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Cassini continues its new tour of the Saturn system with the 21-day-long Rev143, the spacecraft's 144th orbit around the Ringed Planet. Cassini begins Rev143 on December 31 at its farthest distance from Saturn, called apoapse. At this point, Cassini is 2.63 million kilometers (1.64 million miles) from Saturn's cloud tops. Cassini's orbit lays nearly exactly within Saturn's ring plane and within the orbital plane of most its major satellites, affording an opportunity to encounter a few of its moons. During Rev143, Cassini will fly by Saturn's second-largest moon, Rhea.
ISS begins its observations for Rev143 and the new year on January 2 as it performs astrometric observations of Saturn's small, inner moons. During this observation, the camera system will image Polydeuces, Telesto, Pallene, Prometheus, Anthe, and Atlas. Shortly before, the wide-angle camera (WAC) will image Saturn. The large, bright storm that formed in December 2010 in the North Temperate Zone of Saturn should be visible during this observation (though slightly farther to the west than shown in the graphic at right as the storm is slowly drifting to the west by 2.5 degrees per day with respect to the IAU longitude system for Saturn). ISS will take a similar observation on January 6, when the narrow-angle camera (NAC) will observe Prometheus, Anthe, Atlas, Methone, Calypso, Pallene, and Polydeuces. The Saturn WAC images should show the western end of the large northern storm. On January 7, ISS will acquire 311 images of distant Tarvos, a 15-kilometer-wide (9-mile-wide) outer moon of Saturn. These images will be used to build up a light-curve of the satellite, which is useful for measuring the length of the moon's day and understanding how its brightness changes depending on lighting conditions. Tarvos will be 12.04 million kilometers (7.48 million miles) away at the time. On January 8, the camera system will image the E ring, a ring created primarily from debris from Enceladus's south polar plume, at high phase angles in order to better understand the ring's vertical structure.
On January 10 at 16:14 UTC, Cassini will reach the periapse of Rev143, its closest point to Saturn in the orbit. At periapse, the spacecraft will be 155,590 kilometers (96,679 miles) above Saturn's cloud tops. During this periapse passage, Cassini will be focused on Saturn observations before the spacecraft flies by Rhea on January 11. On January 9, the Visible and Infrared Mapping Spectrometer (VIMS) and the Ultraviolet Imaging Spectrometer (UVIS) instruments will acquire observations of the nightside of Saturn as well as the thin, sunlit crescent. ISS will ride along with both observations, as it will during the Saturn observations the next day, January 10. The large northern Saturn storm will not be imaged during either observation. On January 10, UVIS will perform another scan of the sunlit crescent of Saturn, this time focusing on the southern hemisphere, rather than a scan of the entire crescent as was taken during the previous day. Next, VIMS will observe two stellar occultations of Saturn's atmosphere: first using the brightest star in the night sky, Sirus, also known as Alpha Canis Majoris; then using 19 Leo, a 6th magnitude star in the constellation Leo. Both stellar occultations will probe the night side of Saturn. Finally, VIMS will acquire a regional map of Saturn's daylit equatorial region. Again, unfortunately, the large northern storm will not be visible in the day's observations.
Also during this run up to periapse, ISS will image Titan from a distance of 934,490 kilometers (580,665 miles). Imaging scientists will be looking for clouds across the sub-Saturn hemisphere of Titan, as well as additional surface changes that may have occurred as a result of the large "Arrow Storm" that was seen in late September.
On January 11 at 04:53 UTC, Cassini will perform a targeted encounter of Rhea, its fourth flyby of this moon at an altitude of 6,000 kilometers (3,700 miles) or less. In the 50 minutes prior to the encounter, the Cassini Plasma Spectrometer (CAPS) will measure magnetospheric plasmas in the vicinity of Rhea to see how the icy satellite affects them. Data from earlier encounters suggested the presence of a thin ring of material orbiting around Rhea's equator, but that observation has not been backed up by later imaging of Rhea, though color imaging of its surface has revealed a line of dark spots that coincides with the moon's equator. At closest approach, Cassini will fly just 75.9 kilometers (47.2 miles) above terrain at 76 degrees south latitude, 95 degrees west longitude on Rhea. During closest approach, CAPS will still be taking magnetospheric observations, however Rhea will pass through the field-of-view of Cassini's cameras. ISS will snap two pairs of NAC and WAC images during this observation. The first pair will be taken while Cassini is at an altitude of 110 kilometers (68 miles) above Rhea's surface. This pair will cover terrain near 72 degrees south latitude, 185 degrees west longitude on Rhea. The other pair will capture cratered terrain 50 kilometers (31 miles) to the east of Wende Crater. Afterward, ISS will acquire several mosaics featuring Rhea's anti-Saturn hemisphere. The first is a four-image strip covering a small bright crater at 15 degrees south latitude, 169 degrees west longitude and Parun Fossa further to the south. Next, ISS will acquire a seven-image strip across Rhea's equatorial region in order to study the line of dark spots that had been seen there, again in earlier data. The camera system also will image the small, bright ray crater again as well as take a seven-image mosaic of Rhea's southern hemisphere. Finally, two "sit-and-stares" will be performed: first over the south pole as Dione and Prometheus pass in the background and then over the center of Rhea. Afterward, ISS will frame a view off the dark limb of Rhea in an attempt to image the possible ring around Rhea, this time at a high enough resolution that larger clumps might be visible. Finally, VIMS will again look at Saturn's dayside. ISS WACs taken during this observation may catch a glimpse of Saturn's large northern storm.
Finishing up Rev143, on January 15 ISS will image a half-phase Titan at a distance of 835,000 kilometers (519,000 miles). This observation will allow researchers to monitor clouds and look for surface changes across the anti-Saturn hemisphere of Titan, and the southern part of Shangri-la in particular. Much later in the day, ISS will observe Titan again, this time from a distance of 996,000 kilometers (619,000 miles). Titan will have rotated enough between these two observations to allow researchers to observe the unofficially named "Okavango" region on Titan, which has become a subject of great interest since it was first seen in October 2010. This observation is the first opportunity to observe the region since then as high resolution observations of "Okavango" during T73 were lost due to Cassini's safing event in November 2010. Afterward, ISS will take astrometric observations of Saturn's small, inner moons. This time, ISS will observe Polydeuces, Telesto, Methone, Pandora, Atlas, Anthe, and Epimetheus. Saturn will also be imaged using the WAC, but the large northern storm will not visible (unless it has expanded by this time). Finally, on January 20, ISS will image Rhea as Dione passes behind it. Rhea will be 2.23 million kilometers (1.39 million miles) away at the time, while Dione will be 2.84 million kilometers (1.77 million miles) away.
On January 20, Cassini will reach apoapse on this orbit, bringing it to a close and starting Rev144. Rev144 includes non-targeted encounters with Mimas, Helene, and Enceladus.
Image products created in Celestia. Rhea basemap by Steve Albers. All dates in Coordinated Universal Time (UTC).