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After an exciting flyby of the yin-yang moon Iapetus, Cassini's journey at Saturn continues with Rev 50, its 51st orbit of the ringed planet. Cassini has a full plate during this month-long orbit as the spacecraft observes Saturn, its ring system, and four of its satellites: Titan, Tethys, Dione, and Enceladus. Cassini begins Rev50, on September 14 at its farthest distance from Saturn, called apoapsis. At this point, Cassini is 3.6 million km (2.2 million mi) from Saturn. Due to a spacecraft safing event during one of the downlinks following the September 10 Iapetus flyby, the science observations scheduled for the first few days of Rev50 were canceled. Once science observations begin again on September 16, Cassini's main target is Saturn's atmosphere. Four separate observations are planned between September 16 and 19. These 10-hour-long observations are designed to examine cloud motions over a full Saturnian day and will allow for a better understanding of circulation in the planet's atmosphere.
In addition to observations of Saturn's atmosphere, the first couple of weeks of Rev50 are filled with sequences covering Iapetus and Saturn's ring system, as well as calibration observations. The Saturn ring observations include movie sequences of the F-ring, A-ring photometry, and movies to look at structures in the B-ring. Distant observations are planned for Iapetus, coming off the targeted encounter in the previous orbit. Multi-color observations planned for September 17, with a resolution of 7.9 km/pixel, should reveal topographic information about the so-called "moat" craters, large impact basins with dark floors and bright central massifs in the western part of Iapetus' equatorial bright terrain. Another, much more distant observation is planned for September 27. This observation may reveal details about a series of grooves in the south-eastern part of Iapetus' dark terrain. Finally, calibration observations are planned to look for possible changes in the camera system. These observations are planned periodically since calibration parameters can change gradually throughout a mission. Cassini's closest approach distance to Saturn during Rev50 occurs on September 30 at a distance of 278,000 km (173,000 mi). Also on September 30, Cassini performs non-targeted encounters of three of Saturn's inner icy satellites: Dione, Enceladus, and Tethys. Dione is up first with five separate observations planned over a 12-hour encounter period. The observations start on September 29 with Dione seen at high-phase, meaning only a sliver of a sun-lit crescent is visible. As the encounter progresses, more and more of Dione's surface becomes visible. Cassini reaches closest approach to Dione at a distance of 46,700 km (29,000 mi). During this time, the Imaging Science Subsystem's narrow-angle camera will acquire a 20-frame mosaic covering the sub-Saturnian hemisphere-from about 270 to 60 degrees west longitude-an area opposite that that imaged during the targeted flyby in October 2005. Peak resolution should reach 270 m/pixel. Additional observations are planned for Dione's leading hemisphere. Next up is Enceladus. On September 30, Cassini approaches to within 98,000 km (61,000 mi) of the geologically active satellite. This encounter will provide Cassini's best view yet of Enceladus' leading hemisphere. More distant observations of this hemisphere have suggested that this area might be similar geologically to the active south polar region, with crisscrossing sets of grooves and ridges. These observations may help to pin down the surface age of the leading hemisphere. Distant observations and Saturn-shine observations from July 2005 suggested that this area is lightly cratered. A few hours before the encounter, a very high phase observation of the moon is planned. This sequence is designed to examine the state of Enceladus' south-polar jets, first seen in 2005.
The final non-targeted encounter is with the moon Tethys. This encounter, at a distance of 119,000 km (74,000 mi), will allow for additional observations of the area west of Ithaca Chasma, a region examined several times over the last few orbits. Two additional observations are also planned for September 29: the first will be of the B-ring and the other is of the boundary of Saturn's shadow on the rings. Cassini encounters Titan for the 37th time on October 2, with a close approach distance of 975 km (606 mi). Like the previous encounter, and many of the encounters so far this year, this flyby (known as T36) will allow for imaging of the anti-Saturnian hemisphere, centered near the bright, equatorial region named Adiri. ISS and the Visual and Infrared Mapping Spectrometer (VIMS) will be observing Titan starting around 6 hours after closest approach with periodic observations until around 15 hours after. Prior to the encounter, Cassini will be observing Titan at high-phase angles using the Composite Infrared Spectrometer (CIRS) and Ultraviolet Imaging Spectrometer (UVIS). These observations are intended to measure the composition of Titan's thick atmosphere at different altitudes. The ISS and VIMS observations will allow for global mapping of the anti-Saturnian hemisphere and examination of cloud evolution, if clouds are visible.
The RADAR instrument controls spacecraft pointing for the central 10 hours of the encounter, except for the short time immediately surrounding closest approach when the Ion and Neutral Mass Spectrometer (INMS) will be running the show. During the time RADAR is prime, that instrument will acquire radiometry and scatterometry over Fensal, Aztlan, Quivira, and eastern Tsegihi during the inbound leg and Adiri, eastern Belet, and western Shangri-la during the outbound leg. During closest approach, INMS will look for complex hydrocarbons in Titan's upper atmosphere as it flies through it. RADAR will be riding along with INMS. This will allow RADAR to acquire a SAR swath over the southern leading hemisphere, reaching as far south as 70° south latitude. RADAR will also acquire altimetry over the Huygens landing site and "HiSAR" over Shiwanni Virgae and western Tsegihi on Titan's Saturn-facing hemisphere. In the four days following the T36 flyby, Cassini will observe Saturn's ring system, Titan's trailing hemisphere, and two mutual events. The rings observations include sequences covering the F-ring and photometry of the A- and B-rings. On October 4, Cassini will make a global observation of Titan's trailing hemisphere. Finally, Cassini will look at two mutual events, or occultations, between two or more of Saturn's moons. The first involves Mimas and Epimetheus while the second involves Tethys and Calypso.
On October 12, Cassini begins Rev51, its 52nd orbit around Saturn. The next orbit will include numerous non-targeted encounters, include one of Titan that will provide gap-filling coverage over Titan's trailing hemisphere.