Editing Asteroid (section)

=== Size distribution ===
[[File:Asteroids by size and number.svg|thumb|The asteroids of the Solar System, categorized by size and number|300x300px]]
{{image frame
|width=256
 | content = {{Graph:Chart
  | width=75
  | height=75
  | type=pie
  | legend=
  | x=Ceres,Vesta,Pallas,Hygiea,Interamnia,Eunomia,other
  | y1=938,259,204,87,35,30,841<!--total 2394x18kg-->
  | showValues=angle:0,format:.0f
          }}
 | caption = The masses of the largest asteroids in the main belt: [[1 Ceres]] (blue), [[4 Vesta]], [[2 Pallas]], [[10 Hygiea]], [[704 Interamnia]], [[15 Eunomia]] and the remainder of the Main Belt (pink). The unit of mass is {{e|18}} kg.}}
Asteroids vary greatly in size, from almost {{val|1000|u=km}} for the largest down to rocks just 1&nbsp;meter across, below which an object is classified as a [[meteoroid]].{{efn|The definition in the 1995 paper (Beech and Steel) has been updated by a 2010 paper (Rubin and Grossman) and the discovery of 1&nbsp;meter asteroids.}} The three largest are very much like miniature planets: they are roughly spherical, have at least partly differentiated interiors,<ref name=Schmidt2007>{{cite journal |title=Hubble Space Telescope Observations of 2&nbsp;Pallas |journal=Bulletin of the American Astronomical Society |volume=39 |page=485 |date=2007 |display-authors=6 |last1=Schmidt |first1= B. |last2=Russell |first2= C.T. |last3=Bauer |first3= J.M. |last4=Li |first4= J. |last5=McFadden |first5= L.A. |last6=Mutchler |first6= M. |last7=Parker |first7= J.W. |last8=Rivkin |first8= A.S. |last9=Stern |first9= S.A. |author10=Thomas, P.C. |bibcode=2007DPS....39.3519S}}</ref> and are thought to be surviving [[protoplanet]]s. The vast majority, however, are much smaller and are irregularly shaped; they are thought to be either battered [[planetesimal]]s or fragments of larger bodies.

The [[dwarf planet]] [[Ceres (dwarf planet)|Ceres]] is by far the largest asteroid, with a diameter of {{cvt|940|km|-1}}. The next largest are [[4 Vesta]] and [[2 Pallas]], both with diameters of just over {{cvt|500|km|-2}}. Vesta is the brightest of the four main-belt asteroids that can, on occasion, be visible to the naked eye.<ref>{{cite book | title=The Observer's Guide to Astronomy | volume=1 | series=Practical Astronomy Handbooks | editor-first=Patrick | editor-last=Martinez | translator-last1=Dunlop | translator-first1=Storm | publisher=Cambridge University Press | date=1994 | isbn=978-0-521-37945-8 | page=297 | url=https://books.google.com/books?id=k5iUVz7iFTQC&pg=PA297 }}</ref> 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 {{val|2394|6|e=18|u=kg}}, ≈&thinsp;3.25% of the mass of the Moon. Of this, [[Ceres (dwarf planet)|Ceres]] comprises {{val|938|e=18|u=kg}}, about 40% of the total. Adding in the next three most massive objects, [[4 Vesta|Vesta]] (11%), [[2 Pallas|Pallas]] (8.5%), and [[10 Hygiea|Hygiea]] (3–4%), brings this figure up to a bit over 60%, whereas the next seven most-massive asteroids bring the total up to 70%.<ref name="Pitjeva2018"/> The number of asteroids increases rapidly as their individual masses decrease.

