Asteroid spectral types
Encyclopedia
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. For small bodies that are not internally differentiated, the surface and internal compositions are presumably similar, while large bodies such as 1 Ceres
and 4 Vesta
are known to have internal structure.
A list of types can be found at asteroid spectral classes.
for dark carbonaceous objects, S
for stony (silicaceous) objects, and U for those that did not fit into either C or S. This classification has since been expanded and clarified.
A number of classification schemes are currently in existence, and while they strive to retain some mutual consistency, quite a few asteroids are sorted into different classes depending on the particular scheme. This is due to the use of different criteria for each approach. The two most widely used classifications are described below:
, first proposed in 1984. This classification was developed from broad band spectra (between 0.31μm and 1.06μm) obtained during the Eight-Color Asteroid Survey (ECAS) in the 1980s, in combination with albedo
measurements. The original formulation was based on 978 asteroids.
This scheme includes 14 types with the majority of asteroids falling into one of three broad categories, and several smaller types. They are, with their largest exemplars:
and the small classes:
Objects were sometimes assigned a combined type such as e.g. CG when their properties were a combination of those typical for several types.
While attempting to keep to the Tholen taxonomy as much as possible given the differing data, asteroids were sorted into the 24 types given below. The majority of bodies fall again into the three broad C, S, and X categories, with a few unusual bodies categorized into several smaller types:
A significant number of small asteroids were found to fall in the Q
, R
, and V
types, which were represented by only a single body in the Tholen scheme.
In this Bus and Binzel SMASS scheme only a single type was assigned to any particular asteroid.
A few Near-Earth objects have spectra that differ strongly from any of the SMASS classes. This is presumably because these bodies are much smaller than those detected in the Main Belt, and as such may have younger less-altered surfaces or be composed of a less varied mix of minerals.
Some groupings of asteroids have been correlated with meteorite types
:
Emission spectrum
The emission spectrum of a chemical element or chemical compound is the spectrum of frequencies of electromagnetic radiation emitted by the element's atoms or the compound's molecules when they are returned to a lower energy state....
shape, color
Color
Color or colour is the visual perceptual property corresponding in humans to the categories called red, green, blue and others. Color derives from the spectrum of light interacting in the eye with the spectral sensitivities of the light receptors...
, and sometimes albedo
Albedo
Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...
. These types are thought to correspond to an asteroid's surface composition. For small bodies that are not internally differentiated, the surface and internal compositions are presumably similar, while large bodies such as 1 Ceres
1 Ceres
Ceres, formally 1 Ceres, is the smallest identified dwarf planet in the Solar System and the only one in the asteroid belt. With a diameter of about 950 km, Ceres is by far the largest and most-massive asteroid, comprising about a third of the mass of the asteroid belt. Discovered on 1 January 1801...
and 4 Vesta
4 Vesta
Vesta, formally designated 4 Vesta, is one of the largest asteroids, with a mean diameter of about . It was discovered by Heinrich Wilhelm Olbers on March 29, 1807, and is named after the Roman virgin goddess of home and hearth, Vesta....
are known to have internal structure.
A list of types can be found at asteroid spectral classes.
Present-day classifications
The present-day classification was initiated by Clark R. Chapman, David Morrison, and Ben Zellner in 1975 with three categories: CC-type asteroid
C-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 asteroid belt beyond 2.7 AU, which is dominated by this asteroid type...
for dark carbonaceous objects, S
S-type asteroid
S-type asteroids are of a stony composition, hence the name. Approximately 17% of asteroids are of this type, making it the second most common after the C-type.-Characteristics:...
for stony (silicaceous) objects, and U for those that did not fit into either C or S. This classification has since been expanded and clarified.
A number of classification schemes are currently in existence, and while they strive to retain some mutual consistency, quite a few asteroids are sorted into different classes depending on the particular scheme. This is due to the use of different criteria for each approach. The two most widely used classifications are described below:
Tholen classification
The most widely used taxonomy for over a decade has been that of David J. TholenDavid J. Tholen
David James Tholen is an American astronomer at the Institute for Astronomy of the University of Hawaii, who specializes in planetary and solar system astronomy.-Professional life:...
