Now space seems to be truly empty: even Pluto has disappeared behind us and the Sun is an indistinguishable point among the other stars. But perhaps the solar system still has another surprise in store. Something is here, fifteen thousand billion kilometers from the Sun: small clumps of dust and ice which move invisibly and darkly in space. According to the scientist who hypothesized their existence, the Dutch astronomer Jan Oort, there are at least one hundred billion and they form a kind of shell around the entire solar system. According to some scientists, this agglomerate is the residue of interstellar matter, another of the "left-overs" of the formation of the solar system (like, perhaps, some of the outer planetary bodies that we have just seen). Others, instead, believe that it was the Sun that captured them from outer space. In any case, in honor of its "creator", the shell was given the name of "Oort' s cloud". This however is a very special cloud, which acts like a kind of cosmic cradle. In fact, this is where, according to the Dutch astronomer, comets are "born", those luminous apparitions which have always been thought to be omens of misfortune, harbingers of earthquakes, wars and famine, which appear in our skies every so often with their splendid tail of dust that may extend for many degrees in the celestial vault of the heavens. But just what are comets? Defined by a researcher as "the closest thing to nothing that you can imagine", they are formed by a solid nucleus consisting of rock and ice fragments, one of the many which wander in Oort's cloud: a kind of dirty snowball whose diameter may vary from less than one kilometer to a little more than one hundred kilometers. It is only when the comet approaches the Sun that the heat from the star melts a part of the ice, transforming it into a gaseous cloud that may extend into space for millions of kilometers: this is the part that we can't see because it is illuminated by the Sun and because it glows with its own light, thanks to the ionized gases which make it work like a neon bulb. This luminous tail is "pushed" by the solar wind always in the opposite direction to the Sun, as if it were "blown" away from the nucleus: this is why the tail flows behind the comet as it approaches the Sun and precedes it when it moves away. Beyond a certain distance, the tail slowly disappears while the small and dark nucleus can no longer be identified, at least until the next passage close to the Sun. In fact, comets move along an elongated orbit which, from their origin in Oort's cloud, takes them around a very narrow "turn around the buoy" around our star. The mechanism that "releases" a comet every so often from the cloud might be the gravitational influence exerted by a far away star during its motion: once it has abandoned the cloud, the comet moves slowly towards the Sun and, if it does not pass close to any planet, may take several hundreds of years to complete its long orbit. But if it is affected by the gravitational attraction of one of the large outer planets, Saturn or Jupiter for example, the comet, instead of returning to Oort's cloud, "turns" around the planet that captured it: its orbit becomes much shorter and so much so that it can be completed in just a few years.
Now space seems to be truly empty: even Pluto has disappeared behind us and the Sun is an indistinguishable point among the other stars. But perhaps the solar system still has another surprise in store. Something is here, fifteen thousand billion kilometers from the Sun: small clumps of dust and ice which move invisibly and darkly in space. According to the scientist who hypothesized their existence, the Dutch astronomer Jan Oort, there are at least one hundred billion and they form a kind of shell around the entire solar system. According to some scientists, this agglomerate is the residue of interstellar matter, another of the "left-overs" of the formation of the solar system (like, perhaps, some of the outer planetary bodies that we have just seen). Others, instead, believe that it was the Sun that captured them from outer space. In any case, in honor of its "creator", the shell was given the name of "Oort' s cloud". This however is a very special cloud, which acts like a kind of cosmic cradle. In fact, this is where, according to the Dutch astronomer, comets are "born", those luminous apparitions which have always been thought to be omens of misfortune, harbingers of earthquakes, wars and famine, which appear in our skies every so often with their splendid tail of dust that may extend for many degrees in the celestial vault of the heavens. But just what are comets? Defined by a researcher as "the closest thing to nothing that you can imagine", they are formed by a solid nucleus consisting of rock and ice fragments, one of the many which wander in Oort's cloud: a kind of dirty snowball whose diameter may vary from less than one kilometer to a little more than one hundred kilometers. It is only when the comet approaches the Sun that the heat from the star melts a part of the ice, transforming it into a gaseous cloud that may extend into space for millions of kilometers: this is the part that we can't see because it is illuminated by the Sun and because it glows with its own light, thanks to the ionized gases which make it work like a neon bulb. This luminous tail is "pushed" by the solar wind always in the opposite direction to the Sun, as if it were "blown" away from the nucleus: this is why the tail flows behind the comet as it approaches the Sun and precedes it when it moves away. Beyond a certain distance, the tail slowly disappears while the small and dark nucleus can no longer be identified, at least until the next passage close to the Sun. In fact, comets move along an elongated orbit which, from their origin in Oort's cloud, takes them around a very narrow "turn around the buoy" around our star. The mechanism that "releases" a comet every so often from the cloud might be the gravitational influence exerted by a far away star during its motion: once it has abandoned the cloud, the comet moves slowly towards the Sun and, if it does not pass close to any planet, may take several hundreds of years to complete its long orbit. But if it is affected by the gravitational attraction of one of the large outer planets, Saturn or Jupiter for example, the comet, instead of returning to Oort's cloud, "turns" around the planet that captured it: its orbit becomes much shorter and so much so that it can be completed in just a few years.
