 2011[update], it was
estimated that 89% to 96% of near-
Earth asteroids one kilometer or
larger in diameter had been discovered. A list of teams using such
Asteroid Research (LINEAR)
Asteroid Tracking (NEAT)
Lowell Observatory Near-Earth-Object Search
Lowell Observatory Near-Earth-Object Search (LONEOS)
Catalina Sky Survey
Catalina Sky Survey (CSS)
Campo Imperatore Near-
Earth Object Survey (CINEOS)
Asiago-DLR Asteroid Survey (ADAS)
As of 20 September 2013[update], the LINEAR system alone has
discovered 138,393 asteroids. Among all the surveys, 4711
Earth asteroids have been discovered including over 600 more
than 1 km (0.6 mi) in diameter.
Euler diagram showing the types of bodies in the Solar System. (see
Solar System body)
A composite image, to scale, of the asteroids that have been imaged at
high resolution except Ceres. As of 2011[update], they are, from
largest to smallest: 4 Vesta, 21 Lutetia, 253 Mathilde,
243 Ida and
its moon Dactyl, 433 Eros, 951 Gaspra, 2867 Šteins, 25143 Itokawa.
The largest asteroid in the previous image, Vesta (left), with Ceres
(center) and the
Moon (right) shown to scale.
Traditionally, small bodies orbiting the
Sun were classified as
comets, asteroids, or meteoroids, with anything smaller than ten
meters across being called a meteoroid. Beech and Steel's 1995 paper
proposed a meteoroid definition including size limits. The
term "asteroid", from the Greek word for "star-like", never had a
formal definition, with the broader term minor planet being preferred
by the International Astronomical Union.
However, following the discovery of asteroids below ten meters in
size, Rubin and Grossman's 2010 paper revised the previous definition
of meteoroid to objects between 10 µm and 1 meter in size
in order to maintain the distinction between asteroids and
meteoroids. The smallest asteroids discovered (based on absolute
magnitude H) are 2008 TS26 with H = 33.2 and
2011 CQ1 with H = 32.1
both with an estimated size of about 1 meter.
In 2006, the term "small
Solar System body" was also introduced to
cover both most minor planets and comets.[note 4] Other languages
prefer "planetoid" (Greek for "planet-like"), and this term is
occasionally used in English especially for larger minor planets such
as the dwarf planets as well as an alternative for asteroids since
they are not star-like. 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
William Daniel Conybeare to
describe minor planets, but is not in common use. The three
largest objects in the asteroid belt, Ceres, Pallas, and Vesta, grew
to the stage of protoplanets. Ceres is a dwarf planet, the only one in
the inner Solar System.
When found, asteroids were seen as a class of objects distinct from
comets, and there was no unified term for the two until "small Solar
System body" was coined in 2006. The main difference between an
asteroid and a comet is that a comet shows a coma due to sublimation
of 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 asteroid-like. A further distinction is that comets
typically have more eccentric orbits than most asteroids; most
"asteroids" with notably eccentric orbits are probably dormant or
For almost two centuries, from the discovery of Ceres in 1801 until
the discovery of the first centaur, Chiron in 1977, all known
asteroids spent most of their time at or within the orbit of Jupiter,
though a few such as Hidalgo ventured far beyond
Jupiter for part of
their orbit. When astronomers started finding more small bodies that
permanently resided further out than Jupiter, now called centaurs,
they numbered them among the traditional asteroids, though there was
debate over whether they should be considered asteroids or as a new
type of object. Then, when the first trans-Neptunian object (other
than Pluto), Albion, was discovered in 1992, and especially when large
numbers of similar objects started turning up, new terms were invented
to sidestep the issue: Kuiper-belt object, trans-Neptunian object,
scattered-disc object, and so on. These inhabit the cold outer reaches
Solar System where ices remain solid and comet-like bodies are
not expected to exhibit much cometary activity; if centaurs or
trans-Neptunian objects were to venture close to the Sun, their
volatile ices would sublimate, and traditional approaches would
classify them as comets and not asteroids.
The innermost of these are the Kuiper-belt objects, called "objects"
partly to avoid the need to classify them as asteroids or comets.
