I once read a quote in a conference abstract that stated the paper had been written to serve as “a historical guide to the asteroid literature for the perplexed”. That quote sums up my desire to write this piece about the history of asteroid classification and the schemes currently in use.
When asteroid spectral types are quoted in the scientific literature, they are often taken from different classification schemes. The most widely cited and enduring taxonomies are the Bus-DeMeo, Tholen, and Gaffey schemes, but a not insignificant number of other asteroid classification taxonomies have come and gone over the decades.
It is appropriate to point out before we begin that since the initial publication of this review in 2019, the field of asteroid taxonomy has undergone a transformation, moving away from the “curve matching” that defined the Tholen and Bus-DeMeo eras, into machine learning and probabilistic clustering, introduced in Mahlke et al (2022)—basically, a shift from classification labels based on hard boundaries, to likeness probabilities that tell you how much you can trust the label.
This history of asteroid classification runs through all of the earlier classification systems that have informed those currently in use, from those developed in the mid-1970s to the present day. My intention is to provide the lowdown on this high-revision subject for the uninitiated and those in the know.
Note that this 2019 review is undergoing a gradual 2026 update.
The wry quotation at the start of this essay appeared in a XVIII LPSC abstract by Tholen & Bell (1987). You can either read the abstract now, or (recommended) wait until you reach the year 1987 further below when you can appreciate the quote in context, taking into account what took place in the intervening years that might have led to a state of perplexation.
This text is organized into sections: click on a link below to jump straight to that section, although I do recommend you read the whole piece in order from start to finish to appreciate the ever-evolving subtleties of this absorbing—albeit extraordinarily dry—subject of asteroid classification. And I don’t mean dry as in ‘flat across 2.7–3.0 µm’, I mean the subject can be more than a little monotonous at times. So it seems I went ahead and wrote a 9,000 word piece on a subject that the majority of bodies on this planet will probably find rather dreary.
Jump to section:
• The evolution of asteroid classification schemes
• What is it that’s being classified?
• The current asteroid classification schemes
• Interpreting asteroid spectra
• Boundaries between classes
• All asteroid classification schemes: 1973 to date
• The A to Z of classes
• Key spectral features
• Classifying asteroids Bennu and Ryugu
• References
First published online: 19 July 2019.
Last updated: 26 February 2026 (added probabilistic clustering and the new Z class).
The evolution of asteroid classification schemes
The following alluvial diagram attempts to show the evolution of the various asteroid classification systems that define the “A to Z” of asteroid spectral types in use today (meaning up to 2019). Click on the image for a larger view.
The diagram starts on the left-hand side with the three CSU taxonomic classes published in 1975. (Technically, the first classification scheme was the 1973 RMF scheme—but more on that and why it’s not in the diagram, later). The CSU scheme is named for Carbonaceous and Stony-metallic meteorites, with all other types clumped into an “Unclassifiable in the present system” class. The diagram culminates on the right-hand side with the Bus-DeMeo classification scheme (published in 2009, with a minor revision in 2013) and its 25 classes and sub-types. The 25 classes fall into three main groups, or complexes, labelled CSX, plus a collection of smaller classes often described as “end members” or “outliers”.
Note that unequal lengths of the black vertical lines for the three primary strands on the left-hand side of the diagram do not represent the relative proportions of asteroids. Instead, they represent the number of sub-types into which the particular class (C, S, or U) splits as more classes and types are defined in subsequent years. For example, although the majority of discovered asteroids appear to belong to the C-complex (on the right-hand side of the diagram), the early S-class (on the left-hand side) splits into more distinct classes and sub-types than the C-class as the taxonomies evolve. Consequently, the length of the vertical black line is proportional to the number of classes, types, and sub-types within that group—not the number of asteroids.
All classes that appear in the above alluvial diagram from 1975 to 2013 (and the earlier 1973 scheme) are discussed in this review. If you want to see the above diagram described in words, click here, or perhaps read on for a primer before you venture down that rabbit hole.
References to the papers which define the classes are shown in the diagram. Links to all sources of data and information on the different asteroid classification schemes shown in the diagram and throughout this review are provided at the end.
What is it that’s being classified?
The observing regions of the electromagnetic spectrum that will be referred to extensively by abbreviation in this review are:
| Region | Abbreviation | Wavelength |
| Ultraviolet (Far to Near) | UV (FUV–NUV) | 0.1–0.4 µm |
| Visible | V | 0.4–0.75 µm |
| Near Infrared | NIR | 0.75–3 µm |
| 3 µm Band | 3 µm band | 2.5–3.5 µm |
| Mid Infrared | MIR | 3–40 µm |
When sunlight hits the surface of an asteroid, electromagnetic radiation is transmitted through the near-surface minerals which absorb or scatter radiation at certain wavelengths, characteristic of the particular mineral species present. The features in the processed spectrum—such as slope steepness, line curvature, and positions, widths and depths of absorption or emission bands—indicate the specific combination of minerals present. In the VNIR, these features are primarily defined by how sunlight is reflected and absorbed via electronic transitions. In the MIR, the diagnostic focus shifts to the asteroid’s own thermal emission, with spectral features arising from fundamental molecular vibrations.
The measured (inferred) surface composition may or may not be characteristic of the composition of the asteroid as a whole, depending on the asteroid’s geological history—for example, whether it is a primitive solid body, a rubble pile, or differentiated. If the asteroid is a primitive solid body, the inferred surface composition will be characteristic of the asteroid as a whole; if a differentiated body, the inferred surface composition will only be characteristic of the surface of the asteroid. If a rubble pile asteroid, the inferred surface composition is unlikely to represent the whole asteroid because a rubble pile is an aggregate of fragments which have coalesced under gravity—either from break-up and re-accumulation of one asteroid type, or from collision and accumulation of different asteroid types, either of which would provide a non-uniform surface composition.
Current asteroid taxonomies are largely based on the presence of major features in UV to VNIR wavelength reflectance spectra, together with albedo (the total amount of sunlight reflected) and photometric colour indices (the numerical differences in brightness between filters, such as B–V or U–B). Over the last fifty years, the wavelength range utilised to define these classes has expanded from 0.3–1.1 µm in early classification schemes (such as Tholen) to 0.45–2.45 µm in more recent systems (such as Bus-DeMeo and Mahlke). Why this wavelength range? It is effective because the Sun emits most of its energy within these wavelengths, providing the high-quality data required to resolve spectral features. This range also coincides with atmospheric windows where Earth’s atmosphere remains mostly transparent, before becoming increasingly opaque due to water vapour and thermal emission beyond 2.45 µm.
Common spectral features that separate the taxonomic classes include absorptions due to the silicate minerals olivine and pyroxene; hydrated silicates such as phyllosilicates; oxides such as magnetite and spinel; sulphides such as troilite and oldhamite; and other hydrated minerals. Metal asteroids have near featureless VNIR spectra (or weak absorptions if silicates are mixed in) and require radar and NIR observations (e.g. Neeley et al, 2014).
Ongoing surveys may refine the current VNIR asteroid taxonomies, for example the 0.35–0.9 µm wavelength coverage by GAIA of 300,000 asteroids and the James Webb Space Telescope, which will provide NIR to MIR coverage of the 0.7–5.3 µm wavelength region.
In the NUV (0.1–0.3 µm), some spectral features are associated with carbon, as well as iron-bearing silicate materials with varying degrees of space weathering (the effects of which may be evident in the UV/blue region before becoming apparent in the VNIR). NUV features may distinguish primitive asteroids because S-type slopes appear to remain redder than C-types into the NUV (e.g. Waszczak et al, 2015). For discussions of NUV spectra, see Wong et al (2019) and Hendrix et al (2016).
The so-called 3 µm band (2.5–3.5 µm) is associated with water/ice, water-bearing materials, and the hydroxyl (OH) molecule . In particular, the band around 3.1–3.2 µm is associated with water/ice or ice frost and the band around 2.95 µm is associated with water in minerals.
In particular, a 2.7–2.8 µm band is associated with the hydroxyl molecule in phyllosilicate (clay) minerals. This band is often present when the 0.7 µm band is present—and while 0.7 µm indicates water, its absence does not rule out water (because some hydrated magnesium-clays are invisible at 0.7 µm). This means 0.7 µm is used a proxy for hydration when NIR data are unavailable. (Phyllosilicates also have a band minimum in the MIR around 12 µm, the exact position of which may indicate the degree of aqueous alteration.)
For a review of the 3 µm region generally see Rivkin et al (2015; 2019), or related to primitive asteroids see Campins et al (2019). For 3 µm spectral features of Trojans see Brown (2016) and for discussions about the distinction between red or less-red types, see Emery et al (2011).
