As of September 2017, there are 503,850 numbered minor planets with calculated orbits and another 245,833 unnumbered ones.
In 1596, Johannes Kepler noticed that the average radii of planetary orbits from Mercury to Saturn calculated by Copernicus are as 0.38: 0.72: 1.00: 1.52: 5.2: 9.2. The gap between Mars and Jupiter seemed too wide to Kepler, and he suggested that there was another planet there. This hypothesis was confirmed on New Year's Eve in 1801, when the director of the Palermo Observatory, Giuseppe Piazzi, spotted a dim star in the constellation Taurus, shifting in relation to neighboring luminaries. He mistook her for a comet, but soon doubted it. The German astronomer Johann Bode, with whom Piazzi shared his observations, considered this body a new planet, which he announced in a monthly magazine published by the director of the Gotha Observatory, Baron Franz von Zach. Bode and Zach were convinced before that the space between Mars and Jupiter hides an unknown planet;moreover, in September 1800, Zach convinced several German astronomers to take part in a collective search for it. Later, other scientists, including Piazzi, joined this group (calling themselves the "heavenly police").
In addition to the eight planets, the solar suite includes a great variety of bodies of lesser mass and size. Some of them consist of dust and frozen gas (these are comets), the rest are composed of solid matter (minor planets, or planetoids). Some of them, with very rare exceptions, do not go beyond the orbit of Jupiter from the Sun, while others, on the contrary, walk along the periphery of the Solar System. By tradition, the minor planets of the first group are called asteroids.
Piazzi did not have time to collect enough data to calculate the orbit of the alleged planet, which had left the European sky by the fall of 1801. Nevertheless, Bode's note prompted the great mathematician Karl Friedrich Gauss to begin work on a computational method that required fewer observational data than conventional calculations. He sent his results to von Zach, who with their help rediscovered the fugitive on January 1, 1802, exactly one year after Piazzi. On the same night she was observed by another member of the "heavenly police" Heinrich Olbers. At the request of Piazzi, the new celestial body was named after the Roman goddess of fertility Ceres, who was considered the patroness of Sicily.
Olbers continued to observe Ceres and on March 28, 1802 noticed another moving point in the vicinity. She received the name of Pallas, the Greek goddess of wisdom. When Gauss calculated the elements of its orbit, it became obvious that Olbers was fantastically lucky. Pallas revolves around the Sun in almost the same time as Ceres (4.6 Earth years), but its trajectory is tilted to the ecliptic plane by 34 degrees. Had she not been during Olbers' observations near Ceres, she could have been discovered only after several decades. Within five years, two more such celestial bodies were discovered. But after that, the "sky police" broke up. Olbers held out longer than others, but he also left the asteroid hunt in 1816. It resumed only in the middle of the 19th century, when the discoverers were no longer alive.
"Like the Stars"
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In a letter to William Herschel, he suggested that Ceres and Pallas are fragments of a planet that died from an explosion or from a collision with a comet. From this it followed that there would be other solar satellites between Mars and Jupiter. Herschel suggested calling them asteroids, which translated from ancient Greek means "like stars" (he meant that these bodies are much inferior to planets in brightness and therefore it is difficult to distinguish them from most stars). This neologism entered the language of astronomy.
Olbers's hypothesis predicted the existence of new asteroids, so the Sky Police continued their search. Participants of this collective research project (by the way, the first in the history of astronomy) discovered two more asteroids, which also received the names of Roman goddesses. On September 1, 1804, Karl Harding discovered Juno, and on March 29, 1807, Olbers captured Vesta. The right to choose the name of the fourth asteroid was given to Gauss, who calculated its orbit in just a few hours (it is not easy to keep within such a timeframe even with the help of a modern calculator!).
Hunting season
In 1830, the mathematician and astronomer Friedrich Wilhelm Bessel appealed to German observatories to begin mapping the sky in order to search for asteroids. Something was done in this direction, but the first find went not to a professional, but to an amateur, postmaster Karl Henke. On December 8, 1845, after 15 years of fruitless observations, he discovered the fifth asteroid, Astrea. In 1847, the same Henke spotted asteroid number 6 - Hebu, and soon the young English astronomer John Russell Hind discovered the asteroids Iris and Flora. After that, the search for minor planets quickly gained momentum. The first American hunter for these bodies, Christian Peters, discovered 48 asteroids from 1861 to 1889, and the German astronomer Karl Luther - 24. By 1890, about three hundred inhabitants of the space between Mars and Jupiter were included in astronomical catalogs.
And then a new era began. Privat-docent at the University of Heidelberg, Maximilian Wolf, was the first in the world to use photography to search for minor planets. In December 1891, he discovered his first asteroid, and the next year - already 13. In 1902, Wolff headed a new university observatory and turned it into the world center of "minor planetology". His younger colleague Karl Reinmuth discovered 389 asteroids from 1912 to 1957, and no one could beat this record.
