Asteroid belt

Schematic image of the asteroid belt. The main belt, between the orbits of Mars and Jupiter, and the group of the Trojans, is shown in the orbit of Jupiter.

The asteroid belt is a circumstellar disk of the solar system that lies between the orbits of Mars and Jupiter. It houses a multitude of astronomical objects, called asteroids, and the dwarf planet Ceres. This region is also called the main belt in order to distinguish it from other groupings of smaller bodies in the solar system, such as the Kuiper belt or the Oort cloud.

More than half of the total mass of the belt is contained in the five most massive objects that are the dwarf planets: Ceres, Pallas, Vesta, Hygia, and Juno. The most massive of all dwarf planets in the belt is Ceres, it has a diameter of 950 km and a mass twice that of Pallas and Vesta combined. Most of the bodies that make up the belt are much smaller. The belt material, barely 4% of the Moon's mass, is scattered throughout the volume of the orbit, so it would be very difficult to collide with one of these objects if you go through it. However, two large asteroids can collide with each other, forming what are known as asteroid families, which have similar compositions and characteristics. The collisions also produce a dust that forms the major component of zodiacal light. Asteroids can be classified, based on their spectrum and composition, into three main types: carbonaceous (C-type), silicate (S-type), and metallic (M-type).

The asteroid belt formed in the protosolar nebula along with the rest of the solar system. The fragments of material contained in the region of the belt could have formed a planet, but the gravitational perturbations of Jupiter, the most massive planet, caused these fragments to collide with each other at high speeds and could not be grouped, resulting in the rocky residue that currently observe. A consequence of these disturbances are Kirkwood holes, areas where asteroids are not found due to orbital resonances with Jupiter, and their orbits become unstable. If an asteroid happens to occupy this area, it is expelled in most cases out of the solar system, although on occasions it can be sent towards an inner planet, such as Earth, and collide with it. Since its formation, most of the material has been expelled.

The asteroid belt is divided into several regions based on the boundaries set by Jovian resonances. However, not all authors agree. For most it is divided into interior, exterior and middle or main proper, whose limits are the 4:1 and 2:1 resonances. In turn, the main belt is divided into three zones designated with Roman numerals and limited by the 3:1 and 5:2 resonances. A last resonance, 7:3, marks a break in zone III. Some asteroids have orbits so eccentric that they even cross the orbit of Mars (in English, Mars-crossing asteroids).

Description

It occupies a ring-shaped area formed by a large number of astronomical objects. Terms such as belt, ring or disk can be misleading: it is not a dense space in which bodies, asteroids, often collide with each other; on average, each major asteroid is separated from its neighbor by a distance of five million kilometers. The collisions occur with intervals of hundreds or even hundreds of thousands of years (in the largest asteroids). The largest of these bodies, Ceres, is a bumpy sphere 952.4 km in diameter, and the smallest are jagged, pebble-sized remnants.

It corresponds to an area of the solar system located between 2 and 4 AU, in which no planet could have formed due to disturbances caused by Jupiter. For this reason, astronomers think that a good part of these bodies date from the early days of the solar system, that is, from a time when planets did not exist. More than 4.5 billion years ago, only small blocks revolved around the Sun. At more than 3 astronomical units, those bodies were made of rock, but mostly ice, the existence of which was possible thanks to sufficiently low temperatures. At less than 3 astronomical units, the ice could not survive and only the silicates regrouped to create small planetoids. This is how asteroids were born. Most were attracted to bodies with greater mass: the planets in formation. These played the role of gigantic vacuum cleaners that cleared space of asteroids, except for Mars and Jupiter. For this reason, a part of these space rocks constitute vestiges capable of giving testimony of the prevailing conditions in the vicinity of the Sun 4.5 billion years ago.

However, not all asteroids are such primitive bodies. Astronomers have detected differences in their composition. About 6 out of 10 of type C probably date from the genesis of the solar system. The others are rocky (type S) or metallic (type M), and are the result of the fragmentation of larger objects, whose diameter would exceed 200 kilometers. This is the minimum size from which the internal heat generated by the object's own gravity is enough for a differentiation to take place: in magma, heavy elements such as metals slide towards the center to form the nucleus, while light elements, such as stones, float to form the mantle. When, as a result of a collision, the body fragments, the pieces of the core produce M-type asteroids and those of the mantle give rise to S-type asteroids. Some of these small planets follow their own path, moving away from the others, outside the orbit. asteroid belt.

