Ultramontanism

Terminology

The belt of asteroids is sometimes specified “girdles asteroids principal” when it is a question of distinguishing it from other similar belts of the solar system (like, for example, the Ceinture of Kuiper).

All the asteroids of the belt are small bodies of the solar system, except for Cérès, considered as a dwarf Planet.

Characteristics

Number

The belt of asteroids contains several hundreds of thousands of Astéroïde S known, and probably several million, of a size going of the grain of dust to the planetoid of a few hundred kilometers in diameter.

At the end of 2005, more: 100000 asteroids carrying a number (on approximately: 120000) belonged to the belt of asteroids. : 200000 others were listed, but were not numbered. It was estimated that more: 500000 were detectable visually with the means of the time.

In 2007, one knew more than 200 asteroids of more than 100 km while a systematic study of the belt in the infra-reds estimated between: 700000 and: 1700000 the number of asteroids larger than one km. The absolute Magnitude median of these asteroids is of approximately 16^,. The four larger objects, Cérès, Vesta, Pallas and Hygée, include/understand almost half of the total mass of the belt; Cérès counts for a third with him all seul^,. It orbits with: 2.8 UA of the Sun, which is also the distance from the Center of mass of the belt of asteroids. Vesta has in addition an absolute magnitude higher than the other asteroids, with approximately of: 3.20.

Composition

During the beginning of the solar system, the asteroids underwent a certain degree of fusion, making it possible their elements to be partially or completely differentiated by mass. Certain initial bodies could have known one period of explosive Volcanisme and the oceans of Magma. However, because of their small size, this period of fusion was brêve (compared to planets) and generally finished there is: 4.5 billion years after having lasted between a few tens to a hundred of million of years.

The belt of asteroids includes/understands mainly three categories of asteroids. In the external part, close to the Jupiter orbit, the asteroids rich in Carbone prevail. These asteroids of standard C include more than 75% of all the visible asteroids. They are redder than the other asteroids and has a very weak Albédo. Their composition of surface is similar to the carbonaceous Météorite S Chondrite S. From the chemical point of view, their spectrum indicates a composition similar to that of the primitive solar system, without the light and volatile elements (like the ices).

Towards the portion belt interns, in the neighborhoods of: 2.5 UA of the Sun, the asteroids of standard S (Silicate S) are more current. The spectrum of their surface reveals the presence of silicates and some metals, but any significant carbonaceous compound. They thus consist of materials deeply modified since the beginnings of the solar system. Their mechanism of formation supposed includes a phase of fusion which caused a differentiation of mass. They have a relatively high albedo and form 17% of the total.

A third category, gathering 10% of the total, is that of the asteroids of standard M (rich in metals). Their spectrum resembles that of an alloy Fer - Nickel, with a white appearance or slightly red and any characteristic of absorption. It is thought that certain asteroids of the type M were formed in the metal cores of larger objects which were split up by collision. However, some compounds silicates can produce a similar appearance; for example, the asteroid of the type M Kalliope does not seem to be made up mainly of metal. Inside the belt, the distribution of the asteroids of the type M culminates with: 2.7 UA of the Sun. One is unaware of if all the asteroids of the type M have a similar composition or if it is about a label gathering several varieties not belonging to the classes C and S.

The belt of asteroids contains only few asteroids of standard V, Basalt ic, a fact which one does not know the reason. The theories of formation of the asteroids predict that objects of the size of Vesta or larger should form crusts and coats, which would be mainly made up of rock basaltic; more half of the asteroids should then be made up of basalt or Olivine. The observations suggest that 99% of predicted basalt do not exist. Until 2001, one thought that the majority of the basaltic bodies discovered in the belt came from Vesta (from where their name of the type V). However, the discovery of (1459) Magnya revealed a chemical composition slightly different from the other basaltic asteroids, suggesting a distinct origin. For a larger asteroid, its rotation imposes more important variations, its surface being alternatively exposed with the solar radiation and the stellar bottom.

Orbits

General information

The great majority of the asteroids of the principal belt have a eccentricity lower than: 0.4 and a Slope lower than 30°. Their orbital distribution is maximum for an eccentricity of approximately: 0.07 and one slope lower than 4°. In a diagrammatic way, a typical asteroid of the principal belt has a relatively circular orbit located close to the plan of the ecliptic , but there exist exceptions.

The term of “principal belt” is sometimes used to indicate the central area exclusively where the concentration in asteroids is largest. It is located between the gaps of Kirkwood 4:1 and 2:1, between: 2.06 and: 3.27 UA and its components have an eccentricity smaller than: 0.33 and one slope lower than 20°. This area contains: 93.4% of all the numbered asteroids of the solar system or because of disturbances or former collisions) were gradually moved towards orbits having an equatorial different radius.

