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'''Asteroids''', sometimes called minor [[planets]] or ''planetoids'', are small [[Solar System]] bodies in [[orbit]] around the [[Sun]], especially in the inner Solar System; they are smaller than planets but larger than [[meteoroids]]. The term "asteroid" has historically been applied primarily to minor [[planets]] of the inner Solar System, as the outer Solar System was poorly known when it came into common usage. The distinction between asteroids and [[comets]] is made on visual [[appearance]]: Comets show a perceptible [http://en.wikipedia.org/wiki/Coma_(cometary) coma] while asteroids do not.

'''Asteroids''', sometimes called minor [[planets]] or ''planetoids'', are small [[Solar System]] bodies in [[orbit]] around the [[Sun]], especially in the inner Solar System; they are smaller than planets but larger than [[meteoroids]]. The term "asteroid" has historically been applied primarily to minor [[planets]] of the inner Solar System, as the outer Solar System was poorly known when it came into common usage. The distinction between asteroids and [[comets]] is made on visual [[appearance]]: Comets show a perceptible [https://en.wikipedia.org/wiki/Coma_(cometary) coma] while asteroids do not.

==Terminology==

==Terminology==

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

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

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 [[comet]]ary [[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]].

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 [[comet]]ary [[activity]]. Conversely, some (perhaps all) comets are [[eventually]] depleted of their surface volatile ices and become asteroids. A further distinction is that [[comets]] typically have more [[eccentric]] [[orbits]] than most asteroids; most "asteroids" with notably eccentric orbits are probably dormant or extinct [[comets]].

For almost two centuries, from the [[discovery]] of the first asteroid, [http://en.wikipedia.org/wiki/Ceres_(dwarf_planet) Ceres], in 1801 until the discovery of the first http://en.wikipedia.org/wiki/Centaur_(minor_planet) centaur], [http://en.wikipedia.org/wiki/2060_Chiron 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 [http://en.wikipedia.org/wiki/Jupiter Jupiter], now called centaurs, they numbered them among the [[traditional]] ''asteroids'', though there was [[debate]] over whether they should be classified as asteroids or as a new type of object. Then, when the first [http://en.wikipedia.org/wiki/Trans-Neptunian_object trans-Neptunian object], 1992 QB1, was discovered in 1992, and especially when large numbers of similar objects started turning up, new terms were [[invented]] to sidestep the issue: [http://en.wikipedia.org/wiki/Kuiper_belt Kuiper belt object] (KBO), [http://en.wikipedia.org/wiki/Trans-Neptunian_object trans-Neptunian object] (TNO), [http://en.wikipedia.org/wiki/Scattered-disc_object scattered-disc object] (SDO), and so on. These inhabit the cold outer reaches of the [[Solar System]] where ices remain [[solid]] and [[comet]]-like bodies are not expected to exhibit much cometary activity; if centaurs or TNOs were to venture close to the [[Sun]], their volatile ices would sublimate, and [[traditional]] approaches would [[classify]] them as [[comets]] and not ''asteroids''.

For almost two centuries, from the [[discovery]] of the first asteroid, [https://en.wikipedia.org/wiki/Ceres_(dwarf_planet) Ceres], in 1801 until the discovery of the first https://en.wikipedia.org/wiki/Centaur_(minor_planet) centaur], [https://en.wikipedia.org/wiki/2060_Chiron 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 [https://en.wikipedia.org/wiki/Jupiter Jupiter], now called centaurs, they numbered them among the [[traditional]] ''asteroids'', though there was [[debate]] over whether they should be classified as asteroids or as a new type of object. Then, when the first [https://en.wikipedia.org/wiki/Trans-Neptunian_object trans-Neptunian object], 1992 QB1, was discovered in 1992, and especially when large numbers of similar objects started turning up, new terms were [[invented]] to sidestep the issue: [https://en.wikipedia.org/wiki/Kuiper_belt Kuiper belt object] (KBO), [https://en.wikipedia.org/wiki/Trans-Neptunian_object trans-Neptunian object] (TNO), [https://en.wikipedia.org/wiki/Scattered-disc_object scattered-disc object] (SDO), and so on. These inhabit the cold outer reaches of the [[Solar System]] where ices remain [[solid]] and [[comet]]-like bodies are not expected to exhibit much cometary activity; if centaurs or TNOs were to venture close to the [[Sun]], their volatile ices would sublimate, and [[traditional]] approaches would [[classify]] them as [[comets]] and not ''asteroids''.

