Image from John Dollan
Generally any small object that is not a satellite of a planet, and is too small to be considered a planet. Asteroids larger than 50km are classified as Planetoids under the NoLWoCS system.
The Asteroidal Class is the most basic, and the most numerous, of all the classes in the Non Luminary World Classification System (NoLWoCS). There are, of course, even more numerous smaller objects, namely meteoroids and space dust, and these are the ultimate building blocks of any solar system. But only asteroids can be considered worlds in their own right. By NoLWoCS definition, these are worlds from 50 meters to 50 kilometers in diameter. They include the classically separate rocky asteroids and icy comets, mainly because as Man moved out into the Solar System of his birth and investigated these two seemingly disparate bodies, he learned that they often held very similar characteristics.
In a young solar system, gas and dust will come together to form the small meteoroids that, billions of years later, still rain down on all planets. And these meteoroids will come together and form Asteroidal bodies. Most of these, in turn, will continue on to create Planetoids, and eventually full fledged planets. But there are regions where asteroids might survive in their pristine form. The gravitational interactions between giant planets and central suns can conspire to prevent a planet from coalescing, and thus an asteroid belt will be formed, as was one between the orbits of Mars and Jupiter in the Solar System. Further out in space, there will remain the almost untouched reserve of frozen cometary bodies in what is called the Oort Belt. And of course, asteroidal bodies of all types will be nudged into wildly eccentric, often stable orbits that reach throughout a solar system.
According to the NoLWoCS classification as used by the Astrography Institute, an asteroid is a small celestial body of a size range of 5 meters to 50 kilometers, and a planetoid a body from 50 to 1,000 kilometers. However, these terms are often used interchangeably. Asteroids generally cluster together in belts which encircle a star. Barring dust and rock chips, these are the smallest planetary bodies. Despite their small size, these chunks of rock and ice are every bit as varied in their form as are the larger planets.
Asteroids are divided into various classes, each of which is different in nature and value. Two classification systems are used; the NoLWoCS system, and the Old Earth system, mostly derived from Earth-based observations.
Because of the ease with which resources can be extracted and utilized, and habitats created, asteroids are very important in development of new solar systems, and are frequently the first part of a solar system to be settled. Nevertheless, while asteroids in a whole belt are extremely valuable, individual asteroids, because of their small size, are less so, and few belters ever make it rich.
The NoLWoCS classifications
- Carbonic: Almost exclusively of carbon compound construction, resulting in the formation of bodies from pockets of high carbon material around later generation stars choked with heavier elements; common near the galactic core. They may also form in systems where two white dwarfs have spiralled together, and the resulting circumstellar disk coalesces into bodies high in carbon.
- Metallic: more here
- Carbonaceous: More here
- Silicaceous: more here
- Hydronic: Located close to the system's snow-line, these are silicaceous bodies with high instances of subsurface volatiles, typically in the form of water ice. Polar deposits in permanently shadowed regions may also be present.
- Gelidic: Located beyond the snow line of their system, these are bodies with high instances of ice, ranging from water to methane to carbon dioxide, and many other compounds besides, surrounding a core of silicate rock. The smaller bodies may be a nearly homogeneous mixture of ice and rock, due to the lack of a mass great enough to have caused layer differentiation early in the formative period. See also Centaurian Type
- Oortean: More here
- Vulcanian: More here
The Old Earth classification system This overlaps with the NoLWoCS system to a great extent; however note that the OE system makes no distinction between asteroids and planetoids
- C (Carbonaceous) asteroids are the most common in most belts, but are also quite valuable due to the rich composition of elements and building materials they consist of. Rich in carbon, these bodies are the most similar in composition to the star system's primary sun, and thus represent nearly pristine relics from the formation of that solar system. They are also quite rich in water, and can be mined for this item, which is trapped within the rock of the asteroid itself. Space colonies and large stations can make use of the variety of the useful elements - carbon, hydrogen, oxygen, and so on - that can be easily extracted from them.
- S (Silicaceous) asteroids are usually the most numerous in inner belt regions. They have high albedoes (from 15% to 25%) are slightly red in colour and contain metallic elements. Silicaceous asteroids are mainly metallic iron and magnesium-silicates in composition. Quite common, these bodies are sometimes mined, but more often used as bases or refineries for other materials. Many times these are fragments from larger bodies, even to the point of being little more than piles of "rubble", loose aggregates of material that have been drawn back together by gravity after the initial object was broken up.
- M (Metallic) or Nickel-iron asteroids are less common that the preceding two types. They are dense and have very high metallic ratios, and moderate albedo's. Also referred to as the Sideritic Type, these asteroids are composed almost entirely of nickel-iron. They represent a source of great mineral wealth, and are often the sites of mining operations by colonists. Being high-grade sources of metal they are of value to the manufacturing industry; and some large asteroids can be sold to local shipbuilding concerns as slowship and worldship hulls. Because of the relative scarcity of such heavy elements in the pre-solar nebula, these are typically the largest metal rich bodies to be found in the solar system.
- E (Enstatite) are relatively rare asteroids with high albedoes above 40%, they resemble chondrites and have high MgSiO3 (Enstatite) composition.
- D type or Carbonaceous chondrites are the most valuable asteroids of all, due to the large proportion of organic molecules. They generally have a low albedo, reddish colour high clay composition with carbon rich substances (carbon, hydrocarbons and amino acids) and magnetite. Most D types are remote from their parent star, and tend to form in the more outer rather than inner solar system. The inclusion of Ca-Al-Ti minerals found in carbonaceous chondrites suggests these were among the first minerals to condense out of the original protoplanetary nebula.
- A type or Ferromagnesic asteroids are almost pure olivine, resulting from recrystallised melted elements. (Mg,Fe)2SiO4.
- P type are rare forms with very low albedo but spectrally similar to type M, and are high in metallic elements.
- V type asteroids have igneous rock surfaces, and are very rare, Vesta is among the most well-known examples.