Entering the Void

From the book “Asteroids: Born of Flame”

Nothing calms your nerves and puts you in a philosophical mood like reading about what happened billions of years ago with objects on a scale of tens of billions of kilometers. We publish an excerpt from the book of astronomer Leonid Elenin, dedicated to the time before the birth of the Sun.

All of us, since February 24, 2022, have found ourselves in the face of advancing barbarity, violence and lies. In this situation, it is extremely important to preserve at least the remnants of culture and support the values ​​of humanism - including for the sake of the future of Russia. Therefore, the editors of Gorky will continue to talk about books, reminding our readers that there is still a place for thought and fiction in the world.

Leonid Elenin. Asteroids: born of flame / M.: Eksmo, 2025. Contents

Asteroids were lucky: they were discovered before a real revolution in astronomy - the invention of photography and spectroscopy. In contrast to the thousand-year history of understanding the essence of comets, the centuries-old history of asteroids seems to us like a scientific spark. Imagine: humanity soft-landed a spacecraft on the surface of the first near-Earth asteroid, discovered in 1898, just 103 years later! And this despite the fact that comets for more than one and a half thousand years were considered “signal fires of the gods” and were not recognized as cosmic objects equal to planets. In fact, the history of the present study of these two related types of small bodies in the Solar System began almost simultaneously. In this chapter, I will tell you about the even larger history of the formation of these bodies - about where it all began and where it has come to. What are asteroids from a physicochemical point of view? Where were they formed and where did they end up in our era? We will go with you 4.5 billion years ago, to where, in the darkness of the gas and dust cloud, even before the birth of the Sun, there was already the substance from which the main characters of this book were subsequently formed. Time is back!

Black silence reigned in the empty space, sparkling with the lights of distant, perhaps long-dead stars. This is how it would look to an observer if he found himself where, in billions of years, the furious Sun will rage, incinerating the fragile comets falling on it with its heat. In fact, around our observer there would be a lot of gas and some dust thrown into the vast space by long-dead stars, but this matter would be so rarefied that only a few kilograms of it would fit in the volume of our planet. Yes, now we would say that this is a vacuum. All the more amazing is the fact that from this “nothing” our entire planetary system emerged, from the Sun to the unimaginably distant asteroids of the trans-Neptunian belt and the icy bodies of the Oort cloud.

If the matter of this nebula were illuminated by a nearby star, then distant astronomers of worlds unknown to us, with their powerful telescopes, would be able to examine interesting processes occurring in deep space, where a new planetary system was just beginning to emerge. This cloud was not static: it slowly swirled under the influence of the stellar winds blowing in outer space and the powerful magnetic field that our gigantic “emptiness” already possessed. The gas flowed, inhomogeneities formed in it - clots of matter. As soon as these compactions gained a certain mass, Her Majesty gravity came into play. This was a turning point: there was no turning back. The “nodules” of gas became more massive, which means their gravitational force also increased. Larger clots “ate” smaller ones and became even stronger. In the end, only one remained - the one that would become the Sun.

The clot became more and more massive, drawing more and more surrounding gas into its circulation. We observe this rotation now: its angular momentum has been preserved to this day. The overwhelming majority of the mass of the modern Solar System rotates in one direction - counterclockwise, when viewed from the “north pole” of our planetary island, lost in one of the small arms of the Galaxy. So, the funnel grew, and the substance in its center became denser - the pressure and temperature increased. In some areas of the clump, when the temperature exceeded 3 million degrees, hydrogen began to ignite, but this was not yet the birth of a star. The gravitational collapse continued: the protostar seethed until the temperature at its center exceeded about 15 million degrees. It was then that the self-sustaining thermonuclear reaction of converting hydrogen into helium began: a star was born, illuminating the hot protoplanetary disk rotating around it with its first light...

