Editing Asteroid (section)

== Classification ==
Asteroids are commonly categorized according to two criteria: the characteristics of their orbits, and features of their reflectance [[visible spectrum|spectrum]].

=== Orbital classification ===
{{Main|Asteroid group|Asteroid family}}
[[File:Animation_of_(419624)_2010_SO16_orbit.gif|thumb|A complex horseshoe orbit (the vertical looping is due to inclination of the smaller body's orbit to that of the Earth, and would be absent if both orbited in the same plane){{legend2|Yellow|Sun}}{{·}}{{legend2|RoyalBlue|Earth}}{{·}}{{legend2|Magenta|[[(419624) 2010 SO16]]}}]]
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.<ref name=AstFams-Icarus-1995/> Families are more common and easier to identify within the main asteroid belt, but several small families have been reported among the [[Jupiter trojan]]s.<ref name="JewittEtal2004"/> Main belt families 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]] {{dp|Haumea}}.

Some asteroids have unusual [[horseshoe orbit]]s that are co-orbital with [[Earth]] or another planet. Examples are [[3753 Cruithne]] and {{mpl|2002 AA|29}}. The first instance of this type of orbital arrangement was discovered between [[Saturn]]'s moons [[Epimetheus (moon)|Epimetheus]] and [[Janus (moon)|Janus]]. Sometimes these horseshoe objects temporarily become [[quasi-satellite]]s 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 (planet)|Mercury]], are a special class of [[Aten asteroid]]s. However, such objects could be associated with the outer planets as well.

=== Spectral classification ===
{{Main|Asteroid spectral types}}

In 1975, an asteroid [[Taxonomy (general)|taxonomic]] system based on [[color]], [[albedo]], and [[spectral line|spectral shape]] was developed by [[Clark R. Chapman|Chapman]], [[David Morrison (astrophysicist)|Morrison]], and [[Ben Zellner|Zellner]].<ref name=CMZ-1975-Icarus/> These properties are thought to correspond to the composition of the asteroid's surface material. The original classification system had three categories: [[C-type asteroid|C-types]] for dark carbonaceous objects (75% of known asteroids), [[S-type asteroid|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&nbsp;asteroid categories.<ref name=Tholen-1989/> In 2002, the Small Main-Belt Asteroid Spectroscopic Survey resulted in a modified version of the Tholen taxonomy with 24&nbsp;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 asteroid|M-type]]. There are also several smaller classes.<ref name=Bus-2002/>

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.<ref name=McSween-1999/> 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 the same (or similar) materials.

=== Active asteroids ===
{{Main|Active asteroid}}[[File:PIA23554-AsteroidBennu-EjectingParticles-20190106.jpg|thumb|Asteroid {{ats|101955|Bennu}} seen ejecting particles by the [[OSIRIS-REx]]]]

Active asteroids are objects that have asteroid-like orbits but show [[comet]]-like visual characteristics. That is, they show [[Coma (cometary)|comae]], [[comet tail|tails]], or other visual evidence of mass-loss (like a comet), but their orbit remains within [[Jupiter]]'s orbit (like an asteroid).<ref name="Jewitt" /><ref name="JHA15">{{cite book|chapter=The Active Asteroids|first1=David|last1=Jewitt|first2=Henry|last2=Hsieh|first3=Jessica|last3=Agarwal|year=2015|title= Asteroids IV|pages=221–241| editor1-last =  Michel| editor1-first = P. | editor2-last = others| display-editors = 1 | publisher=[[University of Arizona]]|doi= 10.2458/azu_uapress_9780816532131-ch012 |arxiv=1502.02361|bibcode=2015aste.book..221J|isbn=978-0-8165-3213-1|s2cid=119209764| chapter-url= http://www2.ess.ucla.edu/~jewitt/papers/2015/JHA15.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www2.ess.ucla.edu/~jewitt/papers/2015/JHA15.pdf |archive-date=2022-10-09 |url-status=live |access-date=2020-01-30}}</ref> These bodies were originally designated '''main-belt comets''' (MBCs) in 2006 by astronomers [[David Jewitt]] and [[Henry Hsieh (astronomer)|Henry Hsieh]], but this name implies they are necessarily icy in composition like a comet and that they only exist within the [[asteroid belt|main-belt]], whereas the growing population of active asteroids shows that this is not always the case.<ref name="Jewitt">{{cite web |title=The Active Asteroids |publisher=[[UCLA]], Department of Earth and Space Sciences |author=David Jewitt |url=http://www2.ess.ucla.edu/~jewitt/mbc.html |access-date=2020-01-26|author-link=David Jewitt }}</ref><ref name="NYT-20190319">{{cite news |last1=Chang |first1=Kenneth |last2=Stirone |first2=Shannon |title=The Asteroid Was Shooting Rocks Into Space. 'Were We Safe in Orbit?' – NASA's Osiris-Rex and Japan's Hayabusa2 spacecraft reached the space rocks they are surveying last year, and scientists from both teams announced early findings on Tuesday (03/19/2019) |url=https://www.nytimes.com/2019/03/19/science/bennu-ryugu-asteroids.html |date=19 March 2019 |work=[[The New York Times]] |access-date=21 March 2019 }}</ref><ref>{{cite web |title=Hubble Observes Six Tails from an Unusual Asteroid|date=14 November 2013 |publisher=Space Telescope Science Institute (STScI), official YouTube channel for the Hubble Space Telescope|url=https://www.youtube.com/watch?v=CGgRNWUFfZ0  |archive-url=https://ghostarchive.org/varchive/youtube/20211222/CGgRNWUFfZ0 |archive-date=2021-12-22 |url-status=live|access-date=2014-11-15}}{{cbignore}}</ref>

