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Cassini continues its exploration of the Saturn system with the 13-day Rev 225, which begins on November 4 at its farthest distance from the planet. This is also called the orbit’s apoapse. At this point, Cassini is 2.00 million kilometers (1.24 million miles) from Saturn’s cloud tops. Rev 225 occurs during the second Equatorial phase of Cassini’s extended-extended mission. During this 10-month phase, Cassini will orbit within the orbital plane of Saturn’s rings, allowing for frequent encounters with Saturn’s icy satellites. This orbit includes the final Titan flyby for 2015 and a close, non-targeted encounter with Tethys. Thirteen ISS observations are planned for Rev 225 with the majority focused on Titan and Saturn’s atmosphere.
For its first observation of Rev 225, on November 7, ISS will ride-along with an Ultraviolet Imaging Spectrometer (UVIS) scan of Saturn. ISS will acquire photometric and polarimetric observations of Saturn during this observation. On November 11 at 09:30 UTC, during a Visual and Infrared Mapping Spectrometer (VIMS) observation of a stellar occultation by Saturn’s rings of the star 30 Piscium, Cassini will reach periapse for Rev 225 at an altitude of 114,600 kilometers (71,210 miles) from Saturn's cloud tops, between the G ring and the orbit of Mimas. Five hours later, at 14:42, Cassini will perform a close, non-targeted encounter with Saturn’s moon Tethys. Cassini will pass 8,356 kilometers (5,192 miles) from the moon’s surface. ISS will acquire an observation around closest approach, with a single footprint centered near 47 degrees North Latitude, 26 degrees West Longitude. This will allow for high-resolution, multispectral imaging of a set of reddish arcs on Tethys’s leading hemisphere. A similar set of streaks were observed at lower resolution during Rev 214 in April 2015. The images will be used to help scientists understand the origin of these mysterious reddish arcs. One possibility is that they are tectonic structures, but so far, they do not seem to be associated with topographic features. The long stare during this observation will allow for regional stereo mapping. Additional images will also be acquired during Cassini’s slew away from Tethys near closest approach.
On November 13 at 05:47 UTC, Cassini will perform at targeted encounter with Titan. This is Cassini’s 115th flyby of Titan, and the final one for 2015. The next encounter is scheduled for January 16. T114 has a relatively-high, close approach altitude of 11,920 kilometers (7,407 miles). On approach, Cassini’s instruments will view the Fensal-Aztlan hemisphere of Titan. Departing Titan Cassini will observe a crescent Titan. The encounter starts with a mid-infrared scan of Titan by the Composite Infrared Spectrometer (CIRS) followed by near-infrared mapping observations by VIMS. ISS will ride-along with both in order to look for methane clouds in Titan’s atmosphere.
ISS will be prime during closest approach. The camera system will first take a mosaic strip of seven, Narrow-Angle-Camera (NAC) footprints. This mosaic will run from east to west, starting with a footprint centered on Doom Mons (named after Mount Doom from Tolkien’s Lord of the Rings), covering the complex boundary between Aztlan and the easternmost extension of Xanadu. This mosaic also runs along portions of the T113 RADAR SAR swath. Afterwards, around closest approach to Titan, ISS will acquire a four-frame mosaic with the Wide-Angle Camera (WAC). This mosaic will run from the Menrva impact basin in the north to Hotei Regio in the south, with the fourth WAC frame over the intersection between the T13 and T43 RADAR SAR swaths in central Xanadu. After closest approach, CIRS, UVIS, and VIMS, will observe a crescent Titan, which is great for observing Titan’s upper haze layers.
On November 15, shortly before an optical navigation observation involving Enceladus, the camera system will acquire a quick observation of Saturn using the Wide-Angle Camera (WAC). This observation is part of a series of “Storm Watch” observation 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 other experiments’ observations. These sequences include blue, clear, two methane band, and one full-frame, continuum band filter images. Two more Storm-watch observations will be acquired on November 15 and November 17. On November 17, ISS will observe a crescent Titan from a distance of 1.91 million kilometers (1.19 million miles) and will then look at the vertical structure of the E ring for 18 hours.
On November 17, Cassini will reach apoapse, bringing Rev 225 to a close and starting up the next orbit, Rev 226, which will include a non-targeted encounter with Tethys.
Image products created in Celestia. Tethys map by Paul Schenk. All dates in Coordinated Universal Time (UTC).