In general terms, the asteroid belt is defined as the relatively thickly populated region approximately between the orbits of the planets Mars and Jupiter. Also known as the main-asteroid belt to differentiate between it and other known asteroid populations in the solar system, such as the near-Earth and Trojan asteroids, the area is populated by millions of oddly shaped objects often referred to as “minor planets”, or “proto-planets” in the case of the largest objects, Ceres, Vesta, Pallas, and Hygiea.
Roughly 50% of the total mass of the asteroid belt, which amounts to only 4% of that of the Moon, is accounted for by these four largest objects, with Ceres having a diameter of 950kms, and the other three Vesta, Pallas and Hygiea having diameters of less than 650kms each. The rest of the mass is taken up by bodies that get progressively smaller, with a significant percentage being no bigger than dust grains.
To put this in some sort of perspective, the area is so thinly populated that despite its vast extent several space craft had crossed it without even running into a dust particle, much less a rock several kms in diameter. However, collisions between larger objects are known to occur since many collisional families have been identified in which the members have identical chemical compositions and closely matching orbits.
Classification of asteroids is done by virtue of their spectra, and three main groups have emerged; C-type objects, which are predominantly carbonaceous, S-type objects that are mainly composed of silicates, and M-type bodies that are rich in various metals.
History of the Asteroid Belt
However, this method only proved effective when the investigator allowed for the gap between Mars and Jupiter and in efforts to resolve this difficulty, the German astronomer, Baron Franz Xaver von Zach formed the “United Astronomical Society” in 1800. This informal gathering had some notable members however, and among them were Heinrich Olbers, Charles Messier, William Herschel, and even the then Astronomer Royal, Nevil Maskelyne.
The object of the Society was to look for a planet in the position the Titius-Bode Law said it must be, and to this end, each member of the group was assigned a 150 section of the sky as his own hunting ground. However, it fell to a non-member of the group, who had by then become known as the “Celestial Police”, to make the first discovery.
This person was the Chair of Astronomy at the prestigious University of Palermo, one Giuseppe Piazzi, who in 1881 found a tiny, fast moving object at exactly the position predicted by the Titius-Bode Law, an object he promptly named after the Roman harvest goddess, Ceres. Piazzi first concluded that the object was cometary in nature, but subsequent observations revealed a lack of a coma, which suggested that the object was rather a planet, or even a star.
Nevertheless, in March of 1882, Heinrich Olbers discovered a second body in the same area, and not to be outdone, he promptly named it Pallas. However, even under the highest magnifications neither Ceres or Pallas could be resolved into planetary discs like the other planets, and despite their rapid movement across the sky the two objects appeared to be stars, a circumstance that prompted William Herschel to propose a unique category for the two objects in 1882- “asteroids”, after the Greek word “asteroeides”, which means “ star-like”, or in some interpretations, “similar to stars”. Herschel was known as a meticulous observer, and once he had completed a long series of observations of the two bodies he wrote the following statement:
“Neither the appellation of planets, nor that of comets, can with any propriety of language be given to these two stars … They resemble small stars so much as hardly to be distinguished from them. From this, their asteroidal appearance, if I take my name, and call them Asteroids; reserving for myself however the liberty of changing that name, if another, more expressive of their nature, should occur.”
It is not known who coined the phrase “asteroid belt”, but its first use in the English language occurs in the English translation by E.C. Otté, of Alexander von Humboldt’s seminal work, “Cosmos”. In translation, the following sentence appears: “[…] and the regular appearance, about the 13th of November and the 11th of August, of shooting stars, which probably form part of a belt of asteroids intersecting the Earth’s orbit and moving with planetary velocity”.
Other early references to “asteroid belt” appear in “A Guide to the Knowledge of the Heavens” by Robert James Mann in the following sentence: “The orbits of the asteroids are placed in a wide belt of space, extending between the extremes of […]”.
Nevertheless, by late 1886, a total of 100 asteroids were known, but by 1891 the introduction by Max Wolf of astrophotography had vastly increased the rate of discovery; in 1921 there were more than 1,000 known asteroids, by 1981 this had increased to 10,000, and by the year 2000 more than 100,000 asteroids had been cataloged. Modern detection methods employ automated telescopes and CCD devices, which are finding and listing new discoveries at the rate of dozens every day.
Formation of the Asteroid Belt
The conventional view regarding the formation of the asteroid belt holds that the belt formed out of the same primordial material that formed the rest of the solar system. However, instead of forming into proto-planets the extreme tidal effects of both Mars and Jupiter prevented the accretion of matter into viable planets because of the high orbital velocities imparted to them by their combined gravitational effects. In effect, this meant that the resulting collisions between the forming proto-planets were too energetic to allow for accretion, which in turn meant that the proto-planets were broken up faster than they formed.
This resulted in the loss of around 99.9% of the collective mass of the asteroid belt within the first 100 million years or so of the solar system’s evolution, which is thought to be origin of the several thousand fragments that bombarded the inner solar system during the period known as the Great Bombardment that ended about 3 billion years ago.
However, the current state of the asteroid belt is by no means stable- whenever the orbits of asteroids around the Sun enter into a state of resonance with that of Jupiter their orbits are severely disrupted, and at those orbital distances asteroids are swept out of their usual orbits in large numbers to form Kirkwood-gaps, which are similar to those in the rings of Saturn, although these separations, or divisions, are not as pronounced. Below is a plot of the main Kirkwood-gaps in the main asteroid belt.
Composition of Asteroids
Comprising in excess of 75% of the total population, C-Type, or carbonaceous asteroids predominate in the outer reaches of the belt. Having a low reflective index, these objects are generally red in color, and have the same chemical make-up as the material that occurred in the early solar system. However, these objects do not have the lighter elements and volatiles present due to the effects of solar radiation.
S-Type, or asteroids that are rich in various silicates, mostly occur within a radius of 2.5 AU, and although they are known to contain varying amounts of metals and silicates they do not contain noteworthy amounts of carbonaceous material. This suggests that these objects have been modified, or altered from their primordial state, most likely through the action of extreme heat. Having relatively high reflective indices, this type of asteroid accounts for roughly 17% of the collective asteroid population.
M-Type, or metal rich asteroids that account for about 10% of the total population, are concentrated at a distance of roughly 2.7 AU and composed mainly of iron-nickel alloys. However, one notable exception to this rule, 22 Kaliope, does not seem to contain notable amounts of metal at all, which goes some way toward casting doubt on the widely held belief that M-Type asteroids are the result of collisions between large, differentiated bodies that broke apart as the result of colliding with each other. It thus seems likely that the M-type asteroids are a group that does not fit the pattern of either the C-, or S-Type asteroids.
The Missing Basalt Asteroids
Given the large diameters of some asteroids, such as Vesta for instance, it would be reasonable to assume that at least a significant percentage of asteroids would contain basalt or olivine as the result of having formed crusts and mantles. However, it turns out that instead of around 50% of all asteroids containing basalt or olivine as expected, there are hardly any, and some estimates put the percentage of “missing” basalt as high as 99%.
Up to 2001, it was thought that all of the basalt observed in the asteroid belt originated from Vesta, hence the designation” V-Type” asteroids, but the discovery of 1459 Magnya revealed a type of basalt that differed from that found on Vesta, which means that 1459 Magnya must have formed independently of Vesta and under different circumstances. To confuse matters further, two more basaltic asteroids, 7472 Kumakiri, and (10537) 1991 RY16, were discovered in the outer reaches of the belt that proved to contain basalt that could not have formed on Vesta. To date, these are the only two basaltic asteroids ever discovered in the outer belt, and the mystery of the missing basalt remains unresolved.