The asteroid belt issolar systemInside BetweenMarsandJupiterBetween tracksasteroidDense area, 98.5%asteroidAll have been found here, and 120437 asteroids have been numbered.Since the asteroid belt is the most densely populated area of asteroids, this area is also calledMain belt。
The asteroid belt is about 2.17-3.64 away from the sunAstronomical unitOfSpace areaThere are more than 500000 asteroids in the area.So many asteroids can be condensed in the asteroid belt. In addition to the gravitational effect of the sun,JupiterThe gravity of also plays a role.
The asteroid belt is composed ofSolar nebulaA group ofStar Child(the predecessor of a planet smaller than a planet) formed.However, due to the gravity of Jupiter, these planetesimals are prevented from forming planets, causing many planetesimals to collide with each other and form many debris and debris.The three largest asteroids in the asteroid belt areZhishen Star、Nuptial god starandVesta, with an average diameter of more than 400 km;Only one in the main banddwarf planet——Ceres, about 950 kilometers in diameter;The rest of the asteroids are smaller, some even the size of dust.The material in the asteroid belt is very thin, and there are severalspacecraftPassed safely without accident.Asteroids in the main belt can be divided into three categories according to their spectra and main forms: carbonsilicateAnd metal.In addition, collisions between asteroids may formasteroid family , these collisions also occurZodiacal lightThe main source of dust.
Discovery History
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Theoretical prediction
Piazzi, who discovered the first asteroid Ceres.
1766 German astronomerTitius(J. Titius) accidentally found a number sequence: (n+4)/10. Substituting n=0,3,6,12,..., the actual distance between each major planet and the sun can be given quite accurately.It didn't attract people's attention at first, but laterBerlinobservatoryChief ofBode(J. Bode) published it after learning it, which was known by the astronomical community.Found in 1781UranusLater, the formula was further confirmed to be valid, and Bode proposed that there might be a planet between the orbits of Mars and Jupiter.
Observational findings
In 1801, Sicily and G. PlazziAstronomical observationThere is a small object at 2.77AU in the middle accidentally, that is, it is named Ceres.
In 1802, astronomerAubers(H. Olbere) Another is found in the same areaasteroidAnd later named Pallas.William Herschel It is believed that these celestial bodies are the remnants of a destroyed planet.In 1807, the third marriage god star and the fourth Vesta star were added in the same area.Because the appearance of these celestial bodies is similar to that of planets, William Herschel adopted the GreekRootAster - (star like) is named asteroid, which is translated into asteroid in Chinese.
Napoleonic Wars The first stage of the discovery of the asteroid belt ended, and the fifth asteroid was discovered only in 1845Yishen Star。Then, the rate of discovery of new asteroids increased rapidly. By 1868, 100 asteroids had been discovered, while in 1891Max Wolf IntroducedAstronomical photography, accelerating the discovery of asteroids.The number of asteroids was 1000 in 1923, 10000 in 1951 and 100000 in 1982.Use of modern asteroid survey systemautomation equipment So that the number of asteroids continues to increase.
Calculation confirmation
After the discovery of asteroid belts, it is necessary to calculate their orbital elements.In 1866,Daniel KirkwoodIt is announced that there is no blank area where asteroids exist at some distance from the sun, and that they orbit the sun in these areasOrbital periodThere is a simple integer ratio to the revolution period of Jupiter.KirkwoodThink it's JupiterPerturbationCausing asteroids to be removed from these orbits.
In 1918, Japanese astronomersKiyotsugu Hirayama It is noted that the orbits of some asteroids in the asteroid belt have similar parameters, and thus the asteroid family is formed.In the 1970s, a classification system was developed by observing the colors of asteroids. The three most common types are C-type (carbon) and S-type（silicate）And M-type (metal).In 2006, astronomers announced the discovery ofcometIt is speculated that these comets may be the source of water in the Earth's oceans.