The innermost of these are the [http://en.wikipedia.org/wiki/Kuiper_belt Kuiper belt objects] (KBOs), called "objects" partly to avoid the need to [[classify]] them as ''asteroids'' or [[comets]]. KBOs are believed to be predominantly [[comet]]-like in [[composition]], though some may be more akin to asteroids. Furthermore, most do not have the highly [[eccentric]] [[orbits]] [[associated]] with [[comets]], and the ones so far [[discovered]] are larger than [[traditional]] [[comet]] [[nuclei]]. (The much more distant [http://en.wikipedia.org/wiki/Oort_cloud 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 [http://en.wikipedia.org/wiki/Stardust_(spacecraft) Stardust] probe, are increasingly blurring the distinction between [[comets]] and asteroids, suggesting "a [[continuum]] between asteroids and comets" rather than a sharp dividing line.

The innermost of these are the [https://en.wikipedia.org/wiki/Kuiper_belt Kuiper belt objects] (KBOs), called "objects" partly to avoid the need to [[classify]] them as ''asteroids'' or [[comets]]. KBOs are believed to be predominantly [[comet]]-like in [[composition]], though some may be more akin to asteroids. Furthermore, most do not have the highly [[eccentric]] [[orbits]] [[associated]] with [[comets]], and the ones so far [[discovered]] are larger than [[traditional]] [[comet]] [[nuclei]]. (The much more distant [https://en.wikipedia.org/wiki/Oort_cloud 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 [https://en.wikipedia.org/wiki/Stardust_(spacecraft) Stardust] probe, are increasingly blurring the distinction between [[comets]] and asteroids, suggesting "a [[continuum]] between asteroids and comets" rather than a sharp dividing line.

The minor planets beyond [http://en.wikipedia.org/wiki/Jupiter Jupiter's] [[orbit]] are rarely called "asteroids", but all are commonly lumped together under the term "asteroid" in popular presentations. For instance, a joint [http://en.wikipedia.org/wiki/NASA NASA]-[http://en.wikipedia.org/wiki/JPL JPL] [[public]]-outreach website states,

The minor planets beyond [https://en.wikipedia.org/wiki/Jupiter Jupiter's] [[orbit]] are rarely called "asteroids", but all are commonly lumped together under the term "asteroid" in popular presentations. For instance, a joint [https://en.wikipedia.org/wiki/NASA NASA]-[https://en.wikipedia.org/wiki/JPL JPL] [[public]]-outreach website states,

<blockquote>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.</blockquote>

<blockquote>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.</blockquote>

<blockquote>Because it is becoming increasingly common for the term "asteroid" to be restricted to minor planets of the inner Solar System,[9] this article will restrict itself for the most part to the classical asteroids: objects of the main asteroid belt, Jupiter trojans, and near-Earth objects.</blockquote>

<blockquote>Because it is becoming increasingly common for the term "asteroid" to be restricted to minor planets of the inner Solar System,[9] this article will restrict itself for the most part to the classical asteroids: objects of the main asteroid belt, Jupiter trojans, and near-Earth objects.</blockquote>

When the IAU introduced the class [http://en.wikipedia.org/wiki/Small_solar_system_body small solar system bodies] in 2006 to include most objects previously [[classified]] as minor planets and [[comets]], they created the class of [http://en.wikipedia.org/wiki/Dwarf_planet 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, [http://en.wikipedia.org/wiki/Ceres_(dwarf_planet) Ceres], at about 950 km (590 mi) across, has been placed in the dwarf planet category, although there are several large asteroids ([http://en.wikipedia.org/wiki/4_Vesta Vesta], [http://en.wikipedia.org/wiki/2_Pallas Pallas], and [http://en.wikipedia.org/wiki/10_Hygiea Hygiea]) that may be [[classified]] as dwarf planets when their shapes are better known.

When the IAU introduced the class [https://en.wikipedia.org/wiki/Small_solar_system_body small solar system bodies] in 2006 to include most objects previously [[classified]] as minor planets and [[comets]], they created the class of [https://en.wikipedia.org/wiki/Dwarf_planet 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, [https://en.wikipedia.org/wiki/Ceres_(dwarf_planet) Ceres], at about 950 km (590 mi) across, has been placed in the dwarf planet category, although there are several large asteroids ([https://en.wikipedia.org/wiki/4_Vesta Vesta], [https://en.wikipedia.org/wiki/2_Pallas Pallas], and [https://en.wikipedia.org/wiki/10_Hygiea Hygiea]) that may be [[classified]] as dwarf planets when their shapes are better known.

