Image comet


The figure shows a comet during its phase of activity, which takes place when it is close to the Sun: the oval on the right of the figure is the comet’s head, which is made up of a solid nucleus. The comet’s tail can be seen on the left of the figure, it is made up of a stream of material that comes from the surface of its nucleus.

This is how such a celestial body is born:

Far beyond Pluto’s orbit, in the cold vastness of the Solar System, there is a densely populated cloud of small frozen bodies, no bigger than a few kilometres each and with a range of shapes. From time to time, some of them are disturbed by a passing star or by mutual collisions, they change paths and are sent hurtling towards the Sun. the nearer they get to the Sun the higher the temperature becomes. Once they reach Jupiter’s orbit the heat of the Sun is enough to stimulate the evaporation of the external frozen layers. A cloud of gas and dust called the coma, forms around its nucleus. The coma and the nucleus form the head of the comet. The Solar wind, a continuous jet of particles released by the Sun and hurtled at high speeds into space, pushes the dust and gas out of the coma thus creating trails of material that go to form its tail. The Solar wind sets the tail in the opposite direction to the Sun. The tail of a comet can be as long as 150,000 kilometres, or rather about 300 billion footsteps, which is practically the same distance from the Earth to the Sun. A comet doesn’t always end up on the Sun but will spin around it and disappear into the darkness from where it came, it may return after years, centuries, millennia or not at all.

Every time a comet gets close to the Sun a part of its surface melts. This means that with every subsequent passage the comet deteriorates and is therefore destined to wear itself out completely.

Next page        Back