They are thought to be predominantly comet-like in composition, though
some may be more akin to asteroids. Furthermore, most do not have
the highly eccentric orbits associated with comets, and the ones so
far discovered are larger than traditional comet nuclei. (The much
Oort cloud is hypothesized to be the main reservoir of
dormant comets.) Other recent observations, such as the analysis of
the cometary dust collected by the Stardust probe, are increasingly
blurring the distinction between comets and asteroids, suggesting
"a continuum between asteroids and comets" rather than a sharp
The minor planets beyond Jupiter's orbit are sometimes also called
"asteroids", especially in popular presentations.[note 5] However, it
is becoming increasingly common for the term "asteroid" to be
restricted to minor planets of the inner Solar System. Therefore,
this article will restrict itself for the most part to the classical
asteroids: objects of the asteroid belt,
Jupiter trojans, and
When the IAU introduced the class small
Solar System bodies in 2006 to
include most objects previously classified as minor planets and
comets, they created the class of dwarf planets for the largest minor
planets—those that have enough mass to have become ellipsoidal under
their own gravity. According to the IAU, "the term 'minor planet' may
still be used, but generally the term 'Small
Solar System Body' will
be preferred." Currently only the largest object in the asteroid
belt, Ceres, at about 975 km (606 mi) across, has been
placed in the dwarf planet category.
Artist’s impression shows how an asteroid is torn apart by the
strong gravity of a white dwarf.
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. Ceres and Vesta grew large enough to melt and
differentiate, with heavy metallic elements sinking to the core,
leaving rocky minerals in the crust.
In the Nice model, many Kuiper-belt objects are captured in the outer
asteroid belt, at distances greater than 2.6 AU. Most were later
ejected by Jupiter, but those that remained may be the D-type
asteroids, and possibly include Ceres.
Distribution within the Solar System
See also: List of minor-planet groups, List of notable asteroids, and
List of minor planets
The asteroid belt (white) and Jupiter's trojan asteroids (green)
Various dynamical groups of asteroids have been discovered orbiting in
the inner Solar System. Their orbits are perturbed by the gravity of
other bodies in the
Solar System and by the Yarkovsky effect.
Significant populations include:
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, 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.
Main article: Trojan (astronomy)
Trojans are populations that share an orbit with a larger planet or
moon, but do not collide with it because they orbit in one of the two
Lagrangian points of stability, L4 and L5, which lie 60° ahead of and
behind the larger body.
The most significant population of trojans are the
Jupiter trojans have been discovered (As of
2010[update]), it is thought that they are as numerous as the
asteroids in the asteroid belt. Trojans have been found in the orbits
of other planets, including Venus, Earth, Mars, Uranus, and Neptune.
Main article: 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
June 2016[update], 14,464 near-
Earth asteroids are known and
the number over one kilometer in diameter is estimated to be
Frequency of bolides, small asteroids roughly 1 to 20 meters in
diameter impacting Earth's atmosphere.
The asteroids of the Solar System, categorized by size and number
Asteroids vary greatly in size, from almost
7006100000000000000♠1000 km for the largest down to rocks just
1 meter across.[note 6] The three largest are very much like
miniature planets: they are roughly spherical, have at least partly
differentiated interiors, and are thought to be surviving
protoplanets. The vast majority, however, are much smaller and are
irregularly shaped; they are thought to be either surviving
planetesimals or fragments of larger bodies.
The dwarf planet Ceres is by far the largest asteroid, with a diameter
of 975 km (606 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
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.
Adding in the next three most massive objects, Vesta (9%), Pallas
(7%), and Hygiea (3%), brings this figure up to 51%; whereas the three
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
Approximate number of asteroids (N) larger than a certain diameter (D)
See also: Largest asteroids
The relative masses of the twelve largest asteroids known,[note 7]
compared to the remaining mass of the asteroid belt.
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. The fourth largest asteroid, Hygiea, has an
undifferentiated interior, like the majority of asteroids. Between
them, the four largest asteroids constitute half the mass of the
Ceres is the only asteroid with a fully ellipsoidal shape and hence
the only one that is a dwarf planet. It has a much higher absolute
magnitude than the other asteroids, of around 3.32, and may
possess a surface layer of ice. Like the planets, Ceres is
differentiated: it has a crust, a mantle and a core. No meteorites
from Ceres have been found on Earth.