Origin and evolution
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Asteroid Ida and its satellite, photographed by Galileo probe
At the early stage of the formation of the solar system, due to the widespread collision in the accretion process, small particles gradually gathered to form larger clusters. Once they gathered to a sufficient mass (the so-called micro star), they could attract the surrounding materials by gravity.These planetesimals can steadily accumulate mass to become rocky planets or giant gas planets.It is unknown when the mystery of the formation of the asteroid belt can be solved.However, more and more astronomers believe that asteroids recordSolar system planetsForm initial information.Therefore, the origin of asteroids is researchOrigin of the solar systemAn important and indivisible part of the problem.
Mainstream view
As for the cause of formation, the common view is that in the early days of the formation of the solar system, for some reason, the gap between Mars and Jupiter failed to accumulate to form aMegaplanets, resulting in a large number of asteroids.
The accepted theory of planet formation is the solar nebula hypothesis, which believes that the materials, dust and gas in the nebula that make up the sun and planets form a rotating disk due to gravitational collapse.In the first few million years of the solar system, due to the collision in the accretion process, small particles gradually gathered to form larger clusters, and the size of particles steadily increased.Once it has gathered enough mass - the so-called micro stars - it can attract nearby matter through gravity.These planetesimals can steadily accumulate mass to become rocky planets or giant gas planets.
stayaverage velocityIf the collision is too high, the star particles will be broken up and the accumulation of mass will be inhibited, preventing the generation of planet sized objects.In the region where the orbital period of the planetesimals is a simple integer ratio to the period of JupiterTrack resonanceThe orbits of these star particles will change due to perturbation.In the space between Mars and Jupiter, there are many places with strong orbital resonance with Jupiter.When Jupiter moves inward during its formation, theseResonance orbitIt will also sweep across the asteroid belt, dynamically excite the scattered star particles, and increase each other'srelative velocity。Stars are too strong in this areaPerturbationTherefore, it cannot become a planet, and it can only continue to revolve around the sun as in the past, and the asteroid belt can be regarded as the original solar systemresidue。
Asteroid Gastra, photographed by Galileo probe
The mass of the asteroid belt should be only a small part of the original asteroid belt. According to the results of computer simulation, the original mass of the asteroid belt should be equal 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 materials are thrown out, and the remaining mass is only about one thousandth of the original.
When the main belt begins to form, the distance between the main belt and the sun is 2.7AUA freezing point line with temperature lower than that of water - "snow line" has been formed at the place where the stars formed outside this line can accumulate ice.Generated in the asteroid beltmain-belt comets They are all outside this line and are the main suppliers of the earth's oceans.
Because about 4 billion years ago, the size and distribution of the asteroid belt had stabilized (relative to the entire solar system), which means that 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, cosmic rays andMicrometeoroidBombardingspace weathering 。Therefore, the asteroid is not primitive, but outsideAncient cypress beltThe asteroids in the solar system experienced less changes.
The inner boundary of the main belt has a 4:1 orbital resonance with Jupiter at 2.06AUWhere, any celestial body here will be removed due to orbital instability.In the early history of the solar system, the celestial bodies in this gap would be caused by Mars（ApohelionThe gravity disturbance at 1.67 AU) is swept or thrown out.
Other interpretations
Asteroid Mathilde, photographed by NEA
The earliest explanation is the explosion theory, which is the solar systemTenth planetThe Big Bang billions of years ago broke down into tens of millions of asteroids.This theory solved two problems at once: the emergence of the asteroid belt and why there is no tenth planet.But the biggest flaw of this assumption is that the cause of the planetary explosion is unclear.It is also believed that there are 5-10 relatively large asteroids similar in size to Ceres in the orbit between Jupiter and Mars.These planets gradually disintegrated through long-term collision, becoming smaller and more divided, forming a large number of debris, that is, the asteroid belt we observed.These explanations have their own reasons, but they cannot justify themselves, so they have not formed a final conclusion.
features
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form
By the 21st century, noManned spacecraftAt least visited many asteroids.According to their shape detection and research results, scientists found that the surface of many asteroids is rugged, covered with large and smallImpact craterOn the surface, there are manyBoulder；Like Earth, asteroids also have many grooves (ridges and valleys on the surface of asteroids),riftAnd cracks (tiny crack lines on the surface of asteroids);Unlike most of the grooves, fractures and fissures on the earth, they are formed by violent collision.