The first active asteroid discovered is [[7968 Elst–Pizarro]]. It was discovered (as an asteroid) in 1979 but then was found to have a tail by [[Eric Walter Elst|Eric Elst]] and Guido Pizarro in 1996 and given the cometary designation 133P/Elst-Pizarro.<ref name="Jewitt"/><ref name=HH133P>{{cite web|last=Hsieh|first=Henry|title=133P/Elst-Pizarro|url=http://www.ifa.hawaii.edu/~hsieh/elstpiz.shtml|publisher=UH Institute for Astronomy|access-date=22 June 2012|date=20 January 2004|url-status=dead|archive-url=https://web.archive.org/web/20111026205338/http://www.ifa.hawaii.edu/~hsieh/elstpiz.shtml|archive-date=26 October 2011}}</ref> Another notable object is [[311P/PanSTARRS]]: observations made by the [[Hubble Space Telescope]] revealed that it had six comet-like tails.<ref name="hubblesite">{{cite web|title=NASA's Hubble Sees Asteroid Spouting Six Comet-Like Tails|url=http://hubblesite.org/newscenter/archive/releases/2013/52/text/|publisher=Hubblesite|date=7 November 2013}}</ref> The tails are suspected to be streams of material ejected by the asteroid as a result of a [[rubble pile]] asteroid spinning fast enough to remove material from it.<ref name=Jewitt2013>{{cite journal
 |last1=Jewitt |first1=D.
 |last2=Agarwal |first2=J.
 |last3=Weaver |first3=H.
 |last4=Mutchler |first4=M.
 |last5=Larson |first5=S.
 |year=2013
 |title=The Extraordinary Multi-Tailed Main-Belt Comet P/2013 P5
 |journal=[[The Astronomical Journal]]
 |volume= 778|issue= 1|pages=L21
 |arxiv=1311.1483
 |bibcode=2013ApJ...778L..21J
 |doi=10.1088/2041-8205/778/1/L21
|s2cid=67795816
 }}</ref>

[[File:Hubble view of Dimorphos ejecta (October 8).png|thumb|Dimorphos and the tail created after the DART impact, photo by the Hubble Space Telescope]]