Vesta, too, has a differentiated interior, though it formed inside the
Solar System's frost line, and so is devoid of water; its
composition is mainly of basaltic rock such as olivine. Aside from
the large crater at its southern pole, Rheasilvia, Vesta also has an
ellipsoidal shape. Vesta is the parent body of the
Vestian family and
other 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.
Its composition is similar to that of Ceres: high in carbon and
silicon, and perhaps partially differentiated. Pallas is the
parent body of the
Palladian family of asteroids.
Hygiea is the largest carbonaceous asteroid and, unlike the other
largest asteroids, lies relatively close to the plane of the
ecliptic. It is the largest member and presumed parent body of the
Hygiean family of asteroids.
Attributes of largest asteroids
(% of Moon)
(% of Ceres)
3.44 ± 0.12
2.12 ± 0.04
2.71 ± 0.11
2.76 ± 1.2
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
inertial force 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.
Cratered terrain on 4 Vesta
The physical composition of asteroids is varied and in most cases
poorly understood. Ceres appears to be composed of a rocky core
covered by an icy mantle, where Vesta is thought to have a nickel-iron
core, olivine mantle, and basaltic crust. 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, or, possibly, a planet.
Asteroids contain traces of amino acids and other organic compounds,
and some speculate that asteroid impacts may have seeded the early
Earth with the chemicals necessary to initiate life, or may have even
brought life itself to
Earth (also see panspermia). In August
2011, a report, based on
NASA studies with meteorites found on Earth,
was published suggesting
RNA components (adenine, guanine and
related organic molecules) may have been formed on asteroids and
comets in outer space.
Asteroid collision – building planets (artist concept).
Composition is calculated from three primary sources: albedo, surface
spectrum, and density. The last can only be determined accurately by
observing the orbits of moons the asteroid might have. So far, every
asteroid with moons has turned out to be a rubble pile, a loose
conglomeration of rock and metal that may be half empty space by
volume. The investigated asteroids are as large as 280 km in
diameter, and include
121 Hermione (268×186×183 km), and 87
Sylvia (384×262×232 km). Only half a dozen asteroids are larger
than 87 Sylvia, though none of them have moons; however, some smaller
asteroids are thought to be more massive, suggesting they may not have
been disrupted, and indeed 511 Davida, the same size as Sylvia to
within measurement error, is estimated to be two and a half times as
massive, though this is highly uncertain. The fact that such large
asteroids as Sylvia can be rubble piles, presumably due to disruptive
impacts, has important consequences for the formation of the Solar
System: Computer simulations of collisions involving solid bodies show
them destroying each other as often as merging, but colliding rubble
piles are more likely to merge. This means that the cores of the
planets could have formed relatively quickly.
On 7 October 2009, the presence of water ice was confirmed on the
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. Scientists hypothesize that some of the first
water brought to
Earth was delivered by asteroid impacts after the
collision that produced the Moon. The presence of ice on 24 Themis
supports this theory.
In October 2013, water was detected on an extrasolar body for the
first time, on an asteroid orbiting the white dwarf GD 61. On 22
European Space Agency
European Space Agency (ESA) scientists reported the
detection, for the first definitive time, of water vapor on Ceres, the
largest object in the asteroid belt. The detection was made by
using the far-infrared abilities of the Herschel Space
Observatory. The finding is unexpected because comets, not
asteroids, are typically considered to "sprout jets and plumes".
According to one of the scientists, "The lines are becoming more and
more blurred between comets and asteroids." In May 2016,
significant asteroid data arising from the Wide-field Infrared Survey
Explorer and NEOWISE missions have been questioned, but
the criticism has yet to undergo peer review.
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.
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
Asteroids become darker and redder with age due to space
weathering. However evidence suggests most of the color change
occurs rapidly, in the first hundred thousands years, limiting the
usefulness of spectral measurement for determining the age of
Asteroids are commonly classified according to two criteria: the
characteristics of their orbits, and features of their reflectance
Asteroid group and
Many asteroids have been placed in groups and families based on their
orbital characteristics. Apart from the broadest divisions, it is
customary to name a group of asteroids after the first member of that
group to be discovered. Groups are relatively loose dynamical
associations, whereas families are tighter and result from the
catastrophic break-up of a large parent asteroid sometime in the
past. Families are more common and easier to identify within the
main asteroid belt, but several small families have been reported
Jupiter trojans. Main belt families were first
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
Venus are known to have
Such objects, if associated with
Venus or even hypothetically
Mercury, are a special class of Aten asteroids. However, such objects
could be associated with outer planets as well.
