M-type asteroids are asteroids of unknown composition; they are moderately bright (albedo 0.10–0.18). Some, but not all, are made of nickel-iron, either pure or mixed with stone. These are thought to be samples of the metallic core of differentiated asteroids that were fragmented in the early history of the Solar system by impacts. They originate in the inner portion of the asteroid belt and are thought to be the source of iron meteorites.

There are also M-types whose composition is uncertain. For example, 22 Kalliope has an accurately known density that is far too low for a solid metallic object or even a metal rubble pile. A rubble pile of iron-nickel metal would need about 70% porosity which is inconsistent with packing considerations. Kalliope and 21 Lutetia have features in their spectra which appear to indicate the presence of hydrated minerals and silicates , anomalously low radar albedos inconsistent with a metallic surface , as well as characteristics more in common with C-type asteroids. A variety of other M-type asteroids do not fit well into a metallic body picture.

Their spectrum is flat to reddish and usually devoid of large features, although subtle absorption features longward of 0.75 µm and shortward of 0.55 µm are sometimes present .

16 Psyche is the largest M-type asteroid, and does appear to be metallic. 21 Lutetia, an anomalous, probably nometallic body, will be the first M-type asteroid to be imaged by a spacecraft when the Rosetta space probe visits it on July 10, 2010. Another M-type, 216 Kleopatra, was the first main belt asteroid to be imaged by radar by the Arecibo Observatory in Puerto Rico.

The M type was one of the three basic asteroid types in early classifications (the others being the S and C types), and was thought to indicate a metallic body. Nowadays it appears likely that it is an assemblage of several possibly unrelated surface types with a featureless spectrum, not all of which are metallic.

You can use the Whatlinkshere/M-type asteroid toolbox link to find more asteroids of this type.

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