The Asteroidbelt is a dense asteroid area in the solar system between Mars and Jupiter's orbits. According to statistics from 120,437 asteroids that have been numbered, 98.5% of asteroids have been discovered here. Since this is the densest area of ​​asteroids, it is estimated that there are as many as 500,000. This area is therefore called the main belt, usually called the asteroid belt. It is about 2.17-3.*Astronomical units, about 500,000 asteroids are gathered in the space area, forming the asteroid belt. So many asteroids can be condensed in the asteroid belt. In addition to the universal gravity of the sun, Jupiter's universal gravity plays a greater role.

The asteroid belt is formed by a group of star subs (predecessors of planets that are smaller than planets) in the primitive solar nebula. However, due to the influence of Jupiter's gravity, these stars hinder the formation of planets, causing many stars to collide with each other and form many wreckage and debris. The three largest asteroids in the asteroid belt are Zhishen, Huishen and Vesta, with an average diameter of more than 400 kilometers; there is only one dwarf planet in the main belt - Ceres, with a diameter of about 950 kilometers.

The remaining asteroids are smaller, some even the size of dust. The asteroid belt is very thin, and several spacecraft have passed safely without accidents. Asteroids in the main belt are divided into three categories according to their spectrum and main forms: carbonaceous, silicates and metals. In addition, collisions between asteroids may form a family of asteroids with similar orbital characteristics and colors, and these collisions are also the main source of dust that produces ecliptic light.

Discover history

Piazzen, who discovered the first asteroid, Ceres. In 1766, German astronomer J. Titius accidentally discovered a sequence: (n+4)/10, substituting n=0, 3, 6, 12,..., which can accurately give the orbit radius of the known planet at that time. This incident did not attract people's attention at first, and later, the director of the Berlin Observatory, J. Bode published it after learning about it, which is known to the astronomical community. After Uranus was discovered in 1781, it was further confirmed that the formula was valid. Pode proposed that there might be a planet between the orbits of Mars and Jupiter. In 1801, Sicily and G. Plazzi accidentally discovered a small celestial body at 2.77 AU in routine astronomical observations, name it Ceres.

In 1802, astronomer H. Olbere discovered another asteroid in the same region, and then named it Pallas. William Herschel suggested that these celestial bodies were remnants of a planet after it was destroyed. By 1807, the third celestial stars and the fourth veterinary stars were added in the same region. Since these celestial bodies looked similar to stars, William Herschel named asteroid using the Greek root asteroid (satellite) and translated as asteroid in Chinese.

The Napoleonic War ended the first phase of the discovery of the asteroid belt, and the fifth asteroid, Yishen, was not discovered until 1845. Immediately afterwards, the speed of discovery of new asteroids increased rapidly. By mid-1868, there were 100 asteroids discovered. In 1891, Max Wolf introduced astrophotography, which accelerated the discovery of asteroids. In 1923, the number of asteroids was 1,000, 10,000 in 1951, and 100,000 in 1982. Modern asteroid patrol systems use automation equipment to continuously increase the number of asteroids.

After the discovery of the asteroid belt, their orbital elements must be calculated. In 1866, Daniel Kirkwood announced that from the sun, there are blank areas without asteroids at certain distances, and the orbital periods revolving around the sun in these areas have a simple integer ratio to the orbital periods of Jupiter. Kirkwood believes that Jupiter's perturbation caused the asteroid to be removed from these orbits.

In 1918, Japanese astronomer Kiyoji Hirayama noticed that the orbits of some asteroids in the asteroid belt had similar parameters, thus forming a family of asteroids. By the 1970s, a classification system was developed to observe the colors of asteroids. The three most common types were C-type (carbonaceous), S-type (silicate) and M-type (metal). In 2006, astronomers announced that the population of comets was discovered in the asteroid belt, and it was speculated that these comets might be the source of water in the Earth's ocean.

Origin and evolution

In the early stages of the formation of the solar system, due to the common collision of the accretion process, small particles gradually gather to form larger clusters. Once they gather enough mass (the so-called MSI), gravity can be used to attract the surrounding matter. These planets can steadily accumulate mass to become rocky planets or huge asteroid Ida and its satellites. The Galileo probe photographed gas planets. I don’t know when the mystery of the formation of the asteroid belt will be solved. However, more and more astronomers believe that asteroids record information about the early stages of the formation of the solar system. Therefore, the origin of asteroids is an important and inseparable link in the study of the origin of the solar system.

