Spectral classes of asteroids

A modern classification was proposed by Clark Chapman, David Morrison, and Ben Zellner in 1975 . It included three types: C - dark carbon objects, S - stone (silicon) objects, and U for asteroids that do not fall into categories C and S. Later this classification was expanded and refined.

Currently, there are a number of classifications, and although they maintain some mutual uniformity, some asteroids in different schemes belong to different classes - due to the use of different criteria when approaching. Most often, two classifications are used: David Tolen and SMASS.

Tolen's classification

Tolen's classification was proposed in 1984 based on broadband spectrum measurements (from 0.31 microns to 1.06 microns) and albedo . 14 types of asteroids belonging to 3 groups were identified:

• Carbon asteroid group (C-group) - in the Tolen classification, class C asteroids are combined with three less numerous classes into a wide group of carbon asteroids. The group includes dark carbon asteroids, which have a very low reflectivity. 75% of all known asteroids belong to it.
  • Class C ( (10) Hygea ) - this class includes the majority of typical group asteroids and up to 75% of the total number of asteroids.
  • Class B ( (2) Pallas )
  • Class F ( (704) Interamnia )
  • Class G ( 1 Ceres )
• Group of silicon asteroids (S-group) - includes silicon (stone) asteroids. About 17% of the total number of asteroids belongs to this class.
  • Class S ( (15) Yevnomiya , (3) Juno )
• Group of iron asteroids (X-group) - several classes of asteroids are combined here, with similar spectra, but probably of different composition. The group includes objects with a high content of metals. This is the third most common asteroid group.
  • Class M ( (16) Psyche) —Most typical group asteroids belong to this class. This is the third most common asteroid group.
  • Class E ( (44) Nisa , (55) Pandora ) - differs from class M in high albedo.
  • Class P ( (259) Aletheia , (190) Ismena ; (324) Bamberg ) - differs from class M in low albedo.

and also there are some more smaller classes:

• Class A ( (446) Eternitas )
• Class D ( (624) Hector )
• Class J ( (3869) Norton )
• Class Q ( (1862) Apollo )
• Class R ( (349) Dembowska )
• Class T ( (96) Aegle )
• Class V ( (4) Vesta )

Sometimes asteroids are attributed to mixed types, for example CG, when their characteristics bear features inherent in different classes.

SMASS Classification

This relatively new classification method was proposed by Shelte Bass and Richard P. Binzel in 2002 based on the results of the Spectral Study of Small Asteroids of the Main Asteroid Belt (SMASS) project, based on a study of 1,447 asteroids. This study of the spectra was carried out with a much higher resolution than ECAS, which allowed us to analyze the spectra of the surfaces of asteroids at narrower wavelengths, thereby revealing many new features in the spectrum. However, the studies were carried out in a small wavelength range (from 0.44 μm to 0.92 μm) and the albedo of asteroids was not taken into account. The new classification was developed in such a way as to keep the connection with the Tolen taxonomy as a result, as a result, taking into account the difference in the data obtained, the asteroids were divided into 24 classes. As a result, most asteroids were distributed into three large classes (C, S, and X), and the rest fell into smaller classes:

• The carbon asteroid group (C-group) - carbon objects, includes the following classes:
  • Class B corresponds in Tolen classification to classes B and F
  • The main class are class C asteroids, which have the most typical spectra for this group.
  • Asteroids of the class Cg, Ch, and Cgh correspond in the Tolen classification to class G
  • Ch class includes asteroids with absorption at a wavelength of 0.7 μm
  • Cb class includes asteroids corresponding to the transition between objects belonging to C and B class in the SMASS classification.
• The group of silicon asteroids (S-group) - silicate (stone) asteroids, includes the following classes:
  • Class A
  • Class K ( (181) Eucharis , (221) Eos )
  • Class L ( (83) Beatrice )
  • Class Q
  • Class R
  • The main class is class S asteroids, which have the most typical spectra for the S-group
  • Sa, Sk, Sl, Sq and Sr classes, which include objects that occupy an intermediate position between the class S and the classes A, K, L, Q and R, respectively.
• The group of iron asteroids (X-group) - asteroids with a high content of metals, includes the following classes:
  • The main class are class X asteroids, which have the most typical spectra for this group.
  • Class Xe asteroids in the spectrum contain a moderately wide absorption band at a wavelength of 0.49 μm. It has been suggested that this indicates the presence of pyrrhotite (FeS). This class roughly corresponds to class E in the Tolen classification.
  • Class Xc and Xk asteroids in the spectrum contain a fairly broad maximum at wavelengths of 0.55 - 0.8 microns, which is caused by high reflection at these wavelengths. These spectra occupy an intermediate position between the spectra of classes X, C, and K.

Except in the Xe class, there is no more correspondence between the SMASS classes and the M , E , P classes in the Tolen classification. All other classes in the group of metal asteroids of the SMASS classification occupy an intermediate position between classes M , E , P.

• Class D
• Class j
• Class Ld : A new subclass of asteroids with more specific spectral characteristics than Class L
• Class O ( (3628) Bozhnemtsov )
• Class t
• Grade V

Some objects in near-Earth space have spectra that are very different from any of the SMASS classes. This is probably because these bodies are much smaller than those found in the main asteroid belt, and their surface can be younger and less altered by various processes or consists of simpler minerals.

Modified SMASS Classification

An extended study of the spectrum of asteroids, including the near infrared, led to a revision of the SMASS classification:

• A = A
• B = b
• C = c
• Cb = cb
• Cg = cg
• Ch = ch
• Cgh = cgh
• D = D
• L = L
• Ld is split between L and D
• K = K
• O = o
• Q is redefined in Qw
• R = R
• Sq divided into Sqw and Srw
• Sr is redefined as Sa
• Sa, Sl, Sk merge into Sw
• S is split between Sw and Svw
• T = T
• V is redefined to Vw
• X = X
• Xc = Xc
• Xk = Xk
• Xe = Xe

In the process of further research, these classifications will be refined and changed / replaced. In any case, for 2017, spectral classifications based on two previous spectroscopic studies of the 1990s are still the standard. Scientists still have not been able to agree on a better taxonomic system, which is largely due to the difficulty of obtaining detailed data when measuring a large number of asteroids. So, for example, conducting high-resolution spectroscopic studies or obtaining data on the density of asteroids, could significantly help in creating a more accurate classification.

At the moment, 3 main classes of asteroids have been precisely identified, depending on the chemical composition of meteorites:

• Class C - Based on High Carbon Meteorites
• Class S - based on stone meteorites
• Class M - based on iron meteorites

• Asteroid family
• Industrial development of asteroids
• Asteroid Classifications
• Asteroid List
• Asteroid belt

1. CR Chapman, D. Morrison, and B. Zellner Surface properties of asteroids: A synthesis of polarimetry, radiometry, and spectrophotometry , Icarus, Vol. 25, pp. 104 (1975).
2. DJ Tholen Asteroid taxonomic classifications in Asteroids II, pp. 1139-1150, University of Arizona Press (1989).
3. SJ Bus, F. Vilas, and MA Barucci Visible-wavelength spectroscopy of asteroids in Asteroids III, pp. 169, University of Arizona Press (2002).
4. SJ Bus and RP Binzel Phase II of the Small Main-belt Asteroid Spectroscopy Survey: A feature-based taxonomy , Icarus, Vol. 158, pp. 146 (2002).

• Types of Asteroids
• Unterteilung von Asteroiden nach S-types, C-types und V-types (English)