S-type asteroids are

asteroid An asteroid is a minor planet of the inner Solar System. Sizes and shapes of asteroids vary significantly, ranging from 1-meter rocks to a dwarf planet almost 1000 km in diameter; they are rocky, metallic or icy bodies with no atmosphere. ...

s with a spectral type that is indicative of a

siliceous Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...

(i.e. stony)

mineralogical Mineralogy is a subject of geology specializing in the scientific study of the chemistry, crystal structure, and physical (including optical) properties of minerals and mineralized artifacts. Specific studies within mineralogy include the proces ...

composition, hence the name. They have relatively high density. Approximately 17% of asteroids are of this type, making it the second most common after the carbonaceous C-type.

Characteristics

S-type asteroids, with an

astronomical albedo Albedo (; ) is the measure of the diffuse reflection of solar radiation out of the total solar radiation and measured on a scale from 0, corresponding to a black body that absorbs all incident radiation, to 1, corresponding to a body that refle ...

of typically 0.20, are moderately bright and consist mainly of iron- and magnesium- silicates. They are dominant in the inner part of the

asteroid belt The asteroid belt is a torus-shaped region in the Solar System, located roughly between the orbits of the planets Jupiter and Mars. It contains a great many solid, irregularly shaped bodies, of many sizes, but much smaller than planets, called ...

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 Eunomia (minor planet designation 15 Eunomia) is a very large asteroid in the inner asteroid belt. It is the largest of the stony ( S-type) asteroids, with 3 Juno as a close second. It is quite a massive asteroid, in 6th to 8th place (to within ...

(230 km), with other large S-types being

29 Amphitrite Amphitrite (minor planet designation: 29 Amphitrite) is one of the largest S-type asteroids, approximately in diameter, and probably fifth largest after Eunomia, Juno, Iris and Herculina. Discovery Amphitrite was discovered by Albert Marth o ...

, 532 Herculina and

7 Iris Iris (minor planet designation: 7 Iris) is a large main-belt asteroid and perhaps remnant planetesimal orbiting the Sun between Mars and Jupiter. It is the fourth-brightest object in the asteroid belt. It is classified as an S-type asteroid, mea ...

. 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, the ...

. Their spectrum has a moderately steep slope at wavelengths shorter than 0.7 micrometres (μm), and has moderate to weak absorption features around 1 μm and 2 μm. The 1 μm absorption is indicative of the presence of silicates (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 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 * 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 and the often metallic

X-group The X-Group Culture was an ancient civilization that existed from ca. 300 CE to ca. 600 CE. It was centered in Nubia stretching from the Dodekaschoinos in the north to Delgo in the south. George A. Reisner coined the term ''X-Group Culture'' for ...

Tholen classification

In the

Tholen classification An asteroid spectral type is assigned to asteroids based on their emission spectrum, color, and sometimes albedo. These types are thought to correspond to an asteroid's surface composition. For small bodies that are not internally differentiated ...

, 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 are the: * Eos family (0.14) * Eunomia family (0.21) *

Flora family The Flora family (''adj. Florian''; ; also known as ''Ariadne family'') is a prominent family of stony asteroids located in the inner region of the asteroid belt. It is one of the largest families with more than 13,000 known members, or approx ...

(0.24) *

Koronis family ] The Koronis or Koronian family (), also known as the Lacrimosa family, is a very large asteroid family of stony asteroids, located in the outer region of the asteroid belt. They are thought to have been formed at least two billion years ago in ...

(0.24) *

Nysa family The Nysa family (adj. ''Nysian''; FIN: 405) is part of the Nysa–Polana complex, the largest cluster of asteroid families in the asteroid belt. It is located in the inner region of the asteroid belt, orbiting the Sun between 2.41 and 2.5 AU ...

(0.20) * Phocaea family (0.23)

References