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Cassini continues its exploration of the Saturn system with the 20-day Rev174, which begins on October 6 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. Rev174 occurs six months into the first inclined phase of the Cassini Solstice Mission, a phase which lasts until March 2015. The inclined phase will allow for polar views of Saturn and Titan as well as better vistas of Saturn's rings than those Cassini had while in the earlier, equatorial phase of the Solstice Mission. Thirty-five ISS observations are planned for Rev174, most focused on Saturn's atmosphere and rings as a well as on a Titan flyby on November 13.
ISS begins its observations for Rev174 the day after apoapse with an observation of a half-phase Titan from a distance of 1.86 million kilometers (1.15 million miles). The observation is an effort to look for clouds in the moon's atmosphere as part of the "Titan Monitoring Campaign"(TMC). This observation is designed to monitor clouds over the moon's southern and trailing hemispheres. ISS will also be taking shorter-wavelength images to study changes in Titan's upper haze layers. ISS will image Titan again on November 2, covering the southern and trailing hemispheres once more. Immediately afterward, ISS will acquire a quick observation 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. Eight more such observations are planned between November 2 and 8.
After the first Saturn storm watch observation, 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 such observation will be taken on November 4. Also on November 2, ISS will acquire a series of images of Saturn rings using the WAC. These images will be tracking spokes -- a ring phenomenon Cassini has monitored throughout the mission -- over the B ring. With Cassini over the unlit side of the rings and with Saturn at a high phase angle, the spokes, if visible, will be brighter than the dark B ring. Additional spoke movies will be taken on November 4 and 7. ISS will observe Titan again on November 4 from a distance of 2.40 million kilometers (1.49 million miles). This observation will focus on the moon's southern and sub-Saturn hemispheres. On November 8, ISS will turn its attention back to Saturn's rings as it acquires a series of narrow-angle-camera (NAC) images of the F ring. These are designed to track changes in various streamers and channels in the ring that are formed by the effects of gravitational pull from the moon Prometheus.
On November 11 at 07:30 UTC, Cassini will reach periapse for Rev174 at an altitude of 414,590 kilometers (257,610 miles) from Saturn. During the periapse period on November 10, ISS will ride along with the Visual and Infrared Mapping Spectrometer (VIMS) as it observes the unlit side of the D and F rings. ISS images will be taken using the WAC. Next, ISS will ride along with VIMS to observe the occultation of the red giant star Alpha Ceti by the F ring. Afterward, ISS will acquire high-resolution images of the D ring followed by a survey of propellers in the outer A ring.
Two days after periapse, Cassini encounters Titan on November 13 at 10:22 UTC for the 88th time. This is the eighth of nine Titan flybys planned for 2012, with the next encounter scheduled for November 29. T87 is a very low-altitude flyby with a close-approach altitude of 973 kilometers (605 miles). This flyby will allow for imaging of the Adiri region and the southern anti-Saturn hemisphere of Titan outbound from the encounter. Before the encounter, the Composite Infrared Spectrometer (CIRS) and VIMS will acquire spectral scans and other data of Titan's night side and narrow crescent. CIRS will scan across Titan in order to map stratospheric temperatures. VIMS will image the north polar region of Titan in order to look for specular reflection from lakes in the region. ISS will ride along to acquire images of Titan's upper haze layers, which are more easily visible at high phase angles.
At closest approach, the Ion and Neutral Mass Spectrometer (INMS) and Navigation teams will be prime. The two teams are collaborating in order to better measure atmospheric density by measuring drag on the spacecraft which results from passing through the moon's upper atmosphere. INMS will be acquiring data at the same time to act as a comparison with the Doppler shift data. Researchers hope to use the information gleaned from this encounter to improve estimates for hydrazine fuel usage during close Titan encounters in the future. As Cassini departs from Titan, CIRS will map surface temperatures across the visible disk to look for diurnal and albedo-related differences while VIMS will perform global mapping at medium resolution. ISS will ride along during these observations, searching for clouds across Adiri and the southern anti-Saturn hemisphere, including over the Huygens landing site.
Late on November 14, ISS will acquire a series of WAC images of Saturn in order to measure wind speeds. A similar set (though only four hours long instead of ten) will be taken on November 17. On November 16 and 18, ISS will ride along with UVIS scans of Saturn.
On November 19, Cassini will reach apoapse on this orbit, bringing it to a close and starting Rev175. Rev175 includes the final Titan flyby for 2012.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).