S-type (stony-type or silicaceous-type) asteroids are

asteroid An asteroid is a minor planet—an object larger than a meteoroid that is neither a planet nor an identified comet—that orbits within the Solar System#Inner Solar System, inner Solar System or is co-orbital with Jupiter (Trojan asteroids). As ...

s with a

spectral type In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the ...

that is indicative of a siliceous (i.e. stony) mineralogical composition, hence the name. They have relatively high density. Approximately 17% of

asteroids An asteroid is a minor planet—an object larger than a meteoroid that is neither a planet nor an identified comet—that orbits within the Solar System#Inner Solar System, inner Solar System or is co-orbital with Jupiter (Trojan asteroids). As ...

are of this type, making it the second-most common after the

carbonaceous Carbonaceous refers to something relating to, containing, or composed of carbon. It is a descriptor used for the attribute of any substance rich in carbon. Particularly, ''carbonaceous hydrocarbons'' are very unsaturated, high- molecular-weight h ...

C-type.

Characteristics

S-type asteroids, with an

astronomical albedo Albedo ( ; ) is the fraction of sunlight that is diffusely reflected by a body. It is measured on a scale from 0 (corresponding to a black body that absorbs all incident radiation) to 1 (corresponding to a body that reflects all incident radia ...

of typically 0.20, are moderately bright and consist mainly of iron- and

magnesium Magnesium is a chemical element; it has Symbol (chemistry), symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 ...

silicate A silicate is any member of a family of polyatomic anions consisting of silicon and oxygen, usually with the general formula , where . The family includes orthosilicate (), metasilicate (), and pyrosilicate (, ). The name is also used ...

s. They are dominant in the inner part of the

asteroid belt The asteroid belt is a torus-shaped region in the Solar System, centered on the Sun and roughly spanning the space between the orbits of the planets Jupiter and Mars. It contains a great many solid, irregularly shaped bodies called asteroids ...

within 2.2 AU, common in the central belt within about 3 AU, but become rare farther out. The largest are 3 Juno (about 240–250 km across) and 15 Eunomia (230 km), with other large S-types being 29 Amphitrite, 532 Herculina and 7 Iris. These largest S-types are visible in 10x50

binoculars Binoculars or field glasses are two refracting telescopes mounted side-by-side and aligned to point in the same direction, allowing the viewer to use both eyes (binocular vision) when viewing distant objects. Most binoculars are sized to be held ...

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 reflective

4 Vesta Vesta (minor-planet designation: 4 Vesta) is one of the largest objects in the asteroid belt, with a mean diameter of . It was discovered by the German astronomer Heinrich Wilhelm Matthias Olbers on 29 March 1807 and is named after Vesta (mytho ...

. Their spectrum has a moderately steep slope at wavelengths shorter than 0.7

micrometre The micrometre (English in the Commonwealth of Nations, Commonwealth English as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American English), also commonly known by the non-SI term micron, is a uni ...

s (μm), and has moderate to weak absorption features around 1 μm and 2 μm. The 1 μm absorption is indicative of the presence of

s (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 stony

meteorites A meteorite is a rock that originated in outer space and has fallen to the surface of a planet or moon. When the original object enters the atmosphere, various factors such as friction, pressure, and chemical interactions with the atmospheri ...

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 of .P. Vernazza et al. (2021) VLT/SPHERE imaging survey of the largest main-belt asteroids: Final results and synthesis. ''Astronomy & Astrophysics'' 54, A56

S-group asteroids

SMASS classification

In the SMASS classification, several generally "stony" types of asteroids are brought together into a wider S-group which contains the following types: * A-type * K-type * L-type * Q-type *

R-type is a horizontally scrolling shooter arcade video game developed and released by Irem in 1987 and the first game in the ''R-Type'' series. The player controls a star ship, the R-9 "Arrowhead", in its efforts to destroy the Bydo, a powerful a ...

* a "core" S-type for asteroids having the most typical spectra for the S-group * Sa, Sk, Sl, Sq, and Sr-types containing transition objects between the core S-type and the A, K, L, Q, and R-types, respectively. The entire "S"-assemblage of asteroids is spectrally quite distinct from the carbonaceous

C-group The C-Group culture is an archaeological culture found in Nubia, Lower Nubia, which dates from 2400 BCE to 1550 BCE. It was named by George A. Reisner. With no central site and no written evidence about what these people called themselves, Re ...

and the often metallic X-group.

Tholen classification

In the Tholen classification, the S-type is a very broad grouping which includes all the types in the SMASS S-group except for the A, Q, and R, which have particularly strong "stony" absorption features around 1 μm.

Stony asteroid families

Prominent stony asteroid families with their typical

albedo Albedo ( ; ) is the fraction of sunlight that is Diffuse reflection, diffusely reflected by a body. It is measured on a scale from 0 (corresponding to a black body that absorbs all incident radiation) to 1 (corresponding to a body that reflects ...

are the: * Eos family (0.14) * Eunomia family (0.21) * Flora family (0.24) * Koronis family (0.24) * Nysa family (0.20) * Phocaea family (0.23)

References