Asteroid Vesta
Image Courtesy NASA

For objects that have been extremely close to Earth on astronomical scales long before humans walked on the planet, there is still quite a large amount of information that is unknown about asteroids. According to one study, “…astronomers did not consider asteroids as subjects worthy of study in their own right…until the latter half of this century.”

Now it seems that not only are people learning that there is a wealth of information to be derived from these space vagabonds in regards to mineralogy and planetary dynamics, but they also realize that some asteroids may be potential threats to Earth; should an asteroid large enough be headed for a collision with it.

Facts about Asteroids

  • Asteroids are rocky objects that orbit the Sun, with diameters of less than 1000 kilometers at their widest point.
  • There are two major types of asteroids when classified by composition: Carbonaceous (C) and Silicaceous (S) types. They can further be subdivided into other groups from these two major types.
  • These phenomena can also be classified by their location in the solar system. Several groups are much closer to the Sun than the main belt, such as the Aten and Apollo groups. Others, like the Trojan and Centaur groups, lie further than the main belt.

What are Asteroids?

Asteroids can be defined as rocky masses that orbit the Sun, with sizes that measure from small dust particles to a maximum of 1000 kilometers wide at their longest width. Most people know of asteroids as the giant ‘belt’ of space rocks that lies between the planet Mars and the planet Jupiter. This is absolutely a true description – but there are more of these space phenomena that lie elsewhere in Earth’s solar system, including a group called the Centaur Group that extend from Jupiter’s orbit to distances as far as Pluto.

One of the prevailing theories regarding the origin of the main asteroid belt between Mars and Jupiter, is that the asteroids would have formed another planet at the same time the other planets of solar system were forming, but Jupiter’s (and possibly the other gas giants’) gravitational pull was so massive that it prevented this additional planet from ever forming.

Classifications of Asteroids

Multiple asteroid classification systems have been developed over the last 40 years. As technology has become more advanced, and individuals have become better able to observe and identify additional features of asteroids, the classification systems have adapted and become more sophisticated. Initially, asteroids were identified by the light they were reflecting from the sun, and were classified only by variations in the visual spectrum. Later, as equipment and processes became more advanced, classification systems began to add the near-infrared part of the spectrum to the already existent visual spectrum classes.

The first classification was developed in 1975, by Clark Chapman, David Morrison, and Ben Zellner. This classification system separated asteroids into 3 classes, a “C” class, for carbonaceous asteroids, an “S” class for silicaceous asteroids, and a “U” class for asteroids which did not fall into either the “C” class or the “S” class. The next major classification system developed was the Tholen classification system, developed by David J. Tholen in 1984. The Tholen classification system
divided the 3 groups defined in the initial classification system into subgroups, and added 6 additional classes; A-type, D-type, T-type, Q-type, R-type, and V-type asteroids. In 2002, Schelte J. Bus and Richard P. Binzel, using data from MIT’s Small Main-belt Asteroid Spectroscopic Survey (SMASS) taken at the Kitt Peak Observatory, created a more advanced classification system. This classification system consisted of 26 classes, and was based on slope values over various sections of the spectral curve

Asteroid Classification by Composition

Group Type Comments
C B Asteroids with low hydrous amounts
C Typical carbonaceous asteroids
Cb, Cg, Cgh, Ch Transitional carbonaceous
S A Silicaceous, rich in Olivine
Q Olivine and Pyroxene are present
R Rich in Olivine and Pyroxene
K Moderate red spectrum less than .75 um
L Strong red spectrum less than .75 um
S Typical silicaceous asteroids
Sr, Sq, Sa, Sv Transitional silicaceous

In 2007, members of that group, along with Francesca E. DeMeo, a student at MIT, developed the Bus-DeMeo classification system (Figure 1) to include asteroids which appeared in the near-infrared part of the spectrum. This is the most recent classification system, but most likely it will not be the last. It is important to note that throughout the last 40 years there have been additional classifications systems to the ones listed in this paper (the one listed in the book is by Hartmann in 2005), however, the classification systems described above serve as landmark classification systems.

Carbonaceous Asteroids (B and C-types)

B and C-type asteroids fall under the category of “carbonaceous asteroids”. Obtaining spectra from carbonaceous chondrites can be challenging, due to the low reflectivity of their surface elements. These asteroids often show absorption lines near 3 um which shows a presence of hydrous minerals such as phyllosilicates. There are variations in the strength or intensity of the absorption lines, which is a main reason for the division of the two classes. Spectral analysis of B-type asteroids shows that surface elements may include anhydrous silicates, hydrated clay minerals and other elements. The albedo of B-type asteroids is generally higher than C-type asteroids, but the albedos of both groups are relatively small compared to other groups of asteroids.

Two examples within these two groups, Ceres and Pallas, were once classified together as C-type asteroids, but are now classified as Cg and B-type asteroids, respectively, due to the strengths of their absorption lines around 3 um. To be specific, Pallas has a much lower 3 um absorption line than Ceres, showing a lower amount of hydrous materials on Pallas.