The number of asteroids decreases markedly with increasing size. Although the size distribution generally follows a [[power law]], there are 'bumps' at about {{val|5|u=km}} and {{val|100|u=km}}, where more asteroids than expected from such a curve are found. Most asteroids larger than approximately 120&nbsp;km in diameter are primordial (surviving from the accretion epoch), whereas most smaller asteroids are products of fragmentation of primordial asteroids. The primordial population of the main belt was probably 200 times what it is today.<ref>{{Cite journal |last1=Bottkejr |first1=W |last2=Durda |first2=D |last3=Nesvorny |first3=D |last4=Jedicke |first4=R |last5=Morbidelli |first5=A |last6=Vokrouhlicky |first6=D |last7=Levison |first7=H |date=May 2005 |title=The fossilized size distribution of the main asteroid belt |url=https://linkinghub.elsevier.com/retrieve/pii/S0019103504003811 |journal=Icarus|volume=175 |issue=1 |pages=111–140 |doi=10.1016/j.icarus.2004.10.026|bibcode=2005Icar..175..111B }}</ref><ref>{{Cite journal |last1=O'Brien |first1=David P. |last2=Sykes |first2=Mark V. |date=December 2011 |title=The Origin and Evolution of the Asteroid Belt{{snd}}Implications for Vesta and Ceres |url=http://link.springer.com/10.1007/s11214-011-9808-6 |journal=Space Science Reviews|volume=163 |issue=1–4 |pages=41–61 |doi=10.1007/s11214-011-9808-6 |bibcode=2011SSRv..163...41O |s2cid=121856071 |issn=0038-6308}}</ref>

====Largest asteroids====
{{See also|Largest asteroids}}

{{Multiple image
| direction         = vertical
| image1            = 42 of the largest objects in the asteroid belt.jpg
| caption1          = 42 of the largest objects in the asteroid belt captured by [[European Southern Observatory|ESO]]'s [[Very Large Telescope]]
| image2            = Eros, Vesta and Ceres size comparison.jpg
| caption2          = Eros, Vesta and Ceres size comparison
| total_width       = 250
}}

Three largest objects in the asteroid belt, [[Ceres (dwarf planet)|Ceres]], [[4 Vesta|Vesta]], and [[2 Pallas|Pallas]], are intact [[protoplanet]]s that share many characteristics common to planets, and are atypical compared to the majority of irregularly shaped asteroids. The fourth-largest asteroid, [[10 Hygiea|Hygiea]], appears nearly spherical although it may have an undifferentiated interior,<ref>{{Cite web|title=Asteroids {{!}} Imaging the Universe|url=http://astro.physics.uiowa.edu/ITU/labs/general-astronomy/asteroids/|access-date=2021-08-31|website=astro.physics.uiowa.edu|archive-date=31 August 2021|archive-url=https://web.archive.org/web/20210831200522/http://astro.physics.uiowa.edu/ITU/labs/general-astronomy/asteroids/|url-status=dead}}</ref> like the majority of asteroids. The four largest asteroids constitute half the mass of the asteroid belt.

Ceres is the only asteroid that appears to have a [[Plasticity (physics)|plastic]] shape under its own gravity and hence the only one that is a [[dwarf planet]].<ref name=IAU-2006/> It has a much higher [[Absolute magnitude#Solar System bodies (H)|absolute magnitude]] than the other asteroids, of around 3.32,<ref name=AstJ-2002-v123-p549/> and may possess a surface layer of ice.<ref name="planetary"/> Like the planets, Ceres is differentiated: it has a crust, a mantle and a core.<ref name="planetary"/> No meteorites from Ceres have been found on Earth.<ref name=satellites/>

Vesta, too, has a differentiated interior, though it formed inside the Solar System's [[Frost line (astrophysics)|frost line]], and so is devoid of water;<ref>{{cite press release |title=Asteroid or mini-planet? Hubble maps the ancient surface of Vesta |date=19 April 1995 |id=STScI-1995-20 |url=http://hubblesite.org/news_release/news/1995-20 |website=Hubble Space Telescope |publisher=Space Telescope Science Institute |access-date=16 December 2017}}<br />{{cite press release |title=Key stages in the evolution of the asteroid Vesta |website=Hubble Space Telescope |publisher=Space Telescope Science Institute |date=19 April 1995 |url=http://hubblesite.org/newscenter/newsdesk/archive/releases/1995/20/image/c |access-date=20 October 2007 |url-status=live |archive-url=https://web.archive.org/web/20080907192327/http://hubblesite.org/newscenter/newsdesk/archive/releases/1995/20/image/c |archive-date=7 September 2008}}</ref><ref>
{{cite journal |last1=Russel |first1=C. |last2=Raymond |first2=C. |last3=Fraschetti |first3=T. |last4=Rayman |first4=M. |last5=Polanskey |first5=C. |last6=Schimmels |first6=K. |last7=Joy |first7=S. |year=2005 |title=Dawn mission and operations |journal=Proceedings of the International Astronomical Union |volume=1 |issue=S229 |pages=97–119 |bibcode=2006IAUS..229...97R |doi=10.1017/S1743921305006691 |doi-access=free }}</ref> its composition is mainly of basaltic rock with minerals such as olivine.<ref>{{cite journal |last=Burbine |first=T.H. |date=July 1994 |title=Where are the olivine asteroids in the main belt? |journal=Meteoritics |volume=29 |issue=4 |page=453 |bibcode-access=free |bibcode=1994Metic..29..453B}}</ref> 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 asteroid]]s, and is the source of the [[HED meteorite]]s, 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.<ref name="Torppa1996"/> Its composition is similar to that of Ceres: high in carbon and silicon, and perhaps partially differentiated.<ref name=Icarus-1983-v56-p398/> Pallas is the parent body of the [[Palladian family]] of asteroids.