Beyond water, the NIR reveals the complex chemistry of primitive surfaces. Carbonates produce distinct features near 3.4 µm and 3.9 µm, implying a history of liquid water and CO2. Organic compounds give signatures around 3.2–3.5 µm, while ammoniated minerals—like those identified on 1 Ceres—produce a diagnostic feature at 3.1 µm, suggesting the asteroid may have formed in the outer Solar System and migrated inwards.
In the MIR (3–40 µm), most spectral features are due to silicate minerals and not only do they indicate an asteroid’s composition, but also how it has been processed. The position of the Christensen feature (7–10 µm) and Reststrahlen bands (10–25 µm) can reveal if an asteroid is primitive or was heated enough to melt and differentiate into layers—like 4 Vesta (see Donaldson Hanna & Sprague, 2009). The 12 µm phyllosilicate band is a direct indicator of aqueous alteration (where liquid water reacted with rock to form clays). The Transparency feature (15–25 µm) indicates surface texture, appearing only when an asteroid is covered in fine-grained regolith rather than coarse rock (where impacts and space weathering have processed boulders over billions of years).
While the VNIR may not always provide enough detail and give ambiguous red slopes or flat lines, the MIR provides unique, diagnostic emission bands—and two different rock types are unlikely to give the same MIR signature. So, the wider the spectrum, the more certain the classification of asteroids will become.
Current asteroid classification systems
The most current taxonomic system of asteroid classification as of 2019 is the Bus-DeMeo system, published in 2009 (with a minor 2013 revision). It covers the wavelength range 0.45–2.45 µm, although the classification schemes of Bus (1999) (0.44–0.92 µm) and Tholen (1984) (0.3–1.04 µm) are still used. Additionally, the scheme devised by Gaffey (1993) (0.34–2.57 µm) remains relevant for classifying asteroids based on silicate mineralogical ratios.
The representative spectra that define the classes in the Tholen, Bus and Bus-DeMeo classification systems are shown below.
Each spectrum shown above represents the flux ratio of sunlight reflected from the asteroid’s surface relative to the sunlight incident on the surface, plotted as a function of wavelength. The faint horizontal lines shown with the Bus/Bus-DeMeo spectra represent a relative reflectance of 1; by convention, all spectra have been normalised to 1 at 0.55 µm. This specific wavelength is chosen because it is the effective wavelength midpoint of a standard V (visible) band photometric filter.
The “A to Z” letter designations are not entirely arbitrary—at least they weren’t in the early days of asteroid taxonomy. Most early letter designations had some meaning, whether related to colour, inferred composition or meteorite analog. This logic relaxed over time as the choice of letters became more limited. Somewhat perplexingly, some letters have been recycled, but every so often in this text I provide the A-to-Z list as it existed at specific times to clarify any duplication.
You may ask yourself why certain decades-old asteroid classification schemes remain in use today, when methods to classify these rocks have changed and later schemes have emerged. Well, there are several reasons for this:
(i) Modern CCD spectrophotometry is often less sensitive to measurements below 0.45 µm, whereas pre-CCD surveys extended further into the UV, therefore providing interesting diagnostic features at the shorter wavelengths.
(ii) The Tholen (1984) scheme was a landmark in asteroid taxonomy that permeated the entire practice of asteroid classification. The Eight-Color Asteroid Survey (ECAS) data used by Tholen analyses the NUV down to 0.32 µm, and even though more recent asteroid taxonomies extend further into the IR (which Tholen doesn’t), the Tholen system contains potentially diagnostic information in the UV drop-off. The Tholen system also uses visual albedo to distinguish between otherwise inseparable spectral types.
(iii) The Gaffey (1993) classification scheme provides a sub-classification for S-type asteroids based on the implied relative abundance of the silicate minerals olivine and pyroxene and is often used to supplement the S-class of more recent classification schemes.
(iv) The Bus (1999) classification scheme was based on the most internally consistent dataset in the 0.45–0.92 µm range and on the largest number of asteroid spectra available at the time. While the extended 0.45–2.45 µm Bus-DeMeo classification largely supersedes it, the original Bus system remains an important reference at visible wavelengths.
Interpreting asteroid spectra
In general, any inferred surface mineral assemblage—or other characteristic such as albedo—of one asteroid in a taxonomic class should be applicable to others in the same class, since the primary purpose of asteroid classification is to group objects with similar characteristics. However, it doesn’t necessarily mean that all asteroids within the same class have the same composition.
The interpretation of asteroid surface composition from a reflectance spectrum can be masked by compositional and non-compositional effects that can alter the spectral features (slope, band position, band shape, band centers, band depths, band widths, band area ratios, and so on), as well as produce spectrally neutral effects on albedo. Spectra can be altered by, for example, phase angle (Sun-asteroid-observer angle), surface temperature, surface particle size, space weathering, the addition of exogenic material from impacts, and impact shock. For a discussion on interpreting asteroid spectra, see Reddy et al. (2015).
For the uninitiated reading this review: the term “redder” or “reddening” means increasing reflectance towards longer wavelengths (i.e., a positive slope with increasing wavelength), and the term “bluer” means increasing towards shorter wavelengths (i.e., a negative slope with increasing wavelength).
Phase angle—which is greater for near-Earth asteroids than for main belt asteroids—alters spectral slope, albedo and band depths. Generally, the spectral slope becomes redder with increasing phase angle, and bluer with decreasing phase angle.
Temperature—affected by distance to the Sun, obviously—also alters band shapes and positions. These effects need to be corrected for to interpret mineralogy. For a discussion of how this is done, see Reddy et al (2015).
Particle size affects spectral slope, albedo and band depth: spectra typically become bluer and darker, and absorption bands deeper, with increasing particle size. Examples of blue-sloped asteroids include 2 Pallas (B-type) , and the regolith-poor asteroid 101955 Bennu (Cb-type)—a sample of which was collected from the asteroid in 2020 for a return to Earth in 2023 (see here).
Fresher, recently impact-excavated material has bluer and darker spectra. Spectrally-neutral darkening can also occur by surface contamination with carbonaceous material, something which is believed to be why the spectrum of the V-type asteroid 4 Vesta appears less red than other V-types (e.g. Buratti et al, 2013).
Conversely, spectra may become redder and darker with increasing exposure to the space environment. The extent of space weathering depends largely on composition, and in general, S-types show more optical alteration than C-types, and certain S- and A-types may show subdued or broadened olivine absorption features. The anomalous Q-type spectrum that long-baffled planetary scientists is now suggested to represent a “fresh” ordinary chondrite-like surface, whereas some S-types may represent space-weathered versions of the same material (e.g. see Binzel et al, 2010).
The space weathering explanation of unusual S-type spectra was supported by laboratory analysis of ordinary chondrite meteorites, observations of S-type asteroids visited by space missions (e.g., 951 Gaspra, 243 Ida, 221 Eros and 25143 Itokawa) and comparison with the samples of Itokawa returned to Earth in 2010—as well as the decades of investigation into space weathering effects on Apollo lunar samples. This led to the introduction of a “weathered” classification suffix for asteroids with spectral features similar to an existing class but with a higher spectral slope. For example, in the Bus-DeMeo classification scheme, Itokawa is an Sq-type but weathered, and is therefore designated as Sqw-type.
For a review of space weathering on asteroids and other airless bodies, see Pieters & Noble (2016) and all references therein, but I also like Chapman (2004) for a history of the debate.
Boundaries between classes
Boundaries between classes in early asteroid classification schemes (i.e. the 1970s, the left-hand side of the alluvial diagram above) were defined by computer programs that sorted spectrophotometric data into similar spectral groups. These programs took albedo and photometric colours into account, with spectra within a group physically examined by overlaying and comparing shapes—was it red or blue, was it steep or flat, and was it very, very very, or very very very steep (yes, that’s a real thing—you’ll understand when you get to the RMF scheme). Spectra with large error bars or those lying close to what were arbitrary boundaries at the time were assigned to one group or another, even though asteroid spectra were continuous across the boundary.
In current (as of 2019) asteroid classification schemes, groupings are decided by multivariate analysis of the data using principal component analysis (PCA) and assigning boundaries between the resulting clusters. Some boundaries between classes represent natural groupings of asteroid dynamical families. For example, the Hungaria family consists mostly of E-type asteroids, like its namesake 434 Hungaria. Similarly, the K class was defined for 221 Eos and the Eos dynamical family (and others resembling them), objects which show less reddening at NIR wavelengths than the S class but had originally been grouped with the S class by expanding the S class boundaries.
