Asteroid classes, explained

The atlas colours and filters asteroids by two unrelated kinds of class: what they are made of and where they orbit. This page explains both, using the bodies in the atlas as examples. Every number in a grey box is computed live from the atlas data.

Two ways to classify

Spectral type (composition)

Letters like C, S, M or V. They describe how the surface reflects sunlight: its colour across visible and near-infrared wavelengths, and how bright it is (albedo). Types hint at composition and link asteroids to meteorite families. In the atlas, choose Colour by → Asteroid type.

Orbit class (dynamics)

Names like main belt, Apollo or Jupiter trojan. They are set purely by the orbit: its size, its shape, and whether it crosses Earth's or Mars's path. A C-type can be in any orbit class. In the atlas, choose Colour by → Orbit class.

How a spectral type is measured

Astronomers measure a reflectance spectrum: the fraction of sunlight reflected at each wavelength from about 0.45 to 2.5 µm. Two features matter most:

Slope. "Red" spectra reflect more at longer wavelengths. Organic-rich and space-weathered surfaces are red. "Flat" or "blue" spectra are neutral.
Absorption bands. Dips near 1 µm and 2 µm come from iron in the silicate minerals olivine and pyroxene. A dip near 0.7 µm points to water-altered clays. A featureless spectrum has no dips.

Spectra alone cannot tell some types apart. Several very different bodies have featureless, slightly red spectra. The albedo, the fraction of light reflected overall, breaks the tie. It runs from about 0.03 (darker than charcoal) to over 0.5 (like fresh snow).

Three classification schemes appear in the atlas

Tholen (1984): 14 classes from 8 colours plus albedo. Albedo splits the featureless X types into E (bright), M (medium) and P (dark).
Bus–DeMeo (2009): 24 classes from visible and near-infrared spectra, without albedo, so E, M and P merge into X, Xc, Xe and Xk. Adds subclasses such as Ch, Sq, Sv.
Mahlke et al. (2022): 17 classes from spectra and albedo together. It brings back E, M and P and adds Z for extremely red bodies. Most types in the atlas use this scheme, via IMCCE's SsODNet best estimate.

Albedo by type, at a glance

Each row shows the middle half of albedos (bar) and the median (dot) for every asteroid of that type in the atlas. Types fall into dark, medium and bright families. That is why albedo alone gives a rough guess when no spectrum exists.

The types

The atlas groups detailed classes into ten families (the legend's colours). Each card lists what the family means, what it is probably made of, and the atlas members.

C-complex — carbonaceous

Classes C, Ch, Cgh, B (and Tholen F, G)

Dark (albedo around 0.03–0.10), with flat, featureless spectra and a drop in the ultraviolet. C-types are the most common asteroids in the outer main belt. They match carbonaceous chondrite meteorites, which are primitive rock rich in carbon, clays and water-bearing minerals that have barely changed since the Solar System formed.

Ch / Cgh: show the 0.7 µm band of water-altered clays (hydrated minerals).
B: "blue" (falling) spectra. Pallas, Bennu, Phaethon and Mathilde are B-types. OSIRIS-REx brought back samples of Bennu rich in water-bearing clays and organics.

P — primitive

Class P (Tholen, Mahlke). Inside Bus–DeMeo's X-complex.

Very dark (albedo under ~0.07), featureless and slightly red. P-types dominate the outer belt beyond ~3 AU and the Hilda group. They are thought to be organic- and ice-rich material, related to C- and D-types. (87) Sylvia, a large outer-belt asteroid with two moons, is a P-type.

D / T / Z — very red, dark

Classes D, T, Z (Mahlke), plus subclasses such as Ds

Dark, with steep red slopes and no absorption bands. Most bodies far out are like this: Jupiter trojans such as (624) Hektor, Hildas, and outer-belt objects. The red colour is usually credited to complex organics. These may be among the most primitive bodies in the inner Solar System, possibly formed beyond Neptune and later scattered inward. T sits between D and the S/X families. Z is the most extreme red.

S-complex — stony

Classes S, Sq, Sv, Q, R, O and other S subclasses

Moderately bright (albedo ~0.15–0.35), with clear 1 µm and 2 µm silicate bands from olivine and pyroxene. S-types dominate the inner main belt and near-Earth space. Hayabusa proved the link to ordinary chondrites, the most common meteorites. Its samples from (25143) Itokawa match LL chondrites. Eros, Ida, Gaspra, Juno and Iris are S-types.

Q: an S-type with a fresh, unweathered surface. Exposure to solar wind (space weathering) reddens S surfaces over time. Q-types appear mostly among near-Earth asteroids, whose surfaces get shaken clean by close planetary encounters.
R, e.g. (349) Dembowska: very strong olivine and pyroxene bands.