Asteroid spectral types
This picture of
433 Eros shows the view looking from one end of the
asteroid across the gouge on its underside and toward the opposite
end. Features as small as 35 m (115 ft) across can be seen.
In 1975, an asteroid taxonomic system based on color, albedo, and
spectral shape was developed by Clark R. Chapman, David Morrison, and
Ben Zellner. These properties are thought to correspond to the
composition of the asteroid's surface material. The original
classification system had three categories: C-types for dark
carbonaceous objects (75% of known asteroids), S-types for stony
(silicaceous) objects (17% of known asteroids) and U for those that
did not fit into either C or S. This classification has since been
expanded to include many other asteroid types. The number of types
continues to grow as more asteroids are studied.
The two most widely used taxonomies now used are the Tholen
classification and SMASS classification. The former was proposed in
1984 by David J. Tholen, and was based on data collected from an
eight-color asteroid survey performed in the 1980s. This resulted in
14 asteroid categories. In 2002, the Small Main-Belt Asteroid
Spectroscopic Survey resulted in a modified version of the Tholen
taxonomy with 24 different types. Both systems have three broad
categories of C, S, and X asteroids, where X consists of mostly
metallic asteroids, such as the M-type. There are also several smaller
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.
Originally, spectral designations were based on inferences of an
asteroid's composition. However, the correspondence between
spectral class and composition is not always very good, and a variety
of classifications are in use. This has led to significant confusion.
Although asteroids of different 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
Minor planet § Naming
2013 EC, shown here in radar images, has a provisional designation
A newly discovered asteroid is given a provisional designation (such
as 2002 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. In
addition, names can be proposed by the asteroid's discoverer, within
guidelines established by the International Astronomical Union.
Main article: Astronomical symbols
The first asteroids to be discovered were assigned iconic symbols like
the ones traditionally used to designate the planets. By 1855 there
were two dozen asteroid symbols, which often occurred in multiple
Ceres' scythe, reversed to double as the letter C
Athena's (Pallas') spear
A star mounted on a scepter, for Juno, the Queen of Heaven
The altar and sacred fire of Vesta
A scale, or an inverted anchor, symbols of justice
A rainbow (iris) and a star
A flower (flora), specifically the Rose of England
The eye of wisdom and a star
Hygiea's serpent and a star, or the Rod of Asclepius
A harp, or a fish and a star; symbols of the sirens
The laurels of victory and a star
A shield, symbol of Egeria's protection, and a star
A dove carrying an olive branch (symbol of irene 'peace')
with a star on its head, or an olive branch, a flag of truce, and
A heart, symbol of good order (eunomia), and a star
A butterfly's wing, symbol of the soul (psyche), and a star
A dolphin, symbol of Thetis, and a star
The dagger of Melpomene, and a star
The wheel of fortune and a star
Bellona's whip and lance
The shell of
Amphitrite and a star
A lighthouse beacon, symbol of Leucothea
The cross of faith (fides)
In 1851, after the fifteenth asteroid (Eunomia) had been
Johann Franz Encke
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
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.[note 8] 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.
See also: Sample return mission,
Asteroid mining, and Colonization of
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
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
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
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
9969 Braille (by
Deep Space 1
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
In September 2007,
NASA launched the Dawn spacecraft, which orbited 4
Vesta from July 2011 to September 2012, and has been orbiting the
dwarf planet 1 Ceres since 2015.
4 Vesta is the second-largest
asteroid visited to date.
On 13 December 2012, China's lunar orbiter
Chang'e 2 flew within
3.2 km (2 mi) of the asteroid
4179 Toutatis on an extended
Planned and future missions
The Japan Aerospace Exploration Agency (JAXA) launched the
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 launched on September 8, 2016.
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. On 19 June 2014,
NASA reported that asteroid
2011 MD was a
prime candidate for capture by a robotic mission, perhaps in the early
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.