Rise of the machines—a note from the future: As referred to above, Mahlke et al. (2022) introduced probabilistic clustering for asteroid classification. As of 2019, if an asteroid spectrum didn’t fit within a class boundary, it was assigned to the closest one or classified as an outlier. Since 2022, machine intelligence has allowed an object to sit between classes, grouping objects based on likelihood rather than forcing them into rigid boxes. It is a shift from ‘either/or’ to ‘percentage of certainty’.
Dynamical groups are a way to study the interiors of fragmented asteroids, as some family members were originally part of the interior of a larger parent body or planetesimal. If members of an asteroid family classify into different taxonomic groups, it could mean that the fragmented parent body was differentiated, exposing mantle or even metallic core material. For example, the planet-wide Rheasilvia basin-forming impact on 4 Vesta: although the impact didn’t break the body apart, it did excavate deep enough to expose what is thought to be a mantle layer (spectrally similar to diogenite meteorites). Alternatively, different taxonomic groups within an asteroid family could simply mean an asteroid with a different composition was dynamically incorporated into the group later.
Although a finite number of parent bodies produced the millions of asteroids thought to exist in the asteroid belt, it is unclear whether there are groups of discrete mineralogical assemblages or a continuum of compositions.
The next section is where the real fun begins.
All asteroid classification schemes: 1973 to date
All asteroid classification schemes shown in the alluvial diagram above are described below:
Note that early attempts (pre-1973) to use photometric UBV colour indices and albedo as discriminators to classify asteroids are not discussed in this 2019 review (although a section on that will be added in the 2026 update (provided I am not broken by the time I finish this).
As mentioned previously, the first generation of asteroid classification schemes (1973–1975), which introduced the RMF classes, are not included in the main alluvial diagram. This is because the RMF and CSU classes are defined using different methodologies, and combining them into one diagram would be like trying to fit a square peg into a round hole. I would need some kind of androgynous docking adapter to make that work—although perhaps a highly convoluted diagram would highlight the perplexity that triggered this review. For now, the evolution from RMF to CSU classes is shown in the standalone diagram below. Simply picture this diagram tagged onto the left-hand side of the main diagram above.
1973: The Chapman–McCord–Johnson classification represents the first attempt at an asteroid taxonomy. It identifies 15 spectral groups based on measuring 32 asteroids using 24-colour filters over UV–V–NIR wavelengths (0.3–1.1 µm). This system is defined primarily by the spectral reflectance slope, with sub-groups based on the position of absorption bands around 0.65 µm and 0.95 µm, as well as the location of the UV drop-off. It defined three “arbitrary” classes (R, M, F), where R = red slope/overall positive, M = medium red slope, and F = neutral/flat to bluish with a UV drop-off. Each class was subdivided into four groups: R1–R4, M1–M4, and F1–F4. Three further R sub-types were defined based on the steepness of the red slope (A = very, B = moderate, C = slight) resulting in sub-types R2A, R2B, R3A, R3B, R3C. While the parameters in this classification system formed the basis for many later taxonomies, the names did not. For example, in this scheme, asteroid 4 Vesta is defined as M3, which has no relation to the M-type (metallic) asteroids in modern asteroid classification schemes. (See Chapman et al., 1973).
1975: The McCord–Chapman classification extends the 1973 study to 98 asteroids. It retained the three main classes (R, M, F) now variously referred to as “groups” but redefined the sub-groups. In total, they defined 27 significantly different spectral groups defined using the nine parameters shown in the green box below. A spectral group means a point or cluster significantly removed from others in classification space, but not necessarily different mineral assemblages. The spectral groups are split into: R (reddish), 16 sub-groups (11 with the 0.95 µm pyroxene band and 5 without); M (intermediate), 6 sub-groups; and F (flat), 5 sub-groups with UV drop-off. The sub-groups are a colour sequence of F, MF, MR, R, VR, VVR and VVVR (where V = very) each being assigned a further colour sequence of f, m, mr and vr, to give a colour matrix of 27 groups. The authors speculated from statistical analysis that they had probably identified “about half” of the distinct compositions in the asteroid belt and expected to find “about 20” new ones. (See McCord & Chapman, 1975a;b).
At this point in time (1975), the parameters used for distinguishing the different classes are:
1. R/B: the ratio of reflectance at 0.7 µm/0.4 µm (i.e., slope steepness), which correlates with the definitions of F, M and R in the 1973 and 1975 systems above.
2. BEND: the positive curvature near 0.56 µm, (R0.56 − R0.4) − (R0.73 − R0.56). Increases with increasing proportions of silicates compared to opaques or metals.
3. IR: intensity of IR to red part of the spectrum, (R1.05 − R0.73). Sensitive to olivine which has a major absorption at 1.05 µm—olivine-rich surfaces yield negative values.
4. UV Drop-off: appears in curves with low R/B (i.e., M and F types).
5. UV Slope.
6. IR Absorption.
7. IR Band Centre.
8. IR Band DEPTH: related to the Fe3+ absorption near 0.95 µm due to pyroxene—ratio of reflectance at band base over highest point on short-wavelength side.
9. 0.65 µm Band Depth.
1975: The Chapman–McCord–Zellner (CMZ) taxonomy redefined the earlier R–M–F groups, introducing the C–S–U classes which formed the basis for many future taxonomies. The naming came from comparisons with meteorite spectra which suggested the C and S groups were compositionally similar to carbonaceous and stony-metallic meteorites, respectively. U means “unclassifiable in this system” and incorporated the distinctive M types from the RMF scheme. The classification used a dataset of 110 asteroids observed in what is now called the 24-Color Asteroid Survey covering the wavelength range 0.32–1.08 µm. Classes were defined used spectrophotometry (R/B, DEPTH), B-V colour, albedo and polarisation, but with less weight on albedo. The distinct spectra of certain M types (of the RMF scheme) were noted at the time, including that of 4 Vesta, but specific classes were not defined for these and they were all classified under U. (See Chapman et al., 1975).
1976: An analysis by Zellner & Gradie is one of the first that introduces a formal M class for metal-rich asteroids and the E class for asteroids with high albedo attributable to pure enstatite. The authors suggested M-types may represent extremely reduced examples of C- or S-types. At this point, the broad defined classes were C, S, M, E, and U. (See Zellner & Gradie, 1976).
1977: The Zellner & Bowell analysis extends the classification to eight groups (C, S, M, E, O, T, I, and U) based on observations of 359 asteroids and an algorithm using parameters related to UBV color, spectrophotometry, albedo and polarisation. The O and T classes are notably short-lived, but later resurrected and then—well, let’s just say, “they’ll be back.” Classes are described in terms of composition: O (ordinary chondritic, metal-poor, i.e., LL chondrites), T (Trojan of unidentified composition), C (carbonaceous), S (silicaceous), M (metal-rich) and E (metal-free, enstatite). They also use I (indeterminate or inadequate data), and U (unclassifiable or unusual) for asteroids that did not fall into any of the other defined classes which at the time included 2 Pallas, 4 Vesta and 221 Eos. (See Zellner & Bowell, 1977).
1978: The Bowell taxonomy drops O and T and introduces a new R class for the reddest UBV colour asteroids. (Note: this R class is not the same as the R class in the earlier RMF scheme and is related to a rebranding of the dropped O class). This establishes six classes (C, S, M, E, R, and U) based on observations of 523 asteroids. The classification uses an algorithm based on seven parameters: spectrophotometry (R/B, BEND, DEPTH), UBV colour, albedo and polarisation. Of the 523 asteroids used to define the classification, C = 36%; S = 27%; M = 2%, E = 0.5%, R = 0.5%, while 34% are ambiguous or unclassifiable in this system. The new R class was defined by extremely red UBV colours and high albedos, and at the time included both 4 Vesta and 349 Dembowska (until 1983/1984). The number of objects classified using this algorithm was extended to 752 asteroids shortly thereafter with no changes to the classes. (See Bowell et al., 1978; Zellner, 1979).
1979: Another Chapman classification also defined the six C, S, M, E, R, and U classes based on observations of 277 asteroids from the 24-Color Asteroid Survey. This includes recalibrations and re-averages of previous data, but still gives less weight to albedo than other systems. Notably, this system recognised about 80 different spectral types among the asteroids surveyed (C = 30 types, S = 22 types, M/E = 7 types, R/U = 20 types). They also identified 221 Eos and family as one of the few groups that might be mixtures of C- and S-types. (See Chapman et al., 1979).
