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Cassini continues its exploration of the Saturn system with the 23-day Rev169, which begins on July 10 at its farthest distance from the planet. This is also called the orbit's apoapse. At this point, Cassini is 2.84 million kilometers (1.76 million miles) from Saturn's cloud tops. This orbit includes a targeted encounter with Titan, Cassini's 86th to date. Rev169 is near the beginning of 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. Twenty-eight ISS observations are planned for Rev169, the vast majority focused on Titan and Saturn's rings.
ISS begins its observations for Rev169 on July 11, the day after Cassini passes apoapse, with an eight-hour light curve observation of the outer irregular satellite Ymir. Cassini will be 15.6 million kilometers (9.67 million miles) away from the 18-kilometer-wide (11.2-mile-wide) satellite. On July 13, 15, and 17, ISS will ride along with the Ultraviolet Imaging Spectrometer (UVIS) to acquire a time-lapse movie of Saturn's south polar aurora. These three observations will be taken over a period of between 12 and 17 hours. On July 16, ISS will take a look at Titan from a distance of 3.04 million kilometers (1.89 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 of a gibbous-phase Titan is designed to monitor clouds over the moon's Senkyo dune field. ISS also will be taking shorter-wavelength images to study changes in Titan's upper haze layers. On July 18, ISS will image Titan's sub-Saturn hemisphere again, this time from a distance of 3.03 million kilometers (1.88 million miles).
On July 22 at 23:03 UTC, Cassini will reach periapse for Rev169 at an altitude of 245,240 kilometers (152,390 miles) from Saturn. ISS observations during the periapse period include a high-phase observation of Saturn's C ring, occultations by the F ring of the stars Sirius and Zeta Canis Majoris, an occultation by Dione of Spica, and a search for moonlets in the Cassini Division. First, ISS will ride along with the Visual and Infrared Mapping Spectrometer to image the C ring at very high phase angles. Portions of this observation will be performed while Cassini is in the shadow of Saturn. Scientists are hoping to catch a few impacts by meteorites in the ring. A few hours later on July 22, ISS will ride along with a pair of UVIS and Visual and Infrared Mapping Spectrometer (VIMS) rings stellar occultation observations. ISS is hoping to catch a pair of F ring occultations of the stars Sirius (which is also called Alpha Canis Majoris) and Zeta Canis Majoris. Next, early on July 23, ISS will ride along with UVIS to observe a stellar occultation of Spica by Dione. The ISS images will provide global color imaging of Dione's leading hemisphere from a distance of 410,000 kilometers (255,000 miles). Afterward, ISS will search for moonlets in the Cassini Division.
Two days after periapse, Cassini encounters Titan on July 24 at 20:03 UTC for the 86th time. This is the sixth of nine Titan flybys planned for 2012, with the next encounter scheduled for September 26. T85 is a low-altitude flyby with a close-approach altitude of 1,012 kilometers (629 miles). This flyby will allow for imaging of the Adiri region and the 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. VIMS will search for specular, or mirror-like, reflections off the northern lakes, particularly at Kivu Lacus. CIRS will scan across Titan using its far-infrared and mid-infrared channels as well as perform a limb integration. In a limb integration, CIRS stares at Titan's sunlit limb, or edge of the visible disk, to build up high resolution spectra, or values that vary along a continuum, of Titan's hazes. 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, VIMS will control spacecraft pointing. The VIMS team will acquire a number of high-resolution infrared observations of Titan's surface. Right at closest approach, the VIMS team will image Kivu Lacus, a northern lake from which scientists hope to see specular reflection a few hours earlier. Next, VIMS acquire a pushbroom, an image strip over Titan's northern mid-latitudes, again on the anti-Saturn side. Afterward, VIMS will image the crater Selk, image the Huygens probe's landing site, and build up a mosaic of the Adiri region. 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 map the anti-Saturn hemisphere of Titan. ISS will ride along during these four observations, searching for clouds across Adiri and western Shangri-La, including over the Huygens landing site.
On July 28, ISS will acquire a pair of dark current calibration observations for both the wide-angle and narrow-angle cameras. Dark current is result of electric current within the cameras' detectors that flows even when very few photons are hitting the detector. These calibration images will provide up-to-date dark current files to remove this source of noise during image processing. On August 2, ISS will take 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. Immediately afterward, ISS will take a TMC observation of Titan, covering the sub-Saturn hemisphere of the large moon from a distance of 3.61 million kilometers (2.24 million miles).
On August 2, Cassini will reach apoapse on this orbit, bringing it to a close and starting Rev170.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC). Dione basemap by Steve Albers.