K / L — the Barbarians

Classes K, L (Mahlke groups them as K and L, plus Kl, Ld)

Medium albedo, with spectra between S and C. A weak 1 µm band (K) or a strong ultraviolet slope with a flat near-infrared (L). K-types make up the Eos family, including (221) Eos, and resemble CV/CO carbonaceous chondrites. L-types include the "Barbarians", named after (234) Barbara. They polarise light unusually and seem rich in calcium-aluminium inclusions (CAIs), the oldest solids in the Solar System.

A — olivine-rich

Class A

Red, with a very broad, deep 1 µm band from nearly pure olivine. Rare. They may be fragments of the olivine mantles of melted, layered bodies, or primitive olivine rock. (246) Asporina is a classic example.

V — basaltic, Vesta-like

Class V

Bright (albedo ~0.3–0.5), with very deep 1 µm and 2 µm pyroxene bands, the signature of basaltic lava. Nearly all V-types are chips of (4) Vesta's crust (the "Vestoids"), blasted off by two giant impacts at its south pole. They are the source of the HED meteorites (howardites, eucrites, diogenites). The Dawn mission confirmed the Vesta link.

M — metallic

Class M (Tholen, Mahlke). Inside Bus–DeMeo's X-complex.

Medium albedo (~0.1–0.3), with featureless red spectra much like iron meteorites in the laboratory. Many reflect radar strongly, as metal does. M-types were long read as the exposed iron cores of shattered protoplanets. (16) Psyche is the best case: its bulk density of about 4 g/cm³ is high for an asteroid, and NASA's Psyche spacecraft arrives in 2029. M is not a guarantee of metal, though. Rosetta found (21) Lutetia (density ~3.4) more like an enstatite chondrite, and many M-types show silicate or hydration bands.

E — enstatite, bright

Class E

The brightest asteroids (albedo often above 0.4), with flat spectra. They match the aubrite meteorites, made of iron-free enstatite pyroxene that formed in very dry, oxygen-poor conditions. Many sit in the Hungaria region at the inner edge of the belt, including (434) Hungaria and (44) Nysa. Rosetta flew past the E-type (2867) Steins.

X-complex — unresolved

Classes X, Xc, Xe, Xk, Xt

Featureless, slightly red spectra without a usable albedo, so the body could be E, M or P. The subclasses add hints: Xe has a 0.49 µm band seen in enstatite-rich bodies, and Xk has a faint 0.9 µm band. Measuring an albedo usually resolves an X into one of the three.

Orbit classes

The orbit classes are JPL's, defined by the semi-major axis a (orbit size), the perihelion distance q (closest approach to the Sun) and the aphelion distance Q (farthest). The chart below plots every atlas body inside 6 AU by size and eccentricity. The curves are the boundaries that separate the classes.

Class	Rule (JPL)	What it means	In atlas

Near-Earth asteroids (NEAs) are those with q < 1.3 AU: the Amors, Apollos and Atens. NEAs are the cheapest targets to reach (see the launch calendar) and the ones watched for impact risk. The Earth MOID in each panel is the closest the two orbits come to each other.

Comets and dwarf planets

Comets

Icy bodies that release gas and dust (a coma and tail) near the Sun. Their nuclei are very dark (albedo ~0.04) and fragile; 67P's density is only about 0.5 g/cm³. Jupiter-family comets (period under 20 years, orbits shaped by Jupiter) came from the Kuiper belt. Halley-type comets (20–200 years, often steeply tilted or retrograde) probably came from the Oort cloud. The atlas has the five comets with spacecraft shape models.

Dwarf planets

The IAU's 2006 definition: a body that orbits the Sun and is massive enough for its gravity to pull it into a round shape, but has not cleared its orbit of other material. There are five official ones: Ceres (the largest asteroid; dark, with water-altered clays, close to C-type), and Pluto, Eris, Haumea and Makemake in the Kuiper belt. Asteroid types do not apply to them.

Caveats

Types describe surfaces, not interiors. A thin weathered layer controls the spectrum. Psyche and Lutetia are both M-types but probably very different inside.
Space weathering changes the colour. The same rock reads as Q when fresh and S when old.
Albedo-only types are guesses. The map shows them in the matching colour, but their panels say "likely".
Densities on the map are often assumed. Without a measured mass, the atlas estimates mass from diameter and a typical density for the type, with roughly 2× uncertainty.

References

Tholen, D. J. (1984). Asteroid taxonomy from cluster analysis of photometry. PhD thesis, University of Arizona.
DeMeo, F. E., Binzel, R. P., Slivan, S. M., Bus, S. J. (2009). An extension of the Bus asteroid taxonomy into the near-infrared. Icarus 202, 160.
Mahlke, M., Carry, B., Mattei, P.-A. (2022). Asteroid taxonomy from cluster analysis of spectrometry and albedo. A&A 665, A26.
DeMeo, F. E., Carry, B. (2014). Solar System evolution from compositional mapping of the asteroid belt. Nature 505, 629.
Carry, B. (2012). Density of asteroids. Planetary and Space Science 73, 98.
Berthier, J. et al. (2023). SsODNet: Solar system Open Database Network. A&A 671, A151.
JPL Small-Body Database: orbit class definitions.