Deflected by the major planets, some occasionally cross the Earth, such as (433) Eros, the largest, a "rugby ball" of 14 × 14 × 40 kilometers, or (2101) Adonis, famous for having grazed —in fiction— Tintin's rocket. Phobos and Deimos, the two satellites of Mars, are captured asteroids during their escape from the main belt. The same must have happened to Amalthea, one of the small Jovian satellites.

History of your observation

Titius-Bode Law

In 1766, Johann Daniel Titius discovered a supposed pattern in the distance of the planets from the Sun. He observed that if to the numerical sequence: 0, 3, 6, 12, 24, 48... (starting with 0, continuing by 3 and doubling the previous number each time) four are added to each figure and divided by 10, it turns out that it agrees quite well with the distances of the planets of the solar system in astronomical units (AU):

0.4

0.7

1.0

1.6

2.8

5.2

But the Titius-Bode law predicts a planet where none are known: at 2.8 astronomical units.

In 1768, the astronomer Johann Elert Bode made reference to this relationship in one of his writings, but he did not credit Titius until 1784, so many authors refer to it as Bode's law. For this reason, it is now known as the Titius-Bode law. This empirical pattern predicted the semimajor axis of the six known planets at the time (Mercury, Venus, Earth, Mars, Jupiter, and Saturn), except that the series predicted a planet at a distance of 2.8 AU from the Sun, corresponding to an area between the orbits of Mars and Jupiter, however, none were observed there. Titius declared: “But would the Creator have left that empty space? Not at all."

When William Herschel discovered Uranus in 1781, the position of the planet coincided almost perfectly with that predicted by this law (it was 19.2 AU, in front of at 19.6 AU predicted by law); this led astronomers to conclude that a planet could exist between the orbits of Mars and Jupiter. The following table shows the actual distance of the planets from the Sun in AU compared to that predicted by the Titius-Bode law, for the planets that were known until then:

Giuseppe Piazzi undertook the search for said planet, and in the new year of 1801 he found it. Or he thought he did. Ceres, at just 950 km in diameter, was on the small side.

But the discovery of Ceres was quickly followed by Pallas in 1802; June in 1804; and Vesta in 1807, and a large number of minor planets or planetoids. He went from euphoria to disappointment. Once Ceres (dwarf planet) was classified as a planet and then as an asteroid, it was reclassified as a dwarf planet in 2006.

Ceres and the Sky Police

Giuseppe Piazzi, discoverer of Ceres, the largest and most massive object of the asteroid belt.

The astronomer Franz Xaver von Zach began in 1787 to search for the planet predicted by the Titius-Bode law. However, he realized that to achieve this he would need the help of other astronomers, and in September 1800 von Zach gathered a group of 24 observers, who divided the band of the zodiac into 24 parts, which corresponded to 15° each. This group called itself the sky police (Himmels polizei), and its members included such renowned astronomers as William Herschel, Charles Messier, Johann Elert Bode, Barnaba Oriani, and Heinrich Olbers.

The celestial police sent an invitation for the Italian Giuseppe Piazzi to join their cause, but before the invitation arrived, Piazzi discovered the "planet" searched on January 1, 1801, which he named Ceres after the Roman goddess of agriculture and patron saint of Sicily. Piazzi, who was not aware of the plans of the group of astronomers, was trying to make observations to complete his catalog of stars, when he located in the constellation Taurus a small luminous point that did not appear in the catalogue. The Italian observed it the following night and found that it had moved against the background of stars. The following days he continued to observe that tiny point of light, and soon became convinced that it was a new object in the solar system. At first, Piazzi believed that it was a comet, but the absence of nebulosity around it and its slow and uniform movement convinced him that it could be a new planet. Ceres was at 2.77 AU, almost exactly at the position predicted by the Titius-Bode law of 2.8 AU.

Pallas and the concept of an asteroid

Fifteen months later, on March 28, 1802, Heinrich Olbers discovered a second object in the same region, which he named Pallas. Its semimajor axis also matched the Titius-Bode law, currently estimated to be 2.78 AU, but its eccentricity and tilt were very different from those of Ceres. The astronomers were puzzled; Ceres fit the predictions of the Titius-Bode law perfectly, but so did Pallas, and this law did not allow two objects in the same region.

In order not to violate the Titius-Bode law, astronomers began to believe that the two bodies that had been discovered were actually fragments of a larger planet that had exploded or been torn apart by successive impacts of comets. On May 6, 1802, and after studying the nature and size of these two new objects, William Herschel proposed naming Ceres and Pallas "asteroids", because of their resemblance to stars when observed. In the words of the astronomer:

As neither the denomination of planets, nor of comets, can apply to these two stars, we must distinguish them by a new name... They look small stars and hardly distinguish themselves from them. By its asteroidal appearance, if I am allowed this expression, I suggest taking this name and calling them "Asteroids". [...] The asteroids are celestial bodies, which move in orbits either of scarce or considerable eccentricity around the Sun, and whose inclination over the ecliptic may be of any angle. Their movement can be direct or retrograde; and they may have or may not have atmospheres, small commas, discs or nuclei.

