C-type asteroids are carbonaceous asteroids. They are the most common variety, forming around 75% of known asteroids, and an even higher percentage in the outer part of the asteroid belt beyond 2.7 AU, which is dominated by this asteroid type. The proportion of C-types may actually be greater than this, because C-types are much darker than most other asteroid types except D-types and others common only at the extreme outer edge of the asteroid belt.
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This type of asteroid has very similar spectra to carbonaceous chondrite meteorites (types CI and CM), whose chemical composition is approximately the same as the Sun and the primitive solar nebula, except that they do not contain hydrogen, helium and other volatiles. Hydrated (water-containing) minerals are present.[1]
C-type asteroids are extremely dark with albedos typically in the 0.03 to 0.10 range. Consequently, whereas a number of S-types can normally be viewed with binoculars at opposition, even the largest C-types require a small telescope. The potentially brightest C-type asteroid is 324 Bamberga, but that object's very high eccentricity means it rarely reaches its maximum magnitude.
Their spectra contain moderately strong ultraviolet absorption at wavelengths below about 0.4 μm to 0.5 μm, while at longer wavelengths they are largely featureless but slightly reddish. The so-called "water" absorption feature around 3 μm, which can be an indication of water content in minerals is also present.
The largest unequivocally C-type asteroid is 10 Hygiea, although the SMASS classification places the largest asteroid, 1 Ceres, here as well, because that scheme lacks a G-type.
See also asteroid spectral types.
In the Tholen classification, the C-type is grouped along with three less numerous types into a wider C-group of carbonaceous asteroids which contains:
In the SMASS classification, the wider C-group contains the types
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