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Cassini continues its extended tour of Saturn on Feb. 20 with Rev104, the spacecraftís 105th orbit around the Ringed Planet. Cassini begins Rev104 at its farthest distance from Saturn, called apoapse. At this point, Cassini is 1.1 million kilometers (706,000 miles) from Saturn. The spacecraft remains in a high-inclination orbit, providing an opportunity to study the rings and the polar regions of Saturn and its satellites.
For the first five days of this revolution, ISS and the other optical remote sensing (ORS) instruments will focus on Saturnís ring system and satellites. Following a downlink session near apoapse, Cassini will spend much of Feb. 20 using the Ultraviolet Imaging Spectrometer (UVIS) to examine the extended plume region of Enceladusís atmosphere. ISS will also track the small outer satellite Skathi for an hour and a half. During the next three days, Feb. 21-24, Cassini will acquire three astrometric observations of several small satellites of Saturn, including Janus, Daphnis, Epimetheus, Pan, Atlas, Prometheus, Anthe, Pandora, and Methone. Astrometric observations are designed to improve our knowledge of the orbits of Saturnís small satellites. On Feb. 24, Cassini ISS will observe Titan at high-phase angles. Although not the best situation for observing Titanís surface, these observations can be useful for examining the large moonís atmospheric haze layers. ISS also will acquire a 13-frame, wide-angle camera mosaic of the unlit side of Saturnís ring system. Finally, the Composite Infrared Spectrometer (CIRS) will take several temperature radial scans of the rings.
On Feb. 25, ISS will take three observations. The first two will be short movie sequences covering portions of the left side of the ring system, including the F ring and Cassini Division. The third observation will cover Dione, which will be visible as a thin crescent. This will be a photometry observation, which, along with other distant images of the anti-Saturn side of this moon, will help refine our understanding of surface properties such as roughness and grain size.
Cassini reaches periapse, its closest point to Saturn on Rev104, early on Feb. 26. At this point, Cassini will be 759,000 kilometers (472,000 miles) from Saturnís center, between the orbits of Rhea and Titan. Following periapse, CIRS will take additional temperature radial scans of the ring system before Cassini downlinks its data to Earth. CIRS will acquire more temperature measurements on Feb. 27, though this time the scans will be limited to Saturnís shadow on the rings. These scans are useful for understanding the thermal inertia of the rings by seeing how the ring particles cool as they spend more and more time in Saturnís shadow. Thermal inertia can vary due to particle size and the amount of dust.
ISS will close out February with a pair of observations. The first is an astrometric observation of several of Saturnís small moons, including: Pandora, Prometheus, Janus, Atlas, Pan, and Pallene. The second observation is a 14-frame, wide-angle camera mosaic of the sunlit face of the ring system to compliment the one taken of the unlit face on Feb. 24. On March 1, CIRS once will again take various radial scans of the ring system, measuring the temperature of the rings at different longitudes.
On March 2, ISS will take another look at Titan, this time looking at the large moonís sunlit side. This observation, acquired from a distance of 2.3 million kilometers (1.5 million miles), will show Titanís sub-Saturn hemisphere -- a region dubbed Senkyo -- near the center of the visible disk. ISS also will take another astrometric observation, this time covering five moons: Thrym (one of the many, small, outer moons of Saturn), Prometheus, Anthe, Janus, and Pallene. ISS will acquire two narrow-angle camera radial scans of both ring ansae. Both will consist of 1-by-12 mosaics.
Cassini reaches apoapse on March 3, bringing Rev104 to an end. But before doing so, Cassini will observe the leading hemisphere of Rhea as the spacecraft crosses above the ring plane. These and other imaging sequences are being used to look into the possibility of a ring system around that icy satellite. Finally, ISS and the Visual and Infrared Mapping Spectrometer will observe the unlit side of the faint E ring.
Image products created in Celestia. All dates in Coordinated Universal Tme (UTC).