William Herschel, Observations on the Two Lately Discovered Celestial Bodies1802.

Thus, Herschel intended to include them within a new class of solar system objects, as long as they did not violate the Titius-Bode law for planets. The definition is intentionally ambiguous, so that, in Herschel's words, it is "broad enough to encompass future discoveries".

However, despite Herschel's efforts, for several decades astronomers continued to frame these objects as planets. Ceres was considered a planet until the 1860s, when it was considered an asteroid, but this classification lasted until 2006, since it is currently part of the so-called dwarf planets along with Pluto and some others.

Asteroid Belt

In a few years, astronomers discovered two more new objects, which matched Herschel's concept. On September 1, Karl Harding found Juno, and on March 29, 1807 Heinrich Olbers discovered Vesta. However, a new object of this nature was not discovered until 1845, with Karl Hencke's discovery of Astrea on December 8. of that year. From then on, a multitude of these objects began to be discovered as telescopes became more powerful, to such an extent that by the early 1850s more than a dozen of them had already been discovered, so the concept of "asteroids" it was gradually replacing that of planets to classify these objects.

With the discovery of the planet Neptune in 1846, the Titus-Bode law began to lose force among the community of astronomers, since this planet did not comply with it. In fact, currently such a law is taken by mere chance without any theoretical justification, although some works show that Kepler's laws could have some correlation with the Titus-Bode law.

The question of nomenclature began to be a problem for astronomers. Whenever one of these objects was discovered, it was usually given the name of some goddess or heroine of mythology and designated with a symbol for short, as is the case with planets. However, the multitude of asteroids discovered made these symbols increasingly complex, to the point that some artistic skill was required to draw them. For this reason, finally in 1867 a new nomenclature was agreed for these objects, which consisted of the name of the asteroid preceded by a number in parentheses, and in order of discovery: (1) Ceres, (2) Pallas, (3) Juno, (4) Vesta, etc. Currently they are usually represented in the same way, including or subtracting the parentheses.

The term "asteroid belt" began to be used in the early 1850s, although it is not known who first referred to it. In the year 1868, a hundred asteroids were already known and in 1891 the use of astrophotography, pioneered by Max Wolf, accelerated the pace of discoveries even more. In 1923 the number of asteroids exceeded 1,000, in 1981 10 000, in 2000 the 100 000 and in 2012 the number of asteroids is around 600,000.

Origin

Training

Artistic representation of a protoplanetary disk around a star, similar to that formed by the planets of the solar system.

In 1802, shortly after the discovery of (2) Pallas, Heinrich Olbers suggested to William Herschel that Ceres and (2) Pallas might be fragments of a much larger planet that in the past might have orbited in that region between Mars and Jupiter. According to this hypothesis, the planet decomposed millions of years ago due to an internal explosion or comet impacts. However, the large amount of energy that would have been required for such an event to occur, combined with the low total mass of the belt of asteroids (only a 4% the mass of the Moon), show that this hypothesis cannot be valid. In addition, the differences in chemical composition between the asteroids in the belt are very difficult to explain if they originated from the same planet. Therefore, today most scientists accept that asteroids were never part of a planet..

It is generally believed that the solar system formed from a primitive nebula, made of gas and dust, which collapsed under the influence of gravity into a rotating disk of material. While in the center, where the Sun would form, the density increased rapidly, in the outer regions of the disk, small solid grains were formed that, over time, were grouped by accretion and collision processes to form the planets.

Planetsimals in the region where the belt is now found were gravitationally perturbed by Jupiter. The planet caused a certain part of the planetesimals to acquire very high eccentricities and inclinations, accelerating at high speeds, which caused them to collide with each other, and therefore, instead of grouping together to form a planet, they disintegrated into a multitude of rocky residues: the asteroids. A large portion were ejected out of the solar system, with only less than 1% of the initial asteroids surviving.

Evolution

Since their formation in the primitive nebula that gave rise to the solar system, asteroids have undergone various changes. These include internal heat during the first few million years, surface melting due to impacts, space erosion due to radiation and solar wind, and bombardment by micrometeorites. Some scientists refer to asteroids as the residual planetesimals, while others consider them distinct due to these processes.