Mainstream viewpoint

Regarding the reason for the formation, a common view is that in the early stages of the formation of the solar system, for some reason, a large planet was not accumulated in this gap between Mars and Jupiter, leaving behind a large number of asteroids.

The currently recognized theory of planet formation is the solar nebula hypothesis, which believes that the materials, dust and gases that constitute the sun and planets in the nebula form a rotating disc due to gravity shrinkage. In the first few million years of the solar system, collisions during the accretion process became viscous, causing small particles to gradually accumulate to form larger clusters, and the size of the particles to stabilize and continue to increase. Once sufficient mass - the so-called MSI - can attract adjacent matter through gravity. These starsons can stably accumulate mass into rocky planets or huge gas planets.

In areas where the average velocity is too high, collision will cause the starson to shatter, inhibit the accumulation of mass, and prevent the generation of celestial bodies of the planet. In areas where the orbital period of the starson is a simple integer ratio to the period of Jupiter, orbital resonance will occur, and the orbits of these starsons will change due to disturbance. In the space between Mars and Jupiter, there are many places where there are strong orbital resonances with Jupiter. When Jupiter moves inward during the formation process, these resonant orbits will also sweep through the asteroid belt, dynamically stimulate the scattered starsons, and increase their relative velocities. Stars are too perturbed in this area (continued until now) and cannot become planets. They can only continue to orbit the sun as before, and the asteroid belt can be regarded as residues of the original solar system.

The asteroid Gaspra and Galileo probe photographed that the mass of the asteroid belt should be only a small part of the original asteroid belt. According to computer simulation, the original mass of the asteroid belt should be comparable to that of the earth. It is mainly due to the disturbance of gravity. During the formation cycle of millions of years, most of the matter is thrown out, and the remaining mass is probably only one thousandth of the original mass.

When the main belt begins to form, a condensate line with a temperature lower than water is formed at 2.7 AU from the sun - a "snow line". The stars formed outside this line can accumulate ice. The main belt comets generated in the asteroid belt are all outside this line and are the main suppliers to the Earth's ocean.

Because about 4 billion years ago, the size and distribution of the asteroid belt had stabilized (relative to the entire solar system), that is, the main belt of the asteroid belt has no significant increase or decrease in size. However, asteroids will still be affected by many subsequent processes, such as internal heating, melting caused by impact, and space weathering caused by cosmic rays and micrometeoroid bombardment. Therefore, asteroids are not primitive, but rather asteroids in the outer Cochypium belt experience less changes when the solar system is formed.

The inner boundary of the main belt is 2.06AU where there is a 4:1 orbital resonance with Jupiter's orbital period. Any celestial body here will be removed due to orbital instability. In this gap, in the early history of the solar system, it will be swept or ejected due to the disturbance of gravity of Mars (the aerial point is 1.67AU).

Other explanations

The earliest explanation for the cause was the explosion theory, which was the 10th largest planet in the solar system, which was decomposed into tens of millions of asteroids billions of years ago. The shooting theory of this asteroid Mathilde, near-Earth asteroid probe, solved two problems at once: the emergence of the asteroid belt and why there was no tenth planet. But the biggest flaw of this idea is that the cause of the planet explosion is unclear. Some people also believe that there are 5-10 relatively large asteroids with similar sizes to Ceres in the orbit between Jupiter and Mars. These planets gradually disintegrate through long-term collisions, getting smaller and smaller, and becoming more and more divided, forming a large number of fragments, which is the asteroid belt we currently observe. These explanations have their own reasons, but they cannot be self-consistent, so they have not formed a conclusion.

Family and Groups

See the entry Asteroid Family.

About one-third of the asteroids in the main belt belong to members of different families. Asteroids in the same family come from fragments of the same parent, sharing similar orbital elements, such as semi-major axis, qiu-rate, orbital inclination, and similar spectrum. The graph of these orbital elements shows that asteroids in the main belt are concentrated into several families, and about 20-30 groups can be determined to be asteroid families and may have common origins. Some may be, but not very certain. Asteroid families can be identified by the characteristics of the spectrum. Smaller asteroid groups are called groups or groups.