In the period between the two world wars, the search for asteroids was extremely intense, and in the 1930s alone brought nearly four hundred discoveries. Then he slowed down - for a long time, about thirty years. Its revival was facilitated by equipping telescopes with semiconductor photometers and other electronic devices and the emergence of powerful computers capable of quickly calculating asteroid orbits. Recently, ground-based robotic telescopes, orbital observatories and distant space probes have been used to study small planets.
Asteroid classes
Information about the structure of asteroids is based on the results of spectral analysis of the reflected sunlight, corrected by geochemical data on the composition of meteorites (since asteroids are their main source). According to this criterion, they are divided into three main classes: C (bodies with a high carbon content), S (silicates with an admixture of metals), and M (mostly iron-nickel asteroids). Class C accounts for three quarters of asteroids in the main belt, class S - 17%. However, there are more detailed classifications with a much larger number of groups.
All asteroids without exception rotate, and their axes are oriented in space quite randomly. Usually, the duration of an asteroid day is from 6 to 13 hours, but there are exceptions. For example, the tiny (about 30 meters across) asteroid 1998 KY26 makes a complete revolution in 10 minutes 42 seconds. Most likely, he gained such a high angular velocity as a result of multiple clashes with his closest relatives.
Main belt
The orbits of almost all asteroids lie within the ring, the inner radius of which is equal to two astronomical units, and the outer one - three and a half (strictly speaking, this is not a ring, but a donut, since the paths of many asteroids go beyond the ecliptic plane). This zone is called the main asteroid belt. It contains about two hundred minor planets, the average diameter of which is more than 100 km. According to rough estimates, there are 1-2 million asteroids at least a kilometer in size. And the total mass of the inhabitants of the main belt is about 25 times less than the mass of the Moon!
The spatial distribution of asteroid trajectories in the main belt is far from uniform. First, there are cracks opened in the 1860s by Indiana University professor Daniel Kirkwood. Based on a study of the trajectories of 97 asteroids, Kirkwood found that these bodies avoid orbits with periods commensurate with the period of Jupiter (for example, if these periods are related as 1: 3). Kirkwood also understood the reason: such bodies periodically approach Jupiter on the same part of their trajectory and, as a result, under the influence of its gravity, they go astray from their previous trajectory (this effect, noted by Laplace at the beginning of the 19th century using the example of Jupiter's moons, is called orbital resonance). In the main belt there are Kirkwood slots (in the Russian-language literature they are also called hatches) and with other resonances - 1: 2, 2: 5, 3: 5, 3: 7. Secondly,no less than a third of the asteroids there are grouped into families with close orbital elements (such as the length of the semi-major axis, eccentricity and inclination of the orbit to the plane of the ecliptic). The first of these families, almost a hundred years ago, was isolated by a professor at the University of Tokyo, Kiyotsugu Hirayama. Hirayama believed that each family consists of fragments of a larger asteroid that disintegrated due to a collision with a smaller body, and this interpretation is still considered the most plausible.disintegrated due to a collision with a smaller body, and this interpretation is still considered the most plausible.disintegrated due to a collision with a smaller body, and this interpretation is still considered the most plausible.
Asteroids of the main belt are likely to collide even now (however, it was not possible to see it live yet), in the past collisions were the most common thing. Many (if not all) asteroids are fragments of their predecessors. This explains why there are not many asteroids in the belt that have their own satellites. As Clark Chapman, a senior researcher at the Southwest Research Institute in Colorado, told PM, their share does not exceed 15% (versus 75% for planets). Most likely, asteroids lose their moons not only during direct collisions, but also due to gravitational disturbances caused by the appearance of neighbors. The chaotic distribution of the axes of rotation of asteroids is also the result of collisions. Only Ceres, Pallas and Vesta have direct rotation inherited from the primary preplanetary swarm,from which both asteroids and planets were formed. They kept this rotation due to the impressive mass, which provides them with a large angular momentum.
Trojan asteroids
Almost all asteroids discovered in the 19th century move within the main belt. The only exceptions are Efra and Eros, which cross the orbit of Mars. There were no other examples of escaping from intra-belt captivity at that time.
The XX century brought changes here too. On February 23, 1906, Wolff photographed a very faint asteroid moving in an almost circular orbit with the same radius as Jupiter's, 55.5 degrees ahead of the planet. He was named Achilles and received the number 588. Soon the Swedish astronomer Carl Charlier realized that in his movement Achilles was tied to one of two points of stable libration predicted in 1772 by Joseph Louis Lagrange. Achilles periodically returns to the vicinity of the libration point L4, which moves 60 degrees ahead of Jupiter. After a while, the asteroid Patroclus was discovered there, and Hector was found near the L5 point moving 60 degrees behind the planet. Soon after this, a tradition arose to name these asteroids in honor of the heroes of the Trojan War - near the libration point L4 by the names of the Achaeans (Achilles, Nestor, Agamemnon, Odysseus, Ajax,Diomedes, Antilochus, Menelaus), and near the libration point L5 - the names of the defenders of Troy (Priam, Aeneas, Antif). However, this tradition did not appear immediately, so that Hector and Patroclus eventually remained in the "enemy camps".
