Point a spectrograph at an asteroid and you see sunlight, filtered by its surface. The way that reflected light rises, dips and darkens across wavelengths sorts asteroids into spectral classes — a first, imperfect but remarkably useful clue to what they are made of.
How classification works#
Astronomers divide the light an asteroid reflects by the spectrum of the Sun itself. What remains is a reflectance spectrum: how much light the surface returns at each wavelength. Three features carry most of the information.
- Slope. A spectrum that rises towards longer wavelengths is called "red"; a flat or falling one is "neutral" or "blue". Primitive, organic-rich surfaces tend to be red; carbon-rich ones are flat.
- Absorption bands. Minerals absorb light at characteristic wavelengths. Olivine and pyroxene, the silicates of stony asteroids, produce bands near 1 and 2 micrometres (µm). A weak band near 0.7 µm in some dark asteroids points to water-altered, clay-like minerals.
- Albedo. The fraction of light a surface reflects. Spectra alone can look alike for very different materials; brightness often separates them.
Two cautions apply to everything on this page. A spectrum describes only the surface, not the interior. And surfaces change: solar wind and micrometeorite impacts slowly darken and redden exposed rock, a process called space weathering.
Taxonomies in brief#
Several classification schemes have been published, each building on the last as instruments improved.
| System | Year | Based on | Classes |
|---|---|---|---|
| Tholen | 1984 | Eight broadband colours from about 0.34 to 1.04 µm, plus albedo | 14 |
| Bus (SMASS II) | 2002 | Visible spectra, about 0.44–0.92 µm | 26 |
| Bus–DeMeo | 2009 | Visible and near-infrared spectra, 0.45–2.45 µm | 24 |
Modern systems group classes into broad complexes — the C-complex, the S-complex and the X-complex — plus a handful of rarer end members such as the D, V and A types. Extending observations into the near-infrared was the decisive step, because that is where the 1 µm and 2 µm silicate bands become clear.
The six classes in our catalogue#
- C-type
Carbonaceous
Clay and silicate rock rich in carbon compounds; hydrated minerals.
Albedo0.03–0.10 - S-type
Silicaceous
Stony silicates (olivine, pyroxene) mixed with nickel-iron metal.
Albedo0.10–0.30 - M-type
Metallic
Spectra consistent with nickel-iron metal; some show hydrated or silicate features.
Albedo0.10–0.20 - V-type
Basaltic
Basaltic crust rich in pyroxene, like the surface of 4 Vesta.
Albedo0.20–0.45 - X-complex
Mixed / enstatite
A spectral grouping that includes E (enstatite), M and P types; composition varies.
Albedo0.04–0.60 - D-type
Primitive red
Very dark, reddish material thought to be rich in organics and ices.
Albedo0.02–0.08
C-type — carbonaceous#
The most common asteroids, dark as charcoal, with flat spectra and geometric albedos of roughly 0.03–0.10. They dominate the outer main belt and are thought to preserve material close to the composition of the early Solar System, including water-bearing clays and organic compounds. In the catalogue: Nyxara, Sable Lantern and Ostara Bloom.
S-type — silicaceous#
Stony bodies made of olivine and pyroxene mixed with nickel-iron metal, with clear 1 µm and 2 µm bands and albedos of about 0.10–0.30. They dominate the inner main belt and are common among near-Earth asteroids. In the catalogue: Vesperine, Corvina, Helion Drift, Meridian Thorn and Lumen Ward.
M-type — metallic candidates#
Moderately bright (albedo about 0.10–0.20) with featureless, slightly red spectra consistent with nickel-iron metal. Some may be exposed cores of shattered protoplanets, but the class is diverse: some members show hydrated or silicate features, and not every M-type is metal-rich. In the catalogue: Aurelis and Kestrel.
V-type — basaltic#
Rare, bright bodies (albedo about 0.20–0.45) with very strong pyroxene bands, the signature of volcanic basalt. Most are fragments of the crust of 4 Vesta, chipped off by giant impacts. In the catalogue: Calyx.
X-complex — mixed#
A grouping of similar, featureless spectra whose members range from very dark to very bright (albedo about 0.04–0.60). The bright E-types, rich in the mineral enstatite, cluster in the Hungaria region at the inner edge of the belt. In the catalogue: Ilyra.
D-type — primitive red#
Very dark (albedo about 0.02–0.08) with steep red spectra, likely rich in organic material and possibly ice beneath the surface. They are common among the Hildas and Jupiter Trojans and probably formed far from the Sun. In the catalogue: Thalassa Veil and Peregrine Ash.
Albedo: a clue, not a verdict#
Albedo ranges overlap, and a single measurement rarely identifies a class on its own. Combined with a spectrum, though, it is powerful: in the Tholen system, the E, M and P types share nearly identical spectra and are told apart by albedo alone — bright, moderate and dark.
| Class | Typical geometric albedo | Catalogue entries (albedo) |
|---|---|---|
| C | 0.03–0.10 | Nyxara 0.05 · Sable Lantern 0.06 · Ostara Bloom 0.07 |
| S | 0.10–0.30 | Lumen Ward 0.21 · Meridian Thorn 0.22 · Vesperine 0.24 · Corvina 0.25 · Helion Drift 0.27 |
| M | 0.10–0.20 | Aurelis 0.16 · Kestrel 0.18 |
| V | 0.20–0.45 | Calyx 0.35 |
| X | 0.04–0.60 | Ilyra 0.38 |
| D | 0.02–0.08 | Peregrine Ash 0.05 · Thalassa Veil 0.06 |
Albedo also changes how big an asteroid appears to be. Two bodies of the same brightness can differ in size by a factor of two or more depending on their surfaces — see absolute magnitude.
The view across the belt#
Spectral classes are not scattered at random. Stony S-types dominate the inner main belt, carbonaceous C-types the outer belt, and dark, red D-types and P-types the Hildas and Trojans beyond. This gradient is one of the strongest clues that the belt preserves a record of how temperature and composition changed with distance from the young Sun — although later migration of the giant planets has mixed the populations considerably.
From asteroids to meteorites#
Most meteorites are fragments of asteroids, and linking a meteorite type to a spectral class lets laboratory analyses speak for bodies millions of kilometres away. Some links are firm; others remain open questions.
| Class | Likely meteorite analogue | Confidence | Main evidence |
|---|---|---|---|
| S | Ordinary chondrites | Strong, for many S-types | Grains returned by Hayabusa from the S-type Itokawa matched ordinary chondrites. |
| V | HED meteorites (howardites, eucrites, diogenites) | Strong | Spectral match with Vesta, confirmed in detail by NASA's Dawn orbiter. |
| C | Carbonaceous chondrites | Strong, for some subtypes | Samples of Ryugu (Hayabusa2) and Bennu (OSIRIS-REx) closely resemble primitive carbonaceous chondrites. |
| M | Iron meteorites; some enstatite chondrites | Debated | Radar and spectra suggest some, but not all, M-types are metal-rich. |
| X (E-types) | Aubrites (enstatite achondrites) | Moderate | Similar spectra and high albedo. |
| D | Possibly rare, organic-rich carbonaceous meteorites | Tentative | Few meteorites match D-type spectra closely. |
How classes are assigned in the catalogue#
Every preview entry carries one of the six labels, chosen to be consistent with its albedo and orbit. For verified entries, the class is planned to come from published spectroscopic surveys, as recorded in public databases such as the JPL Small-Body Database. Where no published spectrum exists, the entry will say so rather than guess. The full field reference is in Data & sources.