Asteroid

An asteroid is a small, solid object in our Solar System, orbiting the Sun. An asteroid is an example of a minor planet (or planetoid), which are much smaller than planets. Most asteroids are believed to be remnants of the protoplanetary disc which were not incorporated into planets during the system's formation. Some asteroids have moons. The vast majority of the asteroids are within the main asteroid belt, with elliptical orbits between those of Mars and Jupiter.

Asteroid classification

Asteroids are commonly classified into groups based on the characteristics of their orbits and on the details of the spectrum of sunlight they reflect.

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Orbit groups and families

Asteroids are divided into groups and families based on their orbital characteristics. It is customary to name a group of asteroids after the first member of that group to be discovered. Groups are relatively loose dynamical associations, whereas families are much "tighter" and result from the catastrophic break-up of a large parent asteroid sometime in the past.

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For a full listing of known asteroid groups and families, see minor planet.

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Spectral classification

In 1975, an asteroid taxonomic system based on colour, albedo, and spectral shape was developed by Clark R. Chapman, David Morrison, and Ben Zellner. These properties are thought to correspond to the composition of the asteroid's surface material. Originally, they classified only three types of asteroids:

Related Topics:
Taxonomic - Colour - Albedo - Spectral shape - Clark R. Chapman - David Morrison - Ben Zellner

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• C-type asteroids - carbonaceous, 75% of known asteroids
• S-type asteroids - silicaceous, 17% of known asteroids
• M-type asteroids - metallic, most of the remaining asteroids

This list has since been expanded to include a number of other asteroid types. The number of types continues to grow as more asteroids are studied. See List of asteroid types for a complete list.

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Note that the proportion of known asteroids falling into the various spectral types does not necessarily reflect the proportion of all asteroids that are of that type; some types are easier to detect than others, biasing the totals.

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Problems with spectral classification

Originally, spectral designations were based on inferences of an asteroid's composition:

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• C - Carbonaceous
• S - Silicaceous
• M - Metallic

This has led to great confusion though in that an asteroid's type is not indicative of its composition. While asteroids of different spectral classifications are likely to be composed of different materials, there are no assurances that asteroids within the same taxonomic class are composed of similar materials.

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However, scientists have been unable to agree on a new taxonomic system for asteroids and as a result, the spectral classification has stuck.

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