M-class asteroid

M-class asteroids are a classification of asteroids characterized by a moderate albedo (reflectivity) and a spectral reflectance that slopes upward into the red. They may contain significant amounts of nickel-iron metal in their surface material, similar to iron meteorites. However, paradoxically, some M-class asteroids also show spectral features indicating the presence of hydrated minerals.1

Initially, in the mid-1970s, asteroids were broadly grouped into three classes: C, S, and M. M-class asteroids were estimated to make up about 5 percent of the total asteroid population at that time. Later, as more data became available, the classification system was expanded, and it was recognized that asteroid populations could be structured by taxonomic class at different distances from the Sun.2