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| Ultimi arrivi - Uranus and His Moons |
Uranus-2005.jpgGoing, Going, Gone: Hubble captures Uranus' Rings on Edge (2)54 visiteCaption NASA:"With further analysis of the Hubble data, Astronomer Mark Showalter of the SETI Institute in Mountain View, Calif., hopes to detect some of the small moons that may shepherd the debris into distinct rings.
Until Voyager 2 flew by Uranus in January 1986, the Rings were only known from the way they temporarily blocked the light of stars passing behind the Planet. Hubble provided some of the first images of the Ring System as viewed from Earth's distance of approx. 2 BMs.
The advent of adaptive optics gave ground-based observers using large telescopes comparatively sharp views.
The Rings were discovered in 1977, so this is the first time for a Uranus Ring crossing to be observed from Earth. Earth's orbit around the Sun permits 3 opportunities to view the Rings in an "edge-on" configuration: Uranus made its first ring crossing as seen from Earth on May 3; it made its second crossing on August 16 and will cross for the third time on February 20, 2008.
Though the last ring crossing relative to Earth will be hidden behind the Sun, most of Earth's premier telescopes, including Keck, Hubble, the European Southern Observatory's Very Large Telescope and the Hale Telescope on Mt. Palomar, plan to focus on the planet again in the days following December 7, 2007. On December 7 the rings will be perfectly edge-on to the Sun".MareKromiumSet 16, 2007
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Uranus-2007-00.jpgGoing, Going, Gone: Hubble captures Uranus's Rings on Edge (3)54 visiteCaption NASA:"Showalter is a member of a team led by Imke de Pater of the University of California, Berkeley, who reported that the Rings of micron-sized dust have changed significantly since the Voyager 2 spacecraft photographed the Uranus System 21 years ago. Observations were also gleaned from near-infrared adaptive optics observations with the Keck II telescope on May 28, 2007, and reported in an article appearing on August 23 in Science Express, the online edition of Science Magazine".MareKromiumSet 16, 2007
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Uranus-2007-01.jpgGoing, Going, Gone: Hubble captures Uranus's Rings on Edge (4)54 visitenessun commentoMareKromiumSet 16, 2007
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Uranus-HST-PIA01282.jpgUranus from HST55 visiteThis NASA HST image of Uranus reveals the Planet's Rings and bright clouds and a high altitude haze above the Planet's South Pole.
Hubble's view was obtained on August 14, 1994, when Uranus was 1,7 BM (about 2,8 BKM) from Earth. These details, as imaged by the Wide Field Planetary Camera 2, were only previously seen by the Voyager 2 spacecraft, which flew by Uranus in 1986. Since then, none of these inner satellites has been further observed, and detailed observations of the Rings have not been possible.
Though Uranus' Rings were discovered indirectly in 1977 (through stellar occultation observations), they have never before been seen in visible light through a ground-based telescope.
Hubble resolves several of Uranus' Rings, including the outermost Epsilon-Ring. The Planet has a total of 11 concentric Rings of dark dust. Uranus is tipped such that its rotation axis lies in the plane of its orbit, so the Rings appear nearly face-on.
Three of Uranus' inner moons each appear as a string of three dots at the bottom of the picture. This is because the picture is a composite of three images, taken about six minutes apart, and then combined to show the moons' orbital motions. The satellites are, from left to right, Cressida, Juliet, and Portia. The moons move much more rapidly than our own Moon does as it moves around the Earth, so they noticeably change position over only a few minutes.
One of the four gas giant planets of our solar system, Uranus is largely featureless. HST does resolve a high altitude haze which appears as a bright "cap" above the planet's south pole, along with clouds at southern latitudes (similar structures were observed by Voyager). Unlike Earth, Uranus' south pole points toward the Sun during part of the planet's 84-year orbit. Thanks to its high resolution and ability to make observations over many years, Hubble can follow seasonal changes in Uranus's atmosphere, which should be unusual given the planet's large tilt.
The Wide Field/Planetary Camera 2 was developed by the Jet Propulsion Laboratory and managed by the Goddard Spaced Flight Center for NASA's Office of Space Science.
MareKromiumLug 23, 2007
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Uranus-HST-PIA01283.jpgUranus' Rotation from HST54 visiteThese 3 NASA HST images of the planet Uranus reveal the motion of a pair of bright clouds in the Planet's Southern Hemisphere, and a high altitude haze that forms a "cap" above the Planet's South Pole.