, first proposed in 1984. This classification was developed from broad band spectra (between 0.31μm and 1.06μm) obtained during the Eight-Color Asteroid Survey (ECAS) in the 1980s, in combination with albedo
Albedo
Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...
measurements. The original formulation was based on 978 asteroids.
This scheme includes 14 types with the majority of asteroids falling into one of three broad categories, and several smaller types. They are, with their largest exemplars:
- C-group dark carbonaceous objects. This group contains about 75% of asteroids in general, divided into several sub-types:
- B-typeB-type asteroidB-type asteroids are a relatively uncommon type of carbonaceous asteroid, falling into the wider C-group. In the asteroid population, B-class objects are abundant in the outer asteroid belt, and also dominate the high-inclination Pallas family which includes the second-largest asteroid 2 Pallas...
(2 Pallas2 PallasPallas, formally designated 2 Pallas, is the second asteroid to have been discovered , and one of the largest. It is estimated to constitute 7% of the mass of the asteroid belt, and its diameter of 530–565 km is comparable to, or slightly larger than, that of 4 Vesta. It is however 20%...
) - F-typeF-type asteroidF-type asteroids are a relatively uncommon type of carbonaceous asteroid, falling into the wider C-group.-Characteristics:Generally similar to the B-type asteroids, but...
(704 Interamnia704 Interamnia704 Interamnia is a very large asteroid, with an estimated diameter of 350 kilometres. Its mean distance from the Sun is 3.067 . It was discovered on October 2, 1910 by Vincenzo Cerulli, and named after the Latin name for Teramo, Italy, where Cerulli worked...
) - G-typeG-type asteroidG-type asteroids are a relatively uncommon type of carbonaceous asteroid. The most notable asteroid in this class is 1 Ceres.- Characteristics :Generally similar to the C-type objects, but containing a strong ultraviolet absorption feature below 0.5 μm...
(1 Ceres1 CeresCeres, formally 1 Ceres, is the smallest identified dwarf planet in the Solar System and the only one in the asteroid belt. With a diameter of about 950 km, Ceres is by far the largest and most-massive asteroid, comprising about a third of the mass of the asteroid belt. Discovered on 1 January 1801...
) - C-typeC-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 asteroid belt beyond 2.7 AU, which is dominated by this asteroid type...
(10 Hygiea10 Hygiea10 Hygiea is an asteroid located in the asteroid belt. With somewhat oblong diameters of 350–500 km, and a mass estimated to be 2.9% of the total mass of the belt, it is the fourth largest asteroid by volume and mass...
) the remaining majority of 'standard' C-type asteroids.
- B-type
- S-typeS-type asteroidS-type asteroids are of a stony composition, hence the name. Approximately 17% of asteroids are of this type, making it the second most common after the C-type.-Characteristics:...
(15 Eunomia15 Eunomia15 Eunomia is a very large asteroid in the inner asteroid belt. It is the largest of the stony asteroids, and somewhere between the 8th-to-12th-largest main-belt asteroid overall...
, 3 Juno3 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...
) silicaceous (or "stony") objects. This class contains about 17% of asteroids in general. - X-groupX-type asteroidThe 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...
- M-typeM-type asteroidM-type asteroids are asteroids of partially known composition; they are moderately bright . Some, but not all, are made of nickel-iron, either pure or mixed with small amounts of stone. These are thought to be pieces of the metallic core of differentiated asteroids that were fragmented by impacts,...
(16 Psyche16 Psyche16 Psyche )is one of the ten most massive main-belt asteroids. It is over 200 kilometers in diameter and contains a little less than 1% of the mass of the entire asteroid belt. It is the most massive of the metallic M-type asteroids....
