Asteroids, the larger of which are sometimes termed planetoids, are in essence the remnants of the many collisions that took place during the formation of the solar system we see today. These rocky/ice and metallic bodies, mainly irregular in shape, are those that orbit the Sun as the planets do, but never became planets themselves. There are many millions of individual asteroids in our solar system.
The majority of asteroids are found orbiting the Sun in the 'asteroid belt' that lies between Mars and Jupiter, but there are other regions or groupings of asteroids that are also significant in number, including 'near-Earth asteroids' and other groups that share orbits with the other major planets.
There are larger asteroid types that share the same orbital path as Jupiter, the 'Jupiter Trojans', although the larger of these Trojans, some of which share orbits with other planets, and another class of minor solar system body called 'Centaurs' also have some of the characteristics of comets, and so are distinguished and defined further in their own groups.
The term asteroid has generally now been limited to those in the inner solar system, out to the orbit of Jupiter, and in astronomy circles the actual preferred term, particularly for the larger asteroids, Trojans and Centaurs, is 'minor planets'.
Astronomers are able to determine the composition of asteroids using three main sources:
1. 'Albedo'. The reflective brightness of the surface.
2. 'Surface spectrum' or 'Spectra'. Individual elements emit specific and unique electromagnetic radiation. Spectroscopy can determine the surface composition from these emissions.
3. 'Density'. Some asteroids have moons themselves, other asteroids or smaller bodies that orbit them. By studying these orbits, an asteroid's density can be determined by calculating the mass that would be required to hold these bodies in its orbit.
Following the development of these means of determining an asteroid's composition, a system developed in 1975 by Clark R. Chapman, David Morrison and Ben Zellner, asteroids are now classified mainly into one of the following classifications:
C-type. Dark, carbon-rich asteroids. 75% of known asteroids.
S-type. Stony asteroids mainly consisting of silicate minerals. 17% of known asteroids.
X-type. Consisting mainly of metallic asteroids, but also any that do not fit into 'C' or 'S' categories.
Many asteroids also contain organic compounds, and even long-chain amino-acids. It is widely believed that asteroids, along with comets that also contain organic compounds, impacting our planet during its long history, may well have seeded the young Earth with the building blocks of life.
This method and system of classification has since been refined and developed further, with many other types of asteroid being catalogued. It should also be noted that the percentages given here are not necessarily truly reliable as a definite ratio of all of the types of asteroid in the solar system. The reason for this is that some kinds of asteroid are more easily detected, hence the percentages can be biased purely by the that fact.
The composition of asteroids does vary considerably though, and new compositions are being discovered all the time. Their composition is often still not completely understood. Some seem to have rocky cores, often covered by an icy crust, and others still, have what appear to be metallic, particularly nickel-iron cores with a basaltic crust. It is believed that many asteroids can be likened simply to piles of rubble that are held together loosely by gravity. The scattered remains from the collisions of larger asteroids.
Asteroids are further grouped according to their orbital characteristics. A group will be based on those that share a loosely linked orbital likeness, and further grouped into 'families', usually a group of smaller asteroids that remain closely together, probably the remnants of one larger asteroid that has been broken up through collisions in the past.
Rarer groups of asteroids are termed 'quasi-satellites' or 'horseshoe objects'. These are asteroids that normally share an orbit with a planet, but for periods of decades or sometimes hundreds of years can also become temporary satellites of those planets before returning to their original and individual orbit of the Sun.