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One of the major classifications of asteroid, based on an analysis of their spectra, and hence their chemical composition. Most asteroids of the carbonaceous C-type, with significant levels of carbon within their structure, while the stony S-type represents the second most common type, representing an estimated 17% of all asteroids.
The relative lack of carbon in S-type asteroids implies that they originated from larger bodies, within which elements had begun to separate into different regions before being broken apart. The resulting asteroids are composed primarily of silicates or stony materials, especially iron or magnesium silicate, and notably olivine (which is also abundant in Earth's mantle). In general they show a higher albedo or reflectivity compared with the dark C-type carbonaceous asteroids. They also tend to be denser than asteroids of other types.
S-type asteroids are not to be confused with S-type stars (evolved orange or red stars with unusual chemistries), or with S-type galaxies (galaxies with an unbarred spiral form).
S-type asteroids are not evenly distributed throughout the Asteroid Belt, but are rather more common along the inner parts of the belt, with other examples in the central parts, but few in its outer reaches. The largest and most massive example of an asteroid of this kind is Eunomia (a long narrow body some 357 km along its longest axis). The more approximately spherical Juno is a close second in terms of mass, and has an average diameter of 254 km.
Other prominent examples of S-type asteroids include Iris, Ariadne, Amphitrite, Bellona and Herculina (along with literally thousands of other examples). Because stony asteroids typically result from the breakup of larger bodies, they are often found in families of 'sibling' asteroids that emerged from the same original object, such as the Eunomia family or the Flora family.
The S-group
S-type asteroids represent typical stony bodies, but there are a variety of other related types that are grouped together with the S-type to collectively form the 'S-group':
| Type |
Description |
A-type |
Found in the inner Asteroid Belt, particular strong in olivine and thus probably originating in the mantle of a larger original asteroid.
Examples: Asporina, Nenetta, Aeternitas
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K-type |
Stony asteroids with darker and redder surfaces than a typical S-type, and showing characteristics of a C-type when observed in infrared.
Examples: Metis, Julia, Celuta
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L-type |
These objects appear similar to K-type asteroids in visible light, but lack the distinctive infrared features of that type. They are typically thought to be largely composed of the mineral spinel.
Examples: Barbara, Aquitania, Anacostia
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Q-type |
Stony asteroids with spectra that suggest the presence of metal as well as silicate material, found predominantly within the inner Asteroid Belt and among those bodies that stray through the inner Solar System.
Examples: Apollo, Tantalus, Cruithne
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R-type |
Asteroids of the stony type showing an unusually high albedo. The bright asteroid Vesta has been classified as R-type (though its unusual nature means that it is more commonly given the rare 'V-type' classification).
Examples: Dembowska, Massevitch, Dimitrov
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Note that the assignment of a classification is based on a deep analysis of an asteroid's spectrum, and the boundaries between types is not always as clear as this simple list might suggest, so different sources can disagree over the classification of particular asteroids (including some of those listed as examples above).
In addition, there are asteroids that represent a transitional state between the typical S-type and one of these subtypes. This is indicated with a lowercase suffix to the main type, so (for example) an object intermediate between a full S-type and an A-type is desginated as 'Sa-type'.
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