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'''M-type asteroids''' are a type of asteroid or minor planet. Asteroids are generally classified into broad categories such as C-type, S-type, or M-type according to their brightness and color.  Brightness, or albedo, is a measurement of how well the object reflects light. Albedo ranges from 0 (perfectly black) to 1 (perfectly reflecting).<ref>http://www.planetary.org/explore/topics/near_earth_objects/asteroids_and_comets/facts.html</ref>

M-type asteroids have [[albedo]]s varying from 0.10 to 0.18.<ref name=Arnett/><ref name=planetsoc/>

These mid-region main belt objects are relatively bright, with albedos ranging from 0.10 to 0.18. Composition is apparently dominated by metallic iron.  

Originally they were thought to be composed of pure [[nickel]] and [[iron]], like the iron-nickel [[meteor]]ites, or at most to have a small portion of silicates added. However, this definition has been in some dispute since 1982, when Lupishko ''et al.'' first reported an unusually high amount of silicates in the asteroid [[Kalliope]].<ref name=Lupishko>Lupishko D.F., Belskaia I.N., Tupieva F.A., and Chernova G.P. "[http://adsabs.harvard.edu/abs/1982AVest..16..101L UBV photometry of the M-type asteroids 16 Psyche and 22 Kalliope]." ''Astronomicheskii Vestnik'' 16:101-108, April-June 1982. Translated into English and republished in ''Solar System Research'' 16(2):75-80, October 1982. Accessed June 21, 2008.</ref> In 2000, Rivkin ''et al.''<ref name=Rivkin>Rivkin A.S., Howell E.S., Lebofsky, L.A., ''et al.'' "[http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WGF-45FC10H-3N&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=7f3e87f014e2bc0e55ee8c8645829cfa The Nature of M-class Asteroids from 3-μm Observations]." ''Icarus'' 145(2):351-368, June 2000. {{doi|10.1006/icar.2000.6354}} Accessed June 21, 2008.</ref> reported that of 27 asteroids of this type that they and others had observed, 35% of them were [[water|hydrated]] and thus could not be composed primarily of iron and nickel. They thus urged their fellow astronomers to abandon the notion that M-type asteroids were metallic fragments of previously differentiated objects that were later struck and shattered. Instead they held that the smaller 10% of all M-type asteroids (likely to be anhydrous) were the likely sources of iron [[meteor]]ites, and the rest were chondrites, including salt-rich carbonaceous chondrites.<ref name=Rivkin/> And in 2001, Magri ''et al.'' reported that many M-type asteroids have too weak a radar signature to have metallic surfaces.<ref name=Magri>Magri C., Consolmagno G.J., Ostro S.J., ''et al.'' "[http://adsabs.harvard.edu/abs/2001M&PS...36.1697M Radar constraints on asteroid regolith compositions using 433 Eros as ground truth]." ''Meteoritics and Planetary Science'' 36(12):1697-1709, December 2001. [http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?2001M%26PS...36.1697M&amp;data_type=PDF_HIGH&amp;whole_paper=YES&amp;type=PRINTER&amp;filetype=.pdf Full text as PDF]. Accessed June 21, 2008.</ref> Today the definition of the M type depends strictly on the spectrum.<ref name=Bus>Bus S.J. and Binzel R.P. "[http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WGF-466CG4S-B&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=44956401613256703ff318fbff9264de Phase II of the Small Main-Belt Asteroid Spectroscopic Survey: A Feature-Based Taxonomy]." ''Icarus'' 158(1):146-177, July 2002. {{doi|10.1006/icar.2002.6856}} Accessed June 21, 2008. Bus and Binzel here rename the M type as X.</ref>

M-type asteroids inhabit the main belt's middle region.

.<ref>http://nssdc.gsfc.nasa.gov/planetary/text/asteroids.txt</ref> They make up some 10% of the total asteroid population.<ref>http://www.spaceguardindia.com/Asteroids.html</ref>

== References ==

M-type asteroids are found in the middle portion of the asteroid belt, between the S-type and C-type asteroids.<ref name=Arnett/><ref name=planetsoc/>

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== Examples ==

The asteroid [[Psyche]] is the largest known M-type asteroid found to date.<ref name=Lupishko/> This class also includes the asteroids [[Lutetia]], [[Klotho]], [[Kleopatra]], and [[Sarita]]<ref name=Magri/>

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[[Category:Astronomy]]

[[Category:Astronomy]]