When Sir William Herschel first used the word asteroid in 1802 he used it to describe an object with a point like appearance like a star which moved through the heavens like a planet. When this description became popular in the second half of the nineteenth century itcame to describe a wide range of objects which had similar observational characteristics. .
Although Herschel’s definition is useful to an experimentalist it has come to mean that the word asteroid is used to describe a wide range of small Solar System bodies that lack structure to the observer. As these objects were first found in the region of space between the orbits of Mars and Jupiter this area of space is often known as the Asteroid Belt. Asteroids that are found within the Asteroid Belt are often called Classic Asteroids.
Over time many asteroids have been found in the Solar System and it is customary to name them by their location. Trojan Asteroids straddle the orbit of Jupiter at points of gravitational equilibrium 60 degrees ahead and behind the orbit of the major planet. Neptune Trojans similarly straddle the orbit of Neptune. Centaurs are small bodies that lie between the orbits of Jupiter and Neptune. Trans Neptune objects lie in orbits beyond that of Neptune principally in the Kuiper belt or the Scattered Disc.
Rather than allow the continued use of vague terminology the International Astronomical Union tidied up the definition of planetary objects in 2006. Rather than use the word asteroid to describe a small astronomical object within the Solar System that is not a comet, meteoroid, planet or natural satellite it introduced the phrase Small Solar System Body. For clarification, a comet is a point like object which has an observable tail due to the sublimation of material, a meteroid is an interplanetary object less than ten metres in diameter and a planet is an object in orbit around the sun that is large enough to be spherical in form through hydrostatic equilibrium and large enough to sweep smaller objects from its orbit. From 2006 the word asteroid should lonely be used in the classic sense to describe SSSBs found in the Asteroid Belt.
The classic asteroids can be subdivided by location and by composition. Most asteroids are found to orbit the sun in a cluster. The clusters are often referred to as Asteroid Families. Most asteroids within a family are believed to have once been part of a larger solar system body that fragmented. The most famous class of asteroids by location are the near Earth orbits. These are of immense astronomical interest because the class could contain objects that are on collision course with Earth. Another class of asteroids are known as the Apollo asteroids because they are located in gravitational equilibrium close to the moon.
Asteroids are commonly thought of as large rocks that do not support an atmosphere. This view reinforces that of Herschell. Observationally an object that was not solid rock would develop a vapour trail on approach to the son and not fulfil his empirical definition. Spectroscopic methods enable astronomers to make a more detailed classification based upon composition. They use the terms C, S and M asteroids then further subdivide the spectrographic detail . A C-type is carbon rich, an S-type in silicate rich whle an M-type is rich in metals.
Asteroids are an abundant class of Small Solar System Body. Unfortunately our knowledge of them is limited, partly because we have treated them as a residual class for so long.