1979: Degewij & Van Houten found that 30% of sampled Hildas, Trojans and outer Jovian moons are reddish and dark compared to C-types, and referred to them as RD types. The RD class has low albedo (2–4%) and steep spectral slopes around 0.7–0.9 µm. Another new but short-lived T class (for Trojan) was referred to in this paper. (See Degewij & Van Houten, 1979).
1981: Zellner et al. introduced a new X class for M-like spectra with low albedo. This X class was briefly referred to by others as DM (for dark-M), later renamed pseudo-M or PM, and and later referred to simply as P. The earlier RD class was renamed D. (See Zellner et al., 1981; Hartmann et al., 1981).
1982: This Gradie & Tedesco classification places greater emphasis on albedo to define the new classes F and P. Albedos were derived from 10 µm and 20 µm radiometry, and spectra were derived from the Eight-Color Asteroid Survey (ECAS) data over the range 0.3–1.1 µm. The F class is a flat spectrum (reviving the F class not used since the 1975 McCord-Chapman “RMF” taxonomy). The P class (renamed from PM or pseudo-M) has spectral characteristics in the 0.3–1.1 µm range that are indistinguishable from M-types, but a lower albedo similar to C-types (< 0.065). They note that 1 Ceres is an unusual C-type and that 2 Pallas and 4 Vesta are still not classifiable in this scheme. Importantly, this paper provides the first clear look at the compositional distribution of the Solar System, identifying at least six distinct regions, with S-types dominating the inner belt, the new P- and D-types dominating the outer belt towards Jupiter, and C-types dominating in between. (See Gradie & Tedesco, 1982).
At this point in time (early 1980s), descriptions of the classes, in terms of albedo and reflectance spectra (slope and absorption bands 0.3–1.1 µm) are:
C – Low albedo (< 0.065). Neutral slope, weak band < 0.4 µm.
D – Low albedo (< 0.065). Very red > 0.7 µm.
E – Very high albedo (> 0.23). Featureless, sloping up into red.
F – Low albedo (< 0.065). Flat.
M – Moderate albedo (0.07–0.23). Featureless, sloping up into red.
P – Low albedo (< 0.065). Featureless, sloping up into red.
R – Very high albedo (> 0.23). Very red, bands deeper than S.
S – Moderate albedo (0.07–0.23). Reddened, band typically 0.9–1.0 µm.
U – Unusual or Unclassifiable in the current system.
I – Indeterminate or inadequate data.
1983: A photometric analysis by Veeder at UBV (0.36–0.55 µm) and JHK (1.25–2.2 µm) wavelengths introduces the A class for very red asteroids. Except for 349 Dembowska, all members previously assigned to the R class were reassigned to the S and A classes based on colour and albedo. See Veeder et al (1983).
1984: A landmark in the history of asteroid classification, the Tholen taxomony uses principal component analysis and minimal tree clustering to define 14 broad classes: A, B, C, D, E, F, G, M, P, Q, R, S, T, V, and I (for Inconsistent). The number of classes is determined by the length of the tree branch to nearest neighbours in the clustering (reducing the branch cut-off size will result in more classes). The taxonomy is based on the highest quality 0.34–1.04 µm wavelength spectra in the Eight-Color Asteroid Survey (ECAS) dataset (405 of 589 asteroids), supplemented by visual albedo to improve separation of the classes (for example, separating between E, M and P, and separating between B and C). There are five new classes (B, G, Q, T, V) and the R class is reintroduced having been dropped by Veeder the year before. The analysis indicates that 95% of the variation in the 8-Color data lies in the first two principal components related to two absorption features (one in the UV region, the other in the NIR). The C class is split into (B, C, F, G) groups based on spectral variation at UV/blue wavelengths, where B, F, G are similar to but outlie C. The T class means Tentative (as opposed to the 1979 T for Trojan class) and lies between S and D. The P class lies between C and D and within the X (EMP) group. Two new classes are introduced for distinct asteroids: Q for 1862 Apollo and V for 4 Vesta, and the R class is resurrected for 349 Dembowska. The I class (Inconsistent) replaces U (Unclassifiable in earlier systems) which is now used for Unusual. See Tholen (1984).
At this point in time (mid 1980s), descriptions of the classes, in terms of albedo and reflectance spectra (slope and absorption bands 0.3–1.1 µm), are:
A – High albedo. Very red <0.7 µm. Strong band near 1.05 µm.
B – Moderately low albedo. Flattish. Weaker absorption <0.4 µm.
C – Low albedo. Flat to reddish >0.4 µm. Absorption <0.4 µm.
D – Low albedo. Featureless. Neutral to reddish <0.55 µm. Very red >0.55 µm, levelling out >0.95 µm.
E – Very high albedo. Spectrum like M and P.
F – Low albedo. Flat to bluish featureless. Weaker absorption <0.4 µm.
G – Low albedo. Flat >0.4 µm. Stronger absorption <0.4μm.
M – Moderate albedo. Spectrum like E and P. Flat to reddish, featureless.
P – Low albedo. Spectrum like E and M. Intermediate to C and D.
Q – Moderately high albedo. Strong absorption <0.7 µm. Strong band near 1 µm.
R – High albedo. Strong band <0.7 µm, strong band near 1 µm but broader than V and deeper than S.
S – Moderate albedo. Absorption strong <0.7 µm, weak or none >0.7 µm.
T – Low albedo. Absorption <0.85 µm. Flat >0.85 µm.
V – High albedo. Strong absorption <0.7 µm. Strong band near 0.95 µm.
X – EMP where no albedo available.
U – Unusual spectrum.
I – Inconsistent data.
1987: Time for a short break. I mentioned at the start that in researching the information for this review on asteroid classification schemes, I came across a paper by Bell which was stated to serve as “a historical guide to the asteroid literature for the perplexed.” I just love that. And it fed my desire to write this piece. The quote—and the following diagram—appeared in a XVIII LPSC abstract (Tholen & Bell, 1987) and in the textbook Asteroids II (p. 299). The diagram contains the same information as in my alluvial diagram from 1975 to 1984—and being far simpler than mine, I suppose it must be better.
1987: The Barucci taxonomy re-analyses 438 asteroids from the Eight-Color data used by Tholen combined with IRAS albedos. The analysis is carried out using G-mode multivariate clustering. The result gives nine classes (containing 18 types): A0, B0-B3, C0, D0-D3, E0, G0, M0, S0-S3, V0. Three of the classes (B, D, S) are each composed of four sub-units, where 0 to 3 represent increasing albedo. The number of classes defined is determined by the confidence level chosen (setting a lower confidence level will result in more classes). The total of 18 groups are obtained by reducing the confidence level from 99.7% to 97.5%. The Tholen class T is sub-unit D3; F is grouped into C0 and B1; R, Q, V are reduced into V0; P is grouped into C0 and considered to be the tail end of C types that reside beyond 3.5 AU. The analysis was re-run in 1990 at 95% confidence resulting in different groupings, the main differences being eight S sub-units, EMP all reduced into a D sub-unit, and T remaining as a separate class. I’ve not shown the re-run groups in my alluvial diagram. See Barucci et al (1987) for the analysis at higher confidence levels, and Barucci & Fulchignoni (1990) for the analysis at the lower confidence level.
1988: Using spectrophotometric data from the 52-Color Asteroid Survey (0.8–2.5 µm), the K class was introduced to account for the atypical spectra of 221 Eos and the Eos dynamical family which have visible spectra similar to S types and flat IR spectra similar to C types. The K class asteroids may be the source of CV/CO chondrites. See Bell (1988) and Bell et al (1988).
1989: This Tedesco classification is a brief detour away from reflectance spectra. It uses visual examination of stereo pairs of 2D projections of 3D data of U-V color indices, ECAS v-x colour indices and IRAS albedos. It aims to make a point that a similar class clustering can be obtained without using spectral reflectance. It characterises 357 previously classified asteroids into 11 taxonomic classes (A, C, D, E, F, G, K, M, P, S, T). Except for the unique classes Q, V, R (and B because of 2 Pallas), it places 96% of the asteroids into the same classes as Tholen’s principal components analysis. It sounds from reading the paper that the analysis was done in part to prove a point about the pitfalls of combining classifications derived from different methods (spectral reflectivity, photometry, radiometry, polarimetry). See Tedesco et al (1989).
1991: In an extension to Tholen’s 1984 analysis, Burbine conducted an 8-Color + 52-Color PCA on a subset of the asteroid spectra used in Tholen’s 8-Color PCA and found no difference in the clustering relationships. This is not plotted in my alluvial diagram because no reclassification of classes was proposed. See Burbine (1991).
