Short and long-period comets

The diversity of comets in the Solar system

The comet (from al-Greek. hairy, shaggy) – a small celestial body having a foggy view, orbiting the Sun in conic section with a very extended orbit. When approaching the Sun, the comet forms a coma and sometimes a tail of gas and dust.

Comets are placed into circulation periods:

1. Short-period

At this time, found more than 400 short-period comets. Of these, about 200 were observed in more than one perihelion passage. Short-period comets (period less than 200 years) come from the region of the outer planets, moving in the forward direction for orbits lying close to the Ecliptic. Away from the Sun, comets usually do not have “tails”, but sometimes have a barely visible “to”, surrounding the core; together they are called the “head” of the comet. Approaching the Sun the head is increased and there is a tail. Many of them belong to so called family. For example, most of the short-period comets (full rotation around the Sun lasts 3-10 years) form the family of Jupiter. A little smaller of a family of Saturn, Uranus and Neptune (the latter, in particular, is the famous comet Halley).


– the family of Jupiter

– the family of Saturn

– a family of Uranium

– the family of Neptune

With the passage of the comet near the Sun its nucleus heats up, and ICES evaporate, forming a gaseous coma and tail. Polenishki hundreds or thousands of such openings in the core remains low-melting substances, and it ceases to be visible. For regular near-Sun short-period comets that means less than a million years their population has become invisible. But we observe, therefore, constantly supplied to the replenishment of “fresh” comets.

The replenishment of short-period comets is the result of their “capture” planets, mainly Jupiter. Previously it was thought that comets are captured from among the long-period, coming from the Oort cloud, but now believe that their source is cometary disk, called the “inner Oort cloud”. In principle, the idea of the Oort cloud has not changed, but the calculations showed that the tidal effect of the Galaxy and the effects of massive interstellar clouds should quickly destroy it. The correct source of its replenishment. This source is now considered an inner Oort cloud, is much more resistant to tidal influence, and contains an order of magnitude more comets than predicted by Aorta external cloud. After each convergence of the Solar system with a massive interstellar cloud of comets from the outer Oort cloud fly away into interstellar space, but they are replaced by comets from the inner cloud.

Transition comets with nearly parabolic orbits to short-period occurs, if it catches up with the planet behind. Usually to capture the comet on a new orbit requires several passages through the planetary system. The resulting orbit of the comet, as a rule, has a small inclination and a large eccentricity. The comet is moving in a forward direction, and the aphelion of its orbit (farthest point from the Sun) lies near the orbit of the planet gripped her. These theoretical considerations are fully confirmed by the statistics of cometary orbits.

2. Long-period

Presumably, long-period comets fly to us from the Oort Cloud, which is a huge number of cometary nuclei. Body, located at the outskirts of the Solar system, as a rule, consist of volatiles (water, methane, and other ICES), evaporating at approach to the Sun. Long-period comets (orbital period greater than 200 years) come from regions located thousands of times further than the most remote planets, and their orbits are tilted at all angles.

This class contains many comets. Because their periods of revolution are millions of years, for a century in the neighborhood of the Sun appears only one ten thousandth part of them. In the 20th century has been observed for about 250 of these comets; therefore, only millions of them. Besides, not all comets approach the Sun enough to be visible: if the perihelion (closest to Sun point) of the comet’s orbit lies beyond the orbit of Jupiter, noting it is almost impossible.

With this in mind, in 1950 Jan Oort suggested that the space around the Sun at a distance of 20-100 thousand.e. (astronomical units: 1 and.e. = 150 million km, the distance from the earth to the Sun) filled with nuclei of comets, the number of which is estimated at 10 12. and the total mass – in 1-100 Earth masses. The outside border of the “cometary cloud” Oort determined that at this distance from the Sun on motion of comets is significantly influenced by the gravity of neighboring stars and other massive objects. Stars are moving relative to the Sun, their perturbing effect on comets is altered, and this leads to the evolution of cometary orbits. So, by chance the comet may be in orbit, passing near the Sun, but in the next development of its orbit will change a little, and the comet away from the Sun. However, instead of the Oort cloud in the neighborhood of the Sun will constantly get “new” comets.

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