The present-day asteroid belt is thought to contain only a fraction of the mass of the early belt. Computer simulations suggest that the original asteroid belt could have contained a mass comparable to that of Earth. Due mainly to gravitational perturbations, most of the material was expelled from the belt during the first million years of its formation, leaving only 0.1% of the original mass. It is believed that some of the ejected material could be found in the Oort cloud, in the outer reaches of the solar system. Since their formation, the typical size of asteroids has remained relatively stable; there have been no significant increases or decreases.

The 4:1 orbital resonance with Jupiter, located around 2.06 AU from the Sun, can be considered the inner boundary of the main belt. Perturbations caused by Jupiter sent asteroids there into unstable orbits, creating a desert area at that distance. Most of the closer bodies were hurled towards Mars (whose aphelion is 1.67 AU) or ejected by gravitational disturbances in the early episodes of the formation of the solar system. The asteroids that make up the Hungaria family are closer to the Sun than the area mentioned above, but they have stable orbits due to their high orbital inclination.

When the asteroid belt was still in formation, at a distance of 2.7 AU from the Sun was the temperature separation line of the point of water condensation. Planetesimals at a greater distance were able to accumulate ice. In 2006 it was postulated that a population of comets located beyond the boundary of that separation may have contributed to the formation of the Earth's oceans.

Features

Contrary to popular belief, the asteroid belt is mostly empty. The asteroids are spread out in such a large volume that it would be very difficult to cross the belt and meet one of them unintentionally. However, and although hundreds of thousands of these celestial bodies are currently known, it is estimated that the belt is home to several million asteroids.

Sizes

Size of the first ten asteroids, in order of discovery, compared to the Moon.

The total mass of the asteroid belt is estimated to be between 3.0×10²¹ and 3.6×10²¹ kg, which is only a 4% of the mass of the Moon, or what is the same, a 0.06% of the land mass. The largest celestial objects in the belt are therefore much smaller and less massive than the Moon. The four main bodies add up to half of the total mass of the belt, and Ceres, the largest of them, represents a third of the total mass. Ceres has a radius of about 475 km, which is one-third the lunar radius, and a mass of 10²¹ kg, which is only about 1.3% of the mass of the Moon. The second largest object in the belt, (4) Vesta, is half the size of Ceres. About 1,000 asteroids are known to have a radius greater than 15 km, and it is estimated that the belt could harbor about half a million asteroids with radii greater than 1.6km.

The sizes of asteroids can be determined in several ways, knowing their distance. One of the methods is by observing its apparent transit in front of a star, which happens due to the Earth's rotation. When this happens, the star is hidden behind the asteroid, and by measuring the time that this occultation lasts, it is possible to find the diameter of the asteroid. With this method, the sizes of the largest asteroids in the belt, such as Ceres or (2) Pallas, have been determined with good precision.

Another method of estimating their sizes is to measure their apparent brightness. The larger an asteroid is, the more sunlight it will reflect due to its larger surface area. However, the apparent brightness also depends on the characteristic albedo of the asteroid, and this is determined by its composition. As an example, (4) Vesta appears somewhat brighter in the sky than Ceres, since the albedo of the former is four times higher. However, the albedo of asteroids can be determined, since the lower the albedo a body has, the more radiation it absorbs and therefore the more it heats up; This heat emits radiation in the infrared, and by comparing the infrared and visible radiation that reaches the Earth's surface, the albedo can be determined, and therefore its size can be calculated. With this method you can even find out the irregularities that a certain asteroid presents in the event that it is in rotation. In this case, the irregularities cause the observed surface to change, also changing its apparent brightness periodically.

Composition

253 Matilde, a type-C or carbonaceous asteroid

Most belt asteroids are classified, based on their composition, into three categories: carbonaceous or C-type asteroids, silicate or S-type asteroids, and metallic or M-type asteroids. Other types of asteroids exist. asteroids, but its population is very small.

There is an important correlation between the composition of asteroids and their distance from the Sun. The closest asteroids are usually rocky, composed of silicates and free of water, while the most distant ones are mostly carbonaceous, composed of clay minerals and with the presence of water. Therefore, the most distant asteroids are also the darkest, and the closest ones reflect the greatest amount of radiation. It is believed that this fact is a consequence of the characteristics of the primitive nebula that gave rise to the solar system. In the most remote regions the temperature was much lower, and therefore the water could condense on the asteroids; Quite the opposite of inland regions, where the higher temperature would probably cause the water to vaporize.

