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Cassini continues its extended tour of Saturn with Rev122, the spacecraft's 123rd orbit around the Ringed Planet. Cassini begins Rev122 on November 30 at its farthest distance from Saturn, called apoapse. At this point, Cassini is 2.5 million kilometers (1.55 million miles) from Saturn's cloud tops. During Rev122, Cassini orbits in the ring and orbital plane of Saturn's main satellites, providing an opportunity for Cassini to encounter some of Saturn's moons as well as observe mutual events between the various satellites.
Cassini ISS starts its observations for Rev122 a few hours after apoapse with an observation of Titan while the satellite is in Saturn's shadow. This will allow scientists to examine atmospheric glows from excited ions like nitrogen in Titan's upper atmosphere. The next day, Cassini will acquire the first of three distant observations of Saturn's outermost, large, icy satellite, Iapetus (the others will be taken on December 4 and 5). These images will be acquired 1.38 to 1.66 million kilometers (0.86 to 1.03 million miles) from Iapetus and are designed to provide a better understanding of Iapetus' shape, which is more irregular than most moons its size. Also on December 1, ISS will image an occultation of Enceladus by Rhea. On December 2, ISS will acquire the first of seven observations of a crescent Saturn using one- or two-frame, wide-angle-camera mosaics of the ringed planet's sunlit side. These observations are taken about once each day between December 2 and December 8. The mosaics are designed to study how the photometry and polarimetry (focusing on the high altitude hazes over the poles) of Saturn's atmosphere has changed since earlier in the Cassini mission. On December 4 and 7, Cassini ISS will acquire two six-frame, north-south strips across Saturn's nightside using the narrow-angle camera. These mosaics are designed to search for lightning within storms in the planet's atmosphere. These observations will also include a wide-angle-camera frame over Saturn's dayside and a narrow-angle frame at different spots on the nightside (usually near the southeast dark limb). On December 6, the Visual and Infrared Mapping Spectrometer (VIMS) and ISS will observe the right ansa of the faint G ring for 12 hours.
On December 10, Cassini will reach periapse for Rev122, passing within 132,600 kilometers (82,400 miles) of Saturn's cloud tops. During this periapse period, on December 8 and 10, ISS and VIMS will map cloud features on Saturn, monitoring their dynamics. VIMS will have the advantage of being able to monitor structures on Saturn's nightside while ISS will have better resolution on the dayside of Saturn's atmosphere. On December 9, ISS will observe Enceladus's plumes from a distance of 630,000 kilometers (390,000 miles) while Cassini is in the shadow of Saturn.
Cassini encounters Titan on December 12 at 01:03 UTC for the 64th time and for the first time in two months (about 3.75 Titan days). The flyby, in addition to the science to be gained, will provide the first push for Cassini out of its current low-inclination orbits and back into the high-inclination orbits that have been typical for much of 2009. The close approach distance is 4,850 kilometers (3,013 miles). This flyby (known as T63) will allow for imaging of the anti-Saturn hemisphere of Titan outbound to the encounter. On approach to Titan, the Composite Infrared Spectrometer (CIRS), ISS, and RADAR teams will be controlling spacecraft pointing, or will be considered "prime." With Cassini approaching over the nightside, sub-Saturn hemisphere, CIRS will map atmospheric temperature and composition in the mid-infrared over Titan's leading hemisphere looking for seasonal variations over this region now that spring has started in the northern hemisphere. CIRS will also have a "point-and-stare" observation over the northern mid-latitudes where spring began a few months ago. ISS will acquire several WAC frames over Titan's visible crescent as part of a monitoring program of Titan's upper atmospheric haze layers. Finally, RADAR will acquire two radiometry scans of Titan's sub-Saturn hemisphere. During the five hours surrounding closest approach, the Cassini Plasma Spectrometer (CAPS) will be prime, acquiring observations of the interaction between Saturn's magnetosphere and Titan's ionosphere on Titan's leading hemisphere, the side of Titan facing the satellite's direction of motion around Saturn.
Following CAPS' close-in observations, CIRS, ISS, and VIMS will be prime during the outbound leg of the encounter. CIRS will acquire two far-infrared composition scans of Titan's southern hemisphere and a mid-infrared temperature map of Titan's anti-Saturn hemisphere. ISS' main observation for T63 will be GLOBMAPC001, a 32-frame global mosaic of Titan's anti-Saturn hemisphere and the area around the bright region Adiri in particular. The area to be covered by this mosaic is shown above. This large mosaic will be acquired from a distance ranging from 80,000 to 165,000 kilometers (50,000 to 103,000 miles). Finally, VIMS will obtain a global observation of Titan, looking for clouds.
On December 13, ISS will observe Titan again during two observation periods. These sequences are designed to help fill out the ISS map of Titan on the satellite's trailing hemisphere and in western Belet, a low-albedo region, in particular. Scientists will also be using these images to monitor clouds that might be visible in this region. Since equinox in August, more clouds have become visible on Titan's northern mid-latitudes, and this region was host to a cloud outburst in April 2008. These two mosaics will be acquired from distances of 685,000 and 750,000 kilometers (425,000 and 466,000 miles). These mosaics will be compared to the GLOBMAPC001 mosaic acquired the day before and another cloud monitoring observation to be acquired on December 14 from a distance of 1.2 million kilometers (745,000 miles). On December 15 and 17, ISS will acquire two astrometric observations known as SATELLORBs of various small, Saturnian moons. The observations will be of the following moons: Methone, Pan, Anthe, Pallene, Polydeuces, and Telesto. On December 15, ISS will image a transit of Janus by Epimetheus. Finally, on December 16, ISS will use three, narrow-angle-camera frames to observe the space around Iapetus, 3.6 million kilometers (2.2 million miles) from Cassini. These images will be used to look for dust in Iapetus' hill sphere, the area around the satellite where Iapetus' gravity dominates over Saturn's.
Cassini reaches apoapse on December 18, bringing Rev122 to an end and starting Rev123. Rev123 includes a flyby of Saturn's largest moon, Titan (T64) and a close, non-targeted encounter of Tethys.
Image products created in Celestia. Rhea All dates in Coordinated Universal Time (UTC).