Asteroid Spectral Types
Asteroids are assigned by asteroid spectral types, based on their spectrum, color, and sometimes based on their albedo (a measure of the diffuse reflection of solar radiation).
Asteroids are assigned a spectral type based on their spectrum , color, and sometimes by their albedo (measure of the diffuse reflection of solar radiation). These types are thought to correspond to the composition of the asteroid’s surface. For small internally undifferentiated bodies, the surface and internal composition are presumably similar, while large objects such as (1) Ceres and (4) Vesta are known to have a distinct internal structure.
|Origin of type||Abundance||Specifications||Subtypes|
|C-type asteroid||“C” means carbon||75% of known asteroids||These asteroids are very dark (albedo coefficient around 0.03) and similar to carbonaceous chondrite meteorites. Their chemical composition is close to that of the early Solar System, without the light and volatile elements like ice. Their spectrum is rather blue and flat.||Type B, type F, type G.|
|Asteroid tipe-S||“S” berarti silika||17% of known asteroids||They are quite bright (albedo 0.10-0.22). They are rich in metal: (mainly iron, nickel and magnesium). Their spectrum is towards the red, similar to that of siderolith meteorites.||Type A, type K, type L, type Q, type R.|
|M-type asteroid||“M” stands for metallic||Most of the rest of the asteroids||They are made of iron-nickel alloy and shiny (albedo 0.10-0.18).|
(1) Ceres photographed by the Dawn probe in May 2015. NASA/JPL-Caltech/UCLA/MPS/DLR/IDA, Public domain, via Wikimedia Commons
Secondary Asteroid spectral types
There are a number of rarer types, a number that increases with new discoveries:
|Type principal||Origin of type||Abundance||Specifications||Subtypes|
|E-type asteroid||X type||“E” for enstatitis (pyroxene rhombic)||About 20 such were listed in 2008, such as (2867) Steins.|
|R-type asteroid||S type||“R” for red (or red)|
|V-type asteroid||“V” for (4) Vesta||About 6% of main belt asteroids are vestoids and most of these are part of the Vesta family.||They are believed to be fragments of Vesta, its largest representative.||J type|
|A-type asteroid||S type||Characteristic of inner-belt asteroids and believed to originate from a fully differentiated mantle.|
|B-type asteroid||C type||A subdivision of type C which is distinguished by differences in ultraviolet absorption.|
|D-type asteroid||(152) Atala, (588) Achille, (624) Hector and (944) Hidalgo.||Characterized by a very low albedo and a uniform reddish spectrum. D-type asteroids are found outside the belt and beyond.|
|F-type asteroid||C type||A subdivision of type C distinguished by differences in ultraviolet absorption and lack of water absorption at 3 µm.|
|G-type asteroid||C type||(1) Ceres||A subdivision of type C which is distinguished by differences in ultraviolet absorption.|
|P-type asteroid||X type||Characterized by a very low albedo and a uniform reddish spectrum. P-type asteroids are found outside the belt and beyond.|
|Q-type asteroid||S type||This type is spectrally very close to ordinary chondrite meteorites, which has allowed scientists to speculate that they are abundant, but only 4 asteroids of this type have been found including (1862) Apollo and (2063) Bacchus.||Characteristic of inner-belt asteroids and possessing broad and intense lines of olivine and pyroxene at 1 µm.|
|T-type asteroid||Rare type. Example: (114) Cassandra.||A rare type of inner-belt asteroid of unknown composition and with a dark, uniform, reddish spectrum and moderate absorption lines around 0.85 µm. As of this date, there is no known analogous meteorite. Although they are thought to be anhydrous, it is assumed to be a type close to type P or D, or even a strongly weathered type C.|
|X-type asteroid||Groups together asteroids of similar spectra, but of different compositions.||Tholen : E, M, P
SMASS : Xe, Xc, Xk
951 Gaspra, an S-type that orbits at the edge of the inner rim of the asteroid belt. Gaspra is the first asteroid to have been closely overflown by a space probe, this happened on October 29, 1991 by the American probe Galileo while on its way to Jupiter. NASA, Public domain, via Wikimedia Commons
Originally, the classification of asteroids was based on assumptions about their composition:
- type C – carbon
- type S – silica
- type M – metallic
This brought a lot of confusion because the spectral type of an asteroid is not indicative of its composition. While asteroids of different types probably don’t have the same composition, there is no guarantee that asteroids of the same type will have similar compositions. However, we could not agree on a new classification system and so we kept the old system.
However, several new systems now exist. Two of these are most often used: the Tholen and SMASS classifications.
In 1984, David J. Tholen established the following classification:
|C Group||Type B, type F, type G, type C.||Objects composed mainly of carbon. Those in this group that are not classified as Type B, F, or G, are classified as “Standard” Type C.|
|S Group||Objects composed primarily of silicates.|
|X Group||Type M, type E, type P.||M-types are metallic, while E-types differ in high albedo and P-types in low albedo.|
|Minor classes||Type A, type D, type T, type Q, type R, type V.|
In 2002, Schelte J. Bus and Richard P. Binzel established the SMASS classification, for Small Main-Belt Asteroid Spectroscopic Survey, for 1447 asteroids.
|C Group||Type B, type C,
Cg, Ch, Cgh et Cb.
|Objects made of carbon.
|S Group||Type A, type K, type L, type Q, type R, type S,
Sa, Sq, Sr, Sk et Sl.
Xe, Xc and Xk.
|Minor classes||Type D, type Ld, type O, type T, type V.|
Better resolution of spectra over a wider range of wavelengths and the identification of previously unknown spectral details, lead to a refinement of the SMASS classification (itself a development of the Tholen classification). The Bus-DeMeo (or Bus-Binzel-DeMeo) classification is a system of 24 classes, including the new type Sv.
Photo description: This is a mosaic of the main belt asteroid 253 Mathilde made up of four images returned by the NEAR spacecraft during its flyby. The images were taken on 27 June 1997 from a distance of 2400 km. The portion of the asteroid visible is about 59 x 47 km and the resolution is roughly 300 m. Three large craters can be discerned, one at lower center, one at the top left viewed edge-on, and one at lower right, also viewed edge-on. The crater at center is estimated to be 10 km deep. These large craters, the abundance of smaller craters, and the angular shape point to a history of heavy bombardment. Lighting is from the upper right. (NEAR Mathilde 199706271400)