To date, about 5,000 Trojans have been discovered near Jupiter. The angular distance between them and Jupiter varies widely - from 45 to 100 degrees. Four more Trojans live near Mars and eight in the orbital zone of Neptune. In July 2011, Canadian astronomers named the first candidate for the title of our planet's Trojan partner. This 300-meter asteroid 2010 TK7 was captured by the WISE infrared telescope, which operated in low-Earth orbit from January to October 2010.
Near-Earth asteroids
Another phase of discovery began in the spring of 1932. On March 12, the Belgian astronomer Eugene Delport discovered the asteroid Amur, which approaches the Sun at 1.08 AU at perihelion. and therefore almost touches the outer side of the earth's orbit. And just six weeks later, Karl Reinmuth stumbled upon the asteroid Apollo, whose orbit crosses both Earth and Venus and at perihelion is only 0.65 AU from the Sun.
Cupid and Apollo became the ancestors of two families of minor planets that visit the inner regions of the solar system. They have a common name - Near-Earth Asteroids (NEAs). The perihelion of the Amor-type asteroids ranges from 1.3 AU. up to the maximum radius of the earth's orbit equal to 1.017 AU. Apollo-type asteroids include bodies with a perihelion less than 1.017 AU. and a semi-major axis exceeding 1 AU. There is also a family of near-Earth asteroids, whose semi-major axis is less than one astronomical unit. Approximately 50% of such asteroids, the first of which was discovered in 1976 and named after the Egyptian god Aton, still move away from the Sun more than the Earth, since they move along ellipses with a large eccentricity. Among the atons, a subfamily of asteroids is distinguished,whose apogee is less than the minimum radius of the earth's orbit, 0.983 AU. These bodies, naturally, are always closer to the Sun than our planet.
The orbits of near-earth asteroids are very diverse. Some of them periodically return to the main belt and sometimes even go much further, while others invariably keep closer to the Sun. Such, for example, is the asteroid 1685 Toro with an apogee of 1.96 AU. and perihelion 0.77 AU. It crosses the orbits of Earth and Mars, and it lacks only 0.05 AU. e, to get to the orbit of Venus. It takes him 8 Earth and 13 Venus years to make five revolutions around the Sun, so Toro is in orbital resonance with both planets. There are even asteroids daring to approach the Sun closer to Mercury. Such is the asteroid 1566 Icarus from the Apollo family, discovered in 1949 by the American astronomer Walter Baade.
Unfinished planets
Asteroids are, in a sense, unfinished planets. Both were once formed from colliding and merging planetesimals, solid bodies ranging in size from a meter to a kilometer, orbiting a newborn Sun. These bodies, in turn, arose due to the adhesion of particles of the primary gas and dust cloud, from which the solar system was formed. In the zone beyond the orbit of Mars, planetesimals were unable to unite into a large planet. Most likely, this was due to gravitational disturbances from Jupiter, although other mechanisms could have worked. In particular, it is possible that Jupiter more than once ejected large bodies towards the Sun, which also destabilized the asteroid belt.
The first asteroids, arising directly from the planesimals, moved in the plane of the ecliptic along almost circular orbits and had low relative speeds. That is why they did not split in collisions, but stuck together and grew. However, the gravity of Jupiter gradually forced the asteroids to move to inclined orbits with a large eccentricity, because of which their relative speed increased to 5 km / s (this is what it is now). When colliding at such a speed, the asteroids shattered into fragments that had no chance of starting a real planet.
These processes have radically changed the asteroid belt. Its initial mass is not known exactly, however, according to model calculations, it could be 2200 times the current mass and approximately equal the mass of the Earth. The same calculations demonstrate that there were hundreds of bodies, in mass and size not inferior to Ceres. These bodies died in the collisions, and their debris went into unstable orbits and left the belt. In the end, it thinned so much that collisions became rare, and the surviving asteroids remained on fairly stable trajectories. So the current main belt is a pale shadow of its former splendor.
Clark Chapman noted that, according to a number of planetary scientists, at one time another belt could exist between the Earth and Venus. However, these asteroids were much more difficult to survive. It can be assumed that almost all of them split after collisions, and their fragments were thrown away from the Sun.
Nickel iron fever
Science fiction writers have long predicted, so to speak, the national economic development of asteroids - recall, for example, Azimov's story "The Way of the Martians." This is understandable. The asteroid belt contains gigantic reserves of the purest water ice and a great variety of minerals. One cubic kilometer of the substance of a typical M-class asteroid contains 7 billion tons of iron, a billion tons of nickel and millions of tons of cobalt. The total cost of these metals at today's prices is over $ 5 trillion. It is hoped that if humanity gets to these resources, it will dispose of them wisely and with real benefit.
Alexey Levin