The famous asteroids in the main zone (sorted by semi-major axis) include the Flower God Star Clan, the Judicial Star Clan, the Crow Girl Star Clan, the Dawn Star Clan, and the Sijian Star Clan. The largest asteroids are the Vesta (Ceres is an intruder belonging to the Gefion clan). They are believed to be caused by the impact of forming a crater on Vesta, and the HED meteorite may also originate from this impact.

Three obvious dust belts were also found in the main belt. They had similar orbital inclinations to the Dawn Star, the Crow Star, and the Manager Star, so they may also belong to these families.

edge

There are Hungarian asteroids at the inner edge of the asteroid belt (distance between 1.78 and 2.0 astronomical units, average concept map, Dawn and the semi-major axis of the asteroid belt 1.9 astronomical units). They are mainly Hungary and contain at least 52 well-known asteroids. The orbits of the Hungarians have high inclinations and are separated from the main belt by the Kirkwood gap of 4:1. Some members are asteroids that cross the orbits of Mars, and it may be that the disturbance of Mars has reduced the members of this family.

Another high-inclination family at the outer edge of the main belt of the asteroid is the Fuhou family, with orbits between 2.25 and 2.5 astronomical units from the sun. It is mainly composed of S-type asteroids, and there are some E-type asteroids near the Hungarians.

One of the largest families, the Flower God Star family, has more than 800 known members, probably formed after an impact a billion years ago, and is mainly distributed on the inner edge of the main belt.

On the outer edge of the main belt are asteroids of the Genshin Impact family, with orbits between 3.3 and 3.5 astronomical units and a 7:4 orbit with Jupiter. The Hilda family's orbits are between 3.5 and 4.2 astronomical units and a 3:2 orbit with Jupiter. Relatively speaking, outside of 4.2 astronomical units, there are still a small number of asteroids between Trojan asteroids that are common orbit with Jupiter.

New family

Evidence shows that new asteroids are still in formation (on astronomy time scale), and Karin Cluster was apparently generated 5.7 million years ago after a parent asteroid collision of about 16 kilometers in diameter. Veritas was formed 8.3 million years ago, and the evidence comes from interplanetary dust deposited in the ocean.

In the longer period of time, the Mandala tribe was born in the main zone 450 million years ago, but age estimates are based on the current orbital elements of the possible members, not all physical characteristics. However, this group can serve as a material source for zodiac dust. Other recently formed groups include the Ianini group (about 1.5 million years ago), which can provide another source of dust in the asteroid belt.

Physical features

Conceptual diagram, Dawn and Vesta and Ceres structure

The current asteroid belt contains two main types of asteroids. At the outer edge of the asteroid belt, close to Jupiter's orbit, it is mainly C-type asteroids rich in carbon values. Such asteroids account for more than 75% of the total. Compared with other asteroids, they are redder in color and have very low albedo. Their surface composition is similar to that of carbon chondrites, and their chemical composition and spectral characteristics are all in the early states of the solar system, but they lack some lighter and volatile substances (such as ice).

The part close to the inside is 2.5 astronomical units away from the sun. S-type asteroids containing silicon are more common. The spectrum shows that their surface contains silicates and some metals, but the composition of carbonaceous compounds is not obvious. This shows that they are significantly different from the composition of the original solar system, which may be due to the early melting mechanism of the solar system. Compared with C-type asteroids, such asteroids have a high reflectivity. They account for about 17% of the entire population of the asteroid belt.

There is also a third type of asteroid, with a total number of M-type asteroids accounting for about 10%. Their spectrum contains spectral lines similar to iron-nickel, which appear white or slightly red, but does not have the characteristics of absorption lines. M-type asteroids are supposedly formed by devastating impacts of iron-nickel as the main body. In the main zone, M-type asteroids are mainly distributed in orbits with a half-major radius of 2.7 astronomical units.

Note: In the 1970s, classification systems were developed by observing the spectrum of asteroids. The three most common types were C-type (carbonaceous), S-type (silicate) and M-type (metal).

collision

Measuring the rotation period of huge asteroids in the asteroid belt shows that there is a lower limit. Asteroids with a diameter greater than 100 meters have a rotation period of more than 2.2 hours. Although a strong object can rotate at a higher rate, when the rotation period of the asteroid is faster than this value, the centrifugal force on the surface will be greater than the gravity, so all the loose matter on the surface will be thrown away. This also means that asteroids with a diameter greater than 100 meters are actually formed in rubble piles after collision.

