S-type asteroids are of a siliceous (
stony) composition, hence the name. Approximately 17% of
asteroidthumb|260px|right|[[253 Mathilde]], a [[C-type asteroid]] measuring about across. Photograph taken in 1997 by the [[NEAR Shoemaker]] probe.Asteroids, sometimes called minor planets or planetoids, are small Solar System bodies in orbit around the Sun, especially in the inner Solar System; they are...
s are of this type, making it the second most common after the
C-typethumb|right|[[253 Mathilde]], a C-type asteroidC-type asteroids are carbonaceous asteroids. They are the most common variety forming around 75% of known asteroids, and an even higher percentage in the outer part of the belt beyond 2.7 AU, which is dominated by this asteroid type...
.
S-types are moderately bright (with an
albedoThe albedo of an object is the extent to which it diffusely reflects light from light sources such as the Sun. It is therefore a more specific form of the term reflectivity. Albedo is defined as the ratio of diffusely reflected to incident electromagnetic radiation. It is a unitless measure...
of 0.10 to 0.22) and consist mainly of iron- and
magnesiumMagnesium is a chemical element with the symbol Mg, atomic number 12 and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the earth's crust by mass, although ninth in the Universe as a whole...
-
silicatesA silicate is a compound containing an ion in which one or more central silicon atoms are surrounded by electronegative ligands. This definition is broad enough to include species such as hexafluorosilicate , [SiF6]2−, but the silicate species that are encountered most often...
. They are dominant in the inner main belt within 2.2
AUAn astronomical unit is a unit of length roughly equal to the mean distance between the Earth and the Sun. It is approximately 150 million kilometres ....
, common in the central belt within about 3
AUAn astronomical unit is a unit of length roughly equal to the mean distance between the Earth and the Sun. It is approximately 150 million kilometres ....
, but become rare further out.
S-type asteroids are of a siliceous (
stony) composition, hence the name. Approximately 17% of
asteroidthumb|260px|right|[[253 Mathilde]], a [[C-type asteroid]] measuring about across. Photograph taken in 1997 by the [[NEAR Shoemaker]] probe.Asteroids, sometimes called minor planets or planetoids, are small Solar System bodies in orbit around the Sun, especially in the inner Solar System; they are...
s are of this type, making it the second most common after the
C-typethumb|right|[[253 Mathilde]], a C-type asteroidC-type asteroids are carbonaceous asteroids. They are the most common variety forming around 75% of known asteroids, and an even higher percentage in the outer part of the belt beyond 2.7 AU, which is dominated by this asteroid type...
.
Characteristics
S-types are moderately bright (with an
albedoThe albedo of an object is the extent to which it diffusely reflects light from light sources such as the Sun. It is therefore a more specific form of the term reflectivity. Albedo is defined as the ratio of diffusely reflected to incident electromagnetic radiation. It is a unitless measure...
of 0.10 to 0.22) and consist mainly of iron- and
magnesiumMagnesium is a chemical element with the symbol Mg, atomic number 12 and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the earth's crust by mass, although ninth in the Universe as a whole...
-
silicatesA silicate is a compound containing an ion in which one or more central silicon atoms are surrounded by electronegative ligands. This definition is broad enough to include species such as hexafluorosilicate , [SiF6]2−, but the silicate species that are encountered most often...
. They are dominant in the inner main belt within 2.2
AUAn astronomical unit is a unit of length roughly equal to the mean distance between the Earth and the Sun. It is approximately 150 million kilometres ....
, common in the central belt within about 3
AUAn astronomical unit is a unit of length roughly equal to the mean distance between the Earth and the Sun. It is approximately 150 million kilometres ....
, but become rare further out. The largest is
15 Eunomia15 Eunomia is a very large asteroid in the inner main asteroid belt. It is the largest of the stony asteroids, and somewhere between the 8th to 12th largest Main Belt asteroid overall...
(about 330 km wide across its longest dimension), with the next largest members by diameter being
3 JunoJuno , formal designation 3 Juno in the Minor Planet Center catalogue system, was the third asteroid to be discovered and is one of the larger main belt asteroids, being one of the two largest stony asteroids, along with 15 Eunomia. Juno is estimated to contain 1% of the total mass of the asteroid...
,
29 Amphitrite29 Amphitrite is one of the largest S-type asteroids, probably third in diameter after Eunomia and Juno, although Iris and Herculina are similar in size....