1993: The Gaffey system for S-type asteroids is based on the relationship between the 1 µm absorption band centre and the 2 µm to 1 µm band area ratio, providing sub-groups for the S class based on the olivine/pyroxene ratio and pyroxene composition. This asteroid classification scheme is often still quoted today. Seven classes S(I) to S(VII) are defined for olivine-dominated to pyroxene-dominated compositions, respectively. It is based on 39 of the 144 S-type asteroids used in the Tholen classification analysed over the spectral range 0.35–2.55 µm by combining datasets from three surveys: 24-Color (0.33–1.1 µm), 8-Color (0.32–1.1 µm) and 52-Color (0.8–2.5 µm). See Gaffey et al (1993).
1994: The Howell taxonomy used an artificial neural network with a self-organising map (SOM) hidden layer to cluster reflectance spectra of 539 asteroids over the range 0.34–2.57 µm. The analysis combined 8-Color (0.34–1.1 µm) and 52-Color (0.8–2.57 µm) survey data, with each combined spectrum resampled to 65 data points or colours. After unsupervised training and convergence of data into clusters for well-characterised spectra, the network was supervised pre-assigning Tholen’s class names to clusters, with further classes post-assigned based on the network cluster suggestions. The neural network approach is less sensitive to data noise than some of the other methods mentioned so far in this review. Using lower signal-to-noise data, Howell’s 8-Color SOM results are still consistent with Tholen’s 8-Color principal component analysis, issues mainly arising for classes only separable by albedo (i.e. E,M,P and C,B). With the combined data (65 colours), the Howell taxonomy shows two compositionally meaningful S sub-groups: So and Sp, for olivine-rich/reddest and olivine-poor/least red, respectively. The C class is split into two sub-groups: Cv and Cx based on continuum slope curvature (v=concave down, reflectance weakly increasing or strongly decreasing with wavelength; x=convex down, reflectance strongly increasing with wavelength). The B and F classes are combined into a single B+F class. Indeterminate classifications include BCv (2 Pallas), CvB (1 Ceres), CvP (some C types), and SoT andTSo. See Howell et al (1994).
1995: Around this time, Rivkin proposed a new W class for hydrated M-type asteroids. An IRTF spectrophotometric survey of 16 EMP class asteroids showed absorption features diagnostic of hydrated minerals in the 3 µm region across all three EMP classes. A later observation of 20 M-type asteroids suggested that larger M-types may be relatively primitive material (E chondrites or salty C chondrites) with iron meteorites derived from smaller anhydrous M-type asteroids. See Rivkin et al (1995; 2000).
1995: The SMASS survey (Small Main-Belt Asteroid Spectroscopic Survey) observed 316 asteroids over the wavelength range 0.44–0.92 µm. Initial analysis of 80 asteroid CCD spectra suggested two new classes: J and O. The J class was proposed for some Vesta family members which had VNIR reflectance spectra separated from 4 Vesta in PCA space, similar to the HED diogenite meteorite Johnstown (although future confirmation was needed that there was no NIR feldspar absorption). The O class was proposed for 3628 Božněmcová which was found to have a VNIR spectrum similar to L6/LL6 ordinary chondrite meteorites. See Binzel & Xu (1993); Binzel et al (1993); Xu et al (1995).
1999–2002: The Bus taxonomy builds on the framework of the Tholen taxonomy to define 26 classes but without considering albedo. Importantly, it uses the internally consistent SMASSII (Small Main-Belt Asteroid Spectroscopic Survey II) dataset of visible wavelength CCD spectra (0.44–0.92 µm) of 1,447 asteroids. Classes were defined using principal component analysis of spectra formed of 48 data points (input channels) analysed with a correlation matrix. The larger sample size and higher resolution revealed more structure in the data producing more sub-classes. The problematic EMP classes, previously only separable by albedo in the Tholen system, are now split into sub-classes. Overall, the 26 classes are the dark C-complex: C, Cb, Cg, Ch, Cgh; the brighter S-complex: S, Sa, Sk, Sl, Sq, Sr; the X-complex: X, Xc, Xe, Xk; and everything else, which are generally referred to as end members and outliers: A, B, D, K, L, Ld, O, Q, R, T, V. See Bus (1999) and Bus & Binzel (2002).
2009-2013: The Bus-DeMeo taxomony extends the Bus taxonomy into the NIR covering a range of 0.45–2.45 µm. It is based on the measurements of 321 asteroids from the SMASSII dataset (0.44–0.92 µm) and IRTF/SpeX observations of 371 asteroids over the range 0.8–2.45 µm. Classes were defined using principal component analysis of spectra formed of 40 data points (input channels) analysed with a covariance matrix. This taxonomy has 25 classes, getting rid of some Bus classes and adding new classes. Out are Ld, Sk, Sl. In are Sv, Xn, and a ‘w‘ subscript added for reddened spectra indicative of space weathering. The C-complex remains as C, Cb, Cg, Ch, Cgh; the S-complex is now S, Sa, Sq, Sr, Sv; the X-complex is now X, Xc, Xe, Xk, Xn; and the end members and outliers are now A, B, D, K, L, O, Q, R, T, V. See DeMeo et al (2009; and revision 2013). If you have spectral data (VNIR or NIR) and want to classify an object in this taxonomy, the classification web tool is available here.
The following diagram is a condensed version of that given at the start of this review, showing the evolution of classes in the most widely-adopted taxonomies, from the original (1975) CSU types, through the (1984) Tholen classes and (1999/2000) Bus classes, to the (2009/2013) Bus-DeMeo classes, sub-types and complexes.
Come back at the end of February 2026 for an update on changes from 2022 onwards.
The A to Z of classes
The following descriptions of the classes in the VNIR region (0.4–2.45 µm) are based on information in the papers referred to throughout this review but in particular that given in DeMeo et al (2009; and rev. 2013):
A – High albedo. Very red slope longward of 0.7 µm. Strong absorption at 1 µm +/- shallow 2 µm band. Originally named VR (very red). Analog: olivine-rich achondrites, brachinites.
C-Complex: B, Cb, C, Cg, Cgh, Ch:
B – Moderately low albedo. Flat or Blue slope in the visible, overall negative into the NIR. Weak absorption at ~0.4 µm, bump at ~0.6 µm. Some show 1μm absorption attributed to magnetite. Some show 1–2 µm concave up curvature. Has subsumed the old Tholen F class.
Cb – Low albedo. Flat with slight positive slope beginning at 1.1 µm.
C – Low albedo. Flat to reddish longward of 0.4 µm, absorption shortward of 0.4 µm, +/- bump at ~0.6 µm, +/- feature at 1 µm and slightly positive slope beginning at 1.3 µm. Originally named for assumed similarity to Carbonaceous chondrites.
Cg – Low albedo. Slight positive slope beginning at 1.3 µm with a pronounced UV drop off (i.e. shortward of 0.4 µm). Naming signifies like a C-type, with similarity to the old G-type, the founding member of which was 1 Ceres.
Cgh – Low albedo. Pronounced UV drop off, slight positive slope beginning at 1 µm. Broad, shallow absorption centered near 0.7 µm. Naming signifies C-type with similarity to the old G-type and hydrated.
Ch – Low albedo. Slight UV drop off, slight positive slope beginning at 1.1 µm. Broad, shallow absorption near 0.7 µm. Naming signifies hydrated C-type.
D – Low albedo. Linear and featureless. Neutral to reddish shortward of 0.55 µm, very steep red slope longward of 0.55 µm, levelling out around 0.95 µm +/- slight curvature around 1.5 µm. Originally named RD (red and dark) (not to be confused the current R class). Includes the red Trojans.
I – Not a class: refers to Inconsistent with the particular data.
J – High albedo. Sharper peak than V, and deeper, narrower 0.9 µm absorption shifted to shorter wavelength. Analog: Johnstown HED diogenite (from deeper in the crust than V-types).
K – Between C and S classes (lying in the dividing gap between C/X and S in principal component space). Low to moderate albedo. Deep absorption band shortward of 0.75 µm, shallow and wide band at 1 µm with straight sides. No 2 µm band. Analog: CV/CO chondrites, like the Eos dynamical family.
L – Between K and S classes. Low to moderate albedo. Steep red visible slope levelling out after 0.7 µm, sometimes a concave down curvature at 1.5 µm, +/- 2 µm band. Analog: CAI-rich, spinel-bearing meteorites.
O – Flat shortward of 0.8 µm, deep, broad rounded absorption at 1 µm and another absorption feature at at 2 µm. Named for Ordinary chondrites with class specifically defined for 3628 Božněmcová. Analog: L6/LL6 ordinary chondrites.
