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Cassini continues its extended tour of Saturn on March 3 with Rev105, the spacecraftís 106th orbit around the Ringed Planet. Cassini begins Rev105 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 March 4 using the Ultraviolet Imaging Spectrometer (UVIS) to examine the extended plume region of Enceladusí atmosphere. ISS will also take a movie of the F ring, staring at the right ansa.
On March 5, ISS will start the day with a downlink session followed by tracking the small outer satellite Siarnaq for two hours. This observation will provide additional data points for a much longer observation of this small moon during Rev106. Very little is known about Siarnaq, a 40-kilometer wide moon discovered in 2000. These observations will help constrain the length of Siarnaqís day by measuring variations in the brightness of the satellite resulting from albedo markings and the moonís irregular shape. Similar bodies around Jupiter and Saturn have non-synchronous rotations with day lengths ranging from 8 to 14 hours. ISS will also observe Titanís sub-Saturn hemisphere from a distance of 2.3 million kilometers (1.4 million miles). The Composite Infrared Spectrometer (CIRS) will acquire several radial scans of the unlit face of Saturnís ring system, using this and similar observations to measure the thermal inertia and grain sizes across the system. Between March 5 and 8, Cassini will acquire three astrometric observations of several of Saturnís small satellites, including Prometheus, Anthe, the newfound moonlet S/2008 S 1, Daphnis, Janus, Pandora, Atlas, Epimetheus, Pan, and Pallene. Astrometric observations are designed to improve our knowledge of the orbits of Saturnís small satellites.
On March 6 and 7, the UVIS and the Visual/Infrared Mapping Spectrometer (VIMS) will observe four stellar occultations of the ring system and Saturnís atmosphere. Two of these involve the B-type star Beta Centauri being blocked, or occulted, by the rings. Another occultation involves the G-type binary Alpha Centauri (ISS has resolved the two components of the binary star during earlier occultations, though no support imaging is planned in this case. The fourth occultation involves the B-type star Alpha Arae. On March 6, CIRS will also observe the shadow of Saturn on the rings, looking at the infrared reflection of Saturn on the rings. Finally on March 7, ISS will observe a half-phase Titan, again looking at that moonís sub-Saturn hemisphere. On March 8, CIRS will take more temperature radial scans of the ring system, while ISS will take a 9-hour movie of the outer C ring.
Cassini reaches periapse, its closest point to Saturn on Rev105, late on March 9. At this point, Cassini will be 759,000 kilometers (472,000 miles) from Saturnís center, between the orbits of Rhea and Titan. Shortly before periapse, ISS will observe the inner Cassini Division and then Rhea as the spacecraft crosses below the ring plane. The camera system will then acquire another 7-hour movie of the outer C ring, this time looking at the sunlit side. Early on March 10, CIRS will acquire several more temperature radial scans of the rings as well as several stares pointing at particular areas of the rings rather than scanning across them. On March 11, ISS will take a 14-hour movie of the sunlit F ring. On March 12, the camera system will take another astrometric observation, this time including Methone, Prometheus, Janus, Atlas, Pan, and Pandora.
On March 13, ISS will take a 6-hour movie of the B ring looking for dust spokes. On March 14, Cassini will calibrate the gyroscopes that help control spacecraft pointing and stability. During this calibration procedure, ISS will acquire dark current calibration files, for the narrow-angle camera. Taken while the shutter is closed, these files are used to measure darkness levels on the cameraís charged-coupled device (CCD) sensor and maintain the related calibration software.
Cassini reaches apoapse on March 15, bringing Rev105 to an end. ISS and the Visual and Infrared Mapping Spectrometer (VIMS) will observe faint E and G rings. ISS will also acquire one more astrometric observation of Saturnís small satellites, including: Epimetheus, Telesto, Atlas, Daphnis, and Pan.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).