Main article: Asteroids in fiction
Asteroids and the asteroid belt are a staple of science fiction
stories. Asteroids play several potential roles in science fiction: as
places human beings might colonize, resources for extracting minerals,
hazards encountered by spacecraft traveling between two other points,
and as a threat to life on
Earth or other inhabited planets, dwarf
planets and natural satellites by potential impact.
951 Gaspra is the first asteroid to be imaged in close-up, imaged by
Galileo on 29 October 1991 (enhanced color)
Several views of
433 Eros in natural color, imaged by NEAR on 12
Vesta imaged by Dawn on 9 July 2011
Ceres imaged by Dawn on 4 February 2015
Asteroid impact avoidance
BOOTES (Burst Observer and Optical Transient Exploring System)
Asteroid groups and families
Centaur (minor planet)
Chang'e 2 lunar orbiter
List of asteroid close approaches to Earth
List of minor planets
List of minor planets named after people
List of minor planets
List of minor planets named after places
List of minor planets
List of notable asteroids
List of impact craters on Earth
List of unconfirmed impact craters on Earth
Marco Polo (spacecraft)
Meanings of minor planet names
Earth Object Surveillance Satellite) Canada's new
Pioneer 10 spacecraft
^ 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
^ In an oral presentation, Clifford Cunningham presented his
finding that the word was coined by Charles Burney, Jr., the son of a
friend of Herschel,
^ 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".
^ The definition of "small
Solar System bodies" says that they
"include most of the
Solar System asteroids, most trans-Neptunian
objects, comets, and other small bodies".
^ 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."
^ Below 1 meter, these are considered to be meteoroids. The
definition in the 1995 paper (Beech and Steel) has been updated by a
2010 paper (Rubin and Grossman) and the discovery of 1-meter
^ The values of Juno and Herculina may be off by as much as 16%, and
Euphrosyne by a third. The order of the lower eight may change as
better data is acquired, but the values do not overlap with any known
asteroid outside these twelve.
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See also: Asteroids in fiction
Asteroids in astrology
Geology of Ceres
Definition of planet
2006 definition of planet
Carl Friedrich Gauss
Franz Xaver von Zach
Johann Elert Bode
(185105) 2006 SV23
Ceres Polar Lander
Ceres Polar Lander (proposed)
Ceres in fiction
Solar System bodies formerly considered to be planets
Naming of comets
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Past and current
Deep Space 1
C/2016 U1 (NEOWISE)
C/2015 G2 (MASTER)
C/2015 F5 (SWAN-XingMing)
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)
Comets in fiction
Lists of comets
Lists of comets (more)
P/1997 B1 (Kobayashi)
P/2010 B2 (WISE)
P/2011 NO1 (Elenin)
2060 Chiron (95P)
4015 Wilson–Harrington (107P)
7968 Elst–Pizarro (133P)
60558 Echeclus (174P)
118401 LINEAR (176P)
P/2010 A2 (LINEAR)
P/2012 F5 (Gibbs)
P/2012 T1 (PANSTARRS)
P/2013 R3 (Catalina-PANSTARRS)
(300163) 2006 VW139
D/1993 F2 (Shoemaker–Levy 9)
D/1770 L1 (Lexell)
9P/Tempel (2005, 2011)
C/2006 P1 (2007)
C/-43 K1 (Caesar's Comet)
X/1106 C1 (Great
Comet of 1106)
C/1577 V1 (Great
Comet of 1577)
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,
C/1743 X1 (Great
Comet of 1744,
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 (
C/1861 G1 (
C/1861 J1 (Great
Comet of 1861)
C/1865 B1 (Great Southern
Comet of 1865)
X/1872 X1 (Pogson's Comet)
C/1874 H1 (
C/1881 K1 (
C/1882 R1 (Great
Comet of 1882)
C/1887 B1 (Great Southern
Comet of 1887)
C/1890 V1 (
C/1901 G1 (Great
Comet of 1901)
C/1910 A1 (Great January
Comet of 1910)
C/1911 O1 (Brooks)
C/1911 S3 (Beljawsky)
C/1927 X1 (Skjellerup–Maristany)
C/1931 P1 (Ryves)
C/1941 B2 (de Kock-Paraskevopoulos) (de)
C/1947 X1 (Southern Comet) (de)
C/1948 V1 (Eclipse)
C/1956 R1 (Arend–Roland)
C/1957 P1 (Mrkos) (de)
C/1961 O1 (Wilson-Hubbard) (de)
C/1961 R1 (Humason)
C/1962 C1 (Seki-Lines) (de)
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/1998 J1 (SOHO)
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 F4 (Bradfield) (de)
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/2008 Q1 (Matičič)
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)
C/2015 V2 (Johnson)
de Kock–Paraskevopoulos (de)
The Solar System
S/2015 (136472) 1
Solar System objects
By discovery date
Gravitationally rounded objects
Possible dwarf planets
first discovered: Ceres
Planets beyond Neptune
List of crewed spacecraft
List of probes
Outline of the Solar System
Solar System → Local Interstellar Cloud → Local
Bubble → Gould Belt → Orion Arm → Milky
Milky Way subgroup → Local Group → Virgo
Supercluster → Laniakea Supercluster → Observable
universe → Universe
Each arrow (→) may be read as "within" or "part of".
