Origin

Most asteroids are believed to be the remnants of the material from the circumstellar disk that coalesced to form the planets. Former theories involving the break-up of a planet between Mars and Jupiter have now been discounted. They therefore represent samples of the primordial solar system, largely unchanged since its formation 4500 million years ago. In addition, there is evidence that some asteroids are related to comets, being either debris ejected from the parent body, or inert comet nuclei.

Size and Composition

The sizes of asteroids range from dust particles to significant bodies up to the largest, Ceres, which is 913 kilometres in diameter. Their composition can be determined by the spectrum of the sunlight that they reflect. There are four main types:

C-Type.These dark, carbonaceous bodies are mainly found in the outer regions of the main asteroid belt. The class includes Carbonaceous Chondrites that have a similar chemical composition as the Sun (without hydrogen or helium) and that are thought to be primitive unprocessed material from the formation of the Solar System. The class may also include old, inactive comet nuclei. With albedos of around 5%, C - type asteroids are dark and difficult to detect optically. They form about 75% of the total population.

S-Type. S-type asteroids tend to be grey in colour, with intermediate albedos. They are composed mainly of silicaceous material, and are commonly known as stony asteroids. Found mainly in the inner regions of the main asteroid belt, S-types form about 15% of the total population.

M-Type. M-type asteroids are metallic in nature with moderate albedos. They make up some 10% of the total asteroid population.

Composite. These bodies display the characteristics of more than one type, and may be the result of coalescence or collision.

Orbital Characteristics

The majority of asteroids are confined to the main asteroid belt, orbiting the sun between Mars and Jupiter. The orbits of these main belt asteroids are generally stable, but mutual interaction or the gravitational influences of Mars or Jupiter can perturb them. However, there are significant groups of asteroids that are found elsewhere, particularly the Aten and Apollo families that have orbits that cross that of the Earth. Another group, the Amors, do not currently have Earth crossing orbits, but orbital evolution will make impacts possible in the future. The estimated numbers of such asteroids with diameters over one kilometre, and those discovered (of any size) up to December 1992, are shown in Table 2.

Discovery rates have increased exponentially since the establishment of projects such as Spacewatch and NEAT and LINEAR.

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