The gaps of Kirkwood appear only in the examination of the distribution of equatorial the radii of the asteroids. In practice, the orbit of those being elliptic, many asteroids cross the distance to the Sun corresponding to the gaps; at one unspecified time, the density of asteroids in the gaps is not significantly different from that of the close areas.

The central area of the belt of asteroids is sometimes subdivided in three zones, based on the most important gaps of Kirkwood. Zone I extends from resonance 4:1 (: 2.06 UA) with resonance 3:1 (: 2.5 UA). Zone II leaves the end of zone I until resonance 5:2 (: 2.82 UA). Zone III begins in extreme cases external from zone II until the gap from resonance 2:1 (: 3.28 UA).

The principal belt can also be divided into belt interns and girdles external. The internal belt is made up of the asteroids orbiting more close to Mars that the gap of Kirkwood of resonance 3:1 (: 2.5 UA) and the external belt of the remainder of the asteroids. Certain authors define these two belts starting from resonance 2:1 (: 3.3 UA). Others push the subdivision by defining the belts interns, average and external.

Collisions

No asteroid larger than 100 m has a Period of rotation lower than: 2.2 h. On a more quickly turning asteroid, any material of surface slightly fixed would be ejected. However, a solid object would be able to turn more quickly without breaking. This suggests that the majority of the asteroids of more than 100 m are remains stackings formed by accumulation after collisions between asteroids.

Because of the great number of objects which it contains, the belt of asteroids is a very active environment and the collisions frequently occur there (on an astronomical scale). It is estimated that a collision between two bodies of a diameter higher than 10 km produced there all the 10 million years. A collision can split up an asteroid in several smaller pieces (and if required to train a new family) and some of these remains can form Météoroïdes.

Conversely, the collisions which occur at low relative speeds can amalgamate two asteroids.

The belt of asteroids contains bands of dust (particles of less than one hundredth of µm) coming partly from collisions between asteroids and impacts of micrométéorites. Because of the Effect Poynting-Robertson, the pressure of the solar Rayonnement slowly leads this dust to spiraler towards the Sun.

The combination of this dust and material ejected by the Comet S causes the Lumière zodiacale. This gleam can be perceived the night in the direction of the Sun along the plan of the ecliptic . The particles which produce it measure on average 40 µm and have one lifespan about: 700000 years. In order to maintain the bands of dust, of new particles must be produced regularly in the belt of asteroids. Smaller associations of asteroids are called groups.

By order of equatorial increasing radius, the principal families are those of Flore, Eunomie, Koronis, Éos and THEMIS.

Vesta is the largest asteroid to belong to a family. The Famille of Vesta would have been formed at the time of an impact having formed a crater on the asteroid. The Météorite HED would come from this impact.

Three band of principal dust were observed in the belt of asteroids, sharing a slope similar to the family of Éos, Koronis and THEMIS and could be to them associated.

Periphery

The Famille of Hungaria extends on the edge interns belt, between: 1.78 and: 2.0 UA. It is named according to its principal member, (434) Hungaria, and contains at least 52 bodies. The family of Hungaria is separated from the remainder of the belt by the Lacune of Kirkwood 4:1 and its orbits have a strong slope. Certain members of this group cross the orbit of Mars and it is possible that gravitational disturbances of this planet reduce the total population of it.

The Famille of Phocée is another group of the internal part of the belt of asteroids whose members present a strong slope. It is made up mainly of asteroids of the type S, while the family close to Hungaria includes some asteroids of the type E. It orbits between: 2.25 and: 2.5 UA.

The group of Cybèle occupies the external part of the principal belt, between: 3.3 and: 3.5 UA, with a resonance 7:5 with Jupiter. The Family of Hilda orbit between: 3.5 and: 4.2 UA on relatively circular orbits and an orbital resonance stable 3:2 with Jupiter.

There are relatively few asteroids beyond: 4.2 UA until the Jupiter orbit. The groupings of following asteroids are the two groups of Trojan asteroids, but they are not regarded as members of the belt of asteroids.

Recent families

Certain families of asteroids were formed recently from the astronomical point of view. The Groupe of Karin seems to be formed there is: 5.7 million years following a collision with an asteroid of 16 km ray. The family of Veritas was formed there is: 8.3 million years and evidence of this event took the shape of an interplanetary dust recovered in oceanic sediments.

The group of Datura east is seemed it formed 450 million years ago per collision. This estimate is based on the probability that its members have their current orbit rather than a physical proof. It could have contributed to dust zodiacale. Other recent groups, as that of Iannini (between 1 to 5 million years) could have contributed to this dust.