Minor planets navigator
Solar System bodies
Meanings of names
Distant minor planet
Spacecraft missions to minor planets
including dwarf planets and asteroids
Hayabusa2 (en route)
OSIRIS-REx (en route)
MASCOT (en route)
MINERVA-II 1A, 1B, 2 (en route)
Deep Space 1
NEA Scout (probe, 2019)
Lucy (multiple flybys, 2021)
DESTINY+ (multiple flybys, 2022)
Psyche (orbiter, 2022)
Asteroid Redirect Mission
New Horizons 2
Human mission to an asteroid
Colonization of asteroids
Solar System bodies
Spacecraft missions to comets
List of artificial objects on extra-terrestrial surfaces
Italics indicate active current missions.
Modern impact events
1783 Great Meteor
1860 Great Meteor
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
2002 Eastern Mediterranean
2008 TC3 impact
2009 Sulawesi superbolide
2012 Sutter's Mill meteorite
2012 UK meteoroid
2012 Novato meteorite
2013 Chelyabinsk meteor
2014 AA impact
2014 Ontario fireball
2015 Kerala meteorite
2015 Thailand bolide
2017 China bolide
Comet Shoemaker–Levy 9
Asteroid close approaches to Earth
Meteor air bursts
crossing Earth's orbit
Asteroid impact avoidance
Potentially hazardous object
List of meteor air bursts
Potentially hazardous object
Asteroid impact avoidance
Asteroid close approaches
Earth-crossing minor planets
Ion Beam Shepherd
Catalina Sky Survey
Sentinel Space Telescope
Space Situational Awareness Programme
Planetary Defense Coordination Office
(410777) 2009 FD
Films / video
The End of the World (1916)
When Worlds Collide (1951)
The Green Slime
The Green Slime (1968)
The Paradise Syndrome
The Paradise Syndrome (1968)
A Fire in the Sky
A Fire in the Sky (1978)
Deep Impact (1998)
Avengers: Age of Ultron (2015)
Judgment Day (1999)
Post Impact (2004)
Deadly Skies (2006)
Super Comet: After the Impact (2007)
Meteor Storm (2010)
HORIZON: Asteroids–The Good, the Bad and the Ugly (2010)
NOVA: Meteor Strike (2013)
NOVA: Asteroid: Doomsday or Payday? (2013)
Global catastrophic risks
Future of the Earth
Ultimate fate of the universe
Mutual assured destruction
Synthetic intelligence / Artificial intelligence
Existential risk from artificial intelligence
New World Order (conspiracy theory)
World War III
Extinction risk from global warming
Runaway climate change
Global terrestrial stilling
Human impact on the environment
Earth Overshoot Day
Coronal mass ejection
Potentially hazardous object
2011 end times prediction
Blood moon prophecy
Book of Revelation
List of dates predicted for apocalyptic events
Revelation 12 sign prophecy
Year 2000 problem
Apocalyptic and post-apocalyptic fiction
List of apocalyptic and post-apocalyptic fiction
List of disaster films
List of fictional doomsday devices
BNF: cb11976093k (d