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Cassini closes out the first month of its two-year extended mission with Rev 78, its 79th orbit around Saturn. During this orbit, Cassiniís observations focus on Saturnís icy satellites and Titan, with a few observations of the rings and atmosphere. Cassini begins Rev78 early on July 25 at its farthest distance from Saturn, called apoapsis. At this point, Cassini is 1.25 million km (779,000 mi) from Saturn. Cassiniís first observations of the orbit involve Saturnís atmosphere. On July 25 and 26, the Ultraviolet Imaging Spectrometer (UVIS) performs several scans of the north polar region of Saturn, looking at the aurora there. ISS will ride along with both of these observations, acquiring wide-angle camera images of Saturnís high northern latitudes.
Starting early on July 27 and running through July 30, Cassiniís instrument pointing will be controlled by the Rings Targeting Working Team, though most of the ISS observations during this period focus on Saturnís icy satellites. Following an orbital trim maneuver to set up the Titan flyby on July 30, the first of these observations on July 27 involves a sequence at Tethys from a distance of 544,000 km (338,000 mi). This sequence is part of Cassiniís campaign to map the north polar regions of Saturnís satellites (this time the northern anti-Saturn hemisphere of Tethys). Following the Tethys observation, the Visual and Infrared Mapping Spectrometer (VIMS) and Composite Infrared Spectrometer (CIRS) observe stellar occultations of the rings, using Gamma Crucis and Eta Carinae, respectively. Early on July 28, VIMS and the Ultraviolet Imaging Spectrometer (UVIS) will observe two additional stellar occultations of the rings, the first with UVIS using Beta Centauri and VIMS using Beta Crucis. ISS will also acquire a partial azimuthal scan of the Maxwell ringlet in the C ring. Cassini also reaches periapse on July 28, its closest point to Saturn on Rev78. At that point, Cassini will be 162,274 km (100,832 mi) from Saturnís cloud tops. Near periapse, Cassini will quickly pass high over the north polar region of Saturn before descending below the ring plane 23 minutes before closest approach to the planet. Before periapse, ISS will observe the northern sub-Saturn hemisphere of Tethys from a distance of 175,000 km (109,000 mi). About 30 minutes after periapse, ISS will observe the outer A ring at high resolution using five color filters. Following this, CIRS and ISS will observe Tethys as it leaves Saturnís shadow over the course of almost three hours.
On July 29, Cassini will turn its instruments back to the rings. ISS will observe the B ring twice, first to look for ring spokes forming, and then using the narrow-angle camera to look for azimuthally variable structures. Next, ISS will observe Rheaís southern leading hemisphere from a distance of 410,000 km (255,000 mi). This will allow for a great look at a bright ray crater seen at high resolution during a close flyby last year. Cassini encounters Titan for the 46th time (the first time in the extended mission) on July 31, with a closest approach distance of only 1,613 km (1,002 mi). This flyby (known as T45) will allow for imaging of the leading hemisphere of Titan. Inbound to the encounter, when Cassini observes Titan at moderate phase angles over southwestern Xanadu, the CIRS and ISS teams trade control of spacecraft pointing (taking turns being ďprimeĒ) until 12 hours before the encounter, when the Radio Science Subsystem (RSS) team takes over. T45 is a ďgravity passĒ for Cassini. During closest approach, Cassini will essentially act as a test particle, pointing its antenna at Earth. The RSS team will use the Doppler effect on Cassiniís radio signal to further refine our knowledge of Titanís gravity field, measuring Titanís Love number (a measure of the general distribution of mass within Titan, which would be affected by the presence of an internal ocean), and looking for irregular mass distributions (like mass concentrations below the surface). At closest approach, CAPS and RPWS will also focus on the magnetic environment around Titan. On the outbound leg of the encounter, CIRS and RSS trade off prime coverage.
Cassini begins Rev79 on July 31.
Image products created in Celestia. Rhea and Tethys basemaps by Steve Albers.