dwarf planet
Ceres(Ceres) Yessolar systemThe smallest and also located in asteroiddwarf planet。It is the largest object in the asteroid belt, with a diameter of (987 ± 150) km,Rotation period9 h 5 min。
About 101367km away from the earth;13km in diameter
1994GL
Smallest main halfshaft
Average distance from the sun 0.638AU
[1]
Physical characteristics
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structure
The asteroid belt contains two main types of asteroids.At the outer edge of the asteroid belt, near the orbit of JupiterC-type asteroidMost of them are asteroids, accounting for more than 75% of the total.Compared with other asteroids, the color is reddish andAlbedoVery low.Their surface composition andcarbonaceous chondrite be similar,chemical composition、Spectral characteristicsIt's the sun
Concept map, Dawn and Vesta and Ceres
It is an early state, but lacks some lighter and volatile substances (such as ice).
The part near the inner side, 2.5 astronomical units away from the sunS-type asteroid It is relatively common. The spectrum shows that its surface contains silicate 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, leading to the result of differentiation.Compared with C-type asteroids, such asteroids are higherreflectivity。It accounts for about 17% of the entire population of the asteroid belt.
There are also the third type of asteroids, which account for about 10% of the total M-type asteroids.Their spectra contain iron nickel likeSpectral line, white or slightly red, but notAbsorption lineCharacteristics of.M-type asteroid is speculated to be formed by destructive impact with iron nickel as the main parent body.In the main belt, M-type asteroids are mainly distributed inSemimajor diameter2.7 Astronomical units in orbit.
Rotation period
Measuring theRotation periodIt shows that there is a lower limit. Asteroids with a diameter of more than 100 meters have a rotation period of more than 2.2 hours.Although a solid object can rotate at a higher speed, when the rotation period of the asteroid is faster than this valuecentrifugal forceWill be greater than gravity, so all loose materials on the surface will be thrown away.This also shows that asteroids with a diameter of more than 100 meters were actually formed in the rubble after collision.
Revolution collision
Asteroid belthigh-densityThe distribution of the celestial bodies makes collisions between them frequent (astronomical time scale).In the asteroid belt, objects with a radius of 10 km will occur every 10 million years on averageA collision。 The collision will produce many asteroid fragments (leading to the generation of new asteroid families), and some collision debris may enter the earthatmosphereAnd become meteorite.But when the asteroid collides at low speed, the two asteroids may combine.In the past 4 billion years, some members of the asteroid belt still maintain 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 come from the collision between asteroids (or the impact of small meteorites on asteroids).becausePoyntine Robertson resistance, fromsolar radiationThe pressure will make these particles move slowly towards the sun in a spiral path.
These fine particles drivecometThe ejected material produces ecliptic light, which can be observed along the plane of the ecliptic in the twilight after the sun sets.The radius of particles that generate zodiacal light is about 40 microns, and the life span of such particles is usually 700000 years. Therefore, new particles must be continuously generated from the asteroid belt.
About one-third of the asteroids in the main belt belong to members of different families.Asteroids of the same family come from fragments of the same parent body, sharing similar orbital elements, such asSemimajor axis、Eccentricity、Rail inclinationThere are similar spectra.According to the patterns of these orbital elements, the asteroids in the main belt are concentrated into several families, and about 20 – 30 groups can be identified as asteroid families, and may have a common origin.Some may be, but they are not sure.Asteroid families can be identified by spectral features.Smaller asteroid groups are called groups or groups.
At the inner edge of the asteroid belt (distance between 1.78 and 2.0Astronomical unitAmong them, the average semi major axis is 1.9 astronomical units), and there are Hungarian asteroids.They are mainly Hungary, including at least 52 well-known asteroids.The orbits of the Hungarian nationality have a high inclination angle and are characterized by a 4:1Kirkwood gapSeparate from the main belt.Some members belong to asteroids passing through the orbit of Mars, and it may be because of the disturbance of Mars that the members of this family are reduced.
Another inmain asteroid belt The high inclination family on the outer edge is the Fuhou star family, and the orbit is in the distancesunlight2.25 to 2.5 astronomical units.It is mainly composed of S-type asteroids, and there are some E-type asteroids near the Hungarian nationality.