By smashing into the asteroid [[Dimorphos]], NASA's [[Double Asteroid Redirection Test]] spacecraft made it an active asteroid. Scientists had proposed that some active asteroids are the result of impact events, but no one had ever observed the activation of an asteroid. The DART mission activated Dimorphos under precisely known and carefully observed impact conditions, enabling the detailed study of the formation of an active asteroid for the first time.<ref name="nasa-march2023">{{cite web |last1=Furfaro |first1=Emily |title=NASA's DART Data Validates Kinetic Impact as Planetary Defense Method |url=https://www.nasa.gov/feature/nasa-s-dart-data-validates-kinetic-impact-as-planetary-defense-method |website=NASA |access-date=9 March 2023 |date=28 February 2023}} {{PD-notice}}</ref><ref>{{cite journal |last1=Li |first1=Jian-Yang |last2=Hirabayashi |first2=Masatoshi |last3=Farnham |first3=Tony L. |last4=Sunshine |first4=Jessica M. |last5=Knight |first5=Matthew M. |last6=Tancredi |first6=Gonzalo |last7=Moreno |first7=Fernando |last8=Murphy |first8=Brian |last9=Opitom |first9=Cyrielle |last10=Chesley |first10=Steve |last11=Scheeres |first11=Daniel J. |last12=Thomas |first12=Cristina A. |last13=Fahnestock |first13=Eugene G. |last14=Cheng |first14=Andrew F. |last15=Dressel |first15=Linda |last16=Ernst |first16=Carolyn M. |last17=Ferrari |first17=Fabio |last18=Fitzsimmons |first18=Alan |last19=Ieva |first19=Simone |last20=Ivanovski |first20=Stavro L. |last21=Kareta |first21=Teddy |last22=Kolokolova |first22=Ludmilla |last23=Lister |first23=Tim |last24=Raducan |first24=Sabina D. |last25=Rivkin |first25=Andrew S. |last26=Rossi |first26=Alessandro |last27=Soldini |first27=Stefania |last28=Stickle |first28=Angela M. |last29=Vick |first29=Alison |last30=Vincent |first30=Jean-Baptiste |last31=Weaver |first31=Harold A. |last32=Bagnulo |first32=Stefano |last33=Bannister |first33=Michele T. |last34=Cambioni |first34=Saverio |last35=Bagatin |first35=Adriano Campo |last36=Chabot |first36=Nancy L. |last37=Cremonese |first37=Gabriele |last38=Daly |first38=R. Terik |last39=Dotto |first39=Elisabetta |last40=Glenar |first40=David A. |last41=Granvik |first41=Mikael |last42=Hasselmann |first42=Pedro H. |last43=Herreros |first43=Isabel |last44=Jacobson |first44=Seth |last45=Jutzi |first45=Martin |last46=Kohout |first46=Tomas |last47=La Forgia |first47=Fiorangela |last48=Lazzarin |first48=Monica |last49=Lin |first49=Zhong-Yi |last50=Lolachi |first50=Ramin |last51=Lucchetti |first51=Alice |last52=Makadia |first52=Rahil |last53=Epifani |first53=Elena Mazzotta |last54=Michel |first54=Patrick |last55=Migliorini |first55=Alessandra |last56=Moskovitz |first56=Nicholas A. |last57=Ormö |first57=Jens |last58=Pajola |first58=Maurizio |last59=Sánchez |first59=Paul |last60=Schwartz |first60=Stephen R. |last61=Snodgrass |first61=Colin |last62=Steckloff |first62=Jordan |last63=Stubbs |first63=Timothy J. |last64=Trigo-Rodríguez |first64=Josep M. |title=Ejecta from the DART-produced active asteroid Dimorphos |journal=Nature |date=1 March 2023 |volume=616 |issue=7957 |pages=452–456 |doi=10.1038/s41586-023-05811-4 |pmid=36858074 |pmc=10115637 |arxiv=2303.01700 |bibcode=2023Natur.616..452L |s2cid=257282549|issn=1476-4687 |display-authors=3}}</ref> Observations show that Dimorphos lost approximately 1 million kilograms after the collision.<ref>{{cite journal |last1=Witze |first1=Alexandra |title=Asteroid lost 1 million kilograms after collision with DART spacecraft |journal=Nature |date=1 March 2023 |volume=615 |issue=7951 |pages=195 |doi=10.1038/d41586-023-00601-4 |pmid=36859675 |bibcode=2023Natur.615..195W |s2cid=257282080 |url=https://www.nature.com/articles/d41586-023-00601-4 |access-date=9 March 2023}}</ref> Impact produced a dust plume that temporarily brightened the Didymos system and developed a {{convert|10000|km|mi|adj=on|sp=us}}-long [[comet tail|dust tail]] that persisted for several months.<ref name="NOIRLab-20221003">{{cite web
 |title = SOAR Telescope Catches Dimorphos's Expanding Comet-like Tail After DART Impact
 |url = https://noirlab.edu/public/news/noirlab2223/
 |first = Charles |last= Blue
 |publisher = NOIRLab
 |date = 3 October 2022
 |accessdate = 4 February 2023}}</ref><ref name="NASA-20221215">{{cite web
 |title = Early Results from NASA's DART Mission
 |url = https://www.nasa.gov/feature/early-results-from-nasa-s-dart-mission
 |first = Jessica |last= Merzdorf
 |publisher = NASA
 |date = 15 December 2022
 |accessdate = 4 February 2023}}</ref><ref name="Li2023">{{cite journal
  |display-authors = etal
  |first1 = Jian-Yang
  |last1 = Li
  |first2 = Masatoshi
  |last2 = Hirabayashi
  |first3 = Tony
  |last3 = Farnham
  |first4 = Matthew
  |last4 = Knight
  |first5 = Gonzalo
  |last5 = Tancredi
  |first6 = Fernando
  |last6 = Moreno
  |title = Ejecta from the DART-produced active asteroid Dimorphos
  |url = https://assets.researchsquare.com/files/rs-2292349/v1/34562254-db7b-4289-a7ff-238159687528.pdf?c=1669139626
  |journal = Nature
  |date = March 2022
  |volume = 616
  |issue = 7957
  |pages = 452–456
  |doi = 10.1038/s41586-023-05811-4
  |pmid = 36858074
  |pmc = 10115637
  |arxiv = 2303.01700
  |bibcode = 2023Natur.616..452L
  |s2cid = 257282549
  |access-date = 11 March 2023
  |archive-date = 7 March 2023
  |archive-url = https://web.archive.org/web/20230307161053/https://assets.researchsquare.com/files/rs-2292349/v1/34562254-db7b-4289-a7ff-238159687528.pdf?c=1669139626
  |url-status = dead
  }}</ref>