Origin

Formation

The majority of the scientists consider that the belt of asteroids is made up of residues of the primitive solar system which never made of planet.

In the beginning, it had been advanced that the belt would come from the fragmentation of a planet (named Phaéton). This assumption fell in disuse because of a certain number of problems. The first relates to enormous energy necessary. Another is the low total mass of the belt, which is only one fraction of that of the Moon. Lastly, the differences in chemical composition between the asteroids are difficult to explain if all come from the same body.

It is thought that the formation of planets follows a process similar to the assumption of the solar Nébuleuse, which supposes that a cloud of dust and interplanetary gas crumbled under the influence of its own garvity to form a disc in rotation which condensed to form the sun and planets. During the first million years of the solar system, a process of Accrétion gradually increased the size of the bodies, until forming various planets.

In the areas where the mean velocity of the collisions was too high, the dislocation of planétésimaux tended to dominate accretion, preventing the formation of sufficiently large bodies. Moreover, the effects of orbital Résonance with Jupiter tend to disturb the small bodies towards other orbits. The area located between Mars and Jupiter contains several strong resonances. Jupiter having migrated towards the Sun following its formation, these resonances swept the belt of asteroids, exciting the population of planétésimaux, increasing their relative speed. The planétésimaux ones of this area were (and continue to be) too disturbed to form a planet. They continue to orbit the Sun independently and to return in collision in an occasional way. The belt of asteroids can be regarded as a relic of the primitive solar system.

Evolution

The current belt of asteroids would not contain that a small portion of the mass of the paramount belt. On the basis of data-processing simulation, this belt would have had a mass équivalement with that of the Earth. Because of gravitational disturbances, the major part of materials were hardly ejected a million of years after their formation, leaving with final only less: 0.1% of the mass of origin. However, they were affected by various later processes like the internal warming (during first tens of million years), the cast iron of their surface after impacts or crumbling by radiation and bombardment of micrométéorites. The asteroids in themselves are thus not undisturbed samples of the primitive solar system. By contrast, the objects of the external Ceinture of Kuiper would have undergone much less transformations.

Orbital resonance 4:1 with Jupiter, towards: 2.06 UA, can be regarded as the limit interns belt. The Jupiter disturbances move the bodies there towards unstable orbits. Moreover, the majority of the bodies which were formed there were ejected per Mars (of which the Aphélie is located at: 1.67 UA) or by gravitational disturbances at the beginning of the solar system. The exceptions include the Famille of Hungaria, of the asteroids located on very tilted orbits and which were thus protected from the disturbances.

Discovered and exploration

Telescopic observations

The first asteroid was discovered by Giuseppe Piazzi on January 1st 1801. Calculation made it possible to reveal that it was about a circulating star on average with 2,8 astronomical units of the Sun. It was named (1) Cérès. Other asteroids were then discovered, (2) Pallas in 1802, (3) Junon in 1804, (4) Vesta in 1807. During about fifty years, these four bodies were regarded as small planets, coming to replace “missing planet” announced by Bode in 1772. Nevertheless, the important differences of orbits and luminosity between these four objects and their positionings compared to planet known as missing gave birth to an intense debate as for their statute.

The discovery of (5) Astrée in 1845 as well as tens of other asteroids located between Mars and Jupiter during the following decade made it possible to put an end to the debate and definitively to establish the existence of a belt of asteroids between the orbits of Mars and Jupiter. The discovery of a new asteroid in the principal belt is a banal event today since one discovered of them on average several tens per day between 1995 and 2005 thanks to programs such as LINEAR, NEAT or Spacewatch. Concerning this research field, even if million discoveries remains to be made, the major discoveries were carried out right now (binary, satellite asteroids of asteroids, asteroids with several satellite asteroids…).

Space exploration

The first Spaceship to have crossed the Belt of asteroids was Pioneer 10 , which entered there the July 16th 1972. At that time, it was not known with certainty if the remains of the belt were going to cause or not damage with the probe. Pioneer 10 however crossed it without damage. Since, the belt of asteroids was crossed by nine other probes: Pioneer 11 , To travel 1 , to travel 2 , Galileo , Cassini , NEAR , Ulysses and New Horizons , without incident. From now on the probability is estimated that a probe meets an asteroid with less one on a billion.

To at the end of 2007, three probes were specifically dedicated to the observation of the asteroids. NEAR and Hayabusa were devoted to asteroids close to the Earth. Only the purpose of Dawn, launched in July 2007, is the belt of asteroids, specifically Vesta and Cérès. If the probe is always operational after having achieved this work, it is envisaged to employ it to continue to explore the belt of asteroids.

See too

Internal bonds

Asteroid
System astéroïdal
List of the principal asteroids of the principal belt classified by mass

External bonds

References

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