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
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 t