Family matters
In the past few decades, astronomers have been actively monitoring asteroids and conducting a kind of "census" among them. All large objects, whose total number reaches about two million, are well known to scientists, while small bodies the size of the Chelyabinsk meteorite are still 99% unexplored.
For example, now we know only about five thousand asteroids about a hundred meters in size, approaching the Earth, while their total number is estimated at several tens of thousands. The number of smaller objects within the main asteroid belt can be even larger and reach several tens of millions.
Scientists combine all this countless number of objects into the so-called "families of asteroids" - a group of small celestial bodies with similar orbits, origin and chemical composition. Today, astronomers distinguish nine "large" and about a hundred small similar "groups".
Some of them are believed to be the source of various meteorites that periodically fall to Earth. For example, rocky meteorites, almost free of iron, the so-called L-type chondrites, were considered traces of the collision of the asteroid Gefien and some other celestial body at the time of the appearance of the first vertebrates on Earth.
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Jenniskens and his colleagues accidentally discovered that these "celestial stones" have not one, but at least two "homelands" within the asteroid belt, checking the work of automated cameras of the meteorite tracking system, which they developed four years ago.
This story, as the scientist notes, actually began in the spring of 2012, when his team witnessed the fall of a fairly large meteor, Sater Mill, on California. This event attracted the attention of hundreds of astronomy enthusiasts and professional scientists who began to systematically monitor the night sky of the southern United States.
Their efforts were rewarded in the fall of 2012, when another "heavenly stone" exploded over North America, dubbed Novato. Analysis of its rocks showed that it belongs to the L-type chondrites, but at the same time it has a rather unusual structure and composition.
Secrets of the Solar System
The oddities of Novato made Jenniskens wonder if all such meteorites come from the same source. He unexpectedly received an answer to this question at the end of October 2015, when the automated cameras of the Global Fireball Observatory system, which at that time were launched by the staff of the SETI Institute, recorded an outbreak over California.
The new meteorite, named Creston, was also one of the L-type chondrites. This allowed scientists to check whether they are related by comparing their physical structure, isotope fractions, and calculating the orbits of their progenitors.
It turned out that both "heavenly stones" have a similar composition, but different origins - "Novato" was born in the middle of the asteroid belt, while "Creston" lived in its regions close to the Earth. In this case, the progenitor of the first meteorite relatively recently collided with another object and disintegrated into fragments, while the forerunner of the second one collapsed in the first days of the life of the solar system and after that did not get into "cosmic accidents".
Several things speak in favor of this. For example, the rocks "Novato" are colored in dark colors and contain almost no noble gases, which indicates a relatively recent "rendezvous" with another asteroid. The similarity of their chemical composition can be explained by the fact that both progenitors of meteorites were generated by the same protoplanetary body, some of the fragments of which "migrated" to a new orbit, while others remained in place.
All this, as Jenniskens notes, suggests that the Gefien family cannot be the only progenitor of these chondrites, which account for about half of all meteorites falling to Earth. Something similar, in his opinion, may be characteristic of other families of "heavenly stones", which makes their study more difficult and interesting.