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Cassini continues its exploration of the Saturn system with the 8.7-day Rev 248, which begins on November 6 at its farthest distance from the planet. This is also called the orbit’s apoapse. At this point, Cassini is 1.34 million kilometers (0.83 million miles) from Saturn’s cloud tops. Rev 248 occurs toward the end of the second inclined phase of the Cassini Solstice Mission. During this phase, Cassini will use encounters with Titan to gradually increase the inclination of its orbit. Fifteen ISS observations are planned for Rev 248 with the majority focused on Saturn’s moon Titan.
For its first observation for Rev 248, on November 7, ISS will acquire a quick observation of Saturn using the Wide-Angle Camera (WAC). This observation is part of a series of “Storm Watch” sequences designed to take advantage of short, two-minute segments when the spacecraft turns to point the optical remote sensing (ORS) instruments back at Saturn, as a waypoint between observations. This observation will be taken at the very end of a Composite Infrared Spectrometer (CIRS) observation of Saturn’s atmosphere. They include blue, clear, two methane band, and one full-frame, continuum band filter images. On November 9 and 10, ISS will acquire a pair of Titan cloud monitoring observations. The closest of these will be taken on November 10, acquired from a distance of 1.71 million kilometers (1.06 million miles), and will cover Titan’s sub-Saturn hemisphere.
On November 11 at 06:45 UTC, Cassini will reach periapse for Rev 248 at an altitude of 278,080 kilometers (172,790 miles) above Saturn's cloud tops, between the orbits of Tethys and Dione. During the periapse period, ISS and the rest of Cassini’s instrument suite will be focused on Saturn’s magnetic field. ISS will ride along with an Ultraviolet Imaging Spectrometer (UVIS) observation of Saturn’s south polar aurorae. On November 10, ISS will observe Saturn’s moon Tethys from a distance of 367,700 kilometers (228,500 miles), in order to map reddish streaks previously seen east of the Odysseus impact basin on the moon’s leading hemisphere. The Radio Science Sub-system (RSS) will perform a radio occultation observation of Saturn’s rings on November 12.
On November 13 at 23:56 UTC, Cassini will perform a targeted encounter of Titan. This is Cassini’s 125th flyby of Titan, the tenth of eleven planned for 2016. The next encounter is planned for November 29 during Rev 250. T124 has a close approach flyby has a close approach altitude of 1,584 kilometers (984 miles). This encounter will increase the inclination of Cassini’s orbit from 57.8 degrees to 61.4 degrees and will shave 1.6 days from the spacecraft’s orbital period. Inbound, ISS will be able to observe the southern, sub-Saturn hemisphere, while outbound, ISS will observe a crescent Titan and the north polar region of Titan.
During the T124 encounter, ISS will ride-along with observations by CIRS and the Visual and Infrared Mapping Spectrometer (VIMS). ISS will acquire WAC images during mid-infrared spectral scans performed by CIRS at the start of the encounter as well as during VIMS mapping observations as Cassini gets closer to Titan. ISS will use the Narrow-Angle Camera (NAC) to acquire a nine-frame, full-disk mosaic of Titan which will include the Fensal-Aztlan dark region, the Tsegihi bright region, as well as Hotei Arcus.
RSS will be prime during closest approach. The experiment will acquire a bistatic observation of portions of Titan’s north polar region, including portions of two large seas, Punga Mare and Kraken Mare. Bistatic observations are acquired by sending a radio signal from the spacecraft to Titan’s surface, where it can bounce off and be received back on Earth. These types of observations are useful for detecting the presence of surface liquids and waves within those surface liquids as well as measuring their composition.
Outbound, CIRS will perform a series of limb- and nadir-pointed observations. These will be used to look at the structure of Titan’s atmosphere, looking for differences in composition, temperature, and aerosols with altitudes at different latitudes on Titan. ISS will also look for clouds across Titan’s north polar region and northern mid-latitudes. If clouds are present, the repeating nature of this two-frame mosaic, with four tracking periods for both footprints, will allow for monitoring of their motion and evolution over a two-hour period. In the day after the encounter, on November 15 and into the next orbit, ISS will acquire a set of longer-range, cloud monitoring observations of Titan’s north polar region.
On November 15, Cassini will reach apoapse, bringing Rev 248 to a close and starting up the next orbit, Rev 249, when Cassini will perform targeted encounter with Titan.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).