Q – Between V and S. Moderately high albedo. Strong absorption shortward of 0.7 µm. Strong band near 1 µm, others near 1.3 µm and 2 µm. Type example is the near-Earth asteroid 1862 Apollo. Current analog: fresh (non-space-weathered) ordinary chondrites.
R – High albedo. Strong band shortward of 0.7 µm. Strong bands near 1 µm (narrower than Q, broader than V, deeper than S) and near 2 µm. Originally named for Red slope, originally VR (very red). Class defined for 349 Dembowska.
S-Complex: S, Sa, Sq, Sr, Sv (and weathered types Sqw, Srw, Svw):
S – Moderate albedo. Moderate absorption at 1 µm, and at 2 µm which varies in depth between objects. Originally named for Silicaceous or Stony-iron meteorites. Current analog: ordinary chondrites. Subsumes some of the old Sk class objects now that spectra are extended into the NIR.
Sa – Moderate albedo. Deep, broad absorption at 1 µm. Features similar to A class but less red. This class subsumes the old Sl class now that spectra are extended into the NIR.
Sq – Moderate albedo. Wide absorption at 1 µm. Features at 1 µm and 1.3 µm similar to Q class, but shallower 1 µm band. Subsumes some of the old Sk class objects now that spectra are extended into the NIR. Current analog: ordinary chondrites,
Sr – Moderate albedo. Features at 1 µm and 2 µm similar to R class, but shallower 1 µm band.
Sv – Moderate albedo. Features at 1 µm and 2 µm similar to V class, but shallower 1 µm band.
T – Low albedo, linear and featureless. Moderately red slope, absorption feature shortwards of 0.85 µm, flat longward 0.85 µm, often gently concave down. Originally referred to Trojan before being merged into the D–P–C classes, and later named for Tentative or requiring follow-up. In 2022, the T-class is terminated.
U – Not a class: refers to Unusual, and previously referred to Unclassifiable in the particular system of the time.
V – High albedo. Strong band near 0.95 µm and narrow band near 1 µm. Class defined for 4 Vesta. Analog: HED achondrites.
W – Moderate albedo. Spectrum like M but with a 3 µm hydration feature.
w – Sub-types of S and V with redder slopes due to space weathering: Sw, Sqw, Srw, Svw, and Vw defined so far.
X-Complex: X, Xc, Xe, Xk, Xn:
X – Linear with medium to high red slope. Previously referred to the old EMP types when no albedo was available. Other than some E types, there is no particular correlation between the new X classes and the old EMP classes. Analogs include iron meteorites, enstatite meteorites, and other primitive types.
Xc – Low to medium slope, slightly curved, concave down. Low albedo. Transitional between X and C types. Some of the old P-types (which have a spectrum intermediate to C and D and included the less-red type Trojans) are now classified as Xc now that spectra are extended into the NIR.
Xe – Low to medium slope, similar to Xc and Xk, and transitional between X and E types. Absorption feature around 0.49 µm, like the old E-type. High albedo. Analog: enstatite chondrites.
Xk – Low to medium slope, slightly curved, concave down, similar to Xc. Weak feature 0.8-1 µm. Transitional between X and K types.
Xn – Low to medium slope. Class defined for 44 Nysa (which was previously classified as E and Xc).
Y – Not used.
Z – Not used (but in 2022, probabilistic clustering will assign the reddest of the red Main Belt objects here).
Key spectral features
The key features that are used to classify asteroids under the different asteroid classification schemes (based on information extracted from the papers referenced in this review) are as follows:
UV drop-off. Absorption feature due to strong Fe2+–Fe3+ intervalence charge transfer transitions.
NUV band: May be a proxy for hydrated silicates in primitive asteroids since correlation has been observed with the 0.7 µm band and with the 2.7 µm band: the 2.7 µm band and shallow NUV absorption implies Mg-rich phyllosilicates as found in CI chondrites; the 0.7 µm band and strong NUV absorption implies Fe-bearing phyllosilicates as found in CM chondrites.
0.49 µm band: Associated with sulphides in E- or Xe-type asteroids.
Slope longward of 0.55 µm µm: Magnitude depends of the presence or absence of reddening agents such as Fe-Ni metal or organics.
0.60-0.65 µm band: Absorption due to Fe3+ in Fe alteration minerals.
0.7 µm band: Absorption due to Fe2+–Fe3+ intervalence charge transfer transition in oxidized iron in phyllosilicate minerals, with band center varying, for example, from 0.59–0.67 µm for saponite to 0.70–0.75 µm for serpentine. Strong correlation with the 2.7 µm band means 0.7μm may be a proxy for the 2.7μm overtone when NIR data are unavailable.
Slope longward of 0.7 µm: Calculated over the region 0.7–1.5 µm. May be used to distinguish between two types of Trojans: red (like D-types) and less red (intermediate C- to P-types).
0.8-0.9 µm band: Absorption due to Fe3+ in Fe alteration minerals.
1 µm band: Olivine. A composite of three absorption bands associated with electronic transitions of Fe2+ between structural sites. The band centre moves to longer wavelengths with increasing Fe2+ content.
1 µm (Band I), 2 µm (Band II): Pyroxene. Associated with crystal field transitions of Fe2+ between structural sites, the band centres being at:
0.9 µm, 1.9 µm: low-Ca pyroxenes (orthopyroxenes).
1.05 µm, 2.35 µm: type B high-Ca pyroxenes (lower % wollasonite).
0.9 µm, 1.15 µm: type A high-Ca pyroxenes (higher % wollastonite).
1.4 µm band. Absorption due to H2O vibrational overtone in hydrated minerals.
1.9 µm band. Absorption due to OH vibrational overtone in hydrated minerals.
2 µm grand divide (in principal component space): A gap between the S-complex (spectra that have a 2 µm absorption) and the C- and X- complexes (spectra that don’t).
2.7 µm band: Fe2+–Fe3+intervalence charge transitions due to structural OH in phyllosilicates. The band center is not observable from Earth. When NIR data are not available, the 0.7 µm band may be a proxy for the 2.7 µm band.
2.5 µm to 3.5 µm. Absorptions due to bound H2O and structural OH in hydrated minerals.
3 µm band. Absorptions at 2.9 µm and 3.1 µm due to interlayer and adsorbed H2O stretch modes, and H2O ice.
Classifying asteroids Bennu and Ryugu as at 2019
The near-Earth asteroid 101955 Bennu was visited by NASA’s OSIRIS-REx mission from December 2018 to May 2021.
Prior to the spacecraft’s arrival in December 2018, the asteroid was classified as F/B-type from ground-based observations (as summarised in Lauretta et al, 2015). The F classification is from the Tholen asteroid taxonomy; the B classification is from the Tholen and Bus/Bus-DeMeo taxonomies (the F class having been dropped in favour of B and Cb types in the Bus/Bus-DeMeo taxonomies).
Measurements by the OSIRIS-REx on-board instruments indicate that the asteroid has a negative slope and featureless visible spectrum, except for a possible magnetite absorption at 0.55 µm (see Lauretta et al, 2019). At NIR wavelengths, Bennu shows a broad 2.74 µm absorption attributed to hydrated clay minerals (Hamilton et al, 2019).
As of 2019, Bennu remains classified as B-type. But check for more recent updates, as the returned samples are analysed.
The near-Earth asteroid 162173 Ryugu (below) was visited by JAXA’s Hayabusa2 spacecraft from June 2018 to November 2019.
Prior to the spacecraft’s arrival, the asteroid had been classified as Ch/Cgh-type due to its slightly positive spectral slope and a weak 0.7 µm hydration feature observed by one ground-based observation. Other ground-based observations did not observe the hydration feature and classified it as C/Cg-type (see Perna et al, 2017).
After arrival, the globally averaged data obtained by Hayabusa2’s on-board instruments found only weak hydration features in the VNIR spectra, with no 0.7 µm absorption and a narrow 2.72 µm absorption attributed to hydrated clay minerals (see Watanabe et al, 2019; Sugita et al, 2019; Kitazato et al, 2019).
As of 2019, Ryugu had been re-classified as Cb-type. But check for more recent updates, as the returned samples are analysed.
Both Ryugu and Bennu may originate from the same dynamical family of asteroids (Polana or Eulalia) (see de León et al, 2018) although perhaps from different generations as their spectra indicate different histories. The spectra of both asteroids show similarities with laboratory spectra of carbonaceous chondrite meteorites, however Ryugu may have experienced more heating and shock than Bennu. Ryugu shows similarity to thermally metamorphosed CI-type and shocked CM-type chondrites, whereas Bennu shows similarity to aqueously altered CM-type chondrites. This is discussed in the 2019 papers cited above for each asteroid.
