M-type asteroid

[2] Asteroids are classified as M-type based upon their generally featureless and flat to red-sloped absorption spectra in the visible to near-infrared and their moderate optical albedo.

[10] For the M-types, the proposed analogs have bulk densities that range from ~3 g/cm3 for some types of carbonaceous chondrites up to nearly 8 g/cm3 for the iron-nickel present in iron-meteorites.

[10] To calculate the bulk density of an asteroid requires an accurate estimate of its mass and volume; both of these are difficult to obtain given their small size relative to other solar system objects.

In most cases, these are estimated from the visual albedo (brightness) of the asteroid, chord-lengths during occultations, or their thermal emissions (e.g. IRAS mission).

Ephemeris estimates are based on the subtle gravitational pull of other objects on that asteroid, or vice versa, and are considered less reliable.

[12] Size estimates based on shape models (usually derived from adaptive optics, occultations, and radar imaging) are the most reliable.

If these objects are porous (aka rubble-piles), then that interpretation may still hold; this is unlikely for Psyche,[12] because of its large size.

[21] First, it must have started as a Vesta-sized (~500 km) protoplanet; statistically, it is unlikely that Psyche was completely disrupted while Vesta remained intact.

Instead, it has been argued that Psyche is the remnant of a protoplanet that was shattered and gravitationally re-accumulated into a well-mixed iron-silicate object.

16 Psyche is the largest M-type asteroid with a mean diameter of 222 km, and has a relatively high mean radar albedo of

21 Lutetia has a mean diameter of 100 km,[1] and was the first M-type asteroid to have been imaged by a spacecraft when the Rosetta space probe visited it on 10 July 2010.

[1] Analysis using data from the Rosetta spectrometer (VIRTIS) was consistent with estatitic or iron-rich carbonaceous chondritic materials.

[19] Kleopatra is also notable for the presence of two small moons, named Alexhelios and Cleoselena, which have allowed its mass and bulk density to be accurately computed.