Hubble's view was obtained on August 14, 1994, when Uranus was 1,7 BM (about 2,8 BKM) from Earth. These atmospheric details were only previously seen by the Voyager 2 spacecraft, which flew by Uranus in 1986. Since then, detailed observations of Uranus's atmospheric features have not been possible because the Planet is at the resolution limit of ground-based telescopes.
Hubble's Wide Field Planetary Camera 2 observed Uranus through a filter that is sensitive to light reflected by a pair of high altitude clouds. This makes a high altitude haze over Uranus' south polar region clearly visible, along with a pair of high altitude clouds or plume-type features that are 2500 and 1800 miles (4300 and 3100 kilometers) across, respectively. This sequence of images shows how the clouds (labeled A and B) rotate with the planet during the three hours that elapsed between the first two observations (left and center picture) and the five hours that elapsed between the second pair of observations (center and right picture). Some cloud motion might be due to high altitude winds on the planet. (Observations are indicated in Universal Time.)
By tracking the motion of high-altitude clouds, the new Hubble observations will allow astronomers to make new measurements of Uranus' rotation period. Based on the previous Voyager observations, Uranus spins on its axis at a faster rate than Earth does, completing one rotation every 7 hours, 14 minutes.
One of the four gas giant planets of our solar system, Uranus is largely featureless. Unlike Earth, Uranus' south pole points toward the Sun during part of the planet's 84-year orbit. Thanks to its high resolution and ability to make observations over many years, Hubble can follow seasonal changes in Uranus's atmosphere, which should be unusual given the planet's large tilt.
The Wide Field/Planetary Camera 2 was developed by the Jet Propulsion Laboratory and managed by the Goddard Spaced Flight Center for NASA's Office of Space Science.
MareKromiumLug 23, 2007
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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.MareKromiumLug 23, 2007
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Uranus-Hurricane02.jpgUranian Hurricane (detail 2)54 visitenessun commentoOtt 01, 2006
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Uranus-Hurricane01.jpgUranian Hurricane (detail 1)55 visitenessun commentoOtt 01, 2006
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Uranus-Hurricane00.jpgUranian Hurricane (context image)54 visiteCaption NASA originale:"Just as we near the end of the hurricane season in the Atlantic Ocean, winds whirl and clouds churn 2 BMs away in the atmosphere of Uranus, forming a dark vortex large enough to engulf two-thirds of the United States.
Lawrence Sromovsky of the University of Wisconsin-Madison leads a team that used NASA's HST to take the first definitive images of a dark spot on Uranus. The elongated feature measures 1.100 by 1.900 miles (such as about 1.700 by 3.000 Km).
There have been prior unconfirmed sightings of dark spots on Uranus, including sketches made in the early 1900s, low-contrast ultraviolet Voyager spacecraft flyby images in 1986, and near-infrared observations taken from a ground-based Observatoryin 1993. However, no other HST images taken almost every Summer from 1994 up to 2006 have shown such a dark spot.
This indicates that the current dark disturbance probably formed very recently (...)".
Ott 01, 2006
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UranusinTime.jpgUranus along the years...54 visiteCaption NASA originale:"This diagram shows how the inclination of orbits of Uranus' moons have appeared increasingly more oblique over the past 12 years due to Uranus's orbit about the Sun. Hubble Space Telescope has been watching Uranus over that period and has traced, in detail, our changing view of the Planet. Uranus is tilted so that its spin axis lies nearly in its orbital plane. This means that only around the time when Uranus's equator is aimed at the Sun (every 42 years) do the orbits of its satellites lie edge-on to the Sun, allowing their shadows to strike the Planet, producing Solar Eclipses on the Planet".Set 03, 2006
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UranusandAriel-Eclypse.jpgUranus and Ariel; Earth and Moon55 visiteCaption NASA originale"This illustrates the relative sizes of Uranus, Earth and Earth's Moon. The images are shown at the proper relative size, but not the correct relative distance from each other. Uranus is approx. 31.000 miles (about 50.000 Km) in diameter, or about 4 times the size of Earth. The Earth is approx. 7.900 miles (about 12.800 Km) in diameter, or about 4 times the diameter of the Moon (2.100 miles, such as about 3.500 Km)".Set 03, 2006
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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".Set 03, 2006
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