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Cassini's journey at Saturn continues with Rev 55, its 56th orbit of the ringed planet. Cassini's slate of observations for this orbit includes a flyby of Titan, a distant encounter with Dione, a stellar occultation of the rings, and numerous observations of Saturn's atmosphere and small moons. Cassini begins Rev55 on December 27 at its farthest distance from Saturn, called apoapsis. At this point, Cassini is 2.2 million km (1.37 million mi) from Saturn. The first week of Rev55 is filled with observations of Saturn's rings and small satellites. Four small satellite sequences are planned for December 27, 28, and 31. These observations are designed to study the orbits of the objects and how the orbits might evolve over short time periods due to perturbations from the other satellites in the system. In addition to the small satellites orbiting in the inner part of the Saturnian system, Cassini will also observe the outer satellite, Albiorix, on December 28. Three sequences, planned for December 27, 29, and 31, are dedicated to Saturn's ring system. The observation on December 29 will consist of a radial scan, looking at different regions of the main ring system, from the inner D ring to the outer Roche Division and the F ring. The other two observations are dedicated to examining clumps in the F ring. The sole observation dedicated to Saturn during this period is a sequence planned for January 2. During this observation, the Ultraviolet Imaging Spectrometer (UVIS) and ISS will be looking for aurorae in Saturn's north polar region. Cassini reaches periapse, the closest point in its orbit, on January 4 when the spacecraft is 241,000 km (150,000 mi) above Saturn's cloud tops. On January 3 and 4, Cassini performs two imaging sequences of the moon Dione during a distant encounter with the satellite. Cassini's closest images of Dione will be acquired from 127,000 km (79,000 mi) away, with the leading hemisphere in sunlight. On January 3, the various instruments on Cassini observe a stellar occultation of the ring system by Antares (Alpha Scorpii). Such observations are designed to understand the fine-scale structure of the rings. Scientists will look at variations in the observed brightness of the star to determine the opacity of the rings in certain areas, particularly in the A ring. Prior stellar occultations have been used to examine gravitational waves induced in the A ring by Saturn's various moons. The Visible and Infrared Mapping Spectrometer (VIMS) will use the high infrared flux of Antares to achieve good quality data about the A ring's opacity. Finally, an observation is planned for January 5 to examine the opposition effect on Tethys, when Cassini is almost directly between that satellite and the Sun. Cassini encounters Titan for the 41st time on January 5, with a close approach distance of only 1,010 km (628 mi). The last Titan encounter until mid-February, this flyby (known as T40) will allow for imaging of the trailing hemisphere of Titan, centered around the bright region named Adiri. Inbound to the encounter, when only a thin, sunlit crescent is visible from Cassini, UVIS, ISS, and the Composite Infrared Spectrometer (CIRS) trade off control of spacecraft pointing (the controlling instrument is said to be "prime"). Their observations of Titan's night side will provide information about the composition of the moon's thick atmosphere. VIMS will be in control of spacecraft pointing during and shortly after closest approach. During this flyby, VIMS will image features in western Xanadu and in Shangri-la, including Tortola Facula (a possible cryovolcano) and the Huygens landing site just off the eastern end of Adiri. After closest approach, CIRS, ISS, and VIMS will trade off spacecraft pointing. ISS' observations of Titan include a regional mosaic over central Adiri and the dark terrain north of Adiri, plus another regional mosaic along the terminator.
During the last few days of Rev55, Cassini will acquire a movie sequence of the far left edge of the rings as seen from Cassini (the left "ansa"), including the edge of the A-ring, the Roche Division, and the F ring. On January 8, an imaging sequence is planned for Titan's trailing hemisphere from a distance of 1.4 million km (879,000 mi). Additional observations of Saturn's small, inner satellites and the F ring are also planned for January 8 and 9.
Cassini begins the following orbit, number 57 ("Rev56"), on January 9.