Besides the division of asteroids into certain orbit types or asteroid families exists another division. This means asteroids are also categorized in spectral types, which are believed to tell something about the asteroids’ composition. The division into spectral types is an even bigger mess than the other categorizations and also quickly developing. Until now had three historical steps been made to constitute different classes of asteroid spectra.

The first historical step was made in 1973 by the astronomers Chapman, Morrison, and Zellner. They looked at the color, albedo, and spectral shape of several asteroids and divided them accordingly into three classes, which the astronomers defined for the asteroids. These classes were called C, S, and U. C stands for carbonaceous. S stands for silicaceous or stony. These two classes were kept in all later classification schemes. U stood for unknown or undefined and was a category for all asteroids, that were neither of the C-type nor of the S-type.

Most widely used until today is the Tholen classification scheme. The astronomer David James Tholen examined 978 asteroids in the early 1980s, adopted mainly the existing scheme and altered it slightly. The C-type remained the C-type. The S-type remained the S-type. The U-type was replaced by the X-type. Then subsequent types were added while other astronomers tried other classification schemes, which resembled the further development of the Tholen classification scheme, but have more complicated designations.

1980 RA or asteroid #3255 was named Tholen after David James Tholen. This asteroid belongs into the Asteroid Main Belt and is a Mars-crosser, a fast rotator (some people see asteroids as special when they are either extraordinarily fast or extraordinarily slow spinning or rotating around themselves), and a common S-type asteroid. The rotation period is 2.95 hours, the diameter 5.1 km, the orbital period 3.65 years. Its orbital inclination is 21.353° high. Tholen is in every classification scheme of the common S-type.

The Small Solar System Objects Spectroscopic Survey, which is abbreviated as S3 O S2, was conducted from 1996 until 2001, examined 820 asteroids, and found that the Tholen-types were insufficient, so introduced some subcategories and grouped 13 % of the examined asteroids in the Caa-type. The resulting classification scheme is known as S3OS2 or the Lazzaro classification scheme.

The astronomers DeMeo, Bus, and Slivan examined 371 asteroids in 2009 and found too that the existing classes were insufficient. So they expanded the classification scheme to 24 classes. This classification scheme is known as the Bus-DeMeo classification scheme.

The Tholen classification had meanwhile developed in the way that C, S, and X became groups, in which subgroups or types are grouped.

Into the C-group fall:

• the common C-type, for example Hygiea (#10)
• the (“bright”) B-type, for example Pallas (#2)
• the F-type, for example Interamnia (#704)
• the G-type, for example Ceres (#1)
• the D-type, typical for the Outer Asteroid Main Belt and for Jupiter trojans, for example Hektor (#624)
• the T-type, rare and found in the Inner Asteroid Main Belt, for example Aegle (#96)

Into the S-group fall:

• the common S-type, examples are Eunomia (#15) and Juno (#3)
• the V-type or vestoids, mainly the Vesta-family, which are believed to have all originated from the parent body Vesta (#4)
• the A-type, for example Asporina (#246)
• the Q-type, for example Apollo (#1862)
• the R-type, for example Dembowska (#349)

Into the X-group fall:

• the (dark “primitive”) P-type, for example Aletheia (#259) or Ismene (#190)
• the (intermediate “metallic”) M-type, for example Psyche (#16)
• the (bright “enstatite”) E-type, mainly the Hungaria-family in the Inner Asteroid Main Belt

The Tholen taxonomy considers that these types are in several cases not sufficient to describe an asteroid correctly. The Tholen taxonomy copes with that by allowing up to four letters to describe an asteroid and allowing an asteroid to be in up to three spectral types at once. Up to three of the up to four letters give the spectral type. The best fitting spectral type is always given first. The additional letter can be an I for “inconsistent” data or an U for “unusual” or a : for “noisy” or a :: for “very noisy” spectral data. Examples are Athalia (#515), that belongs to the Themis-family and shows sometimes the spectrum of a S-type asteroid and sometimes the spectrum of a C-type asteroid, hence is categorized as I, what isn’t a spectral type, hence must be said to be in the class or group I; and the Mars-crosser Wright (#1747), which is categorized as AU:, which means Wright is of the A-type in the S-group, but has an unusual and noisy spectrum. Unusual means to be far from the center of the cluster in the data points. This can of course be caused by the measured data being noisy, which denotes bad quality because of some disturbance in the measurement.

The Small Main-Belt Asteroid Spectroscopic Survey (SMASS) of 1447 asteroids had been conducted in 2002 by the astronomers Bus and Binzel. This survey resulted in the SMASS classification scheme, which ignores the albedo. The SMASS taxonomy develops since 2002 parallel to the Tholen taxonomy, which is more widely used. The Bus-DeMeo taxonomy was a step in the evolution of the SMASS taxonomy. The astronomer Schelte Bus participated in both surveys, but although SMASS was conducted earlier and examined more asteroids, it sorts asteroids into 26 types, while the Bus-DeMeo taxonomy knows only 24 types. Both are meant as expansions of the Tholen taxonomy! The idea of the SMASS taxonomy is actually to put asteroids in only one spectral type instead of in up to three. Hence the SMASS taxonomy adopts all the groups and subgroups of the Tholen classification, but adds the types Cb, Cg, Ch, and Cgh to the C-group, the types K, L, Sa, Sq, Sr, Sk, and Sl to the S-group, and the Xe, Xc, and Xk to the X-group. Furthermore the types T, D, and V are taken out of their groups and sorted together with the new types Ld, and O. All the types, which are designated with more than one letter are intermediary between the spectral types, which are given with these letters. Only h isn’t a type of its own, but stands for hydrated. The new K-type comprises the asteroids Eucharis (#181) and Eos (#221). The new L-type comprises the asteroid Beatrix (#83). The new O-type consists currently only of Božněmcová (#3628) or 1979 WD, located in the middle region of the Asteroid Main Belt.

Some of the types of asteroids are also found in meteorites. C-type: carbonaceous chondrite meteorites; S-type: stony meteorites; M-type: iron meteorites; V-type: HED meteorites. HED stands for “howardite–eucrite–diogenite” and is a clan or subgroup of achondrite meteorites. This clan can be further divided into howardites, eucrites, and diogenites. Achondrite meteorites resemble rocks on Earth.