Asteroid belt
From Wikipedia, the free encyclopedia
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. 98.5% of all numbered minor planets lie in this region.[1]
Sometimes, the term main belt is used to refer only to a 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 AU, and at eccentricities less than roughly 0.33, along with inclinations below about 20°. This "core" region is marked in red in the diagrams below, and contains approximately 93.4% of all numbered minor planets.[1] Several prominent Kirkwood gaps are sometimes used to divide this region into three or four sections.
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[edit] Origin
The currently accepted theory of planetary formation is the nebular hypothesis. During the first few million years of the solar system's history, planets formed by the accretion of smaller planetesimals. Low energy collisions allowed these planetismals to adhere to each other through mutual gravitational attraction. Repeated collisions led to the familiar rocky planets and to the gas giants.
In regions where the average velocity of the collisions is too high, the shattering of planetesimals tends to dominate over accretion, preventing the formation of planet-sized bodies. 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) too strongly perturbed to form a planet. Instead the planetesimals continue to orbit the Sun as before. The inner border of the main belt, at a radius 2.06 AU, is determined by the 4:1 orbital resonance with Jupiter, which sends any bodies straying there onto unstable orbits. Most bodies formed inside the radius 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.
The asteroid belt can be considered a relic of the primitive Solar System. However, it has also been affected by many subsequent processes, such as internal heating, impact melting, and space weathering. Hence, the asteroids themselves are not pristine. By contrast, the objects in the outer Kuiper belt are believed to have experienced much less change since the Solar System's formation.
The current asteroid belt is believed to be only a small fraction (by mass) of the primordial asteroid belt. Based on computer simulations, the original asteroid belt may have contained the equivalent of an Earth mass. Primarily because of gravitational perturbations, most of this material was ejected from the belt within a period of a million years of formation, leaving behind less than 0.1% of the original mass.[2]
An early hypothesis, which has since fallen into disfavor, was that the asteroids in the asteroid belt are the remnants of a destroyed planet called Phaeton. There are two key problems with this hypothesis. One is the large amount of energy which would be required to achieve this kind of effect. The other is the low combined mass of the current asteroid belt, which has less mass than Earth's moon.
The region of the asteroid belt also contains some main-belt comets. Similar comets may have been the source of much of the Earth's oceans.[3]
[edit] 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.
About 220 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, which is about 1000 km across. The total mass of the Asteroid belt is estimated to be 3.0-3.6×1021 kilograms,[4][5] which is 4% of the Earth's Moon. Of that total mass, one-third is accounted for by Ceres alone.
The belt consists primarily of two categories of asteroids. In the middle and outer belt, carbon-rich asteroids predominate.[6] These C-type (carbonaceous) asteroids include over 75% of the visible asteroids. They are redder in hue 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 nearer Mars, within 2.5 A.U. of the Sun, S-type (silicate) chondrite asteroids are more common.[6][7] 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.
At certain radii from the Sun, the main belt contains gaps where relatively few asteroids are to be found. At these radii, the mean orbital period of an asteroid forms an integer fraction with the orbital period of Jupiter. This results in mean-motion resonance with the gas giant that is sufficient to perturb an asteroid to new orbital elements. In effect, the asteroids in these gaps are randomly nudged into different orbits.
These empty rings in the asteroid belt are known as Kirkwood gaps. The main ones occur at the 3:1, 5:2, 7:3 and 2:1 mean-motion resonances with Jupiter. Thus an asteroid in the 3:1 Kirkwood gap would orbit the Sun three times for each Jovian orbit. Weaker resonances occur at other locations, producing narrow gaps at those points. (For example, a 8:3 resonance for asteroids with a semi-major axis of 2.71 A.U.)[8]
Approximately a third of the asteroids in the main belt are members of an asteroid family. These are asteroids that share similar orbital elements, such as semimajor axis, eccentricity, and orbital inclination as well as similar spectral features, all of which indicate a common origin in the breakup of a larger body. Graphical displays of these elements, for members of the main belt, show concentrations indicating the presence of an asteroid family. There are about 20–30 associations that are almost certainly asteroid families, and likely have a common origin. Additional groupings have been found but these are less certain. Asteroid families can be confirmed when the members display common spectral features.