C-type or carbonaceous asteroids are the most abundant in the belt, making up 75% of the total. They reflect very little light (albedo between 0.03 and 0.09) and are therefore very dark, and usually have a slightly bluish cast. These asteroids absorb a lot of infrared radiation due to the presence of water trapped in their structure. They are usually found in the outer regions of the belt. The largest unequivocally C-type asteroid is (10) Hygia.

(433) Eros, type-S asteroid, composed of silicates.

S-type asteroids, made of silicates, account for about 15% of the total. They are located in the part of the belt closest to the Sun. They exhibit a slightly reddish color and have a relatively high albedo (between 0.10 and 0.22). (3) Juno is a good example of this type.

M-type, or metallic, asteroids contain significant amounts of iron and nickel. They make up about 10% of all asteroids, and have an albedo similar to S-types (0.10-0.18). These objects may be the metallic nuclei of earlier larger objects, which ended up fragmenting due to collisions. They are located in the middle of the asteroid belt, around 2.7 AU from the Sun. Although not common, asteroids have been recorded, such as the case of (22) Kalliope, which have very low densities to be of type-M, which implies that they are not mainly composed of metals and have high porosities. Within this type are included asteroids that do not conform to C-types and S, as not all M-type asteroids are made of similar materials or have the same albedo.

One of the unknowns of the asteroid belt is the relative paucity of basaltic, or V-type, asteroids. Asteroid formation theories predict that objects the size of (4) Vesta or larger should form crust and mantle, which would be composed mainly of basaltic rock. Evidence shows, however, that the 99% of the predicted basaltic material is not observed. Until 2001, most of the basaltic objects discovered in the belt were believed to have originated from (4) Vesta. However, the discovery of (1459) Magnya revealed a different chemical composition to previously known basaltic asteroids, suggesting that it originated differently. This hypothesis was reinforced by the 2007 discovery of two asteroids in the outer region of the belt. These are (7472) Kumakiri and (10537) 1991 RY16, which have different basaltic compositions. These two asteroids are the only V-type asteroids discovered to date in the outer belt region.

Orbits

Representation of the eccentricity of asteroids regarding their distance to the Sun. The red and blue points form the main belt. It can be noted that the average eccentricity is around 0.15.

Asteroids orbit in the same direction as planets, with orbital periods from 3.5 to six years, generally. The average eccentricity of asteroids is around 0.15, although some, such as (1862) Apolo and (944) Hidalgo, have very high eccentricities (around 0.6). A few asteroids have orbital inclinations greater than 25°, among them the asteroid (945) Barcelona, discovered by José Comas y Solá in 1921, whose inclination is 32.8°. The asteroid with the most inclined orbit is (1580) Betulia, with 52°.

Kirkwood Hollows

Distribution of distances from asteroid orbits, where you can see the different holes of Kirkwood for the different resonances.

By plotting the distance of asteroids from the Sun, you can see empty regions where there are none. These holes coincide with the orbits where there is orbital resonance with Jupiter, that is, where the period of the orbit is related by a simple fraction to the period of Jupiter. For example, any asteroid at a distance of 3.28 AU, would have a 2:1 resonance with Jupiter; when the asteroid completes two orbits around the Sun, Jupiter completes one. Other important resonances are those corresponding to 3:1, 5:2 and 7:3, at distances of 2.5 AU, 2.82 AU and 2.96 AU, respectively. Other secondary resonances also exist, which are not empty rather, the number of asteroids is smaller, like the 8:3 resonance (semi-major axis of 2.71 AU). The main belt can then be divided into three distinct zones separated by these gaps: Zone I (2.06-2.5 AU), Zone II (2.5-2.82 AU) and Zone III (2.82-3.28 AU).

These empty holes are named after their discoverer, Daniel Kirkwood, who discovered them in 1886. Any asteroid located in these positions would be accelerated by Jupiter and its orbit would be lengthened (eccentricity increased), so the perihelion of its orbit it could approach the orbit of a planet and collide with it or with the Sun, or be ejected out of the solar system. Unlike the holes in Saturn's rings, the Kirkwood holes cannot be directly observed, as asteroids have widely varying eccentricities and are therefore continually passing through them.

Since the formation of the solar system, the planets have undergone variations in their orbit, and specifically have been slowly changing their distance from the Sun. The change in Jupiter's orbit, and therefore the change in position over time of the Kirkwood voids, could explain the small number of asteroids that are harbored by certain regions of the belt.

Changes in orbits

Although the orbital resonances of the planets are the most effective way to modify the orbits of asteroids, there are other means by which this happens. Some evidence, such as the number of NEAs or meteorites near Earth, suggest that resonances alone are not capable of producing them.

