Uranus, the third largest planet in the solar system, with a diameter equal to four times that of Earth, has an absolutely unique motion: instead of being almost perpendicular to the plane of the orbit, like with all the other planets, its axis of rotation is in the same orbital plane, somewhat like a top that "rolls" around the Sun. Since it takes 84 years to complete one orbit, one pole of the planet is illuminated by the Sun for 42 years, while the other pole, enveloped by the night, must wait for its "turn in the sun" for another 42 years. As seen from the Earth with the most powerful telescopes, Uranus appears as a small blue-greenish object surrounded by its largest satellites: Oberon, Titania, Umbriel, Ariel and Miranda (the names refer to characters in Shakespeare's comedy "A Midsummer's Night Dream"). Since this planet receives less than one three hundred sixtieth of the solar energy that reaches the Earth, it has been calculated that the temperature of its atmosphere, which consists of hydrogen, helium and methane, is around -215 degrees Celsius. Uranus' system of rings was discovered very recently, in 1977, when a group of American astronomers noted some anomalies while the planet passed in front of a star: 35 minutes prior to the phenomenon, which is called occultation, the intensity of the light of the star suddenly decreased and then became bright again. This event repeated four times. This could also have been caused by the presence of unknown moons, but when the star reappeared after occultation, the strange phenomenon repeated exactly like before but in the reverse order. That was the proof: only a system of rings could have explained such regular fluctuations. And in fact several years later, a special electronic instrument to intensify pictures took the first photograph of the rings. However, except for this handful of information, Uranus still remained a mysterious object prior to the arrival of Voyager. Even its period of rotation was the subject of debate: some scientists thought it was 24 hours, while others 16. Because of the great anticipation to receive the signals from the probe, Nasa, for the first time, decided to create a live television connection with the Jet Propulsion Laboratory in Pasadena to show the world the pictures that would have arrived from space. However, there was also a certain amount of concern. Everything on board Voyager 2 had to work to perfection, but in the 12 hours prior to the closest approach, the probe would have to work completely independently: adding the 2 and half hours needed for the first signal to arrive to the same time needed for the return "trip", there would have been no chance to make any corrections from the Earth. And finally the great moment arrived: Uranus began to appear on the control monitors from a distance of 18 000 000 kilometers. A homogeneous light-blue globe without any atmospheric details because methane completely absorbs the red component of the solar radiation. In the attempt to understand more, the astronomers tried to "process" the picture with special filters and all of a sudden the view of Uranus changed: the pole appeared with a kind of round and dark spot surrounded by a series of concentric colored bands which gradually become lighter towards the equator. These reddish-brownish bands immediately aroused the interest of the scientists: it might be frost produced by the chemical reactions between the sunlight and the methane in the atmosphere and then transported from the pole by weak circulation. Finally, it was possible to distinguish some cloud formations in the equatorial region, where there is no frost and the luminous radiation penetrates to deeper atmospheric levels. Again special filters were used to highlight such formations: they are very thin, but it was possible to see that they move following the parallels with a speed that differs depending on the latitude. Their counterclockwise motion, which is the same way the planet moves, was used to calculate Uranus' period of rotation, 17 hours and 6 minutes, with an approximation of just a few minutes. Soon, Voyager also communicates some amazing information: Uranus has a magnetic field, with an intensity equal to about a third of the Earth. This field surrounds Uranus with an extensive magnetosphere. But the real news is that it has a truly unique position: in fact, its axis does not coincide with that of the planet, like for Mercury, the Earth, Jupiter and Saturn, but is inclined at 55 degrees with respect to the planet's axis. This discovery, which left scientists dumbfounded, will provide them with material to study for many years. It will also probably make it possible to better understand the internal structure of Uranus, which for the moment is assumed to consist of three parts: a central rocky core with a radius of 7 500 kilometers, a type of liquid "ocean" with a thickness of 10 500 kilometers and a real atmosphere, "fed" by the liquid layer, with a thickness of 7 600 kilometers.
Uranus, the third largest planet in the solar system, with a diameter equal to four times that of Earth, has an absolutely unique motion: instead of being almost perpendicular to the plane of the orbit, like with all the other planets, its axis of rotation is in the same orbital plane, somewhat like a top that "rolls" around the Sun. Since it takes 84 years to complete one orbit, one pole of the planet is illuminated by the Sun for 42 years, while the other pole, enveloped by the night, must wait for its "turn in the sun" for another 42 years. As seen from the Earth with the most powerful telescopes, Uranus appears as a small blue-greenish object surrounded by its largest satellites: Oberon, Titania, Umbriel, Ariel and Miranda (the names refer to characters in Shakespeare's comedy "A Midsummer's Night Dream"). Since this planet receives less than one three hundred sixtieth of the solar energy that reaches the Earth, it has been calculated that the temperature of its atmosphere, which consists of hydrogen, helium and methane, is around -215 degrees Celsius. Uranus' system of rings was discovered very recently, in 1977, when a group of American astronomers noted some anomalies while the planet passed in front of a star: 35 minutes prior to the phenomenon, which is called occultation, the intensity of the light of the star suddenly decreased and then became bright again. This event repeated four times. This could also have been caused by the presence of unknown moons, but when the star reappeared after occultation, the strange phenomenon repeated exactly like before but in the reverse order. That was the proof: only a system of rings could have explained such regular fluctuations. And in fact several years later, a special electronic instrument to intensify pictures took the first photograph of the rings. However, except for this handful of information, Uranus still remained a mysterious object prior to the arrival of Voyager. Even its period of rotation was the subject of debate: some scientists thought it was 24 hours, while others 16. Because of the great anticipation to receive the signals from the probe, Nasa, for the first time, decided to create a live television connection with the Jet Propulsion Laboratory in Pasadena to show the world the pictures that would have arrived from space. However, there was also a certain amount of concern. Everything on board Voyager 2 had to work to perfection, but in the 12 hours prior to the closest approach, the probe would have to work completely independently: adding the 2 and half hours needed for the first signal to arrive to the same time needed for the return "trip", there would have been no chance to make any corrections from the Earth. And finally the great moment arrived: Uranus began to appear on the control monitors from a distance of 18 000 000 kilometers. A homogeneous light-blue globe without any atmospheric details because methane completely absorbs the red component of the solar radiation. In the attempt to understand more, the astronomers tried to "process" the picture with special filters and all of a sudden the view of Uranus changed
