| Piú viste - Uranus and His Moons |
Uranus-HST-PIA01279.jpgUranus from Hubble Space Telescope57 visiteCaption NASA originale:"Uranus is sometimes called the sideways planet, because its rotation axis tipped more than 90° from the planet's orbit around the Sun. The year on Uranus lasts 84 Earth years, which creates extremely long seasons - winter in the Northern Hemisphere has lasted for nearly 20 years. Uranus has also been called bland and boring, because no clouds have been detectable in ground-based images of the planet. Even to the cameras of the Voyager spacecraft in 1986, Uranus presented a nearly uniform blank disk and discrete clouds were detectable only in the Southern Hemisphere. Voyager flew over the Planet's cloud tops near the dead of northern winter (when the northern hemisphere was completely shrouded in darkness).
Two images are shown here. The "aqua" image (on the left) is taken at 5,470 Angstroms, which is near the human eye's peak response to wavelength. Color has been added to the image to show what a person on a spacecraft near Uranus might see. Little structure is evident at this wavelength, though with image-processing techniques, a small cloud can be seen near the planet's northern limb (rightmost edge). The "red" image (on the right) is taken at 6,190 Angstroms, and is sensitive to absorption by methane molecules in the planet's atmosphere. The banded structure of Uranus is evident, and the small cloud near the northern limb is now visible".
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AB-AB-vg2_2686312.gifThe limb of Uranus57 visiteOriginal caption:"Voyager 2 image of the limb of Uranus from about 694.000 Km. This narrow angle view is about 5.100 Km across and was taken 12 hours after closest approach. The image is centered at -10, 212 and North is up. (Voyager 2, FDS 26863.12)".
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ZA-Uranus-IP-V2-PIA00346_modest.jpgFarewell Uranus... (1)56 visiteCaption NASA originale:"This image shows a crescent Uranus, a view that Earthlings never witnessed until Voyager 2 flew near and then beyond Uranus on January 24, 1986. The hazy blue-green atmosphere probably extends to a depth of around 5.400 miles, where it rests above what is believed to be an icy or liquid mixture (an 'ocean') of water, ammonia, methane and other volatiles, which in turn surrounds a rocky core, perhaps a little smaller than Earth".
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ZB-Uranus-HP-V2-PIA01391_modest.jpgFarewell Uranus...56 visiteUna veduta finale di Urano, registrata dalla Sonda Voyager 2 il 27 Gennaio 1986. Come potete vedere chiaramente, Urano si sta facendo sempre più piccolo, quasi che fosse "divorato" dall'oscurità che lo circonda...
Quest'immagine, per Voyager 2 (e per noi tutti), rappresenta l'addio ad Urano ed al suo Sistema di Anelli e di Lune.
Ci torneremo mai? Sicuramente si, ma potrebbero passare tantissimi anni prima che ciò accada davvero...
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VA-Uranus_ Rings-V2-PIA01985_modest.jpgUranus' Rings (2)56 visiteGli Anelli di Urano, come appare del tutto evidente sin dalla prima istantanea, sono molto più sottili e "deboli" rispetto a quelli di Saturno. In realtà, l'effetto "scenico" delle due serie di anelli è incomparabile: quelli di Saturno sono - come possiamo vedere ogni giorno - un autentico capolavoro di Ingegneria Celeste il quale in parte utilizza, per il suo "equilibrio", anche le delicate risonanze di alcune Lune minori. Gli Anelli di Urano (al pari di quelli di Nettuno), invece, appaiono possedere una natura - oltre che un'apparenza - diversa.
Essi, infatti, potrebbero essere effettivamente la semplice (e anche la sola) conseguenza della disintegrazione di un corpo celeste (un satellite? Un asteroide?) avvicinatosi troppo ad Urano ed i cui residui sono rimasti in orbita attorno ad esso. E' questa, a nostro avviso, la "chiave di lettura" (o almeno una di esse) per comprendere una parte dei misteri dei "Signori degli Anelli".
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Oberon-V2-PIA00034_modest.jpgOberon, from 660.000 Km - Voyager 256 visiteCaption NASA originale:"This Voyager 2 picture of Oberon is the best the spacecraft acquired of Uranus' outermost moon. The picture was taken shortly after 3:30 a.m. PST on Jan. 24, 1986, from a distance of 660.000 Km. The color was reconstructed from images taken through the narrow-angle camera's violet, clear and green filters. The picture shows features as small as 12 Km on the moon's surface. Clearly visible are several large impact craters in Oberon's icy surface surrounded by bright rays similar to those seen on Jupiter's moon Callisto. Quite prominent near the center of Oberon's disk is a large crater with a bright central peak and a floor partially covered with very dark material. This may be icy, carbon-rich material erupted onto the crater floor sometime after the crater formed. Another striking topographic feature is a large mountain (vedi il frame "Oberon's Peak"), about 6 km high, peeking out on the lower left limb".
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AB-Ariel-vg2_2684535.jpgCrescent Ariel (HR)56 visitenessun commento
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UMBRIEL-vg2_p29502-B.jpgUmbriel's extremely unusual surface feature: Wunda Crater (HR - detail mgnf)56 visitenessun commento
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Miranda-vg2_2684626.jpgThe "limb" of Miranda56 visitenessun commento
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Uranus_and_Ariel.jpgUranus and Ariel: the Eclypse56 visiteCaption NASA originale:"This image is a never-before-seen astronomical alignment of a moon traversing the face of Uranus, and its accompanying shadow. The white dot near the center of Uranus' blue-green disk is the icy moon Ariel. The 700-mile-diameter satellite is casting a shadow onto the cloud tops of Uranus. To an observer on Uranus, this would appear as a solar eclipse, where the moon briefly blocks out the Sun as its shadow races across Uranus's cloud tops.
Though such "transits" by moons across the disks of their parents are commonplace for some other Gas Giant Planets, such as Jupiter, the satellites of Uranus orbit the planet in such a way that they rarely cast shadows on the Planet's Surface. Uranus is tilted so that its spin axis lies nearly in its orbital plane".
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Uranus-Voyager2-PIA00369.jpgUranus Cloud Movement56 visiteTime-lapse Voyager 2 images of Uranus show the movement of 2 small, bright, streaky clouds - the first such features ever seen on the Planet. The clouds were detected in this series of orange-filtered images taken Jan. 14, 1986, over a 4.6-hour interval (from top to bottom). At the time, the spacecraft was about 12,9 MKM (about 8 MMs) from the Planet, whose pole of rotation is near the center of each disk. Uranus, which is tipped on its side with respect to the other planets, is rotating in a counterclockwise direction, as are the 2 clouds seen here as bright streaks.
(The occasional donut-shaped features that show up are shadows cast by dust in the camera optics. The processing necessary to bring out the faint features on the Planet also brings out these camera blemishes.) The larger of the 2 clouds is at a latitude of 33°; the smaller cloud, seen faintly in the 3 lower images, lies at 26° (a lower latitude and hence closer to the limb). Their counterclockwise periods of rotation are 16.2 and 16.9 hours, respectively. This difference implies that the lower-latitude feature is lagging behind the higher-latitude feature at a speed of almost 100 meters per second (220 mph). Latitudinal bands are also visible in these images. The faint bands, more numerous now than in previous Voyager images from longer range, are concentric with the pole of rotation -- that is, they circle the planet in lines of constant latitude.MareKromium
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UranusfromHST-4.JPGUranus, from HST (Natural, but enhanced, Colors; credits: NASA)56 visitenessun commentoMareKromium
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