At first it was postulated that random collisions between asteroids could cause them to fall into the Kirkwood gaps, and therefore be ejected by the perturbations of the planets. However, computer models have shown that the effects that this produces are several orders of magnitude below what is observed. Therefore, other effects must be more important.

Scheme of the Yarkovsky effect, showing the asymmetry of the infrared radiation emission in an asteroid.

I. O. Yarkovsky proposed at the end of the 19th century that sunlight could cause disturbances in the orbits of asteroids. This effect is known as the Yarkovsky effect, and it is possible because light carries linear momentum. Direct sunlight hitting the asteroid does not change its orbit, since the light hits it in the same direction as the Sun's gravitational pull, and for practical purposes it is as if it were being pulled by an object slightly less massive than the Sun. Yarkovsky's key idea is that an asteroid has different temperatures on its surface depending on its orientation to the Sun. The bodies emit infrared radiation, the greater the higher the temperature they are, and these emitted photons give the asteroid a quantity of movement in opposite direction to which they were radiated. In this way, there will be an asymmetric emission of photons and the asteroid will move. This effect is greater if there are temperature differences between the aphelion and perihelion of the asteroid. Through the Yarkovsky effect, their densities can be determined, and certain orbital and morphological characteristics that some families of asteroids possess can be explained.

Some scientists developed a variation of Yarkovsky's work, called the YORP effect. This effect predicts changes in asteroid rotations and speeds due to the Yarkovsky effect, and observations made so far are in full agreement with the predictions.

Main Objects

Ceres

Internal composition of Ceres, type-C (carbonace). The ice cap can be seen inside.

Ceres is the largest celestial body in the belt and the only one classified as a dwarf planet, since the redefinition of a planet in 2006. This classification is due to the fact that its gravity has molded it into an almost spherical shape (with a diameter of 940 km approx.) and is therefore said to have hydrostatic equilibrium. Prior to 2006 it was considered the largest asteroid, but currently it is the smallest dwarf planet, since the other objects that share the same classification, such as Pluto or Eris, are larger.

Its absolute magnitude is 3.32, greater than that of any other body in the belt. However, it is still a very dark body, as its albedo is only one 5%. Its internal structure is made up of a core made of silicates and a layer of water in the form of ice surrounded by a thin crust. A very small part of the ice is converted to water vapor by solar radiation, giving it a thin atmosphere. Its mass is almost one-third that of the total belt. It orbits at a distance of between 2.5 and 3 AU, and its eccentricity is only 0.08, so its orbit is quite circular.

Vestido

(4) Vesta, discovered by Olbers in 1807, is the second most massive asteroid, the third largest, and the brightest of all. This is because it has an albedo of 42%, higher even than Earth's albedo (37 %). It makes up 9% of the total mass of the belt, and its mean diameter is 530 km. It orbits at a distance from the Sun very similar to that of Ceres. Vesta has a fairly dense metallic core (iron and nickel), a mantle made of olivine, and a very thin crust just a few kilometers thick.

Image of elevation on the surface of (4) Vesta, where the enormous crater of the collision formed by the fragments of the Vesta family can be seen.

Vesta was hit by another asteroid, leaving a huge crater on its surface and sending a multitude of fragments corresponding to 1% of the asteroid's mass into the belt. In this way, the Vesta family was formed, of V-type (basaltics), but currently only a small part of these fragments continue to orbit the belt, since it is believed that the rest was dissipated when reaching the 3:1 resonance with Jupiter, in one of Kirkwood's holes. Some meteorites that fell on Earth have their origin in this collision.

Shovels

(2) Pallas is the second largest object in the belt, although (4) Vesta is more massive. It represents a 7% of the mass of the belt and its albedo is 12%, since which is type-C. It has the most eccentric orbit of the four, with a value of 0.23, making its closest distance to the Sun (2.1 AU) diste much of the farthest (3.4 AU). Its orbital inclination is also higher, with 34° (those of the other three are less than 10°). It is believed that an impact on its surface formed the Palas family, although the number of members is small.

In 1803, a year after its discovery and due to its repercussion, William Hyde Wollaston named a new element palladium.

Hygia

(10) Hygia is the fourth largest object in the asteroid belt, with a mean diameter of 431 km, although it is quite elongated in shape, and constitutes a 3% of the total mass of the belt. It was discovered by Annibale de Gasparis in 1849. Compositionally, it is a carbonaceous (C-type) asteroid with an albedo of 7%. It is the main member of the homonymous family to which it gives its name. It is, of the four, the outermost asteroid, whose aphelion reaches 3.5 AU, and takes 5.5 years to complete its orbit.