It was huge: its diameter was several tens of billions of kilometers. Due to the enormous temperatures, almost all solid particles formed before the birth of the Sun melted and later completely evaporated. But still something remains. The gas disk, releasing heat into outer space, gradually cooled, and approximately 100 thousand years after the birth of our star - an instant by the standards of the age of the Universe - the gas began to gradually condense into solids: minerals. The first of them - corundum (Al2O3) - contained a mixture of oxygen (O2) and aluminum (Al). Its life was short-lived: the gas that continued to cool began to interact with it, slowly destroying it. But in the Universe nothing disappears without a trace. Corundum gave rise to a new mineral - already with an admixture of calcium (Ca) - hibonite (CaAl12O19), and others followed. Even further from the Sun, where the temperature was lower, dust grains of iron (Fe), magnesium (Mg), silicon (Si) and olivine (MgFe)2[SiO4] began to form. Behind them, where it was even colder, there is potassium (K), sodium (Na) and grains of feldspars (K[AlSi3O8] - Na[AlSi3O8] - Ca[Al2Si2O8]) - rock-forming minerals that make up half the mass of the crust of our planet. About 500 million kilometers from the Sun, where the Main Asteroid Belt now ends, particles of water ice (H2O) condensed, and even further, where the light of the young Sun was very weak, the main component of all organic matter, carbon, and tiny floes of frozen ammonia (NH3) and methane (CH4).

Just as in our era the solar wind and the pressure of sunlight flutter the long tails of comets, so during the time of the newborn Sun everything was approximately similar, however, there were no comets themselves, as well as other inhabitants of the solar system. The light of our star was already driving away tiny particles of dust: mixing of high-temperature and low-temperature condensates, which initially formed at different distances from the star, began to occur. Turbulent gas flows from the very young Sun also mixed up the dust. The force of static electricity attracted tiny specks of dust, and they began to gather into shapeless flakes several millimeters and even centimeters in size. These flakes collided, but often did not collapse, but rather joined together. Even larger clumps of solid matter began to appear. When the largest of them reached a certain mass, gravity again came to the fore. Everything was repeated again, but now on a much smaller scale.

This process took millions of years - not so much by the standards of space. As a result, the first truly large objects appeared in the Solar System - planetesimals. Those that formed closer to the Sun consisted exclusively of rock; behind the snow line, water-containing minerals and ice were added to the dried stone. Now these bodies, kilometers and tens of kilometers in size, were joined together during inelastic collisions. And those that were destroyed eventually became part of their more fortunate brothers. They grew, and the force of their gravity also grew, collecting unused dust and fragments of destroyed bodies. When the “successful” planetesimals reached a size of 250–300 kilometers, under the force of self-gravity they began to take a spherical shape. These were already the embryos of future planets, which were repeatedly united or destroyed, but were later brought together again to eventually form planets. Before the snow line there are stone ones, and behind it there are less dense, but much more massive gas giants.

When the planets absorbed all the planetesimals that they could “reach” by the force of their powerful gravity, they stopped growing rapidly. Although, of course, collisions in the solar system were commonplace at that time. The planets themselves actively migrated, which I described in detail in the book “Comets. Wanderers of the Solar System,” so I won’t repeat myself here. Jupiter was originally further from the Sun than it is now. Small bodies constantly approached him, which he threw out by the force of his gravity to the periphery of the Solar system and even away from it, and thus he himself slowly moved in the opposite direction, closer to the Sun, to where there was still a lot of unspent “building material”. This material also stuck together and increased in size, but all attempts to build another planet were thwarted by the powerful gravity of Jupiter, which prevented another large body from coming together.

We can study such an ancient dynamic history of our planetary system only through mathematical modeling. With its help, scientists concluded that in the region between Mars and Jupiter, dozens of “planetary embryos” with a diameter of 500 to 1000 kilometers could have formed, and the total mass of matter, including smaller bodies, was about 4–5 Earth masses. But in the end, most of the matter was scattered by Jupiter and Saturn, who fought in the “great gravitational battle,” and entered orbital resonance. The chaos that reigned then, in which bodies constantly changed their orbits and collided with each other, led to the fact that the belt of small bodies between the orbits of Mars and Jupiter lost up to 99% of its total mass, and large “embryos” of planets were simply destroyed in terrible cosmic collisions. Now we know this belt of “surviving” relict bodies that did not collide with the Sun, did not become part of one of the planets of the Solar System and were not thrown onto its periphery or into interstellar space as the Main Asteroid Belt.

