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Cassini closes out 2009 and brings in 2010 with Rev123, the spacecraft's 124th orbit around the Ringed Planet. Cassini begins Rev123 on December 18 at its farthest distance from Saturn, called apoapse. At this point, Cassini is 2.23 million kilometers (1.39 million miles) from Saturn's cloud tops.
Cassini ISS starts its observations for Rev123 a few hours after apoapse with a calibration sequence as part of a ride-along observation with the Ultraviolet Imaging Spectrometer (UVIS). During this observation, the wide-angle camera will acquire numerous images of the star Spica (Alpha Virginis) using different filters and exposure times. The next day, Cassini will image Saturn's largest moon, Titan, from a distance of 3.16 million kilometers (1.96 million miles). This observation will provide an opportunity to monitor clouds in Titan's troposphere on the satellite's sub-Saturn hemisphere. On December 21, ISS will observe the small, outer satellite Siarnaq in order to better understand the physical properties of this distant moon, 25.8 million kilometers (16.1 million miles) from Cassini. Two days later, on December 23, ISS and the Visual and Infrared Mapping Spectrometer (VIMS) will observe the sunward (left) ansa of Saturn's faint G ring for 13 hours. On December 24 and 25, ISS will acquire two observations of Enceladus at high phase angles, which are useful for observing the satellite's south polar plume. The second opportunity will be performed while Cassini is in Saturn's shadow, making it an excellent opportunity to observe Enceladus at very high phase angles. These phase angles (greater than 162 degrees) would normally be a violation of Cassini flight rules due to the risk of exposing the sensitive sensors on the spacecraft's remote-sensing instruments to too much sunlight. Also on December 25, ISS will take two mosaics, consisting of 15 and 22 frames each using multiple filters and exposure times, of the boundary of Saturn's shadow on the planet's ring plane.
On December 26, Cassini will reach periapse for Rev123, passing within 95,981 kilometers (59,640 miles) of Saturn's cloudtops. During periapse, Cassini will perform a non-targeted encounter with the small satellite Prometheus at a distance of 56,467 kilometers (35,087 miles). This encounter provides an opportunity for ISS to acquire its highest resolution images of this satellite to date with a best resolution of around 340 meters (1,120 feet) per pixel. ISS will image Prometheus's leading hemisphere as shown at right. During this same sequence, ISS will also take images of several propeller features in Saturn's A ring. Following this observation, ISS and VIMS will acquire two distant satellite observations, the first covering Enceladus's sub-Saturn hemisphere from a distance of 482,000 kilometers (300,000 miles) and the second covering Dione's sub-Saturn hemisphere from a distance of 675,000 kilometers (420,000 miles). VIMS and ISS will then acquire two observations of the lit face of Saturn's ring system. The first, higher-resolution observation will focus on the left ansa of the main system, while the second will consist of a 15-frame, red-green-blue, WAC mosaic across Saturn's rings and the planet itself. The view that will be presented in the 15-frame mosaic is shown at the top left of the page.
Cassini encounters Titan on December 28 at 00:17 UTC for the 65th time and for the second time in December. In addition to the science to be gained, this flyby is part of a series of Titan encounters that will help push Cassini back to high-inclination orbits that have been typical for much of 2009. The close approach distance is 955 kilometers (593 miles), close to the lowest safe flyby distance for Cassini. This flyby (known as T64) 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. 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 altimetry and scatterometry data from near Elpis Macula on Titan's northern mid-latitudes. Scatterometry measures the large-scale roughness of Titan's surface.
During closest approach, the Ion and Neutral Mass Spectrometer (INMS) will have prime pointing during the encounter, allowing the instrument to gather high resolution spectral data of Titan's upper atmosphere. During T64, INMS will sample Titan's northern polar atmosphere. The INMS team will compare data acquired during the encounter to data acquired two to three years ago when Cassini repeatedly flew over the north polar region. In particular, the team will be looking for seasonal changes in the upper reaches of Titan's atmosphere. The RADAR team will also be looking for surface changes within the north polar seas Punga Mare and Ligeia Mare during a ride-along observation at closest approach. These seas were also last seen two years ago. Since then, changes in the weather patterns both in the tropospheric methane clouds and higher altitude ethane clouds may have produced changes in the shoreline of these two large lakes. The T64 RADAR SAR swath will stretch from near the north pole south across Titan's anti-Saturn hemisphere down to just north of a bright region known as Adiri.
Following INMS and RADAR's close-in observations, CIRS, ISS, and VIMS will be prime during the outbound leg of the encounter. First, VIMS will acquire a mosaic across the Adiri region of Titan, helping fill in gaps in their map of Titan. ISS's main observation for T64 will be MONITORNA001, a 12-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 to the right. This large mosaic will be acquired from a distance ranging from 240,000 to 270,000 kilometers (150,000 to 170,000 miles). Finally, VIMS will obtain a global observation of Titan looking for clouds.
On December 29, ISS will observe Titan again during two observation periods. These sequences are designed to monitor Titan for clouds and for changes in Titan's north polar haze layers. These two observations will be acquired from distances of 700,000 and 775,000 kilometers (430,000 and 480,000 miles). These mosaics will be compared to the MONITORNA001 mosaic acquired the day before and two more cloud monitoring observation to be acquired on December 30 and 31 from 1.2 and 1.7 million kilometers (0.75 and 1.05 million miles) away. On December 30, January 1, and January 2, ISS will acquire three astrometric observations known as SATELLORBs of several small, Saturnian moons including Pandora, Atlas, Helene, Calypso, Janus, Epimetheus, Anthe, Telesto, and Pallene. On January 1, ISS will image 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. Finally, on January 2, ISS will acquire three mosaics of Saturn and the unlit-face of its main ring system. These include a 3-frame, wide-angle-camera mosaic of Saturn and the main ring system and two narrow-angle-camera radial scans across both ansae. The geometry for the WAC mosaic is shown at left.
Cassini reaches apoapse on January 3, bringing Rev123 to an end and starting Rev124. Rev124 includes a flyby of Saturn's largest moon, Titan (T65) and a non-targeted encounter of Enceladus.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).