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Cassini Snaps Io Among The Clouds

The Galilean satellite Io floats above the cloudtops of Jupiter in this image captured on the dawn of the new millennium, January 1, 2001 10:00 UTC (spacecraft time), two days after Cassini's closest approach. The image is deceiving: there are 350,000 kilometers -- roughly 2.5 Jupiters -- between Io and Jupiter's clouds. Io is the size of our Moon, and Jupiter is very big. Credit: NASA/JPL/University of Arizona Cassini Imaging Science Team
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  • Tucson - Jan. 22, 2001
    The Galilean satellite Io floats above the cloudtops of Jupiter in a stunning image captured on the dawn of the new millennium, January 1, 2001 10:00 UTC (spacecraft time), and only two days after Cassini's closest approach. But the image is deceiving with over 350,000 kilometers -- roughly 2.5 Jupiters -- between Io and Jupiter's cloud tops.

    Making for a stunning Jupiter flyby season is the that compared with the Voyager 1 and 2 flybys in the early 1980s, Cassini is passing at a leisurely rate through the Jovian as ut slews a path that is nearly equatorial.

    Data collection begins with the spacecraft 3.8 degrees above Jupiter's equator plane and approaching the planet from a phase (Sun-Jupiter-spacecraft) angle of 20 deg and a distance of 84.7 million kilometers.

    From this viewing geometry, Jupiter looks only slightly different than it does from Earth. By the middle of November, the phase angle drops to 18 deg, and the distance decreases to the point where 4 narrow angle camera images will be required to cover the planet. All throughout this period Cassini will be making repeated observations of the atmosphere, and searching for previously undiscovered satellites in the region around Jupiter containing the Galilean satellites.

    By the middle of December, the phase angle drops to zero, repeated monitoring of the atmosphere ceases, and we begin our observations of the rings and satellites. On December 18, we make our closest approach to Himalia, a small outer satellite of Jupiter.

    As Cassini sweeps through a large range of phase angle during the rest of the encounter, we monitor the light scattering behavior of the rings and Galilean satellites in a suite of spectral and polarimetric filters. (For a brief time surrounding closest approach, Jupiter is large enough to require 9 images to cover the planet.) And we will be on the lookout for time-variability -- in the rings and in the expected diffuse glows from the tenuous atmospheres of Io, Europa and Ganymede as they pass into Jupiter's shadow. (The Galileo spacecraft first observed such glows, as well as high temperature hot spots, from volcanically active Io.)

    Ring, satellite and occasional atmospheric observations continue through closest approach and out to January 15, at which point the spacecraft is looking back on a crescent Jupiter from a distance of 18 million km and 3 degrees below the equatorplane. At this time, we return to repeated imaging of the planet as we depart. The last Jupiter images are taken on March 22, 2001.

    The closest approach distance to the planet is not very close: 136 RJ, or 9.72 million kilometers. Thus, Cassini images will not have the exquisitely high resolution of either Voyager or Galileo images. But the slow pace of the flyby, the large data collection and downlinking capability of the spacecraft, and the wide spectral range and fine photometric precision of the Imaging Science System (ISS), make it possible to acquire high quality time-lapse CCD imagery of Jupiter's ever changing atmosphere extending over several months in a large suite of atmosphere-probing wavelengths, and to search for time-variability in other Jovian targets ... something no previous Jupiter-bound spacecraft has ever done before.

    To follow the Cassini Jupiter Imaging project visit the Lunar and Planetary Laboratory at the University of Arizona.

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    Cassini Captures Jupiter As Earth Team Tracks Storm Progression
    Pasadena - Oct. 24, 2000
    As Cassini continues to approach Jupiter on-route to Saturn, the spacecraft will provide breathtaking views of the gas giant. Meanwhile, scientists using the Hubble space telescope have been tracking the interplay of storms across Jupiter.