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Cassini's journey at Saturn continues with Rev 54, its 55th orbit of the ringed planet. Cassini's slate of observations this orbit includes a flyby of Titan, distant encounters with Rhea and Dione, and numerous observations of Saturn's atmosphere and small moons. Cassini begins Rev54 on December 11 at its farthest distance from Saturn, called apoapsis. At this point, Cassini is 2.26 million km (1.4 million mi) from Saturn. The first week of Rev54 is filled with observations of Saturn's atmosphere and small satellites. Two small satellite sequences are planned for December 12 and 13. The observations are designed to study the orbits of these objects and how they might evolve over short time periods due to perturbations from the other satellites in the system. Between December 16 and 18, four observations focused on Saturn itself are planned. Most of these imaging sequences ride along with scans by the Composite Infrared Spectrometer (CIRS) and Ultraviolet Imaging Spectrometer (UVIS) that are designed to better understand the composition and structure of Saturn's upper atmosphere. During these observations, Cassini is, unfortunately, poorly positioned to image the new storm that cropped up in Saturn's southern hemisphere during the last couple of orbits. A distant observation of Rhea is also planned for December 17. Cassini reaches periapse, the closest point in its orbit, on December 19 when it is 181,000 km (112,000 mi) above Saturn's cloud-tops. Two imaging sequences are planned for that date, targeting Rhea and Dione. Cassini observes Rhea first from a distance of 295,000 km (183,000 mi). Images from this observation will cover that satellite's sub-Saturn hemisphere (Rhea's equivalent of our Moon's "near-side"). While this region has been covered before by Cassini at higher resolution, the lower phase angle will help with examining color variations in this region. Later in the day, Cassini will observe Dione from a distance of 226,000 km (140,000 mi). Cassini will observe Dione's anti-Saturn hemisphere (Dione's equivalent of our Moon's "far-side"). Cassini encounters Titan for the 40th time on December 20, with a close approach distance of only 970 km (602 mi). Like many of the encounters this year, the flyby (known as T39) 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, the Visual and Infrared Spectrometer (VIMS) and CIRS teams trade off control spacecraft pointing (or are "prime"). Their observations of Titan's night side will provide information on the composition of the moon's atmosphere. The RADAR instrument will be in control of spacecraft pointing for the five hours before and after closest approach. During this flyby, RADAR will use all of its observation modes: synthetic aperture radar (SAR) at closest approach, altimetry before and after the SAR swath, scatterometry, and radiometry. The altimetry swaths, designed to study regional topography, will cover central Tsegihi when inbound, and the terrain north of Adiri while outbound. The SAR swath, which will provide RADAR imaging at up to 300 m/pixel resolution, will cover Titan's South Pole and the terrain along 30 degrees and 210 degrees west longitude, as far north as 40 degrees south latitude. This swath will cover a few of the possible lakes observed by ISS in June 2005, including a large, W-shaped dark feature near 86 degrees south, 35 degrees west.
Following closest approach, ISS, VIMS, and CIRS will trade off control of spacecraft pointing. The only ISS mosaic planned is a 15-frame, full-disk mosaic taken from a distance of 234,000 km (145,000 mi). VIMS will acquire distant observations of Titan designed to study cloud evolution (assuming clouds are visible at the time). Before returning all of its Titan data back to Earth, Cassini will also observe Rhea from a distance of 887,000 km (551,000 mi) in an observation designed to study the photometric properties of Rhea's north polar region. Cassini will image Titan again on December 23, for a distant observation of that moon's trailing hemisphere.
During the last week of Rev54, Cassini will perform numerous observations of small, inner moons, the ring system, and the small, outer moon, Kiviuq.
Cassini begins the following orbit, number 56 ("Rev55"), on December 27, during which it will encounter Titan for the 41st time and perform distant encounters with Tethys and Dione.