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Rev 95: November 28 – December 5, 2008
Cassini continues its extended tour of the Saturn system with Rev95, the spacecraft’s 96th orbit around the Ringed Planet. Rev95 includes a flyby of Saturn’s largest moon, Titan.
Cassini begins Rev95 on November 28 at its farthest distance from Saturn, called apoapse. At this point, Cassini is 1.22 million km (760,000 mi) from Saturn. The spacecraft is in a high-inclination orbit here, as it is for most of 2008, providing opportunities to view the ring system and satellites from high above the ring plane. During Rev95, Cassini will use this high-inclination orbit to study Saturn’s polar regions. On November 29 and 30, Cassini will acquire two observations of Saturn’s north polar region. The first, planned by the Ultraviolet Imaging Spectrometer (UVIS) team, will focus on Saturn’s aurorae. The second, planned by the Visual and Infrared Mapping Spectrometer (VIMS) team, will examine cloud dynamics. During both observations, ISS will make movies of Saturn’s high-northern latitudes. On November 29, Cassini will also take a look at several of Saturn’s small, inner satellites, including Prometheus, Pallene, Atlas, Methone, and Pan, as part of an observation campaign to study the orbital motions of these objects.
On December 2, Cassini reaches periapse, its closest point to Saturn in Rev95. At that point, Cassini will be 313,000 km (195,000 mi) from Saturn’s center. Near periapse, Cassini will quickly pass high over the north polar region of Saturn before descending below the ring plane 77 minutes before periapse. Shortly before periapse, Cassini will perform a non-targeted encounter of Enceladus at distance of 122,430 km (15,200 mi). A two-and-a-half-hour long observation will coincide with this flyby. Images acquired during this encounter can later be combined to create stereo images of the northern trailing hemisphere of this icy satellite, helping scientists better understand the topography of this region of Enceladus. The Composite Infrared Spectrometer (CIRS) will also take a mid-infrared temperature scan across Enceladus’ north polar region.
Following periapse, on December 2 and 3, Cassini will turn its cameras to Saturn’s south polar region. Like the spacecraft’s observations of the north before periapse, Cassini will examine aurorae and cloud dynamics. Cloud dynamics observations are of particular importance for ISS as the sun will be setting on the south pole of Saturn in August 2009.
Cassini encounters Titan on December 5 for the 49th time and the first of two flybys planned for December. The close approach distance is only 960 km (596 mi), close to the lowest safe altitude for a Titan flyby. This flyby (known as T48) will allow for imaging of the leading hemisphere of Titan inbound to the encounter. In the day-and-a-half before close 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 one hour before the encounter when the RADAR team will take over. During this approach period, ISS will acquire three mosaics covering this region. The first, GLOBMAP001, will be taken between 117,000 and 188,000 km (73,000 and 117,000 mi) from Titan and consists of 24 footprints covering the entire sunlit region visible to Cassini. The second, REGMAP001, will be taken between 66,000 and 104,000 km (41,000 and 65,000 mi) from Titan and consists of 13 footprints covering Hotei Arcus and the terrain just to the south. The third mosaic is a three-footprint, high-resolution observation covering the terrain just to the south of Hotei Arcus. VIMS will acquire several global mapping observations as well as high-resolution observations corresponding to the ISS three-frame mosaic. Finally, CIRS will stare at Titan, measuring the composition of the moon’s atmosphere.
During closest approach on T48, the RADAR and Ion and Neutral Mass Spectrometer (INMS) will be prime, taking Synthetic Aperture Radar (SAR)imaging and altimetry of Titan’s surface and measuring the composition of its upper atmosphere. Starting about one hour before closest approach, RADAR will acquire a distant SAR swath over Titan’s southern mid-latitudes (55° South latitude, 120°-175° West longitude) (see map of Titan). Following this pass, RADAR will switch to altimetry mode, and will take measurements covering a small swath of southern Xanadu. Finally, starting about 20 minutes before closest approach, RADAR will acquire a SAR swath. This swath will be broken up into two sections. During the first, RADAR will control spacecraft pointing as Cassini approaches Titan. RADAR SAR will sweep across Eir Macula and Tui Regio from east to west (20°-30° South latitude, 95°-135° West longitude). Tui Regio is thought to be a cryovolcanic region and observations of this feature will be compared to those of Hotei Arcus, a similar feature observed by RADAR earlier in the year. Within 10 minutes of closest approach, spacecraft pointing control will switch to INMS. Given the low altitude of the encounter, T48 is an important flyby for INMS, allowing the instrument team to measure chemical species in Titan’s dayside ionosphere near the equator. RADAR will ride along with INMS near closest approach, acquiring a SAR swath from 15° South latitude, 140° West longitude to 20° North, 195° West longitude. This includes portions of southeastern Xanadu, central Shangri-la, and southeastern Dilmun and features such as Bacab Virgae and Shikoku Facula.
Following closest approach, the CIRS, UVIS, VIMS, and ISS control spacecraft pointing. CIRS will measure the distribution of carbon monoxide, water, methane, and other chemical species in Titan's atmosphere. UVIS will also measure atmospheric composition by observing an occultation of Titan’s atmosphere by the star Epsilon Canis Majoris (Adhara). VIMS and ISS will conduct cloud monitoring observations of Titan’s north polar region.
Near the end of the Titan encounter, Cassini will reach apoapse, beginning Rev96.
Image products created in Celestia. Enceladus basemap by Steve Albers. All dates in Coordinated Universal Time (UTC).