The high density of celestial bodies in the asteroid belt makes collisions frequently (astronomical time scale). In celestial bodies with a radius of 10 kilometers in the asteroid belt, an average collision occurs every ten million years. Collisions will produce many asteroid fragments (which lead to the generation of new asteroid families), and some collision wreckage may enter the Earth's atmosphere and become meteorites. But when asteroids collide at low speeds, the two asteroids may combine. Over the past 4 billion years, some members of the asteroid belt still retain their original characteristics.

Other substances

In addition to the main body of the asteroid, the asteroid belt also contains dust particles with a radius of only a few hundred microns. At least part of these fine particles are from collisions between asteroids (or impacts of tiny meteorite bodies on asteroids). Due to Pointin Robertson's resistance, the pressure from the solar radiation will cause these particles to slowly move towards the sun in a spiral path.

These tiny particles drive the material thrown by comets and produce ecliptic light. This faint glow can be observed along the plane of the ecliptic plane in the twilight after the sun sets in the west. The radius of the particles that produce ecliptic light is about 40 microns, and the life span of this particle can usually be maintained at 700,000 years. Therefore, newly generated particles must be continuously coming from the asteroid belt.

Kirkwood Breakout

See Kirkwood Breakout

The semi-major axis distribution diagram of asteroids is mainly used to describe the range of asteroids near the sun. Its value lies in the inferred orbital period of asteroids. As for the semimajor axes of all asteroids, eye-catching voids will appear in the main zone. On these radii, the average orbital period of asteroids is an integer ratio to the orbital period of Jupiter. This resonance with the average motion of gas giants is enough to cause changes in the orbital elements of the asteroid. The actual effect is that asteroids at these void positions will be pushed into different orbits with larger or smaller semimajor axes. However, because the orbits of asteroids are usually elliptical, there are still many asteroids that will pass through these voids, so in actual space density, the asteroids in these voids will not be lower than those in the neighboring areas.

These arrows point out the famous Kirkwood void in the asteroid belt. The main void resonates with Jupiter at an average motion of 3:1, 5:2, 7:3 and 2:1. That is to say, the asteroid at the Kirkwood void at 3:1 will orbit the sun three times when Jupiter resonates one circle. At other orbital positions with lower resonance, fewer asteroids can be found than adjacent areas. (For example, the semi-major axis of the 8:3 resonance asteroid is 2.71 astronomical units.)

The Kirkwood void obviously divides the asteroid belt into three regions: the first zone is a gap of 4:1 (2.06 astronomical units) and 3:1 (2.5 astronomical units); the second zone connects the resonance gap of the end point of the first zone to 5:2 (2.82 astronomical units); the third zone starts from the outside of the second zone to 2:1 (3.28 astronomical units).

The main belt is also obviously divided into two inner and outer zones. The inner zone belt is from the area close to Mars to the gap resonance of 3:1 (2.5 astronomical units), and the outer zone extends to the vicinity close to Jupiter's orbit. (Some people also use the 2:1 resonance gap as the boundary between the inner and outer zones, or divide it into three inner, middle and outer zones.)

Other information

·The mass currently possesses only a small part of the original asteroid belt. Computer simulation results show that the original mass of the asteroid belt may be comparable to that of the earth. However, due to gravity interference, during the formation cycle of millions of years, most of the matter is ejected, and the remaining mass is probably only one thousandth of the original.

Asteroid belt·When the main belt begins to form, a condensate line (snow line) with a temperature lower than water has been formed in the area 2.7 AU away from the sun. The stars formed outside this line can accumulate ice. The main belt comets generated in the asteroid belt are all outside this line, which has become the main factor causing the Earth's oceans.

·Because the size and distribution of the asteroid belt had stabilized 4 billion years ago (relative to the entire solar system), that is, the main belt of the asteroid belt has no significant increase or decrease in size. However, asteroids will still be affected by many subsequent processes, such as internal heating, melting caused by impact, and space weathering caused by cosmic rays and micrometeoroid bombardment.
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