) metallic objects, the third most populous group. - E-typeE-type asteroidE-type asteroids are asteroids thought to have enstatite achondrite surfaces. They form a large proportion of asteroids inward of the main belt known as Hungaria asteroids, but rapidly become very rare as the main belt proper is entered. There are, however, some that are quite far from the inner...
(44 Nysa44 Nysa44 Nysa is a large and very bright main-belt asteroid, and the brightest member of the Nysian asteroid family. It is classified as a rare class E asteroid and is probably the largest of this type .-Discovery:...
, 55 Pandora55 Pandora55 Pandora is a fairly large and very bright main belt asteroid. Pandora was discovered by George Mary Searle on September 10, 1858 from the Dudley Observatory near Albany, NY. It was his first and only asteroid discovery....
) differ from M-type mostly by high albedo - P-typeP-type asteroidP-type asteroids have low albedo and a featureless reddish electromagnetic spectrum. It has been suggested that they have a composition of organic rich silicates, carbon and anhydrous silicates, possibly with water ice in their interior...
(259 Aletheia259 Aletheia259 Aletheia is a very large main-belt asteroid. It is composed of primitive carbonaceous materials and is very dark in colour, darker than coal.It was discovered by C. H. F. Peters on June 28, 1886, in Clinton, New York....
, 190 Ismene190 Ismene190 Ismene is a very large main belt asteroid. Being a P-type asteroid, it has a very dark surface and may contain water.Ismene orbits near the outer edge of the asteroid belt. It is one of the Hilda asteroids, which are locked in 3:2 resonance with the planet Jupiter.It was discovered by C. H. F...
; CP: 324 Bamberga324 Bamberga324 Bamberga is one of the largest asteroids in the asteroid belt. It was discovered by Johann Palisa on February 25, 1892 in Vienna, making it one of the last large asteroids discovered...
) differ from M-type mostly by low albedo
- M-type
and the small classes:
- A-typeA-type asteroidA-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....
(446 Aeternitas446 Aeternitas446 Aeternitas is a main belt asteroid. It is classified as an A-type asteroid.It was discovered by Max Wolf and A. Schwassmann on October 27, 1899 in Heidelberg....
) - D-typeD-type asteroidD-type asteroids have a very low albedo and a featureless reddish electromagnetic spectrum. It has been suggested that they have a composition of organic rich silicates, carbon and anhydrous silicates, possibly with water ice in their interiors...
(624 Hektor624 Hektor624 Hektor is the largest Jupiter Trojan. It was discovered in 1907 by August Kopff.Hektor is a D-type asteroid, dark and reddish in colour. It lies in Jupiter's leading Lagrangian point, , called the 'Greek' node after one of the two sides in the legendary Trojan War...
) - T-typeT-type asteroidT-type asteroids are rare inner-belt asteroids of unknown composition with dark, featureless and moderately red spectra, and a moderate absorption feature shortwards of 0.85 µm. No direct meteorite analog has been found to date. Thought to be anhydrous, they are believed to be related to...
(96 Aegle96 Aegle96 Aegle is a very large main-belt asteroid. It has a dark-colored surface and probably a primitive carbonaceous composition. It was discovered by Jérôme Coggia on February 17, 1868, and named after one of the three Aegles in Greek mythology. Aegle has been observed occulting three stars.-...
) - Q-typeQ-type asteroidQ-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...
(1862 Apollo1862 Apollo1862 Apollo is a Q-type asteroid, discovered by Karl Reinmuth in 1932, but lost and not recovered until 1973. It is named after the Greek god Apollo....
) - R-typeR-type asteroidR-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...
(349 Dembowska349 DembowskaAsteroid 349 Dembowska is named in honor of the Baron Hercules Dembowski, an Italian astronomer who made significant contributions to research on double and multiple stars. The asteroid was discovered over a century ago on December 9, 1892, by the French astronomer Auguste Charlois while working at...
) - V-typeV-type asteroidThe spectra of the V-type asteroids or Vestoids are similar to that of 4 Vesta, by far the largest asteroid in this class ....
