Asteroid belt: Difference between revisions
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''For the Velvet Chain album, see [[Asteroid Belt (album)]]''. |
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[[Image:Asteroid Belt.jpg|300px|thumb|Image of the main asteroid belt between the orbits of [[Mars]] and [[Jupiter]].]] |
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The '''asteroid belt''' is a region of the [[solar system]] falling roughly between the [[planet]]s [[Mars (planet)|Mars]] and [[Jupiter (planet)|Jupiter]] where the greatest concentration of [[asteroid]] [[orbit]]s can be found. |
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It is termed the '''main belt''' when contrasted with other concentrations of [[minor planet]]s, since these may also be termed '''asteroid belts'''. |
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In this usage, it often refers only to the greatest concentration of bodies with [[semi-major axis|semi-major axes]] between the 1:4 and 2:1 [[Kirkwood gap]]s at 2.06 and 3.27 [[Astronomical unit|AU]], with [[eccentricity|eccentricities]] less than about 0.33, and with [[inclination]]s below about 30°. This region can be clearly seen in plots of these orbital elements e.g. [http://hamilton.dm.unipi.it/astdys/propsynth/allae.gif here] and [http://hamilton.dm.unipi.it/astdys/propsynth/allai.gif here]. |
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== Origin == |
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A common theory agreed upon by many astronomers is that during the first few million years of the solar system's history, planets formed by accretion of [[planetesimal]]s. Repeated collisions led to the familiar rocky planets and to the [[gas giant]]s. |
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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 resonance]]s 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. 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. |
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== Asteroid belt environment == |
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[[Image:Kirkwood Gaps.png|250px|thumb|Distribution of asteroid semi-major axes in the vicinity of the main belt.]] |
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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. |
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Nonetheless, tens of thousands of asteroids are currently known, and estimates of the total number range in the millions. About 220 of them are larger than 100 [[kilometre|km]]. The biggest asteroid belt member is [[1 Ceres|Ceres]], which is about 1000 km across. The total mass of the Asteroid belt is estimated to be 2.3{{e|21}} kilograms, which is 1/35th that of the Earth's [[Moon]]. And of that total mass, one-third is accounted for by Ceres alone. |
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The high population makes for a very active environment, where collisions between asteroids occur very often (in astronomical terms). 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. |
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Asteroid belts are a staple of [[science fiction]] stories less concerned with realism than with drama, since they are always 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 probe]]s 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. The movie ''[[2001: A Space Odyssey]]'' is unusual in that it does portray realistically the ship's "encounter" with a lone asteroid pair. |
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== The Asteroid belt in fiction and film == |
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See [[Asteroid#Asteroids in fiction and film|Asteroid]] |
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== Extra-solar belts == |
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Belts of dust or debris have also been detected around stars other than the Sun, including the following: |
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{| class="wikitable" |
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!Star |
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!Distance<br />([[Light Year|ly]]) |
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!Orbit<br />([[Astronomical Unit|AU]]) |
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|- |
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|[[Epsilon Eridani]] |
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|style="text-align:right;"| 10.5 |
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|style="text-align:right;"| 35-75 |
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|- |
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|[[Vega]] |
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|style="text-align:right;"| 25 |
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|style="text-align:right;"| 86-200 |
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|- |
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|[[AU Microscopii]] |
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|style="text-align:right;"| 33 |
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|style="text-align:right;"| 210 |
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|- |
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|[[HD 69830]] |
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|style="text-align:right;"| 41 |
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|style="text-align:right;"| <1 |
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|- |
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|[[55 Cancri]] |
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|style="text-align:right;"| 41 |
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|style="text-align:right;"| 27-50 |
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|- |
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|[[HD 139664]] |
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|style="text-align:right;"| 57 |
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|style="text-align:right;"| 60-109 |
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|- |
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|[[HD 53143]] |
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|style="text-align:right;"| 60 |
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|style="text-align:right;"| ? |
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|- |
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|[[Beta Pictoris]] |
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|style="text-align:right;"| 63 |
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|style="text-align:right;"| 25-550 |
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|- |
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|[[Zeta Leporis]] |
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|style="text-align:right;"| 70 |
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|style="text-align:right;"| 2.5-12.2 |
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|- |
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|[[HD 107146]] |
|[[HD 107146]] |
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|style="text-align:right;"| 88 |
|style="text-align:right;"| 88 |
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Revision as of 17:56, 23 January 2006
|HD 107146 |style="text-align:right;"| 88 |style="text-align:right;"| 130 |- |Fomalhaut |style="text-align:right;"| 133 |style="text-align:right;"| 25 |- |HD 12039 |style="text-align:right;"| 137 |style="text-align:right;"| 5 |- |HR 4796 A |style="text-align:right;"| 220 |style="text-align:right;"| 200 |- |HD 141569 |style="text-align:right;"| 320 |style="text-align:right;"| 400 |- |HD 113766 |style="text-align:right;"| 430 |style="text-align:right;"| 0.35-5.8 |}
The orbital distance of the belt is an estimated mean distance or range, based either on direct measurement from imaging or derived from the temperature of the belt. The Earth has an average distance from the Sun of 1 AU.
See also
External links
- Plots of eccentricity vs. semi-major axis and inclination vs. semi-major axis at Asteroid Dynamic Site
