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Cassini continues its extended tour of Saturn with Rev119, the spacecraft's 120th orbit around the Ringed Planet. Cassini begins Rev119 on October 2 at its farthest distance from Saturn, called apoapse. At this point, Cassini is 2.88 million kilometers (1.79 million miles) from Saturn's cloud tops. This orbit is only slightly inclined compared to orbital plane of Saturn's main satellites, providing an opportunity for Cassini to encounter some of Saturn's moons as well as to look at mutual events between the various satellites.
Cassini ISS starts its observations for Rev119 the day after apoapse. On October 3, ISS will take a calibration observation using three of the filters on its wide-angle camera by acquiring several images of the B-type star Spica. Between October 5 and 8, the Cassini Plasma Spectrometer (CAPS), the Ultraviolet Imaging Spectrometer (UVIS), and ISS will take four observations of the auroral oval over Saturn's north polar region. These observations will help to gauge the height of the auroral activity in Saturn's atmosphere and monitor motions in the aurora borealis of Saturn. On October 9, ISS will observe the crescent of Titan, 1.43 million kilometers (891,000 miles) distant, as part of a monitoring campaign of Titan's upper atmospheric haze layers. The Visual and Infrared Spectrometer (VIMS) and ISS will also examine the gossamer E ring of Saturn during a 12-hour-long observation designed to help scientists understand the particle size distribution within this faint ring.
Cassini encounters Titan on October 12 at 08:36 UTC for the 63rd time and for the first time since late August. The flyby, in addition to the science to be gained, will provide the final push for Cassini to reach a very low inclination orbit and will set up next month's Enceladus flybys. The close approach distance is 1,300 kilometers (808 miles. This flyby (known as T62) will allow for imaging of the sub-Saturn hemisphere of Titan outbound to the encounter. On approach to Titan, the Composite Infrared Spectrometer (CIRS) and UVIS teams will be controlling spacecraft pointing, or will be considered "prime." With Cassini approaching over the night side, anti-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 stare at the south polar region where fall began a couple of months ago. Following, UVIS will conduct a six-hour, extreme and far-ultraviolet spectrum scan of Titan's upper atmosphere over the satellite's southern leading hemisphere. Finally, CIRS will perform one more far-infrared scan of Titan's atmosphere of far-southern latitudes.
During the hour before and after closest approach to Titan for T62, UVIS and CIRS will be prime. During the approach, Cassini will pass into the shadow of Titan. As Cassini leaves this shadow, the spacecraft will point the UVIS solar port at the sun for a solar occultation observation of Titan's atmosphere. This occultation will allow UVIS to measure haze layers in Titan's atmosphere at high resolution. Like the CIRS observation preceding it, this occultation will be over far-southern latitudes near the south pole (83 degrees South). As Cassini recedes from Titan and the satellite appears more fully illuminated (11 degrees phase angle), CIRS will again conduct a far-infrared scan over these same far-southern latitudes. So, for this encounter, Titan's early fall south polar atmosphere will be observed by CIRS twice -- at both low and high phase angles -- and by UVIS during a solar occultation.
Following UVIS and CIRS's close-in observations, UVIS, CIRS, ISS, and VIMS will be prime during the outbound leg of the encounter. UVIS will acquire more far- and extreme-ultraviolet scans of Titan during another six-hour observation of the satellite. CIRS will then conduct scans of the northern limb of Titan in the far-infrared for three hours. Next, ISS will obtain its only prime observation for this encounter, MONITORNA001. This observation will cover with ten footprints much of the visible terrain of Titan, as shown in the figure above-left. The least blurry frames will cover western Senkyo, though some of the far northern and southern frames may show continued cloud activity in these regions. During MONITORNA001, Cassini will be roughly 250,000 to 275,000 kilometers (155,000 to 171,000 miles) from Titan. Finally, VIMS will obtain a similar observation of Titan, again monitoring Titan and looking for clouds.
On October 13, with Cassini continuing to approach Saturn, ISS will image Saturn, Rhea, and Enceladus. The Saturn observation, along with similar observations conducted during each of the next three days, is designed to examine cloud dynamics on the giant planet, as part of a series of ride-along observations with VIMS. Rhea will be observed twice on October 13 during a non-targeted encounter with Saturn's second largest moon. During this flyby, Cassini will pass within 40,379 kilometers (25,090 miles) of the satellite, though Rhea will be in eclipse at the time of closest approach. The first observation of this encounter will be conducted by CIRS and ISS while the satellite is in the shadow of Saturn. This observation is designed to study temperature changes on the satellite as a result of the eclipse. It is possible that Titan may appear in the background of some of the wide-angle camera images of Rhea during this observation. The second sequence is a ride-along observation with VIMS shortly after this eclipse. In this case, Cassini ISS will observe Rhea's sub-Saturn hemisphere from a distance of 200,000 kilometers (124,000 miles). Finally, ISS will observe Enceladus at high phase angles from a distance of 425,000 kilometers (264,000 miles). This observation is designed to gain a better understanding of Enceladus' south polar plume activity prior to next month's encounters.