Having gone through the crucible of the formation and evolution of the Solar System, it became completely different than it was then, when there was still a chance for the formation of another planet between the orbits of Mars and Jupiter. What does it represent from a dynamic point of view in our era? After the end of the migration of the giant planets, when they all found their equilibrium position, the structure of this belt remained almost unchanged, although it gradually lost and is still slowly losing its mass. How does this happen? If we look at the distribution of the Main Belt asteroids known to us by their average distance from the Sun (the semimajor axis of their orbits), we will see that, although it does not have clearly defined boundaries, the vast majority of cataloged objects are located in the range from 2.1 to 3.3 AU. that is, from the Sun, and they are clearly separated by some kind of barriers invisible to the human eye. These barriers are built by gravity, and each of them corresponds to a certain orbital resonance with the king of all planets - Jupiter.

Resonances are the determining force in the dynamic evolution of the Solar System: once upon a time they “forced” entire planets to migrate! After Jupiter took its position, some of the asteroids orbiting in front of its orbit were caught in strong integer resonances, when for one revolution of the giant there were several revolutions of the small body. In this book I often say: “Jupiter ejected some object.” But how does this happen if gravity is an attractive force? This is not a dipole interaction, as in magnetism, when unlike poles attract and equal poles, on the contrary, repel each other.

The thing is that when the Sun, asteroid and Jupiter line up on the same line (for simplicity, we neglect the inclination of their orbits), scientifically this is called opposition. At this moment, the asteroid and the planet are at a minimum distance from each other (we remember that the orbits of planets, especially asteroids, are not circles, but ellipses). At this moment, Jupiter’s gravitational influence on the asteroid is maximum: it attracts it towards itself, gradually “stretching” its orbit - increasing its eccentricity. From the two simplest formulas for calculating the distances of perihelion and aphelion using the Keplerian elements of their orbits, it follows that the planet thereby reduces the perihelion distance and increases the aphelion distance of the orbit of an asteroid that is in orbital resonance with it. With each opposition, this gravitational influence increases, because at aphelion the asteroid comes closer and closer to the giant planet, and its orbit elongates so much that the asteroid begins to cross the orbit of Mars, which, in turn, “helps” guide the space guest to the Earth’s orbit and beyond. As a result, the asteroid's orbit may change completely, moving so close to the Sun that it will end up inside the Earth's orbit.

This is how, over time, Jupiter “threw out” almost all objects from the resonance zones, creating desert areas where there are almost no asteroids - Kirkwood hatches, named after the American scientist Daniel Kirkwood. He expressed his brilliant hypothesis back in 1857, when the region between the orbits of Mars and Jupiter was just beginning to be called the “asteroid belt,” and about fifty of them were known in total. Over a dozen resonances have now been discovered and studied. Some of them are classified as strong, and in these areas of space there are almost no asteroids, and the bodies that get there as a result of slow migration or collisions, by cosmic standards, are quickly, over thousands of years, “thrown out” to the outside. Such resonances include orbits with an average distance from the Sun of 1.78 AU. e. (5:1), 2.07 a. e. (4:1), 2.5 a. e. (3:1), 2.83 a. e. (5:2), 2.96 a. e. (7:3) and 3.28 a. e. (2:1).

One of the strongest resonances, which scientists consider the main mechanism for feeding groups of asteroids approaching the Earth, is the 5:2 resonance (for every 5 revolutions of the asteroid there are exactly 2 revolutions of Jupiter). Once Jupiter increases the eccentricity of its nearby objects so much that they begin to approach Mars and Earth, these two planets also begin to contribute to the migration of asteroids towards the Sun. As a result, most of them fall on our star. These are real “space moths”. The characteristic time during which a Main Belt asteroid caught in a 5:2 resonance will become, according to the modern classification, a near-Earth asteroid (Near-Earth asteroid, NEA) is only about 10 thousand years—an instant by the standards of the lifetime of the Solar System.

* Top image: NASA/JPL-Caltech