Astronomy Picture of the Day
August 24, 2012

Farewell to Uranus
Farewell to Uranus

Credits: NASA - Voyager 2 Project; Credits fo the additional process. and color.: Dr Paolo C. Fienga/LXTT/IPF

Today's APOD is a view of Uranus that was recorded by the NASA - Voyager 2 Probe on January 25, 1986, as the Spacecraft left the Gas-Giant Planet and its many moons behind, and then set forth on the cruise towards the last Gas-Giant Planet of our Solar System, Neptune. The Voyager 2 Probe was about 1 Million Kilometers (such as approx. 600.000 miles) away from Uranus when it acquired this wide-angle view. The picture - which is a color composite of blue, green and orange frames - has a resolution of approx. 140 Km (such as about 90 miles) per pixel. Note that some very light (and yet colorful) Cloud-Bands can be seen in the lowest visible portion of the Planet.

Uranus possesses the third-largest Planetary Radius and the fourth-largest Planetary Mass in the Solar System. It was named after the ancient Greek Deity of the Sky, Uranus, the father of Cronus (such as Saturn) and grandfather of Zeus (such as Jupiter). Even though Uranus is (just theoretically) visible to the naked eye, like the other 5 (five) so-called "Classical Planets" (such as Mercury, Venus, Mars, Jupiter and Saturn, all clearly visible and easy to recognize in the Sky even to the unaided eye), it was never recognized as a Planet by ancient observers because of its dimness and slow orbit. As a matter of fact, it was Sir William Herschel who announced its discovery, on March 13, of the AD 1781, thus expanding the known boundaries of the Solar System for the first time in Modern history. Uranus was also the first Planet discovered by using a telescope.

Uranus is similar in composition to Neptune, and both Planets show different chemical composition as to the other two (larger) Gas-Giants Planets, Jupiter and Saturn. Astronomers sometimes place them (such as Uranus and Neptune) in a separate category called "Ice Giants" and that is because the Atmosphere of both Uranus and Neptune, while similar to the one of Jupiter and Saturn in its Primary Composition of Hydrogen and Helium, actually contains more "Ices", such as Water, Ammonia and Methane, along with traces of Hydrocarbons than the Atmospheres of Jupiter and Saturn. In addition to that, the Atmosphere of Uranus appears to be the coldest Planetary Atmosphere in the Solar System, with a minimum temperature of 49 K (such as − 224° Celsius). Uranus has also shown, sometimes in its Atmosphere, the presence of complex, layered Cloud Structures, with Water Ice Particles - which are thought to make up the lowest Clouds' Layers - and Methane, on the other hand, that is thought to form their uppermost Layer. In contrast, the interior of Uranus is thought to be mainly composed of Ices and Rock.

Like the other Gas-Giant Planets, Uranus possesses a Ring-System, a Magnetosphere, and a remarkable number of moons. The Uranian System has a unique configuration among the Planets because its Axis of Rotation is tilted sideways, nearly into the Plane of its Revolution about the Sun. This means that its Poles, therefore, lie where the other Planets of the Solar System have their Equators. In the AD 1986, images from NASA - Voyager 2 Probe showed Uranus as a virtually featureless Planet in Visible Light (note that such a total lack of visible reliefs occurs all the times when its Upper Atmosphere does not show the presence of any Weather-related Phenomena, like the Cloud-Bands and huge Storms which are often associated with all the other three Gas-Giant Planets). Furthermore, throughout the analysis of several pictures taken by the NASA - Hubble Space Telescope, a few Astronomers have seen signs of seasonal change and increased Weather Activity in recent years as Uranus approached its Equinox. Last but not least (and just out of curiosity), the speed of the Uranian Winds (as it has been calculated by observing the movements of the Clouds located in its Upper Atmosphere along the Disc of the Planet) apparently can reach up to 250 meters per second (such as approx. 900 Km-per-hour, or either approx. 560 miles-per-hour).

This frame has been additionally processed and then re-colorized, according to an informed speculation carried out by Dr Paolo C. Fienga (LXTT-IPF), in Absolute Natural Colors (such as the colors that a human eye would actually perceive if someone were onboard the NASA - Voyager 2 Probe and then looked outside, towards the Planet Uranus), by using an original technique created - and, in time, dramatically improved - by the Lunar Explorer Italia Team. Different colors, as well as different shades of the same color, mean, among others, the existence of different Elements in the Upper Atmosphere of Uranus, each having a different Albedo and Chemical Composition.

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