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 (i.e. stony) mineralogical 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-

silicate In chemistry, 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 al ...

s. They are dominant in the inner part of the asteroid belt within 2.2 AU, common in the central belt within about 3 AU, but become rare farther out. The largest are

3 Juno ) , mp_category=Main belt (Juno clump) , orbit_ref = , epoch= JD 2457000.5 (9 December 2014) , semimajor=2.67070 AU , perihelion=1.98847 AU , aphelion=3.35293 AU , eccentricity=0.25545 , period=4.36463 yr , inclination=12.9817° , asc ...

(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 withi ...

(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. 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

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 solid piece of debris from an object, such as a comet, asteroid, or meteoroid, that originates in outer space and survives its passage through the atmosphere to reach the surface of a planet or moon. When the original object en ...

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 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 differentiat ...

, 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.

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 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. ...

with their typical albedo are the: *

Eos family The Eos family (''adj. Eoan'' ; ) is a very large asteroid family located in the outer region of the asteroid belt. The family of K-type asteroids is believed to have formed as a result of an ancient catastrophic collision. The family's parent body ...

(0.14) * Eunomia family (0.21) * Flora family (0.24) *

Koronis family ] The Koronis or Koronian family (), also known as the Lacrimosa family, is a very large asteroid family of S-type asteroid, stony asteroids, located in the Kirkwood gap, outer region of the asteroid belt. They are thought to have been formed at le ...

(0.24) * Nysa family (0.20) *

Phocaea family The Phocaea family ( ; adj. ''Phocaean''; ) is a collisional family of asteroids located between 2.25 and 2.5 AU in the inner region of the asteroid belt. Phocaea asteroids are of stony S-type composition and have orbits with eccentricities ...

(0.23)

References