(4 Vesta4 VestaVesta, formally designated 4 Vesta, is one of the largest asteroids, with a mean diameter of about . It was discovered by Heinrich Wilhelm Olbers on March 29, 1807, and is named after the Roman virgin goddess of home and hearth, Vesta....
)
Objects were sometimes assigned a combined type such as e.g. CG when their properties were a combination of those typical for several types.
SMASS classification
This is a more recent taxonomy introduced by Schelte J. Bus and Richard P. Binzel in 2002, based on the Small Main-Belt Asteroid Spectroscopic Survey (SMASS) of 1447 asteroids. This survey produced spectra of a far higher resolution than ECAS, and was able to resolve a variety of narrow spectral features. However, a somewhat smaller range of wavelengths (0.44μm to 0.92μm) was observed. Also, albedos were not considered.While attempting to keep to the Tholen taxonomy as much as possible given the differing data, asteroids were sorted into the 24 types given below. The majority of bodies fall again into the three broad C, S, and X categories, with a few unusual bodies categorized into several smaller types:
- C-group of carbonaceous objects including:
- B-typeB-type asteroidB-type asteroids are a relatively uncommon type of carbonaceous asteroid, falling into the wider C-group. In the asteroid population, B-class objects are abundant in the outer asteroid belt, and also dominate the high-inclination Pallas family which includes the second-largest asteroid 2 Pallas...
largely overlapping with the Tholen BB-type asteroidB-type asteroids are a relatively uncommon type of carbonaceous asteroid, falling into the wider C-group. In the asteroid population, B-class objects are abundant in the outer asteroid belt, and also dominate the high-inclination Pallas family which includes the second-largest asteroid 2 Pallas...
and FF-type asteroidF-type asteroids are a relatively uncommon type of carbonaceous asteroid, falling into the wider C-group.-Characteristics:Generally similar to the B-type asteroids, but...
types. - C-typeC-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 asteroid belt beyond 2.7 AU, which is dominated by this asteroid type...
the most 'standard' of the non-B carbonaceous objects - Cg Ch Cgh somewhat related to the Tholen GG-type asteroidG-type asteroids are a relatively uncommon type of carbonaceous asteroid. The most notable asteroid in this class is 1 Ceres.- Characteristics :Generally similar to the C-type objects, but containing a strong ultraviolet absorption feature below 0.5 μm...
type - Cb transition objects between plain C and B types.
- B-type
- S-groupS-type asteroidS-type asteroids are of a stony composition, hence the name. Approximately 17% of asteroids are of this type, making it the second most common after the C-type.-Characteristics:...
of silicaceous (stony) objects including:- A-typeA-type asteroidA-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....
- Q-typeQ-type asteroidQ-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-typeR-type asteroidR-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...
- K-typeK-type asteroidK-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....
a new category (181 Eucharis181 Eucharis181 Eucharis is a large, slowly rotating main-belt asteroid. It is classified as a K-type asteroid. It is named after Eucharis, a Greek nymph....
, 221 Eos221 Eos221 Eos is a rather large main-belt asteroid. It is classified as a K-type asteroid. It is a member of the extensive Eos asteroid family, which is named after it.Eos was discovered by Johann Palisa on January 18, 1882, in Vienna...
) - L-typeL-type asteroidL-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...
a new category (83 Beatrix83 Beatrix83 Beatrix is a quite large asteroid orbiting in the inner part of the main asteroid belt. It was discovered by Annibale de Gasparis on April 26, 1865. It was his last asteroid discovery. A diameter of at least 68 km was determined from the Beatrician stellar occultation observed on June 15,...
) - S-typeS-type asteroidS-type asteroids are of a stony composition, hence the name. Approximately 17% of asteroids are of this type, making it the second most common after the C-type.-Characteristics:...
the most 'standard' of the S group - Sa, Sq, Sr, Sk, and Sl transition objects between plain S and the other types in the group.
