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=^= 0.3.5 - Asteroid Classification

Created by on 09 Jan 2013 @ 7:45pm

What are asteroids?

Asteroids are essentially small solar bodies in orbit around a system's sun. The term 'planetoid' can also apply, especially toward larger objects. The term asteroid was usually in reference to any astronomical object within a sun's orbit that did not show either the characteristics of a comet or a planet. The discovery of comets and smaller planets have restricted the use of 'asteroid' to refer only to small bodies which are usually rocky or metallic in composition.

Asteroid Classification

Asteroids are classed by three main criteria: the way their orbit presents, and their reflectance spectrum. These diverge further into asteroid groups, and belts, which are comprised of asteroids that take a shared and common pathway across the solar system.

Class C

Class-C asteroids are carbonaceous asteroids, and are among the most common. They share broad orbits, though most class-C asteroids are unrelated to one another. C-type asteroids tend to be darker in luminosity (0.03 to 0.10), and can resemble type CI and CM meteorites. Due to this, viewing type-C asteroids requires a telescope. C-type asteroids are grouped along with types B, C, F and G.


Class-S types are the second most common asteroid type after C, and are fairly bright at an albedo of 0.10 to 0.22. These asteroids are comprised of stone, hence their name. Class-S asteroids are grouped together with A, K, L, Q and R types, also made of stone.


Class-M asteroids are so because of their partially-unknown status. Most are made out of nickel and iron, either purely or mixed with stone, but not all. There are a few recorded who have either too-high or too-low densities to be completely comprised of known metals. At an albedo of 0.1-0.2, they are moderately bright. M-type spectra are flat, reddish and lacking in any distinctive features, though absorption features longward of 0.75 m and shortward of 0.55 m have been detected.


Class-Bs are a very uncommon form of class-C asteroid, and are distinct enough to warrant their own class, but they do fall into the wider C cateogry. Though they are carbonaceous, their spectrum is blue rather than red and the ultraviolet absorption below 0.5 m is very small, to nearly undetectable. The albedo tends to be lighter than those of specific C-class asteroids. Spectroscopy of B-class bodies have suggested anhydrous silicates, hydrated clay minerals, organic polymers, magnetite, and sulfides. Class-F asteroids share the same characteristics as Class-B's, though they lack the water absorption feature indicative of hydrated minerals.


Class-X asteroids are grouped together because of their similar spectra, but they have different compositions. Common X-group classes are E, M, and P types. Type-Xe asteroids have spectra containing moderately broad absorption at 0.49 m. This could possibly indicate the presence of troilite (FeS). The Tholen-E group is similar in makeup. Xc and Xk-type asteroids contain a broad convex spectral feature, at 0.55 m to 0.8 m.


Class-E asteroids are thought to have enstatite (MgSiO3) achondrite surfaces. Their higher albedo rating (0.3 or higher) distinguishes them from M-type asteroids. E-type asteroids are small, with none recorded over 50 kilometers.


P-type asteroids are low on the albedo scale, featureless and with a reddish EM spectrum. They may have a composition of organic rich silicates, carbon and anhydrous silicates, even ice, though this is only a postulation. P-type asteroids are found on the outer edges of asteroid belts and beyond. P-types are some of the darkest planetary bodies in the solar system with albedos ov pv<0.1 and are organic-rich, the same as carbonaceous chondrites. More red than S-type astroids, and they do not show spectral features. Kerogen may be a reason for this reddish hue. Notable P-type asteroids include 46 Hestia, 65 Cybele, 76 Freia, 87 Sylvia, 153 Hilda and 476 Hedwig.


Vestoids, or class-V asteroids, are similar to 4 Vesta, the largest asteroid in this class. Approximately 6% of main-belt asteroids are vestoids. Size is the biggest distinguishing factor of V-class asteroids. V-type asteroids are moderately bright, and similar to more common S-types which are also made of stony irons, and chondrites. The V-class contains more pyroxene than the S-type. The EM spectrum has an absorption rate longward of 0.75 m.


K-class asteroids are uncommon asteroids reddish in spectrum shortwards of 0.75 m, and blueish longwards of this. They have a low albedo rating, resembling that of CV and CO meteorites. These were originally described as 'featureless' S-types in Tholen classification. The K-type was postulated by J.F. Bell and his coworkers in 1988, having a particularly shallow 1 m feature, but lacking the 2 m absorption, found in the Eos family of asteroids.


L-class asteroids are uncommon reddish spectrum asteroids shortwards of 0.75 m, and featureless after this. In comparison to the K-type, they display a much more reddish hue along the visible spectrum.


Class-G types are uncommon carbonaceous asteroids. The most notable asteroid in this class is 1 Ceres, displayed above. They are similar to C-type objects, but contain a strong UV absorption feature below 0.5 to 0.7 m, indicative of phyllosilicate minerals like clays and mica.


Class-A asteroids are uncommon inner-belt asteroids with a strong 1 m olivine feature, and are red shortwards of 0.7 m. They are thought to come from the completely differentiated mantle of an asteroid. A-types are so rare than only a few dozen have been recorded thus far.


Class-T asteroids are rare inner-belt asteroids of unknown composition. They are dark, featureless (with some red) and moderate absorption shortwards of 0.85 m. No direct meteorite comparison has been found. They are thought to be anhydrous, and related to P-types, D-types, or highly altered C-types (114 Kassandra).


D-class asteroids are very low in the albedo scale, featureless and reddish on the EM spectrum. They may have a comopsition of organic rich silicates, carbon and anhydrous silicates, possibly with water ice in their interior. D-types are found in the outer asteroid belt and beyond. Examples are 152 Atala, 944 Hidalgo and most of Jupiter's Trojans.


R-class asteroids are moderately bright, uncommon inner-belt objects which are spectrally intermediate between V and A-class asteroids. This spectrum shows distinct olivine and pyroxene features at 1 and 2 micrometers. There is a possibility of plagioclase. Shortwards of 0.7 m the spectrum is reddish. The IRAS mission has determined 4 Vesta, 148 Gallia, 246 Asporina, 349 Dembowska, 571 Dulcinea and 937 Bethgea as type-R, but the reclassification of Vesta as a V-type has made this debatable. Only 349 Dembowska is fully recognized as being type-R when all wavelengths are taken into account.

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