June

(3) Juno was the third asteroid to be discovered and is one of the largest in the main asteroid belt, being the second heaviest among the S-type asteroids. It was discovered on September 1, 1804 by the astronomer German Karl Ludwig Harding and named after the goddess Juno. At first it was considered a planet, like Ceres, Pallas, and Vesta. It was reclassified as an asteroid, along with the other three, when many more asteroids were discovered. Juno's small size and irregular shape excluded it from being considered a dwarf planet according to the International Astronomical Union's classification.

Location

Although most asteroids are found in the main belt, there are other groups of asteroids as well. Three asteroid regions can be distinguished, depending on their distance from the Sun:

• Main belt: is located between 2.06 and 3.65 UA, in a region between Mars and Jupiter. In turn, families of asteroids, such as Hungaria, Hilda, Eos, Themis, Cibeles, Koronis, can be classified.
• Asteroids near Earth (or NEA, English Near-Earth Asteroid): are asteroids very close to Earth orbit, located less than 1.3 UA from the Sun. They can be subdivided into three groups:
  • Aton asteroids: they have minor semies 1 UA, and larger shaves 0.983 UA.
  • Apolo asteroids: they have higher semies more distant than 1 UA, and perihelios less than 1.017 UA.
  • Asteroids Love: They possess perihelios between 1.017 UA and 1.3 UA. The asteroid (1036) Ganymed is the largest discovered NEA.
• Trojans: are located near the Lagrange points of Jupiter (situated to 60° of the line that unites the Sun and Jupiter). They know about 4,000. Some asteroids located at the Lagrange points of Neptune or Mars are also classified within this group, as is the case of (5261) Eureka. They receive this name due to the first asteroid of this discovered group, (588) Achilles, hero of the Trojan war.

Asteroid Families

Chart that represents orbital inclination regarding eccentricity. Regions where there is a greater accumulation of asteroids can be seen; it is the so-called families.

When the number of asteroids discovered began to be large, astronomers observed that some of them shared certain characteristics, such as eccentricity or orbital inclination. This is how the Japanese Kiyotsugu Hirayama proposed in 1918 the existence of five families of asteroids, a list that has grown over time.

About a third of belt asteroids are part of a family. The families have similar orbital elements and spectra, indicating that they originate from the fragmentation of a larger object. There are 20-30 associations that can certainly be considered asteroid families, although there are many others whose family names are not so clear. Associations with fewer members than families are called asteroid clusters.

Some of the most important families are (in order of distance): Flora, Eunomia, Coronis, Eos and Themis. The Flora family, one of the most numerous, could have its origin in a collision that occurred less than a thousand years ago. million years. The largest asteroid that is part of a family is (4) Vesta. The Vesta family is believed to have originated from a collision on its surface. As a result of the same collision, the so-called HED meteorites were also formed.

Three dust bands have been found within the main belt. It is possible that they are associated with the families Eos, Koronis, and Themis, since their orbits are similar to those of these bands.

Periphery

Bordering the inner limit of the asteroid belt is the Hungaria group, between 1.78 and 2.0 AU, and with semimajor axes around 1.9 AU. The asteroid that gives its name to this family made up of 52 known asteroids is (434) Hungaria. This group of asteroids is separated from the main belt by the Kirkwood gap corresponding to the 4:1 resonance, and its members have very high inclinations. Some cross the orbit of Mars, whose gravitational perturbations are probably the most notable cause of the population decline of this group.

Another group of asteroids with inclined orbits in the inner part of the belt is the family of Phocea. The vast majority of its members are S-type, unlike the Hungaria family, which has some E-type (with enstatite surfaces). The family Phocea orbits between 2.25 AU and 2.5 AU from the Sun.

At the outer limit of the belt is the Cybele family, orbiting between 3.3 and 3.5 AU, in the 7:4 resonance with Jupiter. The Hilda group orbits between 3.5 and 4.2 AU, with fairly circular and stable orbits in the 3:2 Jupiter resonance. Very few asteroids are found beyond 4.2 AU, up to the orbit of Jupiter (5.2 UA), where the Trojan asteroids are found. Trojans can be divided into two groups, according to the Lagrange point of Jupiter they occupy: those located at point L₄ and those located on the opposite side L₅. It is unknown why point L₄ is so much more populated.

New families

Some families have formed recently, in astronomical times. The Karin cluster formed 5.8 million years ago as a result of a collision with an asteroid of 16 km radius. The Veritas family formed 8.7 million years ago; evidence includes interplanetary dust collected from ocean sediments.

