An asteroid is a small solar system body which has no atmosphere, is smaller than a planet, and orbits the sun. Most of the known asteroids in our solar system are located in an asteroid belt between the orbits of the planets Mars and Jupiter. They vary in size from ten or meters to over one thousand kilometers along the longest dimension. These orbiting bodies are thought to be the remnants of the developing solar system that never grew large enough to become planets. Asteroids are classified into three main groups based on the emission spectrum properties of the asteroid and they can also be classified into another three main groups based upon the location of the asteroid.
The first of asteroids into specific groups is done according to the spectra and albedo. Spectra refers to the frequencies of electromagnetic radiation that are emitted from an element or compound and reflect the composition as each element and compound has a different measurement. Albedo is a measure of how an object reflects light. A measurement of zero reflects a perfectly opaque object that is completely black while a measurement of one indicates a perfectly white surface. Using results from both measurements, asteroids are classified into the three main designations which are C-type, S-type, and M-type.
C-type –About 75% of all known asteroids fall into this category. The composition is carbon, iron, and some other compounds which make it similar to stony type meteors. The albeda of this type is between 0.03 – 0.1 which indicates that this type of asteroid is extremely dark.
S-type – This type of asteroid includes about 17% of the known asteroids in the solar system. Its surface is composed of nickel-iron with iron-silicates, and magnesium-silicates. The albedo of this type is lighter, at around 0.1 – 0.22.
M- Type – The last of the three main types includes the remaining percentage of known asteroids in the solar system. It is usually composed of pure nickel-iron with an albedo of 0.1-0.18 which is very similar to the S-type.
In addition to the three main types, there also exist multiple subtypes and other types depending on the classification system that is used. The two most widely used systems are the Tholen classification system and the SMASS classification system.
The second classification of asteroids is dependant on the location of the asteroid in the solar system. The three location classifications are the main belt, the near-Earth asteroids, and the Trojans.
Main belt asteroids – These include the majority of asteroids in the solar system and are between the planets of Mars and Jupiter. It is estimated that over one million asteroids in the belt are large than one kilometer while millions more exist that are much smaller.
Near-Earth asteroids – This type is aptly names as these asteroids have orbits which approach the Earth and are outside the main asteroid belt. About 7,000 asteroids have been discovered or are known and between 500 and 1,000 of these asteroids have diameters over a kilometer.
Trojans – Asteroids that are located in specific areas of Jupiter’s orbit. Although few have been discovered, it is estimated that millions could be in these locations around Jupiter’s orbit.
Asteroids can be classified in two ways, by the composition and by the location. The spectra and albedo are used to classify the surface composition of asteroids but the exact composition of the entire asteroid remains unknown as the core or lower layers could be different. While most asteroids are located in the main belt between Mars and Jupiter, some are located in orbit around Jupiter or are closer to Earth. This classification system isn’t perfect though as scientists still don’t exactly know how accurate the spectra is in determining the exact composition and no one can agree on a better system.