The high population of the main belt makes for a very active environment, where collisions between asteroids occur frequently (on astronomical time scales). A collision may fragment an asteroid in numerous small pieces (leading to the formation of a new asteroid family), or may glue two asteroids together if it occurs at low relative speeds. After five billion years, the current Asteroid belt population bears little resemblance to the original one.
[edit] In fiction and film
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. One of the best-known examples of this is the Hoth system in Star Wars: The Empire Strikes Back. Proto-planets in the process of formation and planetary rings may look like that, but the Sun's asteroid belt does not. (The asteroid belt in the HD 69830 system may, however.) 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 rendez-vous missions have elaborate targeting procedures.
The inaccurate image of an overcrowded Asteroid Belt is especially frequent in science fiction films, apparently because it makes for dramatic visual images which the true nearly empty space does not provide. (The movie 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 science fiction theme since the late 1940s—are more often scientifically accurate. In Ben Bova's Asteroid Wars series, for example, men live in the asteroid belt and mine the asteroids for metals.
[edit] See also
- Asteroid
- Asteroid family
- Main-belt comet
- Debris disk
- Minor planet
- Centaur
- Kirkwood Gap
- Colonization of the asteroids
- Trojan asteroid
[edit] References
- ^ a b This value is obtained by a simple count up of all asteroids in that region using data for 120437 numbered minor planets from the Minor Planet Center orbit database, dated 8 Feb 2006.
- ^ Petit, J.-M.; Morbidelli, A.; Chambers, J. (2001). "The Primordial Excitation and Clearing of the Asteroid Belt" (PDF). Icarus 153: 338-347. Retrieved on 2007-03-22.
- ^ Berardelli, Phil. "Main-Belt Comets May Have Been Source Of Earths Water", Space Daily, March 23, 2006. Retrieved on 2007-03-22.
- ^ Krasinsky, G. A.; Pitjeva, E. V.; Vasilyev, M. V.; Yagudina, E. I. (July 2002). "Hidden Mass in the Asteroid Belt". Icarus 158 (1): 98-105. DOI:10.1006/icar.2002.6837.
- ^ Pitjeva, E. V. (2005). "High-Precision Ephemerides of Planets—EPM and Determination of Some Astronomical Constants" (PDF). Solar System Research 39 (3): 176. DOI:10.1007/s11208-005-0033-2.
- ^ a b Wiegert, P.; Balam, D.; Moss, A.; Veillet, C.; Connors, M.; Shelton, I. (2007). "Evidence for a Color Dependence in the Size Distribution of Main-Belt Asteroids". The Astronomical Journal 133: 1609–1614. Retrieved on 2007-03-27.
- ^ Clark, B. E. (1996). "New News and the Competing Views of Asteroid Belt Geology". Lunar and Planetary Science 27: 225-226. Retrieved on 2007-03-27.
- ^ Ferraz-Mello, S. (June 14-18, 1993). "Kirkwood Gaps and Resonant Groups". Proceedings of the 160th International Astronomical Union: 175-188, Belgirate, Italy: Kluwer Academic Publishers. Retrieved on 2007-03-28.
[edit] External links
- Plots of eccentricity vs. semi-major axis and inclination vs. semi-major axis at Asteroid Dynamic Site
Vulcanoids · Near-Earth asteroids · Main belt · Jupiter Trojans · Centaurs · Damocloids · Comets · Trans-Neptunians (Kuiper belt • Scattered disc • Oort cloud)
For other objects and regions, see Asteroid groups and families, Binary asteroids, Asteroid moons and the Solar System.
For a complete listing, see List of asteroids. See also Pronunciation of asteroid names and Meanings of asteroid names.
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