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Cassiniís journey at Saturn continues with Rev 53, its 54th orbit of the ringed planet. Cassiniís slate of observations for this orbit includes flybys of Titan and Epimetheus and numerous observations of Saturnís ring system and small moons. Cassini begins Rev53 on November 25 at its farthest distance from Saturn, called apoapsis. At this point, Cassini is 2.29 million km (1.42 million mi) from Saturn. The first week of Rev53 is filled with observations of Saturnís small satellites as well as larger Iapetus. A thin, sunlit crescent on Iapetus will be visible from Cassini, but some images will be acquired using longer exposure times to reveal the night-side hemisphere illuminated by Saturn.
Several observations of Saturnís small, icy satellites, as well as the ring system, are planned between November 26 and 30. These 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. These observations also include several of Saturnís small, outer satellites; moons discovered only within the last decade. The moons to be imaged include Ymir, Erriapus, Kiviuq, Siarnaq, and Paaliaq. All of these moons are smaller than 40 km (25 mi), and given Cassiniís distance to them, they will not be resolved as anything more than points of light. However, like the inner, small satellite observations, these images will help pin down the orbital elements for the moons. For Paaliaq, images will be spaced out over a period of nine hours. Measurement of the slight variations in Paaliaqís brightness over the course of the sequence could provide an estimate of Paaliaqís rotational period. Moons at Paaliaqís distance do not have one side always facing Saturn, and thus their rotational period is not equal to their orbital period, like Saturnís closer-orbiting moons. For example, Phoebe (the largest member of Saturnís menagerie of small, outer satellites) takes 550 days to orbit Saturn once but just nine hours to rotate once on its axis. A three-image, wide-angle camera mosaic of Saturnís ring system is also planned for November 29. In the three days prior to periapse, Cassini will observe the north face of Saturnís rings, the side not currently illuminated by the Sun. First up is a 14-hour-long observation of the Encke Gap with the narrow-angle camera. This gap in the outer part of Saturnís A ring is carved out by the small moon Pan and contains a ringlet at its core made of dust shed from Pan. This observation is designed to study the variations in the brightness and thickness of that ringlet (see PIA09777 for a similar observation of the Encke Gap). The second observation is a radial scan of the dark face of the rings using the Composite Infrared Spectrometer (CIRS). Finally, on December 2, Cassini will observe the F ring at high resolution. Cassini reaches periapse, the closest point in its orbit, on December 3 when the spacecraft is 151,000 km (94,000 mi) above Saturnís cloud tops. Shortly before periapse, Cassini will observe Mimas from a distance of 700,000 km (440,000 mi) to study the surface properties of that small moonís leading hemisphere, the area surrounding the giant crater Herschel. On December 3, Cassini will fly by Epimetheus at a distance of 8,711 km (5,412 mi). This is Cassiniís closest targeted encounter with one of Saturnís small moons since the flyby of Phoebe in June 2004. At only 114 km (71 mi), Epimetheus is not large enough to pull itself into a sphere, so in addition to studying the geology of this small body, observations during the encounter will be used to study its shape. The closest approach to Epimetheus will occur when the moon is nearly between Cassini and the Sun, so all images in this encounter will be taken after closest approach at distances greater than 36,000 km. Following the Epimetheus encounter, another CIRS-ISS radial scan of the rings is planned--this time of the sunlit face. Cassini encounters Titan for the 39th time on December 5, with a close approach distance of 1,300 km (807 mi). Like many of the encounters this year, the flyby (known as T38) 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 Radio Science System (RSS) and CIRS teams trade off control spacecraft pointing (or are ďprimeĒ). The CIRS observations of Titanís night side will provide information on the composition of Titanís atmosphere. The RSS team will use their time to examine Titanís gravitational field. These measurements, combined with similar ones taken during other passes in the mission, will be used to put together a model of Titanís interior. During closest approach, the Visual and Infrared Mapping Spectrometer (VIMS) team will be in control of spacecraft pointing. The closest approach point of the spacecraft over the surface of Titan is near the south pole, providing an excellent opportunity to observe the possible lakes discovered in the region by the ISS team in June 2005. The VIMS team will image the dark feature known as Ontario Lacus, a possible lake 250 km (155 mi) across (see PIA06240 for ISSí highest resolution observation of this feature). The VIMS team intends to study the spectra obtained in this observation to look for the presence of liquid methane. ISS intends to ride along with this observation, though observations taken below ~40,000 km from Titan tend to be smeared.
Following closest approach, ISS and VIMS will examine the trailing hemisphere of Titan. ISSí highest resolution mosaic will cover the northwestern part of the dark region named Shangri-la, north of the Huygens landing site. This region includes a 95-kilometer-wide, dark, circular feature that is thought to be an impact crater. A lower resolution mosaic covering much of the visible surface is also planned from a distance of around 120,000 km (75,000 mi). A third mosaic covering the entire visible surface of Titan is planned when Cassini is 240,000 km (150,000 mi) from Titan. Additional CIRS and RSS observations are also planned following closest approach, as well as a VIMS global observation designed to study cloud evolution (if clouds are visible during the encounter).
During the last week of Rev53, Cassini will undertake numerous observations of Saturnís ring system and small, inner moons. One of the larger rings observations, planned for December 6, is designed to look at the dustier portions of the rings from just above the ring plane on the dark side. These regions include the D ring, the Cassini Division, the F ring, the G ring, and the E ring. Throughout this period, Cassini will continue to observe Saturnís small inner satellites in order to refine their orbital elements.
Cassini begins the following orbit, number 55 (ďRev54Ē), on December 11, during which it will encounter Titan for the 40th time and perform distant encounters with Rhea and Dione.