M-type asteroid

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.

Characteristics and composition
M-type asteroids have albedos varying from 0.10 to 0.18.

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. In 2000, Rivkin et al. 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. And in 2001, Magri et al. reported that many M-type asteroids have too weak a radar signature to have metallic surfaces. Today the definition of the M type depends strictly on the spectrum.

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

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

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.

Related link

 * M-type asteroid by Wikipedia

Astéroïdes M-type Asteróides tipo-M