One believes that comets originate from the extreme confinements of the solar system, where they orb in a spherical cloud around the sun in number of several millions. They are essentially composed from ice and rock and in consequence of gravitational perturbations they abandon their orbit to throw themselves towards the inside of the solar system. As soon as they draw near to the sun they start to get warm and the materials of which they are composed begin to turn themselves into gas, forming the so-called head of hair of the diameter of some hundred of thousands of kms, that makes to increase the brightness of the comet notably under the solar irradiation. The nucleus, of the diameter of few kms, is the only solid part of the comet and in our case it is of 40 kms: an extraordinary value. A part of the tail is constituted by gas blown away by the solar wind while the rest is constituted by dust's particles freed by the evaporation of the gas. Its length can arrive to more than 100 millions of kms. A lot of comets put also in evidence a tail formed by ions, that is atoms that have lost some electrons because of the bombardment of the solar wind.