Asteroid spectral class
A label, such as C, S or V, given to an asteroid based on the pattern of sunlight it reflects, which hints at what it is made of.
Asteroids are too far away to sample directly, but the sunlight they reflect carries clues. Minerals absorb certain wavelengths, so splitting the light into a spectrum reveals dips and slopes. Astronomers sort these patterns into spectral classes. The most common are C-type (dark, carbon-rich, with flat spectra), S-type (stony, with absorptions from olivine and pyroxene) and the X group, which includes some metal-rich bodies. Rarer classes include V-type, which matches the lava rocks of Vesta.
Spectral classes link meteorites to their sources. S-types were long suspected to be the parents of ordinary chondrites, but their spectra didn't quite match. The Hayabusa mission settled this: grains returned from the S-type Itokawa in 2010 proved to be LL chondrite material, its spectrum altered by space weathering. Likewise, V-type asteroids with the same spectral signature as eucrites and diogenites support the link between Vesta and the HED meteorites.
The asteroid 2008 TC3 was classed before it struck Sudan, and the recovered Almahata Sitta fragments turned out to be mostly ureilites with a mix of other types. Classes are assigned from telescope spectra, not by eye; colour alone tells you little.
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