In the Tholen (1984) classification system, the C class was split into B, C, G and F classes. In the Bus-DeMeo system, G-type asteroids are again classified as sub-types of C-type asteroids, as the categories labeled Cg and Cgh-types. The F class and B class cannot be differentiated in the new Bus system so they are grouped together under the B-type class. The C-type class of asteroids is currently the largest class of identified asteroids, with the B types being less common. C-types may actually comprise a larger percentage of the entire asteroid population than is currently thought, but due to the low albedo of the class discovery is more difficult than other classes. C-type asteroids are located throughout the main belt and outer belt.

Silicaceous Asteroids (S, A, Q, R and K-types)

S-type Asteroids

S-type asteroids are often referred to as typical S-group asteroids. The “S” stands for silicaceous, which means stony. Scientists are not yet entirely sure of the total compositions of S-type asteroids; however, they are classified as being silicates. Some S-type asteroids are stony-irons, while others may be composed mostly of silicaceous materials. Chapman describes the S-class of asteroids as a “…grab bag of silicate-bearing objects, whatever the final proportions may turn out to be.” In the Bus-DeMeo classification system, Sa, Sl, Sk, and S-type asteroids have been condensed into 2 classes; the S and Sv classes. The Sq and Sr classes have been expanded into 3 classes; the Sr, Sq and Sa classes. In general, silicaceous asteroids have much higher albedos than the previously discussed Carbonaceous Asteroids.

Eros is a well known S-type asteroid. It was the first Near-Earth Asteroid (NEA) discovered and is one of the largest asteroids discovered to date. In 2001, the NEAR Shoemaker probe orbited, photographed, and then landed on Eros’ surface. Some theories state that millions of tones of aluminum, gold and platinum are lodged in Eros.

A-type Asteroids

A-type asteroids are most often located in the main asteroid belt. A-type asteroids are classified as asteroids that are rich in the mineral Olivene (4), showing an asymmetric absorption band with a minimum near 1 um (1). Olivene is a material that is often found at the bottom of magma chambers (4), which may be a clue to the origin of A-type asteroids. A-type asteroids represent a very small percentage of the total asteroid belt. To date, very few asteroids have been classified as being an A-type asteroid. One of those is Asteroid (1951) Lick, discovered by C.A. Wirtanen in 1949.

R-type Asteroids

As of 2004, only 4 asteroids have been classified as R-type asteroids. R-type asteroids are spectrally similar to V-type asteroids, with absorption lines around 1 and 2 um (1), and reflectance around .75 – .92 um (8). R-type asteroids are likely rich in olivine and pyroxene.

K-type Asteroids

K-type asteroids are also rare. They have a moderately red spectrum less than .75 um (12). K-type asteroids have a shallow absorption feature at 1 um and no absorption feature at 2 um (12).

X-type Asteroids

Little is currently known about the compositions of X-type Asteroids. The X-type Asteroid class consists of 3 different classes defined by Tholen in 1984, the E class, the P class, and the M class, now grouped together in the Bus-DeMeo system. X-type asteroids are defined as “indistinguishable on the basis of their visible-wavelength spectral properties as found in principle components analysis of the Eight Color Asteroid Survey data of 589 asteroids.” (10) One of the defining characteristics that separates these classes is their albedo. E class asteroids are high-albedo objects, P-types are low-albedo objects, and M-types are intermediate-albedo objects (10). E-type asteroids possibly have Enstatite achondrite surfaces (15), and are most prominent in the inner belt. Enstatite achondrites are theorized to possibly be fragments of larger, highly differentiated asteroids.

Xe, Xc, and Xk-type Asteroids

Essentially, the Xe, Xc and Xk classifications are based entirely on spectral features. Xe asteroids have an absorption feature at .49 um. Xc objects have a broad, convex curvature from .55 – .80 um. Xk asteroids have similar curvatures, with the difference being a slope of greater than .26 from .55 – .70 um (10). The labels Xc, Xk and Xe describe potential relationships with asteroids from the related classes (e.g. Xc would have similarities with asteroids from the C-type class of asteroids).

Kuiper Belt and Oort Cloud objects

In 1992, the Kuiper Belt was discovered by David Dewitt and Jane Luu when they found a small planetesimal lying beyond the known solar system. Many more objects in this outer region have since been discovered, but their composition is still somewhat of a mystery. It is entirely possible that these objects are composed of more ice than rock, and as asteroids are defined as ‘rocky’ objects, it is difficult to know if KBOs (Kuiper Belt Objects) are to be considered ‘asteroids’ or something else.

Similarly, even less is known about the Oort Cloud, a region that lies beyond the Kuiper Belt and is theorized to envelope the solar system in a ‘sphere’ of icy objects. These objects may eventually be classified as comets or again, something else entirely.