That’s it for my review of the history of asteroid classification schemes and taxonomies. As mentioned above, not much will change until 2022.
See the section below for a list of all the papers quoted throughout this review, including a link to the software used to produce the alluvial diagrams.
For a visual overview of all the asteroids (and comets) that have been visited by spacecraft, see the post Asteroids and Comets Visited by Spacecraft, and for a discussion on the difference between asteroids and comets, read the article Asteroid vs Comet.
For a non-rigorous analysis of the asteroid on steroids in the 1998 film Armageddon, read the article It’s the Size of Texas, and for a list of every (well, not every) asteroid and comet movie ever made, read Making an Impact: Lights, Camera, Asteroid!
References
1. Taxonomic Definitions — The primary sources (Chronological)
2. General Literature — Supporting science and context (Alphabetical)
3. Technical Tools — Alluvial diagrams
4. Further Reading — Foundational textbooks
1. Taxonomic Definitiions:
Chapman, C.R., McCord, T.B., & Johnson, T.V. (1973). Asteroid spectral reflectivities. The Astronomical Journal, 78, pp.126-140. http://adsabs.harvard.edu/full/1973AJ…..78..126C.
McCord, T.B. & Chapman, C.R. (1975a). Asteroids: Spectral reflectance and color characteristics. The Astrophysical Journal, 195, pp. 553-562.
http://articles.adsabs.harvard.edu/full/1975ApJ…195..553M.
McCord, T.B. & Chapman, C.R. (1975b). Asteroids: Spectral reflectance and color characteristics II. The Astrophysical Journal, 197, pp. 781-790. http://articles.adsabs.harvard.edu/full/1975ApJ…197..781M.
Chapman, C.R., Morrison, D., & Zellner, B. (1975). Surface properties of asteroids: A synthesis of polarimetry, radiometry, and spectrophotometry Icarus, 25(1), pp. 104-130. https://doi.org/10.1016/0019-1035(75)90191-8.
Zellner, B., & Gradie, J. (1976). Minor planets and related objects. XX: Polarimetric evidence for the albedos and compositions of 94 asteroids. The Astronomical Journal, 81, pp. 262-280. http://articles.adsabs.harvard.edu/full/1976AJ…..81..262Z
Zellner, B., & Bowell, E. (1977). 2: Asteroid Compositional Types and their Distributions. International Astronomical Union Colloquium, 39, pp. 185-197. https://doi.org/10.1017/S0252921100070093.
Bowell E., Chapman, C.R., Gradie, J.C., Morrison, D., & Zellner, B. (1978). Taxonomy of asteroids. Icarus, 35(3), pp. 313-335. https://doi.org/10.1016/0019-1035(78)90085-4.
Zellner, B.T. (1979). Asteroid taxonomy and the distribution of the compositional types. In: Asteroids, pp. 783-806. http://adsabs.harvard.edu/abs/1979aste.book..783Z.
Chapman, C.R. & Gaffey, M.J. (1979). Reflectance spectra for 277 asteroids. In: Asteroids. (A80-24551 08-91) Tucson, Ariz., Univ. Ariz. Press, pp. 655-687. NASA-supported research. https://ui.adsabs.harvard.edu/abs/1979aste.book..655C/abstract.
Degewij, J. & Van Houten, C.J. (1979). Distant asteroids and outer Jovian satellites. In: Asteroids. (A80-24551 08-91) Tucson, Ariz., Univ. Ariz. Press, pp. 417-435. NASA-supported research. https://ui.adsabs.harvard.edu/abs/1979aste.book..417D/abstract.
Zellner, B., Tedesco, E.F. & Tholen, D.J. (1981). Highlights from the Eight-Color Asteroid Survey. In: Bulletin of the American Astronomical Society, 13, p. 717. http://adsabs.harvard.edu/full/1981BAAS…13..717Z.
Hartmann, W.K., Cruikshank, D.P., Degewij, J. & Capps, R.W. (1981). Surface materials on unusual planetary object Chiron. Icarus, 47(3), pp 333-341. https://doi.org/10.1016/0019-1035(81)90181-0
Gradie, J. & Tedesco, E. (1982). Compositional structure of the asteroid belt. Science, 216(4553), pp. 1405-1407. https://doi.org/10.1126/science.216.4553.1405. Note: The paper that introduces F and P classes is referenced as Tedesco & Gradie (1982) submitted to Icarus, but it doesn’t look like it was ever published. The two classes are however referred to in Gradie & Tedesco (1982).
Veeder G.J., Matson D.L., & Tedesco E.F. (1983). The R asteroids reconsidered. Icarus, 55(1), pp. 177-180. https://doi.org/10.1016/0019-1035(83)90058-1.
Tholen, D.J. (1984). Asteroid taxonomy from cluster analysis of photometry. Doctoral thesis, University of Arizona. https://repository.arizona.edu/handle/10150/187738.
Tholen, D.J. & Bell, J. F. (1987). Evolution of Asteroid Taxonomy. In Lunar and Planetary Science Conference, 18, pp. 1008-1009. http://articles.adsabs.harvard.edu/full/1987LPI….18.1008T.
Barucci, M.A., Capria, M.T., Coradini, A. & Fulchignoni, M. (1987). Classification of asteroids using G-mode analysis. Icarus, 72(2), pp. 304-324. https://doi.org/10.1016/0019-1035(87)90177-1. There is an extension to this paper (not included in the alluvial diagram as the taxonomy is unchanged from the 1987 version) which can be found at Fulchignoni, M., Birlan, M., & Barucci, M.A. (2000). The extension of the G-mode asteroid taxonomy. Icarus, 146(1), pp. 204-212. https://doi.org/10.1006/icar.2000.6381
Bell, J.F., Owensby, P.D., Hawke, B.R., & Gaffey, M.J. (1988). The 52-Color Asteroid Survey: Final Results and Interpretation, Abstracts of the Lunar and Planetary Science Conference, 19, p. 57. http://articles.adsabs.harvard.edu/pdf/1988LPI….19…57B.
Bell, J.F. (1988). A probable asteroidal parent body for the CO or CV chondrites. Meteoritics, 23, pp. 256-257. http://articles.adsabs.harvard.edu/pdf/1988Metic..23..256B
Tedesco, E.F., Williams, J.G., Matson, D.L., Weeder, G.J., Gradie, J.C., & Lebofsky, L.A. (1989). A three-parameter asteroid taxonomy. Astronomical Journal, 97, pp. 580-606. Research supported by USAF. http://articles.adsabs.harvard.edu/full/1989AJ…..97..580T
Burbine, T. (1991). Principal component analysis of asteroid and meteorite spectra from 0.3 to 2.5 microns. Master’s thesis, Univ. of Pittsburgh, Pittsburgh, Pa.
Gaffey, M.J., Bell, J.F., Brown, R.H., Burbine, T.H., Piatek, J.L., Reed, K.L. & Chaky, D.A. (1993). Mineralogical variations within the S-type asteroid class. Icarus, 106(2), pp. 573-602. https://doi.org/10.1006/icar.1993.1194.
Binzel, R.P. & Xu, S. (1993). Chips off of asteroid 4 Vesta: Evidence for the parent body of basaltic achondrite meteorites. Science, 260(5105), pp. 186-191. https://doi.org/10.1126/science.260.5105.186.
Binzel, R.P., Xu, S., Bus, S.J., Skrutskie, M.F., Meyer, M.R., Knezek, P. & Barker, E.S. (1993). Discovery of a main-belt asteroid resembling ordinary chondrite meteorites. Science, 262(5139), pp. 1541-1543. https://doi.org/10.1126/science.262.5139.1541.
Howell, E.S., Merenyi, E., & Lebofsky, L.A. (1994). Classification of asteroid spectra using a neural network. J. Geophys. Res. : Planets, 99(E5), pp. 10847-10865. https://doi.org/10.1029/93JE03575.
Xu, S., Binzel, R.P., Burbine, T.H. & Bus, S.J. (1995). Small main-belt asteroid spectroscopic survey: Initial results. Icarus, 115(1), pp. 1-35. https://doi.org/10.1006/icar.1995.1075.
Rivkin, A.S., Howell, E.S., Britt, D.T., Lebofsky, L.A., Nolan, M.C. & Branston, D.D. (1995). 3-µm spectrophotometric survey of M-and E-class asteroids.Icarus, 117(1), pp. 90-100. https://doi.org/10.1006/icar.1995.1144.
Bus, S.J. (1999). Compositional Structure in the Asteroid Belt: Results of a Spectroscopic Survey. Doctoral thesis, Massachusetts Institute of Technology. https://dspace.mit.edu/handle/1721.1/9527.