Much more recent is the Datura cluster which formed 450,000 years ago from a main belt asteroid. The estimate of its age is based on the statistical probability that its members have the current orbits, and not on solid physical evidence. The Datura cluster is thought to have been a source of zodiacal dust and material. Other recent formations, such as the Iannini cluster (circa 5 million years ago) or the Seinäjoki cluster, could also have contributed to the formation of this dust.

Collisions

Zodiacal light, created in part by dust originated in collisions between asteroids.

Due to the high population of the main belt, collisions between asteroids happen frequently, on astronomical time scales. It is estimated that every 10 million years a collision occurs between asteroids whose radii exceed 10 km. Collisions sometimes cause the asteroid to break up into objects smaller, forming a new family of asteroids. It can also happen that two asteroids collide at very low speeds, in which case they stick together. Due to these collision processes, the objects that formed the primitive asteroid belt are hardly related to those of today.

Tycho, a lunar crater originated by an asteroid belt meteorite.

In addition to asteroids, the belt also contains dust lanes made up of particles with radii of a few hundred micrometers. This material is produced, at least in part, by collisions between asteroids, and by the impact of micrometeorites on asteroids. In addition, the Poynting-Robertson effect causes this dust to spiral slowly towards the Sun due to solar radiation.

The combination of this dust with material ejected from comets produces zodiacal light. The glow it produces, though faint, can be seen at night in the direction of the Sun along the ecliptic. The particles that produce visible zodiacal light have, on average, radii of 40 micrometers. The characteristic lifetime of these particles is of the order of 700,000 years. Therefore, to maintain the dust lanes, new particles must be created at a constant rate in the asteroid belt.

Meteors

Debris from collisions can form meteoroids that eventually reach Earth's atmosphere. More than the 99.8% of the 30,000 meteorites found to date on Earth it is believed to have originated in the asteroid belt. In September 2007, a study was published suggesting that the asteroid (298) Baptistina suffered a collision that caused a considerable amount of fragments to be sent into the inner solar system. It is believed that the impacts of these fragments created the Tycho and Chicxulub craters, located on the Moon and in Mexico respectively, and the latter could have caused the extinction of the dinosaurs 65 million years ago.

Exploration

Artistic representation of the spacecraft of the Dawn mission, with Vesta on the left and Ceres on the right.

The first spacecraft to pass through the asteroid belt was Pioneer 10 on July 16, 1972. At the time there was some concern that the debris there would pose a danger to the spacecraft, but so far they have passed the belt without incident a dozen different ships. The Pioneer 11, Voyager 1 and 2, and Ulysses probes passed through the belt without taking images. The Galileo mission took images of (951) Gaspra in 1991 and of (243) Ida (and its satellite Dactyl) in 1993, NEAR Shoemaker of (253) Matilda in 1997 and (433) Eros in 2000, Cassini-Huygens of (2685) Masursky in 2000, Stardust from (5535) Annefrank in 2002, and New Horizons from (132524) APL in 2006.

The Hayabusa mission, which returned to Earth in June 2010, photographed and landed on the surface of (25143) Itokawa in 2005, for two months. The Dawn mission was launched in 2007, on July 18, 2011 it was confirmed that the probe entered Vesta's orbit, and on March 6, 2015 it entered orbit around Ceres. The WISE mission was launched on December 14, 2009 and will search by infrared radiation detection for all asteroids whose diameter is greater than 3 km. The launch of another mission, OSIRIS-REx, is scheduled to take place in 2016, and it will return samples of material from the surface of an asteroid to Earth.

Most of the photographs taken of asteroids were taken during the brief passage through the belt of space probes heading towards other targets, with the exception of NEAR and the Hayabusa probe, which explored certain nearby asteroids (NEAs).. Only the Dawn mission has as its primary objective the study of objects in the main asteroid belt, and if these are successful it is possible that an extension of the mission will be developed to allow additional explorations.

Future source of resources

Asteroids are the most accessible bodies in the solar system. It has been suggested that in the future, material from near-Earth asteroids (NEA) could be exploitable. The most economically important materials are water (C-type asteroids possess it, usually in the form of ice) and various metals, such as iron, nickel, cobalt or platinum (S and M-type asteroids). The possible methods for doing so and the economic costs involved have already been speculated on, and it is believed that for every ton of terrestrial material used for the construction of ships, up to a thousand tons of material can be obtained on asteroids. This would lower the cost of the materials in question, and they could be used for the construction of structures necessary in future space explorations.

Additional bibliography

• Gibilisco, Stan (1991). Comets, meteors and asteroids: how they affect Earth (1st edition). McGraw-Hill / Inter-American of Spain, S. A. ISBN 978-84-7615-727-5.