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Cassini continues its extended tour of the Saturn system with Rev97, the spacecraft's 98th orbit around the Ringed Planet. Rev97 includes a flyby of Saturn's largest moon, Titan.
Cassini begins Rev97 on December 13 at its farthest distance from Saturn, called apoapse. At this point, Cassini is 1.19 million km (737,000 mi) from Saturn. The spacecraft is in a high-inclination orbit here, as it is for most of 2008, providing opportunities to study the rings and polar regions of Saturn and its satellites. On December 16, Cassini ISS will obtain six observations of Saturn's north polar region. The Visual and Infrared Mapping Spectrometer (VIMS) and ISS will acquire three observations intended to map the dynamics of weather systems on Saturn, including the vortex at the North Pole. These three observations are broken up by two stellar occultations of the nightside of Saturn. The first occultation will use Alpha Crucis, while the second uses Beta Centauri. Both are bright, B-type, blue-white giant stars in the southern sky, useful for ultraviolet occultations. UVIS stellar occultations are used to measure the density of stratospheric haze layers on Saturn as well as Titan.
During the days before periapse, Cassini will also image several of Saturn's moons. On December 14, Cassini will look at several of Saturn's small satellites ―including Telesto, Prometheus, Pan, Anthe, Epimetheus, and Pandora ― as part of an observation campaign to study the orbital motions of these objects. On December 15, ISS will look at Titan's northern sub-Saturn hemisphere, from a distance of 2.02 million km (1.26 million mi), in order to look for clouds.
On December 17, Cassini reaches periapse, its closest point to Saturn on Rev97. At that point, Cassini will be 346,000 km (215,000 mi) from Saturn's center. Before periapse, ISS will observe Saturn's second largest moon, Rhea. During this observation, acquired from a distance of 770,000 km (478,000 mi), Cassini will see only a thin, sunlit crescent, though much of the visible surface will be illuminated by sunlight reflected off of Saturn, also known as "Saturnshine". Following this sequence, ISS will make a three-frame mosaic of the dark limb of Saturn. While not illuminated by the Sun, the planet's upper atmosphere will be backlit by the rings, enabling a measurement of the haze layers in Saturn's stratosphere, much like the stellar occultations observed earlier in the orbit.
On December 18, ISS and VIMS will monitor cloud dynamics near Saturn's south pole. With this and other observations over the next year, the teams hope to find evidence for seasonal variations as Saturn's southern hemisphere approaches fall. ISS will also observe Prometheus, a shepherd moon of the F ring, from a distance of 829,000 km (515,000 mi). The next day, ISS will create a two-frame mosaic over Titan's southern leading hemisphere, hoping to find clouds and to study the geology of western Tsegihi. Recent observations have revealed numerous, east-west trending dark lineaments, which may represent mountains ranges or, at the very least, tectonic control of dark albedo materials.
Cassini encounters Titan on December 21 for the 50th time,the second of two flybys planned for December. The close approach distance is only 970 km (603 mi), close to the lowest safe altitude for a Titan flyby. This flyby (known as T49) will allow imaging of the southern leading hemisphere of Titan inbound to the encounter. In the 12 hours before closest approach, when Cassini will observe Titan at moderate phase angles over southwestern Xanadu, the VIMS, CIRS, and ISS teams will control pointing (or be "prime") until 45 minutes before the encounter when the RADAR team will take over. During this approach period, ISS will make two mosaics covering this region. The first, GLOBMAP001, will be taken between 115,000 and 185,000 km (71,000 and 115,000 mi) from Titan and consists of 24 footprints covering the entire sunlit region visible to Cassini. The second, REGMAP001, will be taken between 65,000 and 100,000 km (40,000 and 62,000 mi) from Titan and consists of 13 footprints covering Hotei Arcus and the terrain just to the south. This mosaic covers the same area as the REGMAP001 mosaic from T48. In the two hours before closest approach, VIMS will acquire a four-frame mosaic of Hotei Arcus, a possible cryovolcanic region in the southeastern portion of Xanadu. In near-infrared light, Hotei appears bright at 5 microns, a spectral feature thought to be diagnostic of recent cryovolcanism on Titan. VIMS will also attempt to observe Ontario Lacus, a lake observed by VIMS late last year. VIMS will attempt to look for changes in the lake's level between the two observations. During the approach period, the Composite Infrared Spectrometer (CIRS) will acquire several observations of Titan's atmosphere, including a mid-infrared temperature map, a far-infrared compositional map, and a far-infrared limb integration, designed to look at compositional variations at different altitudes in Titan's atmosphere.
During closest approach on T49, RADAR will be prime. Starting shortly before closest approach, RADAR will acquire a Synthetic Aperture Radar (SAR) swath over Titan's southern high-latitudes (60-80° South latitude, 70-145° West longitude). One high-priority target for ISS in this swath is a dark feature that appeared near 75° South latitude, 120° West longitude between July 2004 and the new dark feature to the upper right of red cross in July 2005. This dark albedo feature is thought to have been formed during a rainstorm during the 11 months between observations, possibly during a large storm observed by ISS and ground-based observers in October 2004. RADAR observations during this flyby might be able to show whether any liquid remains within this proposed playa. However, given that four years have passed since the floods occurred, it is likely that the playa has dried up already. Still, RADAR observations of this feature might be able to tell us more about the geologic setting of this observed surface change. Following this pass, RADAR will switch to altimetry mode, acquiring a long swath during closest approach that includes Ontario Lacus. Altimetry observations would allow RADAR to confirm the presence of liquid within Ontario. RADAR altimetry will run roughly southeast to northwest from 75° South latitude, 165° West longitude to 65° South latitude, 215° West longitude. Following this altimetry swath, RADAR will switch back to SAR mode. This second SAR swath will run north-south across Titan's trailing hemisphere, from 60° South latitude, 245° West longitude to 25° North latitude, 275° West longitude. This swath will cover portions of western Belet as well as the intermediate albedo terrain to the south of Belet.
Shortly after closest approach, Cassini will reach apoapse, beginning Rev98. Following its SAR and altimetry observations, RADAR will acquire scatterometry and radiometry data over Titan's northern trailing hemisphere. Five hours after the flyby, CIRS will perform another mid-infrared limb integration. ISS and VIMS will search for clouds over the northern hemisphere of Titan.
Image products created in Celestia. Rhea basemap by Steve Albers. All dates in Coordinated Universal Time (UTC).