Rivkin, A.S., Howell, E.S., Lebofsky, L.A., Clark, B.E. & Britt, D.T. (2000). The nature of M-class asteroids from 3-µm observations. Icarus, 145(2), pp. 351-368. https://doi.org/10.1006/icar.2000.6354.
Bus, S.J. & Binzel, R.P. (2002). Phase II of the small main-belt asteroid spectroscopic survey: A feature-based taxonomy. Icarus, 158(1), pp. 146-177. https://doi.org/10.1006/icar.2002.6857.
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(1), pp. 160-180. https://doi.org/10.1016/j.icarus.2009.02.005, https://hal.archives-ouvertes.fr/hal-00545286. Bus-DeMeo Taxonomy Classification Web Tool by Slivan, S.M. (MIT)(2009): http://smass.mit.edu/busdemeoclass.html. Flowchart revision (2013): http://smass.mit.edu/_documents/v4flowchartmods.pdf.
2. General Literature:
Binzel, R.P., Rivkin, A.S., Stuart, J.S., Harris, A.W., Bus, S.J. & Burbine, T.H. (2010). Observed spectral properties of near-Earth objects: results for population distribution, source regions, and space weathering processes. Icarus, 170(2), pp. 259-294. https://doi.org/10.1016/j.icarus.2004.04.004.
Buratti, B.J., Dalba, P.A., Hicks, M.D., Reddy, V., Sykes, M.V., McCord, T.B., O’Brien, D.P., Pieters, C.M., Prettyman, T.H., McFadden, L.A. & Nathues, A. (2013). Vesta, vestoids, and the HED meteorites: Interconnections and differences based on Dawn Framing Camera observations. Journal of Geophysical Research: Planets, 118(10), pp. 1991-2003. https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/jgre.20152.
Campins, H., de León, J., Licandro, J., Hendrix, A., Sánchez, J.A. & Ali-Lagoa, V. (2018). Compositional diversity among primitive asteroids. In: Primitive Meteorites and Asteroids, pp. 345-369. Elsevier. https://doi.org/10.1016/B978-0-12-813325-5.00005-7.
Clark, B.E., Hapke, B., Pieters, C. & Britt, D., (2002). Asteroid space weathering and regolith evolution. Asteroids III, 585, pp. 90086-2.
De León, J., Campins, H., Morate, D., De Prá, M., Alí-Lagoa, V., Licandro, J., Rizos, J.L., Pinilla-Alonso, N., DellaGiustina, D.N., Lauretta, D.S. and Popescu, M. (2018). Expected spectral characteristics of (101955) Bennu and (162173) Ryugu, targets of the OSIRIS-REx and Hayabusa2 missions. Icarus, 313, pp. 25-37. https://doi.org/10.1016/j.icarus.2018.05.009
Donaldson Hanna, K.D. & Sprague, A.L. (2009). Vesta and the HED meteorites: Mid‐infrared modeling of minerals and their abundances. Meteoritics & Planetary Science, 44(11), pp. 1755-1770. https://doi.org/10.1111/j.1945-5100.2009.tb01205.x.
Emery, J.P., Burr, D.M. & Cruikshank, D.P. (2011). Near-infrared spectroscopy of Trojan asteroids: Evidence for two compositional groups. The Astronomical Journal, 141(1), pp. 25. https://doi.org/10.1088/0004-6256/141/1/25.
Hamilton, V.E., Simon, A.A., Christensen, P.R., Reuter, D.C., Clark, B.E., Barucci, M.A., Bowles, N.E., Boynton, W.V., Brucato, J.R., Cloutis, E.A., & Connolly, H.C. (2019). Evidence for widespread hydrated minerals on asteroid (101955) Bennu. Nature Astronomy, 3(4), p. 332. https://doi.org/10.1038/s41550-019-0722-2
Hendrix, A.R., Vilas, F. & Li, J.Y. (2016). The UV signature of carbon in the solar system. Meteoritics & Planetary Science, 51(1), pp. 105-115. https://doi.org/10.1111/maps.12575.
Kitazato, K., Milliken, R.E., Iwata, T., Abe, M., Ohtake, M., Matsuura, S., Arai, T., Nakauchi, Y., Nakamura, T., Matsuoka, M., & Senshu, H. (2019). The surface composition of asteroid 162173 Ryugu from Hayabusa2 near-infrared spectroscopy. Science, p. eaav7432. https://doi.org/10.1126/science.aav7432
Lauretta, D.S., Bartels, A.E., Barucci, M.A., Bierhaus, E.B., Binzel, R.P., Bottke, W.F., Campins, H., Chesley, S.R., Clark, B.C., Clark, B.E., & Cloutis, E.A. (2015). The OSIRIS‐REx target asteroid (101955) Bennu: Constraints on its physical, geological, and dynamical nature from astronomical observations. Meteoritics & Planetary Science, 50(4), pp. 834-849. https://doi.org/10.1111/maps.12353
Neeley, J.R., Clark, B.E., Ockert-Bell, M.E., Shepard, M.K., Conklin, J., Cloutis, E.A., Fornasier, S. & Bus, S.J. (2014). The composition of M-type asteroids II: Synthesis of spectroscopic and radar observations. Icarus, 238, pp. 37-50. https://doi.org/10.1016/j.icarus.2014.05.008.
Perna, D., Barucci, M.A., Ishiguro, M., Alvarez-Candal, A., Kuroda, D., Yoshikawa, M., Kim, M.J., Fornasier, S., Hasegawa, S., Roh, D.G., & Müller, T.G. (2017). Spectral and rotational properties of near-Earth asteroid (162173) Ryugu, target of the Hayabusa2 sample return mission. Astronomy & Astrophysics, 599, p. L1. https://doi.org/10.1051/0004-6361/201630346
Pieters, C.M. & Noble, S.K., (2016). Space weathering on airless bodies. Journal of Geophysical Research: Planets, 121(10), pp. 1865-1884. https://doi.org/10.1002/2016JE005128.
Reddy, V., Dunn, T.L., Thomas, C.A., Moskovitz, N.A. & Burbine, T.H., (2015). Mineralogy and surface composition of asteroids. Asteroids IV, pp. 43-63. https://arxiv.org/ftp/arxiv/papers/1502/1502.05008.pdf.
Rivkin, A.S., Campins, H., Emery, J.P., Howell, E.S., Licandro, J., Takir, D. & Vilas, F. (2015). Astronomical observations of volatiles on asteroids. In: Asteroids IV, pp. 65-87. https://arxiv.org/abs/1502.06442.
Rivkin, A.S., Howell, E.S. & Emery, J.P. (2019). Infrared Spectroscopy of Large, Low‐Albedo Asteroids: Are Ceres and Themis Archetypes or Outliers? Journal of Geophysical Research: Planets. https://doi.org/10.1029/2018JE005833.
Sugita, S., Honda, R., Morota, T., Kameda, S., Sawada, H., Tatsumi, E., Yamada, M., Honda, C., Yokota, Y., Kouyama, T., & Sakatani, N. (2019). The geomorphology, color, and thermal properties of Ryugu: Implications for parent-body processes. Science, 364(6437), p. eaaw0422. https://doi.org/10.1126/science.aaw0422
Waszczak, A., Ofek, E.O. & Kulkarni, S.R. (2015). Asteroids in GALEX: Near-ultraviolet Photometry of the Major Taxonomic Groups. The Astrophysical Journal, 809(1), p.92. https://doi.org/10.1088/0004-637X/809/1/92.
Watanabe, S., Hirabayashi, M., Hirata, N., Hirata, N., Noguchi, R., Shimaki, Y., Ikeda, H., Tatsumi, E., Yoshikawa, M., Kikuchi, S., & Yabuta, H. (2019). Hayabusa2 arrives at the carbonaceous asteroid 162173 Ryugu—A spinning top–shaped rubble pile. Science, 364(6437), pp. 268-272. https://doi.org/10.1126/science.aav8032
Wong, I., Brown, M.E., Blacksberg, J., Ehlmann, B.L. & Mahjoub, A. (2019). Hubble Ultraviolet Spectroscopy of Jupiter Trojans. The Astronomical Journal, 157(4), p.161. https://doi.org/10.3847/1538-3881/ab0e00.
3. Technical Tools:
4. Further Reading:
For further reading about asteroid classification systems, the history of asteroid classification and everything else you could possibly want to know about asteroids:
Physical Studies of Minor Planets (1971)
Asteroids (1979)
Asteroids II (1989)
Asteroids III (2002)
Asteroids IV (2015)
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