Asteroids, or small planets, are much inferior in size to such bodies of the solar system as Earth, Venus, and even Mercury. However, they cannot be considered full-fledged “residents” of our piece of the Galaxy.
Main belt
Asteroids of the Solar System are concentrated in several zones. The most impressive part of them is located between the orbits of Mars and Jupiter. This cluster of small bodies was called the Main Asteroid Belt. The mass of all objects located here by cosmic standards is negligible: it makes up only 4% of the lunar mass. Moreover, the largest asteroids make a decisive contribution to this parameter. Both their movement and the movement of their smaller cousins, as well as parameters such as composition, shape and origin, attracted the attention of astronomers at the beginning of the 19th century: Ceres, previously considered the largest asteroid, and now classified as a dwarf planet, was discovered January 1, 1801.
Beyond neptune
The Kuiper belt, the Horta cloud and the scattered disk began to be considered and studied as the places of accumulation of a large number of small celestial bodies a little later. The first of these is located beyond the orbit of Neptune. It was opened only in 1992. According to researchers, the Kuiper belt is much longer and more massive than a similar formation between Mars and Jupiter. Small bodies located here differ from the objects of the Main Belt in composition: methane, ammonia and water prevail over solid rocks and metals that are characteristic of the “inhabitants” of the Asteroid Belt.
The existence of the Orth cloud is not proven today, but this hypothesis is consistent with many theories describing the solar system. Presumably, the Horta cloud, which is a spherical region, is located beyond the orbits of the planets, at a distance of about a light year from the Sun. Space objects are located here, consisting of ammonia, methane and water ice.
The region of the scattered disk intersects somewhat with the Kuiper belt. Scientists do not yet know its origin. It also houses objects consisting of different types of ice.
Comparison of a comet with an asteroid
For an accurate understanding of the essence of the issue, it is necessary to separate two astronomical concepts: “comet” and “asteroid”. Until 2006, there was no certainty regarding the differences between these facilities. At the MAS General Assembly in that year, the comet and an asteroid were assigned specific signs that made it possible to more or less confidently attribute each cosmic body to a certain category.
A comet is an object moving in a very elongated orbit. When approaching the Sun as a result of sublimation of ice located near the surface, the comet forms a coma - a cloud of dust and gas, which grows as the distance between the object and the body decreases, and is often accompanied by the formation of a “tail”.
Asteroids do not form coma and, as a rule, have less elongated orbits. Those of them that move along trajectories similar to comets are considered the nuclei of the so-called extinct comets (extinct or degenerate comets are objects that have lost all volatile substances and therefore do not form coma).
The largest asteroids and their movement
There are very few really large objects by space standards in the Main belt of asteroids. Most of the mass of all bodies located between Jupiter and Mars falls on four objects - these are Ceres, Vesta, Pallas and Hygea. The first until 2006 was considered the largest asteroid, then it was assigned the status of a dwarf planet. Ceres is an almost round body with a diameter of about 1000 km. Its mass is approximately 32% of the total mass of all known objects of the belt.
The most massive object after Ceres is Vesta. In terms of size of asteroids, only Pallas is ahead of it (after the recognition of Ceres as a dwarf planet). Pallada is distinguished from the rest by an unusually strong tilt of the axis.
Hygea is the fourth largest and largest object in the Main Belt. Despite its size, it was discovered much later than several smaller asteroids. This is due to the fact that Hygea is a very dim object.
All these bodies revolve around the Sun in the same direction as the planets, and do not cross the trajectory of the Earth.
Features of the orbits
The largest asteroids and their motion obey the same laws as the movements of other similar bodies in the belt. Their orbits are constantly affected by the planets, the giant Jupiter has a particularly tangible effect.
All asteroids rotate in weakly eccentric orbits. The movement of asteroids exposed to Jupiter, passes through a few shifting orbits. These displacements can be described as oscillations around a certain middle position. For each such oscillation, the asteroid spends up to several hundred years, therefore, observational data today are not enough to refine and verify theoretical constructions. However, in general, the hypothesis of a change in orbits is generally accepted.
The result of the orbit shift is the increasing possibility of collisions. In 2011, data were obtained suggesting that Ceres and Vesta might collide in the future.
The largest asteroids and their movement are constantly under the scrutiny of scientists. The features of changes in their orbits and other characteristics shed light on some cosmic patterns, which in the process of data analysis are often extrapolated to objects larger than asteroids. The movement of asteroids is studied at the same time with the help of spacecraft, which temporarily become satellites of various objects. One of them entered the orbit of Ceres on March 6, 2015.