M-type asteroid
An M-type asteroid has a moderately bright, featureless spectrum, and some are thought to be rich in metal.
M-type asteroids are moderately bright with fairly featureless, slightly reddish spectra. The letter was chosen because many were thought to be metallic, perhaps the exposed iron cores of broken planetesimals, and so a source of iron meteorites.
Radar observations support this for some of them: metal reflects radio waves strongly, and asteroids such as 16 Psyche and 216 Kleopatra give strong radar echoes. However, telescope spectra show that several M-types also have traces of minerals or water-bearing material, so the class includes bodies of different make-up. Some may resemble enstatite chondrites rather than solid metal.
In newer classification schemes, M-types are often merged into a broader X group, defined by spectrum alone, with the brightness used to separate metal-like bodies from dark ones. NASA's Psyche mission, due to arrive at its target in 2029, should be the first close-up test of what an M-type asteroid is really made of.
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Level: Explorer Ages 10 and up