Asteroid Belt
The asteroid belt is a region of the solar system falling roughly between the planets Mars and Jupiter where the greatest concentration of asteroid orbits can be found.It is termed the main belt when contrasted with other concentrations of minor planets, since these may also be termed asteroid belts. In this usage, it often refers only to the greatest concentration of bodies with semi-major axes between the 1:4 and 2:1 Kirkwood gaps at 2.06 and 3.27 AU, with eccentricities less than about 0.33, and with inclinations below about 20°. This region is marked in red in the diagrams below, and contains approximately 93.4% of all numbered minor planets. The asteroid belt region of space also contains some main-belt comets which may have been the source of Earth's water.
Origin
A common theory agreed upon by most astronomers, called the nebular theory, is that during the first few million years of the solar system's history, planets formed by accretion of planetesimals. Repeated collisions led to the familiar rocky planets and to the gas giants.
However, if the average velocity of the collisions is too high, the shattering of planetesimals dominates over accretion, and planet-sized bodies cannot form. The region lying between the orbits of Mars and Jupiter contains many strong orbital resonances with Jupiter, and planetesimals in this region were (and continue to be) kicked around too strongly to form a planet.
The planetesimals instead continue to orbit the Sun as before. The inner border of the main belt is determined by the 4:1 orbital resonance with Jupiter at 2.06 AU which sends any bodies straying there onto unstable orbits. Most bodies formed interior of this gap were swept up by Mars (which has an aphelion out at 1.67 AU) or ejected by its gravitational perturbations in the early history of the Solar System.
In this sense the asteroid belt can be considered a relic of the primitive Solar System, but it has been affected by many processes active in later periods, such as internal heating, impact melting, and space weathering. Hence, the asteroids themselves are not particularly pristine. Instead, the objects in the outer Kuiper belt are believed to have experienced much less change since the solar system's formation.
An old theory that is much less favoured these days was that the asteroids in the asteroid belt are the remnants of a destroyed planet.
Asteroid Belt Environment
Despite popular imagery, the asteroid belt is mostly empty. The asteroids are spread over such a large volume that it would be highly improbable to reach an asteroid without aiming carefully. Nonetheless, hundreds of thousands of asteroids are currently known, and the total number ranges in the millions or more, depending on the lower size cutoff that is assumed. A survey in the infrared wavelengths shows that the main belt has 700,000 to 1.7 million asteroids with a diameter of 1 km or more.
Over 200 of the asteroids in the belt are larger than 100 km. The biggest asteroid belt member, and the only dwarf planet found there, is Ceres. The total mass of the Asteroid belt is estimated to be 3.0-3.6X1021 kilograms, which is 4% of the Earth's Moon. Of that total mass, one-third is accounted for by Ceres alone. The eleven largest asteroids contain about half the total mass within the main belt.
The center of mass of the asteroid belt occurs at an orbital radius of 2.8 A.U. The large majority of the asteroids within the main belt have orbital eccentricities of less than 0.4, and an inclination of less than 30°. The orbital distribution of the asteroids peak at an eccentricity of around 0.07 and an inclination of under 4°. Thus while a typical asteroid has a relatively circular orbit and lies near the plane of the ecliptic, some asteroid orbits can be highly eccentric or travel well outside the ecliptic plane.
Sometimes, the term main belt is used to refer only to the more compact "core" region where the greatest concentration of bodies is found. This lies between the strong 4:1 and 2:1 Kirkwood gaps at 2.06 and 3.27 A.U., and at orbital eccentricities less than roughly 0.33, along with orbital inclinations below about 20°. This "core" region contains approximately 93.4% of all numbered minor planets within the Solar System.
The absolute magnitudes of most asteroids are 11-19, with the median at about 16. By contrast, Ceres has a much higher absolute magnitude of 3.32. The temperature of the asteroid belt varies with the distance from the Sun. For dust particles within the belt, typical temperatures range from 200 K (-73°C) at 2.2 A.U. down to 165 K (-108°C) at 3.2 A.U. However, due to rotation, the surface temperature of an asteroid can vary considerably as the sides are alternately exposed to solar radiation and then to the stellar background.
Composition
During the early history of the Solar System, minor planets underwent some degree of melting, allowing elements to be partially or completely segregated by mass. Some of the progenitor bodies may even have undergone periods of explosive volcanism and formed magma oceans. However, because of the relatively small size of these bodies, this period of melting was necessarily brief (compared to the much larger planets), and had generally ended about 4.5 billion years ago.
