The X-group of asteroid

Asteroid

Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...

s 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
  E-type asteroid
  E-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...
  
  
• M-type
  M-type asteroid
  M-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,...
  
  , the largest grouping
• P-type
  P-type asteroid
  P-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...
  
  

Since in this scheme the 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...

 is crucial in discriminating between the above types, some objects for which 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...

 information was not available were assigned an X-type. An example of this is 50 Virginia

50 Virginia

50 Virginia is a large, very dark main belt asteroid. It was discovered by James Ferguson on October 4, 1857 from the United States Naval Observatory in Washington, D.C....

.

SMASS classification

The SMASS classification
does not use 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...

, but several spectral types are distinguished on the basis of spectral

Electromagnetic spectrum

The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....

 features which were too subtle to be visible in the broad-band ECAS survey used for the Tholen scheme. The X-group contains the types:

• core X-type containing the asteroids with the most "typical" spectra
• Xe-type of asteroids whose spectra contain a moderately broad absorption band around 0.49 μm. It has been suggested that this indicates the presence of troilite
  Troilite
  Troilite is a rare iron sulfide mineral with the simple formula of FeS. It is the iron rich endmember of the pyrrhotite group. Pyrrhotite has the formula FeS which is iron deficient...
  
   (FeS). There is some correlation between this group and the Tholen E-type
  E-type asteroid
  E-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...
  
  .
• Xc- and Xk-type asteroids, which contain a broad convex spectral
  Electromagnetic spectrum
  The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....
  
   feature in the range 0.55 μm to 0.8 μm (i.e. increased flux in this range). These spectra
  Electromagnetic spectrum
  The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....
  
   tend to be intermediate between the core X-type and the C
  C-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...
  
   and K-type
  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....
  
  .

Apart from the Xe-type, there is no significant correlation between the split into these SMASS types and the Tholen E

E-type asteroid

E-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...

, M

M-type asteroid

M-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,...

, and P-type

P-type asteroid

P-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...

s. All the types in the X-group contain a mixture of asteroids classified as either type E

E-type asteroid

E-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...

, M

M-type asteroid

M-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,...

, or P

P-type asteroid

P-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...

.

See also

• Asteroid spectral types
  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 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...
  
  
• S-type asteroid
  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:...
  
  
• K-type asteroid
  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....