Solar system - Comets

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SOLAR SYSTEM
Jiri W a g n e r , Jr., wag@volny.cz

Comets

Comet

What Are Comets?
A comet is an object in the night sky that resembles a fuzzy star and travels along a definite path through the solar system. Comets have highly elliptical orbits that bring them very close to the Sun and swing them deeply into space, often beyond the orbit of Pluto. They are small, fragile, irregularly shaped bodies composed of a mixture of non-volatile dust, ice and frozen gases. Thus, they are sometimes called dirty snowballs or icy mud-balls.
Structure of a Comet
A comet has a distinct centre called a nucleus that measures less than 20 kilometres in diameter. A hazy cloud called a coma surrounds the nucleus. The coma and nucleus make up the comet's head. When a comet approaches the Sun, it becomes a spectacular sight to behold. The surface of the nucleus begins to warm, and volatiles evaporate. The evaporated molecules boil off and carry small solid particles with them, forming the comet's tail of gas and dust. The Sun's radiation pressure and solar wind accelerate materials away from the comet's head at differing velocities according to the size and mass of the materials. Thus, relatively massive dust tails are accelerated slowly and tend to be curved. The ion tails is much less massive, and is accelerated so greatly that it appears as a nearly straight line extending away from the comet opposite the Sun. Therefore, each time a comet visits the Sun, it loses some of its volatiles. Eventually, it becomes just another rocky mass in the solar system. Many scientists believe that some asteroids are extinct comet nuclei, comet that have lost all of their volatiles.
Paths of Comets
Most comets travel around the Sun in elliptical paths. The time it takes a comet to make a complete orbit is called its period. Some comets have short periods of less than seven years. Others travel in such huge orbits that they pass near the Sun only once in thousands or even millions of years. As of 1995, 878 comets have been catalogued and their orbits at least roughly calculated. Of these 184 are periodic comets (orbital periods less than 200 years); some of the remainder are no doubt periodic as well, but their orbits have not been determined with sufficient accuracy to tell for sure. No comets seem to have approached the Sun from beyond the limits of the solar system. Therefore, all comets seen by astronomers are considered part of the solar system.
Origin
Where do comets come from? Any theory of their origin must first and foremost explain the great predominance of very elongated ellipses among their orbits. According to some early theories, comets were of planetary origin. Other theories ascribe an interstellar origin to comets. Today comets are regarded rather as residues of the primitive solar nebula coming from the outer reaches of the solar system. The most widely accepted theory, developed by the Dutch astronomer Oort, is based on a statistical study of about 50 of the best-known semi-parabolic cometary orbits. This study reveals a remarkable predominance of the orbits in which the aphelion (the furthest point of a comet from the Sun) is at a distance from the Sun ranging from 0.6 to 2.3 light-years. According to Oort, this region would constitute a vast reservoir of as many as 100 billion comets. As a result of the perturbations created by nearby stars, comets would gradually emerge from this reservoir. Some would be sent toward the Sun and would constitute the 'new' comets we observe. The formation of the Oort cloud, however, remains to be explained.
How Often Do We See Comets?
Each year, an average of four to five new comets are discovered and several more old ones make their return visits. Very few comets are bright enough to be visible without a telescope and only rarely is a comet spectacular enough to attract everybody's attention. Most of the time, if you want to see a comet, you have to look for it.
How Are Comets Named?
A comet goes by many names, the most important of which to its discoverer is his or her surname. Many bright comets that frequently return retain their popular names, such as Comet Halley. (Halley did not discover the comet, but he predicted its orbit and return.) Frequently, two or more observers discover a comet simultaneously or independently. The names of the first three observers to report their discovery properly are affixed to the new comet.
The rapid growth in the number of known comets has prompted the International Astronomical Union (IAU) to catalogue them according to precise rules. A new cometary designation system has been adopted with effect from 1 January 1995. The new system resembles that of minor planets, with objects recorded by the half-month. Thus the third comet reported as discovered during the second half of February 1997 would be designated 1997 D3. When appropriate, the nature (or suggested nature) of an object can be indicated by preceding the designation with A/ (for minor planet), C/ (for comet), P/ (for periodic comet), etc. Hence, under the new cometary designation system, Comet Hale-Bopp was designated C/1995 O1.

List of important comets - 75 kB

Comet Wild 2 is shown in this image taken by the Stardust navigation camera during the spacecraft's closest approach to the comet on January 2 2004. The image was taken within a distance of 500 kilometers (about 311 miles) of the comet's nucleus:

Source: JPL NASA Stardust

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