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Cassini continues its exploration of the Saturn system with the 18-day Rev 222, which begins on September 19 at its farthest distance from the planet. This is also called the orbit's apoapse. At this point, Cassini is 2.67 million kilometers (1.66 million miles) from Saturn's cloud tops. Rev 222 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 a targeted flyby of Titan and another non-targeted encounter with Dione. Thirty-four ISS observations are planned for Rev 222, many of which are focused on Titan and Saturn's atmosphere.
For ISS's first observations of Rev 222, on September 22, the Narrow-Angle Camera (NAC) will observe the distant moon Bestla, one of the Norse group of retrograde-orbiting, outer, irregular satellites of Saturn. Bestla is only 7 kilometers (4 miles) across, and at a distance of 6.86 million kilometers (4.26 million miles), it will appear as a point of light even with the NAC. Still, by measuring how Bestla's brightness changes with time and wavelength, estimates of its rotational rate, pole direction, surface composition, and shape can be made. On September 23, the camera system will acquire three quick observations of Saturn using the Wide-Angle Camera (WAC). These observations are part of a series of "Storm Watch" observation sequences designed to take advantage of short, two-minute segments when the spacecraft turns the optical remote sensing (ORS) instruments back to Saturn as a waypoint between other experiments' observations. These sequences include blue, clear, two methane band, and one full-frame, continuum band filter images. Five more storm watch observations will be acquired between September 24 and September 26, while another four will be taken between October 2 and October 6. After the second storm watch observation on September 23, ISS will acquire an astrometric observation of Saturn's small, inner moons. Astrometric observations are used to improve our understanding of the orbits of these small satellites, which can be influenced by Saturn's larger icy moons. Another astrometric observation will be acquired on September 24.
On September 24, ISS will observe a half-phase Titan from a distance of 2.50 million kilometers (1.55 million miles). The observation will be used to look for clouds across Titan's Fensal-Aztlan region. Afterward, ISS will ride along with the Visual and Infrared Mapping Spectrometer (VIMS) to look at the dusty G ring, which was created by micro-meteoroid impacts on the tiny moon Aegaeon. Next, ISS will observe Enceladus as it passes in front of Tethys. On September 26, ISS will acquire another cloud-monitoring observation of Titan, covering the leading hemisphere of the large moon, from a distance of 1.28 million kilometers (796,000 miles). Later in the day, ISS will again image Titan, this time from a distance of 1.00 million kilometers (623,000 miles). This observation will be a photometric calibration observation, designed to better understand how the filters on the NAC may have changed in their photometric response during the Cassini Solstice Mission. This observation will also provide another opportunity to monitor any clouds that might exist across Titan's leading hemisphere.
Cassini encounters Titan on September 28 at 21:37 UTC for the 114th time. This is the sixth, and penultimate, Titan flyby planned for 2015, with the next encounter scheduled for November 13 during Rev 225. T113 has a close-approach altitude of 1,035 kilometers (643 miles). In addition to the science observation acquired during the encounter, the flyby will also decrease the length of Cassini's orbit by eight days. In addition, this encounter sets up a series of flybys of Enceladus, which will occur during the next few orbits. Inbound, ISS will observe Titan's anti-Saturn and leading hemispheres. Outbound, ISS will be able to observe a crescent Titan over its leading hemisphere. Observations for this encounter start with a series of Titan cloud tracking observations by ISS during the day before the encounter period. The camera system will be looking for clouds across Titan's leading hemisphere. The encounter itself starts 12 hours before closest approach with a Composite Infrared Spectrometer (CIRS) observations of Titan, which ISS will ride along with. This observation is designed to look at Titan's atmosphere composition. Afterward, ISS will acquire a short, five-frame mosaic of western Xanadu and eastern Shangri-La. This is ISS's first mosaic of this part of Titan since T12, much earlier in the mission. The images from this mosaic will have a resolution of approximately 1.1 kilometers (0.68 miles) per pixel. Afterward, CIRS will map Titan's limb, after which VIMS will do regional-scale mapping of Titan's surface.
At closest approach, RADAR and the Ion and Neutral Mass Spectrometer (INMS) will control pointing. RADAR will acquire altimetry swaths across parts of western Xanadu and far eastern Aztlan. RADAR will also acquire a SAR imaging swath, running from western Xanadu east to Sotra Patera and ending in southeastern Aztlan (including Elba Facula at the very end). The eastern half of the swath will make up for data lost during the T13 flyby. After closest approach, CIRS will acquire temperature map data of Titan's night side as well as compositional measurements along Titan's limb. ISS will ride along, observing a crescent Titan.
On September 30 at 16:17 UTC, Cassini will reach periapse for Rev 222 at an altitude of 114,660 kilometers (71,250 miles) from Saturn's cloud tops, between the G ring and the orbit of Mimas. A few hours earlier, Cassini will perform a non-targeted encounter with Dione at a distance of 40,819 kilometers (25,363 miles). During this encounter, ISS will acquire a 19-frame mosaic covering the moon's leading hemisphere. This includes the Fidena Fossae fracture system, which runs roughly from Murranus crater south to Erulus crater. Before and after this Dione observation, ISS will observe Saturn's atmosphere, first tracking cloud features in its atmosphere and later, riding along with VIMS to observe cloud features within "Storm Alley", a region of Saturn's atmosphere near 40 degrees North Latitude where major storms frequently spawn. This is the area where the Great White Spot showed up in 2010. VIMS will also observe several stellar occultations by Saturn's ring system. On October 1, ISS will ride along with the Ultraviolet Imaging Spectrometer (UVIS) as it scans across Saturn's atmosphere. On October 2, ISS will acquire a NAC mosaic of the ring system at high-phase angles.
On October 7, Cassini will reach apoapse, bringing Rev 222 to a close and starting up the next orbit, Rev 223, which will include a targeted flyby of Enceladus.
Image products created in Celestia. Dione map by Steve Albers. All dates in Coordinated Universal Time (UTC).