Cassini reaches periapse, its closest point to Saturn on Rev119, on October 14. At this point, Cassini will be 132,440 kilometers (82,294 miles) from Saturn's cloud tops, between the orbits of Mimas and Enceladus. On October 14, Cassini will also perform two non-targeted encounters, with Mimas from a distance of 44,227 kilometers (27,481 miles) and with Tethys from a distance of 85,529 kilometers (52,977 miles). For the Tethys encounter, ISS will acquire a five-frame mosaic of Tethys' trailing hemisphere, including the degraded Penelope impact basin, from a distance of 120,000 kilometers (74,500 miles). ISS will later acquire three observations of Mimas, Enceladus, and Tethys. These images are designed to look at these satellites at very low phase angles (less than 1 degree in the case of Mimas). Such observations are important for refining photometric models of the surfaces of these moons. These models represent the nature of the satellites' surfaces beyond the limits of the cameras' resolutions by modeling the structure of a planetary surface at both the macroscopic (boulder-sized) and microscopic (particle-sized) levels. Finally, VIMS and ISS will acquire an observation, mentioned above, of Saturn's northern hemisphere in order to monitor cloud dynamics.
On October 16, ISS will observe the sub-Saturn hemisphere of Titan from a distance of 2.63 million kilometers (1.47 million miles). While distant, this observation is primarily designed to monitor clouds on this hemisphere of Titan. ISS will also observe Tethys, 1.21 million kilometers (750,000 miles) away, as it passes just above the rings and the small moon Prometheus from Cassini's perspective.
On October 17, ISS will observe a nearly half-phase Titan, over its sub-Saturn hemisphere, this time from a distance of 2.85 million kilometers (1.77 million miles). Combined with a similar observation the previous day, scientists hope to track cloud motions and evolution across this hemisphere, assuming that clouds are present at the time. Next, ISS will acquire a two-frame, wide-angle-camera mosaic of the crescent of Saturn, as part of a photometry campaign to be conducted using five mosaics to be taken between October 17 and 21. These observations are designed to study changes in Saturn's hazy upper atmosphere now that equinox has passed. Differences in the distribution of upper atmospheric haze led to a difference in the color between Saturn's northern and southern hemispheres. Next, ISS will observe a transit of the ice moon Tethys across the disc of Titan. Tethys will appear larger than it normally would against Titan since it is about 1 million kilometers (621,000 miles) closer to Cassini than Titan. Next, ISS will turn its cameras back to Saturn, this time to the planet's night side. For this observation, scientists will be monitoring lightning from storms on the planet. Finally, ISS will acquire an astrometric observation of several of Saturn's small satellites including Atlas, Calypso, Prometheus, Pallene, and Janus. Astrometric observations are used to help provide better orbital calculations for some of these small rocks, which can be effected by gravitational interactions with the larger icy moons. Over the next three days, ISS will acquire three more such observations. These "SATELLORB" sequences will provide distant imaging of the following moons: Epimetheus, Pallene, Calypso, Methone, Pandora, Anthe, Aegaeon, Pan, Atlas, Helene, and Janus.
On October 19, in addition to the Saturn photometry and satellite astrometric observations, Cassini ISS will also observe the transit by the innermost major moon of Saturn, Mimas, across the disk of Rhea. ISS will also conduct another monitoring observation of lightning on Saturn. On October 20, in addition to the aforementioned Saturn photometry and satellite astrometric observations, ISS will ride along with a CIRS compositional scan of Saturn's atmosphere and acquire a wide-angle camera, dayside movie of Saturn's cloud systems and conduct a narrow-angle search for lightning in these same storms on the night side. A similar sequence will be performed the following day, this time during a CIRS mid-infrared temperature map observation.
On October 22, in addition to the Saturn photometry observation, ISS will observe the partial transit of Rhea across Dione, both satellites more than 2 million kilometers (1.24 million miles) away from Cassini. ISS and UVIS will then observe the small outer satellite, Bestla (formerly known as S/ 2004 S 18). The observation is designed to study the photometry of this distant satellite as the moon is too far away and too small to see it as anything more than a point of light in ISS images. On October 22 and 23, ISS and VIMS will acquire two observations of Saturn's faint E ring, again studying the photometry of this feature. Finally, ISS will ride along with a CIRS composition mapping observation of Saturn, again acquiring a dayside movie of Saturn's clouds and searching for lightning on the planet's night side.
Cassini reaches apoapse on October 23, bringing Rev119 to an end and starting Rev120. Rev120 includes a flyby of the active moon Enceladus (E7).
Image products created in Celestia. Rhea, Tethys, and Enceladus basemaps by Steve Albers. All dates in Coordinated Universal Time (UTC).