,
532 Herculina532 Herculina is a very large asteroid, with a diameter of around 225 km.-Discovery:It was discovered on April 20, 1904, by Max Wolf in Heidelberg, and initially catalogued as 1904 NY. The origin of its name is not known; it may be named after the mythical Hercules, or after an unknown woman...
and
7 Iris7 Iris is a large main belt asteroid. Among S-type asteroids it ranks fifth in geometric mean diameter after Eunomia, Juno, Amphitrite and Herculina....
. These largest S-types are visible in 10x50
binocularsBinoculars, field glasses or binocular telescopes are a pair of identical or mirror-symmetrical telescopes mounted side-by-side and aligned to point accurately in the same direction, allowing the viewer to use both eyes with binocular vision when viewing distant objects. Most 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 Vesta4 Vesta is the second most massive object in the asteroid belt, with a mean diameter of about 530 km and an estimated mass of 9% of the mass of the entire asteroid belt...
.
Their spectrum has a moderately steep slope at wavelengths shorter than 0.7 µm, and has moderate to weak absorption features around 1 µm and 2 µm. The 1 µm absorption is indicative of the presence of
silicateA silicate is a compound containing an ion in which one or more central silicon atoms are surrounded by electronegative ligands. This definition is broad enough to include species such as hexafluorosilicate , [SiF6]2−, but the silicate species that are encountered most often...
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 which share similar spectral characteristics
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
A-type asteroids are relatively uncommon inner-belt asteroids that have a strong, broad 1 µm olivine feature and a very reddish spectrum shortwards of 0.7 µm. They are thought to come from the completely differentiated mantle of an asteroid....
- K-type
K-type asteroids are relatively uncommon asteroids with a moderately reddish spectrum shortwards of 0.75 μm, and a slight bluish trend longwards of this. They have a low albedo. Their spectrum resembles that of CV and CO meteorites....
- L-type
L-type asteroids are relatively uncommon asteroids with a strongly reddish spectrum shortwards of 0.75 μm, and a featureless flat spectrum longwards of this. In comparison with the K-type, they exhibit a more reddish spectrum at visible wavelengths and a flat spectrum in the infrared.These...
- Q-type
Q-type asteroids are relatively uncommon inner-belt asteroids with a strong, broad 1 micrometre olivine and pyroxene feature, and a spectral slope that indiciates the presence of metal...
- R-type
R-type asteroids are moderately bright, relatively uncommon inner-belt asteroids that are spectrally intermediate between the V and A-type asteroids. The spectrum shows distinct olivine and pyroxene features at 1 and 2 micrometres, with a possibility of plagioclase...
- 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.
This whole "S" assemblage of asteroids is spectrally quite distinct from the carbonaceous C-group and the mostly metallic
X-groupThe X-group of asteroids collects together several types with similar spectra, but probably quite different compositions.-Tholen classification:In the Tholen classification the X-group contains the types:* E-type* M-type, the largest grouping* P-type...
.
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
AA-type asteroids are relatively uncommon inner-belt asteroids that have a strong, broad 1 µm olivine feature and a very reddish spectrum shortwards of 0.7 µm. They are thought to come from the completely differentiated mantle of an asteroid....
,
QQ-type asteroids are relatively uncommon inner-belt asteroids with a strong, broad 1 micrometre olivine and pyroxene feature, and a spectral slope that indiciates the presence of metal...
, and
RR-type asteroids are moderately bright, relatively uncommon inner-belt asteroids that are spectrally intermediate between the V and A-type asteroids. The spectrum shows distinct olivine and pyroxene features at 1 and 2 micrometres, with a possibility of plagioclase...
, which have particularly strong "stony" absorption features around 1 μm.
See also
- Asteroid spectral types
Asteroids are assigned a type based on spectral shape, color, and sometimes albedo. These types are thought to correspond to an asteroid's surface composition...
- L-type asteroid
L-type asteroids are relatively uncommon asteroids with a strongly reddish spectrum shortwards of 0.75 μm, and a featureless flat spectrum longwards of this. In comparison with the K-type, they exhibit a more reddish spectrum at visible wavelengths and a flat spectrum in the infrared.These...
- K-type asteroid
K-type asteroids are relatively uncommon asteroids with a moderately reddish spectrum shortwards of 0.75 μm, and a slight bluish trend longwards of this. They have a low albedo. Their spectrum resembles that of CV and CO meteorites....
- X-type asteroid
The X-group of asteroids collects together several types with similar spectra, but probably quite different compositions.-Tholen classification:In the Tholen classification the X-group contains the types:* E-type* M-type, the largest grouping* P-type...