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Cassini continues its exploration of the Saturn system with the 13-day Rev176, which begins on December 3 at its farthest distance from the planet. This is also called the orbit's apoapse. At this point, Cassini is 1.65 million kilometers (1.03 million miles) from Saturn's cloud tops. Rev 176 occurs seven months into the first inclined phase of the Cassini Solstice Mission, a phase which lasts until March 2015. The inclined phase will allow for polar views of Saturn and Titan as well as better vistas of Saturn's rings than those Cassini had while in the earlier, equatorial phase of the Solstice Mission. Twenty-four ISS observations are planned for Rev 176, the vast majority of which are focused on Saturn's atmosphere.
ISS begins its observations for Rev 176 the day after apoapse with three quick observations of Saturn using the wide-angle camera (WAC). These observations are part of a series of "Storm Watch" observation sequences designed to take advantage of short, two-minute segments when the spacecraft turns the optical remote sensing (ORS) instruments back to Saturn as a waypoint between other experiments' observations. These sequences include blue, clear, two methane band, and one full-frame, continuum band filter images. Six additional sequences will be taken between December 5 and 7. Also on December 4, ISS will acquire a ten-hour light curve observation of the small, distant moon Kiviuq. The spacecraft will take nearly 100 images of Kiviuq over those ten hours in order to improve the estimate for its rotation period and to determine the position of its north pole. Unlike the moons that orbit closer to Saturn, Kiviuq is not tidally locked, having a rotation period of only 21.82 hours, much shorter than its 449-day orbital period. Finally, on December 4, after the second Saturn storm watch observation, ISS will acquire an astrometric observation of Saturn's small, inner moons. Astrometric observations are used to improve our understanding of the orbits of these small satellites, which can be influenced by Saturn's larger icy moons. Another astrometric observation will be taken on December 12.
On December 6, ISS will acquire a movie of the F ring, observing its various channels and streamers created by the interaction between the ring material and the nearby moon, Prometheus. On December 7 and 8, ISS will ride along with a pair of Ultraviolet Imaging Spectrometer (UVIS) observations of Saturn's south polar aurora. In addition to making a movie of the planet's aurorae, the images will be used to independently measure the rotation period of Saturn's magnetic field. On December 8, ISS will observe a half-phase Titan from a distance of 1.36 million kilometers (0.84 million miles). This observation is designed to look for clouds in the moon's atmosphere as part of the "Titan Monitoring Campaign" (TMC). The observation on December 8 will monitor the clouds over the moon's southern and sub-Saturn hemispheres. ISS also will be taking shorter-wavelength images to study changes in Titan's upper haze layers.
On December 10 at 02:16 UTC, Cassini will reach periapse for Rev 176 at an altitude of 387,780 kilometers (240,960 miles) from Saturn. During the periapse period on December 9, ISS will image the limb of Saturn while the Sun is behind the planet. This will provide an excellent opportunity to observe the various haze layers in the planet's upper atmosphere. Afterward, ISS will monitor the south polar plume of Enceladus from a distance of 700,000 kilometers (435,000 miles). Early on the 10th, ISS will ride along with the Visual and Infrared Mapping Spectrometer (VIMS) to acquire a mosaic of Saturn's north polar region using the Wide-Angle Camera (WAC). Spring has progressed far enough that the entirety of the hexagonal jet stream that lies near 77 degrees North latitude will be in sunlight, as shown in the image at left from Rev175. ISS will be imaging the hexagon with a two-by-two mosaic rather than centering the field-of-view on the north pole like it did during the previous orbit. Later on the 10th, ISS will ride along with VIMS in order to acquire a WAC mosaic of Saturn's mid-northern latitudes, where a major storm raged a couple of years ago. Finally, on December 11, ISS will observe different latitudes of Saturn's atmosphere at low, moderate, and high emission angles to again study Saturn's upper haze layers and their effects on our ability to observe lower altitude cloud structures.
As Cassini recedes from Saturn on December 12, ISS will image the outer A ring, looking at the propellers previously seen in this part of the ring system. Propellers are small voids in Saturn's rings created by the gravitational interaction between large ring particles and the surrounding ring. Following that observation, ISS will observe Methone and the arc of dust that surrounds it for four hours. From late in the day on December 15 to early on December 17, ISS will observe a crescent Titan at a distance of 0.91 to 1.03 million kilometers (564,000 to 638,000 miles). This will last nearly 30 hours, allowing ISS to observe cloud motion and evolution on a long time scale. The field-of-view will also be centered near the south pole, allowing researchers to observe the south polar vortex.
On December 16, Cassini will reach apoapse on this orbit, bringing it to a close and starting Rev177.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).