==Distribution within the Solar System==

==Distribution within the Solar System==

[[File:InnerSolarSystem-en.jpg|right|frame|<center>Main asteroid belt (white), Trojan asteroids (green)</center>]]

[[File:InnerSolarSystem-en.jpg|right|frame|<center>Main asteroid belt (white), Trojan asteroids (green)</center>]]

The [[majority]] of known asteroids [[orbit]] within the [http://en.wikipedia.org/wiki/Main_asteroid_belt main asteroid belt] between the [[orbits]] of [http://en.wikipedia.org/wiki/Mars Mars] and [http://en.wikipedia.org/wiki/Jupiter 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,[20] and millions of smaller ones. These asteroids may be remnants of the [http://en.wikipedia.org/wiki/Protoplanetary_disk 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 [http://en.wikipedia.org/wiki/Jupiter Jupiter]. Although fewer [http://en.wikipedia.org/wiki/Trojan_asteroid Trojan asteroids] sharing Jupiter's orbit are known, it is thought that there are as many as there are asteroids in the main belt.

The [[majority]] of known asteroids [[orbit]] within the [https://en.wikipedia.org/wiki/Main_asteroid_belt main asteroid belt] between the [[orbits]] of [https://en.wikipedia.org/wiki/Mars Mars] and [https://en.wikipedia.org/wiki/Jupiter 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,[20] and millions of smaller ones. These asteroids may be remnants of the [https://en.wikipedia.org/wiki/Protoplanetary_disk 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 [https://en.wikipedia.org/wiki/Jupiter Jupiter]. Although fewer [https://en.wikipedia.org/wiki/Trojan_asteroid Trojan asteroids] sharing Jupiter's orbit are known, it is thought that there are as many as there are asteroids in the main belt.

The [http://en.wikipedia.org/wiki/Dwarf_planet dwarf planet] [http://en.wikipedia.org/wiki/Ceres Ceres] is the largest object in the asteroid belt, with a [[diameter]] of over 975 km (606 mi). The next largest are the asteroids 2 Pallas and 4 Vesta, both with diameters of over 500 km (311 mi). Normally Vesta is the only main belt asteroid that can, on occasion, become visible to the naked eye. However, on some rare occasions, a near-[[Earth]] asteroid may briefly become visible without technical aid; see [http://en.wikipedia.org/wiki/99942_Apophis 99942 Apophis].

The [https://en.wikipedia.org/wiki/Dwarf_planet dwarf planet] [https://en.wikipedia.org/wiki/Ceres Ceres] is the largest object in the asteroid belt, with a [[diameter]] of over 975 km (606 mi). The next largest are the asteroids 2 Pallas and 4 Vesta, both with diameters of over 500 km (311 mi). Normally Vesta is the only main belt asteroid that can, on occasion, become visible to the naked eye. However, on some rare occasions, a near-[[Earth]] asteroid may briefly become visible without technical aid; see [https://en.wikipedia.org/wiki/99942_Apophis 99942 Apophis].

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

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

Various classes of asteroid have been [[discovered]] outside the main asteroid belt. [http://en.wikipedia.org/wiki/Near-Earth_asteroids Near-Earth asteroids] have [[orbits]] near that of [[Earth]]. [http://en.wikipedia.org/wiki/Trojan_asteroids Trojan asteroids] are [[gravitationally]] locked into [[synchronisation]] with [http://en.wikipedia.org/wiki/Jupiter Jupiter], either leading or trailing the [[planet]] in its [[orbit]]. A couple trojans have been found orbiting with [http://en.wikipedia.org/wiki/Mars Mars]. A group of asteroids called [http://en.wikipedia.org/wiki/Vulcanoid_asteroid Vulcanoids] are hypothesised by some to lie very close to the [[Sun]], within the [[orbit]] of [http://en.wikipedia.org/wiki/Mercury Mercury], but none has so far been found.[http://en.wikipedia.org/wiki/Asteroid]

Various classes of asteroid have been [[discovered]] outside the main asteroid belt. [https://en.wikipedia.org/wiki/Near-Earth_asteroids Near-Earth asteroids] have [[orbits]] near that of [[Earth]]. [https://en.wikipedia.org/wiki/Trojan_asteroids Trojan asteroids] are [[gravitationally]] locked into [[synchronisation]] with [https://en.wikipedia.org/wiki/Jupiter Jupiter], either leading or trailing the [[planet]] in its [[orbit]]. A couple trojans have been found orbiting with [https://en.wikipedia.org/wiki/Mars Mars]. A group of asteroids called [https://en.wikipedia.org/wiki/Vulcanoid_asteroid Vulcanoids] are hypothesised by some to lie very close to the [[Sun]], within the [[orbit]] of [https://en.wikipedia.org/wiki/Mercury Mercury], but none has so far been found.[https://en.wikipedia.org/wiki/Asteroid]

[[Category: Astronomy]]

[[Category: Astronomy]]

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