Asteroid families

(ORDO NEWS) — In 1876, when only about 150 asteroids were known, D. Kirkwood tried to understand the “chaos” of asteroid orbits and found about 10 groups of asteroids, each of which consisted of only 2-3 members moving along similar orbits. Among them were, for example, 3 Junos and 97 Clotes.

It seemed that such groups could be seen as connected by a common origin and that the members of the groups were fragments of larger bodies. Kirkwood’s attempts were continued by F. Tisserand, who compiled his list of 417 asteroids in 1891. The number of groups grew as the number of discovered asteroids increased.

In essence, this was a variant of the Olbers hypothesis, only the relationship did not extend to all asteroids, but to some groups. But the matter turned out to be not at all so simple, and the kinship in the groups was doubtful.

This became clear when the Japanese astronomer K. Hirayama in 1918-1919. drew attention to the fact that the similarity of the orbits of asteroids does not mean at all that these asteroids in the past were parts of one, larger body.

With a large number of asteroids, it is not excluded that asteroids can be combined into groups due to the random similarity of their orbits. But the main mistake was that in search of “relatives” modern orbits of asteroids were compared.

Meanwhile, perturbations from the planets, accumulating over time, could gradually change the orbits of those asteroids beyond recognition and in different ways, which really were fragments of the same body and really moved in the past in similar orbits.

On the other hand, the similarity of modern orbits does not yet mean that asteroids moved along similar orbits in the distant past. Therefore, using Kirkwood’s technique, if it is possible to detect real groups of “relatives”, then only those formed quite recently, say, 1000 years ago.

Hirayama raised the question: is it possible to identify groups of asteroids that are related by long-standing kinship, i.e. families of asteroids (as he called them), and how to do it?

The theory of the motion of planetary satellites, taking into account perturbations, developed even earlier by Langrazh, indicated that the eccentricities and inclinations of the orbits of the satellites remain almost unchanged over long periods of time, while the longitudes of the periapsis and node of the orbit continuously change.

This led Hirayama to the idea of ​​”invariant” (unchanging) elements of asteroid orbits, which would also not change (or change slowly) under the influence of planetary perturbations. Such elements could be used to search for a family of asteroids.

Hirayama found such invariant elements and called them proper elements of the orbit, i.e. inherited asteroids from their “parents”. Of course, during the crushing of asteroids, their fragments, having received different, oh small additions to the orbital velocity, move along different orbits with slightly different intrinsic elements.

Generally speaking, the eigenelements are the Keplerian elements of asteroid orbits corrected for secular perturbations. In typical orbits, their own inclinations and eccentricities are almost not subject to secular changes, and they can be considered to have remained unchanged for a billion years.

As for the longitude of the perihelion and the longitude of the node, they change much faster. The proper longitude of the perihelion very slowly (at a rate of tens of seconds to tens of minutes of arc per year), but continuously grows, while the proper longitude of the node decreases at the same rate.

For bodies in the asteroid ring, the periods of revolution of the perihelion and the ascending node of orbits around the Sun are of the order of several thousand years. They increase with a decrease in the size of the orbits.

Thus, asteroids “remember” only the inclination of the orbit and its eccentricity for a long time, but quickly “forget” their node and perihelion.

Hirayama decided to use his own inclination and eccentricity of the orbits to search for families. At first, to simplify the calculations, he took into account only perturbations from Jupiter, neglecting the weaker influence of Saturn and other planets.

He succeeded in identifying three families (the families of Themis, Eos and Koronids, named after one of the members of the families), and then four more and, less confidently, six more. But it soon became clear to Hirayama that it was still necessary to take into account the influence of Saturn and other planets.

Saturn, for example, had a noticeable effect on asteroids with a small average daily motion. Having done this, Hirayama came to the conclusion that there were five families – Themis, Eos, Koronis, Mary and Flora. In 1923, he assigned dozens of known asteroids to these families. Subsequently, they were replenished with asteroids discovered later.

Flora’s family turned out to be the most numerous. D. Bauer, on the basis of the perturbation theory refined by him, divided it into four separate families – I, II, III and IV.

By the 70s, it became clear that “family” is widespread among asteroids: out of 1697 asteroids numbered by that time, 712 (or 42%) were assigned to 37 families. They still “remember” the orbit of the parent body. The situation turned out to be similar for the smaller asteroids of the Palomar-Leiden survey: out of 980 new asteroids, 389 (40%) were included in one or another family, already known or new.

The family reveals itself as a region of increased concentration of points on the distributions of the eigenelements of the orbits. Family boundaries are not always drawn with certainty, and the assignment of an asteroid to one or another family sometimes remains doubtful.

Also, when different researchers account for planetary perturbations with varying degrees of accuracy and select family members using slightly different criteria, they get slightly different results. However, these differences are not fundamental and do not allow one to doubt the very existence of nepotism in asteroids.

By the end of the 1970s, the Japanese researcher I. Kozai attributed about 3/4 of 2125 numbered asteroids to 72 families. American researchers J. Gradier, K. Chapman and J. Williams believe that the number of families exceeds 100. However, one has to be careful not to take a random group of points as a family.