- A-type
- X-groupX-type asteroidThe 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...
of mostly metallic objects including:- X-typeX-type asteroidThe 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...
the most 'standard' of the X group including objects classified by Tholen as M, E, or P-type. - Xe, Xc, and Xk transition types between plain X and the appropriately lettered types.
- X-type
- T-typeT-type asteroidT-type asteroids are rare inner-belt asteroids of unknown composition with dark, featureless and moderately red spectra, and a moderate absorption feature shortwards of 0.85 µm. No direct meteorite analog has been found to date. Thought to be anhydrous, they are believed to be related to...
- D-typeD-type asteroidD-type asteroids have a very low albedo and a featureless reddish electromagnetic spectrum. It has been suggested that they have a composition of organic rich silicates, carbon and anhydrous silicates, possibly with water ice in their interiors...
- Ld-type: a new type with more extreme spectral features than the L-typeL-type asteroidL-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...
- O-typeO-type asteroidThe rare O-type asteroids have spectra similar to the unusual asteroid 3628 Boznemcová, which is the best asteroid match to the spectra of L6 and LL6 ordinary chondrite meteorites. Their spectra have a deep absorption feature longward of 0.75 μm....
a small category (3628 Boznemcová3628 Božnemcová3628 Božněmcová is a main belt asteroid with an orbital period of 1478.2672540 days .The asteroid was discovered on November 25, 1979.-References:* * * See also: List of asteroids...
) - V-typeV-type asteroidThe spectra of the V-type asteroids or Vestoids are similar to that of 4 Vesta, by far the largest asteroid in this class ....
A significant number of small asteroids were found to fall in the Q
Q-type asteroid
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
R-type asteroid
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...
, and V
V-type asteroid
The spectra of the V-type asteroids or Vestoids are similar to that of 4 Vesta, by far the largest asteroid in this class ....
types, which were represented by only a single body in the Tholen scheme.
In this Bus and Binzel SMASS scheme only a single type was assigned to any particular asteroid.
A few Near-Earth objects have spectra that differ strongly from any of the SMASS classes. This is presumably because these bodies are much smaller than those detected in the Main Belt, and as such may have younger less-altered surfaces or be composed of a less varied mix of minerals.
Appraisal
These classification schemes are expected to be refined and/or replaced as further research progresses. However for now the spectral classification based on the two above coarse resolution spectroscopic surveys from the 1990s is still the standard. Scientists have been unable to agree on a better taxonomic system, largely due to the difficulty of obtaining detailed measurements consistently for a large sample of asteroids (e.g. finer resolution spectra, or non-spectral data such as densities would be very useful).Some groupings of asteroids have been correlated with meteorite types
Meteorites classification
The ultimate goal of meteorite classification is to group all meteorite specimens that share a common origin on a single, identifiable parent body. This could be a planet, asteroid, Moon, or other current Solar System object, or one that existed some time in the past...
:
- C-typeC-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 asteroid belt beyond 2.7 AU, which is dominated by this asteroid type...
- Carbonaceous chondriteCarbonaceous chondriteCarbonaceous chondrites or C chondrites are a class of chondritic meteorites comprising at least 7 known groups and many ungrouped meteorites. They include some of the most primitive known meteorites...
meteorites - S-typeS-type asteroidS-type asteroids are of a stony composition, hence the name. Approximately 17% of asteroids are of this type, making it the second most common after the C-type.-Characteristics:...
- Stony meteorites - M-typeM-type asteroidM-type asteroids are asteroids of partially known composition; they are moderately bright . Some, but not all, are made of nickel-iron, either pure or mixed with small amounts of stone. These are thought to be pieces of the metallic core of differentiated asteroids that were fragmented by impacts,...
- Iron meteoriteIron meteoriteIron meteorites are meteorites that consist overwhelmingly of nickel-iron alloys. The metal taken from these meteorites is known as meteoric iron and was one of the earliest sources of usable iron available to humans.-Occurrence:...
s - V-typeV-type asteroidThe spectra of the V-type asteroids or Vestoids are similar to that of 4 Vesta, by far the largest asteroid in this class ....
- HED meteorites