The comets are between more characteristic the celestial bodies and fascinating, thanks to their long luminous tail. They have always fascinated and scared the man for their aspect and their unexpected apparition in sky. According to the ancient popular sideboards, the comets were carriers of misfortunes, plagues and wars. In reality, they are not other that innocuous “dirty snow balls”, composed from cliffs stirred to gas freeze to you, water, methane, ammonia

and powder.

The comet Hyakutake, appeared in spring 1996

The comets come from with of million and million full of rocks bodies, saying “cloud of Oort”. This cloud, to shape of spherical shell, is found to the borders of Solar Sistema and it extends until cinquantamila times the Earth-Sun distance. the comets are found in the cloud of Oort since Solar Sistem has been formed and they have been conserved equal then, like in a great “cosmic refrigerator”. From time to time, when some thing disturbs their orbit, one of these freezed cliff pieces escapes from the cloud and it is approached high velocity in sunlight. It enters in a lengthened orbit a lot and becomes one comet. Some comets cover an orbit sluice, of elliptic shape, therefore ripresentano periodically, while others cover an open orbit and therefore they pass only once next to the Sun.

Not hardly the comet is approached little hundreds of million kilometers from the Sun, the ice that contains begins to do vapour, forming around the full of rocks nucleus a spheroidal cloud of gas and powders, said chioma.

In fact, the name “comet” derives from the Latin “coma” that it means chioma. When the comet is approached less than two-threeundred million than Km from the Sun, the solar cancellation affects powder particles of the chioma, exercises on they a pressure and it pushes to them via along the opposite direction in sunlight. The powder forms therefore one tail, turned from the opposite part of the Sun. the combination of the motion of the comet and the push of the cancellation makes yes that the tail leggermente assumes an arch shape.

A comet can employ migliaia of years to complete an orbit around in sunlight, but he is visible only when it is nearer to it, that is for little weeks or little months.

The cancellation of the Sun ionizza the gas of the chioma, that is tears to atoms of the gas their electrons. The gas becomes therefore a plasma, that is with of atomic nuclei and free electrons. Anch'esso comes pushed via from the pressure of the solar cancellation, in the opposite direction in sunlight, and forms one tail of Ionian. However, being lighter of the powder, “it does not remain behind” while the comet is moved: the tail of Ionian therefore is rectilinear. The reason is this for which two tails separated in comets are observed.

The various aspect of the tail of a comet while one moves along the orbit. As the comet is approached in sunlight, the intensity and the pressure of the solar cancellation increases: the amount of gas and powders that vaporizzano increases, therefore the tail

The comets are not therefore luminous only because they emit own light, but above all because the particles that compong ono chioma and the tail diffuse the sunlight. The tail of a comet can catch up 150 million kilometers, par to the distance Earth - Sun! But hand that the comet goes away, the tail is shortened more and more until not being more visible. The comet returns in the dark deep from

where it has come. The nucleus of a comet has dimensions of little kilometers, but to every successive passage in the vicinities of the Sun, a good part of the material of which it is composed goes dispersed in the space. The comets therefore “are consumed”: they are not eternal, but after a sure number of passages they are disintegrated.

Le Fonti

• Testi: da Wikipedia

• Foto: da Wikipedia

• Autori: Veronica Silvestrini, Viola Russo e Flavia Moroni

• 2 B