The current belt consists primarily of two categories of asteroids. In the outer portion of the belt, closer to Jupiter's orbit, carbon-rich asteroids predominate. These C-type (carbonaceous) asteroids include over 75% of the visible asteroids. They are more red in hue than the other asteroid categories and have a very low albedo. Their surface composition is similar to carbonaceous chondrite meteorites. Chemically, their spectra indicate a match with the primordial composition of the early Solar System, with the lighter elements and volatiles (e.g. ices) removed.
Toward the inner portion of the belt, within 2.5 A.U. of the Sun, S-type (silicate) chondrite asteroids are more common. The spectra of their surfaces reveal the presence of silicates as well as some metal, but no significant carbonaceous compounds. This indicates that they are made of materials that have been significantly modified from the primordial Solar System composition. The expected mechanism was melting early in their history, which caused mass differentiation. They have a relatively high albedo, and form about 17% of the total asteroid population.
A third category of asteroids, forming about 10% of the total population, is the M-type. These have a spectrum that resembles metallic iron-nickel, with a white or slightly red appearance and no absorption features in the spectrum. M-type asteroids are believed to be formed from the metallic cores of differentiated progenitor bodies that were disrupted through collision. However, there are also some silicate compounds that can produce a similar appearance. Thus, the large M-type asteroid 22 Kalliope does not appear to be primarily composed of metal. Within the main belt, the number distribution of M-type asteroids peaks at a semi-major axis of about 2.7 A.U.
Collisions
Measurements of the rotation periods of large asteroids in the main belt show that there is a lower limit to the duration. No asteroid with a diameter larger than 100 metres has a period of rotation of less than once every 2.2 hours. For asteroids rotating faster than this rate, the centripetal force at the surface is greater than the force of gravity, so any loose surface material would become scattered. Yet a solid object should be able to rotate much more rapidly. This suggests that the majority of asteroids with a diameter over 100 metres are actually rubble piles formed through collisions between asteroids.
The high population of the main belt makes for a very active environment, where collisions between asteroids occur frequently (on astronomical time scales). Collisions between main belt bodies with a mean radius of 10-km are expected to occur about once every 10 million years. A collision may fragment an asteroid in numerous small pieces (leading to the formation of a new asteroid family), and some of the debris from these collisions can form meteoroids that enter the Earth's atmosphere. Collisions that occur at low relative speeds may even join two asteroids together. After more than 4 billion years of this process, the members of the asteroid belt now bear little resemblance to the original population.
In addition to the asteroid bodies, the main belt also contains bands of dust with particle radii of up to a few hundred micrometres. This fine material is produced, at least in part, from collisions between asteroids, or by the impact of micrometeorites upon the asteroids. Due to Poynting-Robertson drag, the pressure of solar radiation causes this dust to slowly spiral inward toward the Sun.
The combination of fine asteroid dust, as well as ejected cometary material, produces the zodiacal light. This faint auroral glow can be viewed at night extending from the direction of the Sun along the plane of the ecliptic. Particles that produce the visible zodiacal light average about 40 urm in radius. The typical lifetimes of such particles is on the order of 700,000 years. Thus, in order to maintain the bands of dust, new particles must be steadily produced within the asteroid belt.
Media
Asteroid belts are a staple of science fiction stories less concerned with realism than with drama, since they are frequently portrayed as being so dense that adventurous measures must be taken to avoid an impact. Proto-planets in the process of formation and planetary rings may look like that, but asteroid belts do not.
In reality, the asteroids are spread over such a high volume that it would be highly improbable even to pass close to a random asteroid. For example, the numerous space probes sent to the outer solar system, just across the main asteroid belt, have never had any problems, and asteroid rendezvous missions have elaborate targeting procedures.
Film
The inaccurate image of an overcowded Asteroid Belt is esepecially frequent in science fiction films, apparently because it makes for dramatic visual images which the true nearly empty space does not provide.
The film 2001: A Space Odyssey is unusual in that it does portray realistically the ship's "encounter" with a lone asteroid pair.
On the other hand, written depictions of human encounters with asteroids, their mining and their colonization - an increasingly frequent SF theme since the late 1940s - are more often scientifically accurate.
Asteroid belt estimated to contain over 1 million asteroids with diameter exceeding one kilometer.
The Ceres Asteroid is the largest in the asteroid belt. Asteroids contained within this belt generally orbit the Sun in a stable and essentially circular trajectory. However, a portion of asteroids subject to the gravity of Jupiter undergo a changer in orbit toward the center of the solar system. Asteroids which in this manner draw close to the Earth's orbit are referred to as near Earth objects (NEO), and are considered to pose the danger of potential impact with Earth.
Solar System's Young Twin Has Two Asteroid Belts PhysOrg - October 28, 2008
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