M-type asteroids

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M-type asteroids were once known as metallic asteroids. Today their composition is in dispute, and many astronomers simply define the M type by a reflected spectrum and make no attempt to define a common composition for all of them. About 8% of all asteroids are of this type.[1][2]

Characteristics and composition

M-type asteroids have albedos varying from 0.10 to 0.18.[1][2]

Originally they were thought to be composed of pure nickel and iron, like the iron-nickel meteorites, 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.[3] In 2000, Rivkin et al.[4] reported that of 27 asteroids of this type that they and others had observed, 35% of them were 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 meteorites, and the rest were chondrites, including salt-rich carbonaceous chondrites.[4] And in 2001, Magri et al. reported that many M-type asteroids have too weak a radar signature to have metallic surfaces.[5] Today the definition of the M type depends strictly on the spectrum.[6]

M-type asteroids are found in the middle portion of the asteroid belt, between the S-type and C-type asteroids.[1][2]


The asteroid Psyche is the largest known M-type asteroid found to date.[3] This class also includes the asteroids Lutetia, Klotho, Kleopatra, and Sarita[5]

Observation and exploration

No spacecraft has reconnoitered an M-type asteroid yet. However, the Rosetta probe will make rendezvous with Lutetia on July 10, 2010.


  1. 1.0 1.1 1.2 Arnett, Bill. "Asteroids." The Nine 8 Planets, May 10, 2008. Accessed June 20, 2008.
  2. 2.0 2.1 2.2 "Asteroid Facts." The Planetary Society, n.d. Accessed June 20, 2008.
  3. 3.0 3.1 Lupishko D.F., Belskaia I.N., Tupieva F.A., and Chernova G.P. "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.
  4. 4.0 4.1 Rivkin A.S., Howell E.S., Lebofsky, L.A., et al. "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.
  5. 5.0 5.1 Magri C., Consolmagno G.J., Ostro S.J., et al. "Radar constraints on asteroid regolith compositions using 433 Eros as ground truth." Meteoritics and Planetary Science 36(12):1697-1709, December 2001. Full text as PDF. Accessed June 21, 2008.
  6. Bus S.J. and Binzel R.P. "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.