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S-type (stony-type or siliceous-type) asteroids areasteroids with aspectral type that is indicative of asiliceous (i.e. stony)mineralogical composition, hence the name. They have relatively high density. Approximately 17% ofasteroids are of this type, making it the second-most common after thecarbonaceousC-type.
S-type asteroids, with anastronomical albedo of typically 0.20,[1] are moderately bright and consist mainly of iron- andmagnesium-silicates. They are dominant in theinner part of theasteroid belt within 2.2AU, common in the central belt within about 3AU, but become rare farther out. The largest are3 Juno (about 240–250 km across) and15 Eunomia (230 km), with other large S-types being29 Amphitrite,532 Herculina and7 Iris. These largest S-types are visible in 10x50binoculars at most oppositions; the brightest, 7 Iris, can occasionally become brighter than +7.0, which is a higher magnitude than any asteroid except the unusually reflective4 Vesta.
Their spectrum has a moderately steep slope at wavelengths shorter than 0.7micrometres (μm), and has moderate to weak absorption features around 1 μm and 2 μm. The 1 μm absorption is indicative of the presence ofsilicates (stony minerals). Often there is also a broad but shallow absorption feature centered near 0.63 μm. The composition of these asteroids is similar to a variety of stonymeteorites which share similar spectral characteristics.
Due to their volatile-poor (rocky) composition, S-type asteroids have relatively high density. A survey of 11 S-type asteroids found an average density of3.0 g/cm3.[2]
In theSMASS classification, several generally "stony" types of asteroids are brought together into a wider S-group which contains the following types:
In theTholen classification, the S-type is a very broad grouping which includes all the types in theSMASS S-group except for theA,Q, andR, which have particularly strong "stony" absorption features around 1 μm.
Prominent stonyasteroid families with their typicalalbedo are the:[1]
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