For a long time it was believed that there is a family of Hungary (a=1.8 ae ) and Fokine (a=2.4 ae ) on high inclination orbits (intrinsic inclination 20-25O). However, in reality, these are only groups of random asteroids, isolated from the rest of the ring by empty zones of secular resonances.

Asteroids in them are not connected by a common origin in the same way as members of the groups of Gilda, Apollo, Cupid or Aten. They have only similar dynamical evolution of orbits. Asteroids in them are not connected by a common origin in the same way as members of the groups of Gilda, Apollo, Cupid or Aten.

They have only similar dynamical evolution of orbits. Asteroids in them are not connected by a common origin in the same way as members of the groups of Gilda, Apollo, Cupid or Aten. They have only similar dynamical evolution of orbits.

It is not yet clear whether the Pallas family exists, or whether we are again, as in the case of Hungary and Phocaea, dealing with a group of asteroids isolated by secular resonances.

Many families have dozens or hundreds of known members. It is assumed that the true number of family members is one to two orders of magnitude higher.

In the late 1960s, the astrophysicist H. Alven tried to identify falcons of recent origin in the asteroid ring (more precisely, in already known families).

To do this, he singled out orbits that are similar not in two, but in four proper elements (not counting the semi-major axis), including the proper longitude of the perihelion and the proper longitude of the node.

In the family of Flora I, Alven found 13 such asteroids (out of 23), and in the families of Flora II, III, and IV, he discovered two more groups, consisting of 20 and 28 asteroids. Similar groups were also found in other families. Alfven called them jet streams, or simply jets, or streams.

No matter how close the nodes of the orbits are at the moment of the formation of fragments during the fragmentation of the parent body of the family, due to small differences in the size of the orbits, after several hundred thousand years, the fragments will still be distributed more or less evenly over all longitudes.

Therefore, jet streams can be considered as young formations, indicating recent fragmentation that occurred already in the era of human existence on Earth. True, Alven himself holds a different opinion: he believes that jet streams are structural formations of bodies on the way to accumulation (unification).

Attempts to isolate jet streams were also made by other researchers. Using slightly different selection criteria, they got rather contradictory results: both the streams themselves and their members turned out to be different. This gives reason to doubt both the possibility of detection and the very existence of many of them.

The Soviet astrophysicist B.Yu. Levin showed that a significant part of the families and jets contains only one rather large asteroid, which stands out sharply from the rest of the smaller members of the family or jet.

Of the 54 families and jets considered by him, in 14 (26%) the largest member exceeds the rest in mass by an order of magnitude or more. In four cases (7%), the differences in mass turn out to be simply colossal – 1000 times or more.

This means that the head of the family has a diameter more than 10 times greater than the diameters of the rest of the asteroids. The heads of such families are Ceres and Vesta.

The emergence of such a family or jet stream can be associated with the collision of asteroids that differ greatly in mass, when a larger asteroid does not fall apart completely, but only loses a significant part of its mass in the form of fragments, as well as with oblique, almost tangential collisions of asteroids with comparable masses. in the latter case, the formation of families with two large members is possible. Such a family is containing 19 Fortune and 21 Lutetia.

But most of the families were formed, apparently, during the catastrophic destruction of asteroids that gave rise to these families, and do not contain similar asteroids – giants.

The fragments formed during the crushing of the asteroid, because of their slightly different heliocentric velocities, overtake each other, remaining in the vicinity of the orbit of the parent body. For several years or decades, they stretch along the entire orbit, forming a swarm. It’s funny that the surviving “parents” of families do not tolerate their “children”.

Parent asteroids scoop them out of the swarm, and due to the low relative speed (tens or hundreds of meters per second), the meeting of the asteroid with its fragment does not lead to further fragmentation: the fragment simply burrows into the regolith of its parents (regolith is understood to be the surface layer, ground by the fall of numerous small asteroid fragments).

However, this fate befalls very few. Besides, by gravitational influence, the parents expel their fragments to the periphery of the resulting swarm, reducing the spatial density of bodies in the swarm. A similar effect is exerted on the swarm and planetary perturbations.

However, with the formation of families during the fragmentation of asteroids, the situation is not at all as simple as it might seem.

When, in 1982, employees of the Pasadena Institute of Technology (USA) D. Davis, K. Chapman, R. Greenberg and S. Weidenshilling specifically investigated the issue of the formation of the Eos family, it turned out that the parent asteroid, whose dimensions exceeded, apparently, 180 km, before experiencing a catastrophic collision with a sufficiently large object (as a result of which the family should have formed), should have collided with at least a dozen smaller bodies.

Under the influence of their impacts, the parent asteroid should have “fallen apart” into blocks with characteristic dimensions on the order of 10 km, which were held near each other only by gravitational forces. Meanwhile, an object with a diameter of 98 km has been preserved (this is Eos itself).

It can be assumed that this is a surviving 20% ​​remnant of the mass, consisting of fragments that did not fly apart. But then, as the researchers believe, the next largest body would have to have a diameter of only 5 km. Meanwhile, the second largest member of the family has a diameter of 80 km. It is only by means of a series of highly artificial assumptions that these difficulties can be circumvented.

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