One of the largest familiesFlora family There are more than 800 known members, which may have been formed after the impact a billion years ago, mainly distributed on the inner edge of the main belt.
At the outer edge of the main belt, there are protozoan asteroids with orbits between 3.3 and 3.5 astronomical units, which have a 7:4 orbital resonance with Jupiter.HildaIts orbit is between 3.5 and 4.2 astronomical units, and it has 3:2 orbital resonance with Jupiter.Relatively speaking, beyond 4.2 astronomical unitsTrojan asteroids There are still a few asteroids between them.
New family
Evidence shows that new asteroid families are still formingtime scale ）Karin Cluster was apparently created after the collision of a parent asteroid with a diameter of about 16 kilometers 5.7 million years ago.Veritas family was formed 8.3 million years ago, and the evidence comes from the restoration of sediments in the oceanInterplanetary dust。
In the more distant past, the Datura nationality was born in the collision in the main belt 450 million years ago, but the estimation of age is only based on the orbital elements of possible members, not all physical characteristics.However, this group can act aszodiacA material source of dust.Other groups formed include the Ioanni Group (about 1.5 million years ago), which can provide another source of dust in the asteroid belt.
asteroidSemimajor axisDistribution mapIt is mainly used to describe the range of asteroids near the sun, and its value can infer the orbital period of asteroids.As far as the semi major axis of all asteroids is concerned, there will be noticeable gaps in the main belt.At these radii, the average orbital period of the asteroid is an integer ratio to the orbital period of Jupitergas giant Average motionThe result of resonance is enough to change the orbital elements of asteroids.The actual effect is that the asteroids in these gap positions will be pushed into different orbits with larger or smaller semi major axis.However, because the orbits of asteroids are usually elliptical, there are still many asteroids passing through these gapsSpatial densityOn the other hand, the asteroids in these gaps are not lower than those in the adjacent areas.
These arrows point to the famousKirkwood gapThe average motion resonance between the main gap and Jupiter is 3:1, 5:2, 7:3 and 2:1.That is to say, when the asteroid at the 3:1 Kirkwood gap makes one revolution in Jupiter, it will make three revolutions around the sun.At other locations with low orbital resonance, fewer asteroids can be found than in the adjacent region.(For example, the semi major axis of 8:3 resonant asteroid is 2.71Astronomical unit。）
Kirkwood gap obviously divides the asteroid belt into three regions: the first region is 4:1 (2.06 AU) and 3:1 (2.5 AU) gaps;The second zone connects the end point of the first zone to the 5:2 (2.82 AU) resonant gap;The third zone extends from the outside of the second zone to the 2:1 (3.28 AU) resonance gap.
The main belt is also obviously divided into inner and outer zones. The inner zone extends from the area close to Mars to the 3:1 (2.5 AU) resonance gap, and the outer zone extends to the area close to the orbit of Jupiter.(Some people take the 2:1 resonant gap as the boundary between the inner and outer zones, or divide it into three zones: inner, middle and outer.)
Exploration task
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In November 2018, Dawn spacecraft ended its mission to the asteroid belt because it had exhausted the fuel hydrazine it was pointing to.Nevertheless, it will remain in the orbit of Ceres for decades until it finally ends its magnificent life in a subduction way.During the previous 11 years of work, Dawn spacecraft visited Vesta and Ceres, the two largest objects in the asteroid belt between Mars and Jupiter.[2]
Other information
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The asteroid belt has only a small part of the mass of the original asteroid belt.Computer simulation results show that the original mass of the asteroid belt may be equivalent to that of the Earth.However, due to the interference of gravity, during the formation cycle of millions of years, most of the matter has been ejected, and the remaining mass is only about one thousandth of the original.
Asteroid belt
When the main belt began to form, a line with temperature lower than water had formed in the area 2.7 AU away from the sunCondensation pointLine（Snowline）, hereLineStars formed outside can accumulate ice.But in the asteroid beltmain-belt comets They are all outside this line, which has become the main factor causing the Earth's oceans.
The inner boundary of the main belt is betweenJupiterOfOrbital periodAt 4:1 orbital resonance (2.06 AU), any celestial body will be ejected due to orbital instability.