Hygiea is the largest carbonaceous asteroid<ref name=Icarus-2002-156-p202/> 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. Because there is no sufficiently large crater on the surface to be the source of that family, as there is on Vesta, it is thought that Hygiea may have been completely disrupted in the collision that formed the Hygiean family and recoalesced after losing a bit less than 2% of its mass. Observations taken with the [[Very Large Telescope]]'s [[VLT-SPHERE|SPHERE]] imager in 2017 and 2018, revealed that Hygiea has a nearly spherical shape, which is consistent both with it being in [[hydrostatic equilibrium]], or formerly being in hydrostatic equilibrium, or with being disrupted and recoalescing.<ref name=NatAstr-2019-10-28/><ref name=Strickland2019/>

Internal differentiation of large asteroids is possibly related to their lack of [[natural satellite]]s, as satellites of main belt asteroids are mostly believed to form from collisional disruption, creating a [[rubble pile]] structure.<ref name=satellites>{{cite journal|title=Dawn mission's search for satellites of Ceres: Intact protoplanets don't have satellites|journal=Icarus|volume=316|date=December 2018|pages=191–204|author1-first=Lucy A.|author1-last=McFadden |author2-first=David R. |author2-last=Skillman |author3-first=N |author3-last=Memarsadeghi |doi=10.1016/j.icarus.2018.02.017|bibcode=2018Icar..316..191M |s2cid=125181684 |quote=Examination of the physical properties of the 41 largest and most massive main belt asteroids suggests that large asteroids without satellites are intact and their interiors have internal strength. This is consistent with results from the Dawn mission at both Vesta and Ceres. Ceres' volatile-rich composition also is a likely contributor to both the absence of satellites at Ceres and of Ceres meteorites at Earth. These results suggest that collisional disruption creating rubble pile structure is a necessary condition for formation of satellites around main belt asteroids.}}</ref>

{| class="wikitable"
|+ Attributes of largest asteroids
|- style="font-size: smaller;"
!Name
!Orbital<br />radius<br />([[Astronomical unit|AU]])
![[Orbital period|Orbital<br />period]]<br />(years)
![[Inclination|Inclination<br />to ecliptic]]
![[Orbital eccentricity|Orbital<br />eccentricity]]
! Diameter<br />(km)
! Diameter<br />(% of [[Moon]])
! Mass<br />({{e|18}} kg)
! Mass<br />(% of Ceres)
! Density<br />(g/cm<sup>3</sup>)
! Rotation<br />period<br />(hr)
|- style="text-align:center;"
! style="text-align:left;"| [[Ceres (dwarf planet)|Ceres]]
| 2.77
| 4.60
| 10.6°
| 0.079
| 964×964×892<br />(mean 939.4)
| 27%
| 938
| 100%
| 2.16±0.01
| 9.07
|- style="text-align:center;"
! style="text-align:left;"| [[4 Vesta|Vesta]]
| 2.36
| 3.63
| 7.1°
| 0.089
| 573×557×446<br />(mean 525.4)
| 15%
| 259
| 28%
| 3.46 ± 0.04
| 5.34
|- style="text-align:center;"
! style="text-align:left;"| [[2 Pallas|Pallas]]
| 2.77
| 4.62
| 34.8°
| 0.231
| 550×516×476<br />(mean 511±4)
| 15%
| 204±3
| 21%
| 2.92±0.08
| 7.81
|- style="text-align:center;"
! style="text-align:left;"| [[10 Hygiea|Hygiea]]
| 3.14
| 5.56
| 3.8°
| 0.117
| 450×430×424<br />(mean 433±8)
| 12%
| 87±7
| 9%
| 2.06±0.20
| 13.8
|}