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Cassini continues its exploration of the Saturn system with the 21-day Rev172, which begins on September 13 at its farthest distance from the planet. This is also called the orbit's apoapse. At this point, Cassini is 2.56 million kilometers (1.59 million miles) from Saturn's cloud tops. Rev172 is near the start 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. Fifty ISS observations are planned for Rev172, most focused on Saturn's atmosphere, a Titan flyby and Saturn's rings.
ISS begins its observations for Rev172 on September 17 with 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 are planned between September 18 and 20, while another nine will be taken between October 1 and 4.
On September 17, 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. Similar observations will be taken on September 18 and October 1. On September 18 and 19, ISS will acquire a series of images of Saturn rings using the WAC. These images will be tracking dust spokes 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. On September 20, ISS will image a half-phase Titan from a distance of 2.86 million kilometers (1.78 million miles). Later that day, ISS will acquire a long-range, narrow-angle camera (NAC) movie of the F ring. On September 22, ISS will ride along with a Visual and Infrared Mapping Spectrometer (VIMS) observation of a stellar occultation. During the occultation, the F ring will pass in front of the red giant star Beta Pegasi. Afterward, ISS will acquire a NAC movie of the Encke Gap, which includes the small moon Pan. Early on September 23, after the Encke movie, ISS will re-image several propellers in the A ring in order to improve our knowledge of their orbits. Propellers are small voids formed by large particles in Saturn's main rings.
On September 24 at 14:59 UTC, Cassini will reach periapse for Rev172 at an altitude of 286,850 kilometers (178,240 miles) from Saturn. During the periapse period late on September 23, ISS will search for water ice plumes on Mimas at a distance of 885,000 kilometers (550,000 miles). Next, ISS will ride along with the Ultraviolet Imaging Spectrometer (UVIS) to observe the occultation of the blue-white star Gamma Pegasi by the F ring. Following the UVIS occultation, ISS will monitor the south polar plume of Enceladus from a distance of 740,000 kilometers (460,000 miles). Afterward, ISS will observe the G ring arc and the tiny moon Aegaeon that produces it. Later on September 24, ISS will search for small, embedded moonlets within the C and D rings. Finally, on September 25, ISS will ride along with VIMS, taking a few images with the WAC of the B ring at very low phase angles.
Two days after periapse, Cassini encounters Titan on September 26 at 14:36 UTC for the 87th time. This is the seventh of nine Titan flybys planned for 2012, with the next encounter scheduled for November 13. T86 is a very low-altitude flyby with a close-approach altitude of 956 kilometers (594 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 UVIS will acquire spectral scans and other data of Titan's night side. 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 aerosols. 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) will be prime with RADAR riding along. INMS will measure the composition of Titan's upper atmosphere and will attempt to study how the exobase -- the boundary between the ionosphere and Titan's neutral atmosphere -- varies with time of day and location, both as a response to Titan's interaction (or lack thereof) with Saturn's magnetosphere or season. RADAR will ride along with INMS, acquiring a SAR swath over the north polar region of Titan. This swath will start over the large island Mayda Insula in northern Kraken Mare, continue east over southern Ligeia Mare, crossing a set of dry lakes southeast of that sea, and finishing up near 30 degrees north latitude, 215 degrees west longitude as Cassini heads south. RADAR will also acquire altimetry during this pass. As Cassini departs from Titan, CIRS will map surface temperatures across the visible disk to look for diurnal and albedo-related differences while UVIS will map the anti-Saturn hemisphere of Titan. ISS will ride along during these observations, searching for clouds across Adiri and the southern anti-Saturn hemisphere, including over the Huygens landing site. ISS will cover the same region on September 28 during a set of "caboose" observations designed for tracking clouds across Titan as well as observing the high-altitude south polar vortex.
On October 1, ISS will acquire a five-hour light curve observation of the small, outer moon, Bestla. Later that day, ISS will acquire a 16-hour movie of the Encke Gap in Saturn's A ring. On October 2, ISS will take a look at Titan from a distance of 2.89 million kilometers (1.80 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 region. ISS also will be taking shorter-wavelength images to study changes in Titan's upper haze layers. On October 4, ISS will image Titan again from a distance of 3.37 million kilometers (2.09 million miles).
On October 6, Cassini will reach apoapse on this orbit, bringing it to a close and starting Rev173. Rev173 includes solar conjunction, a period every 12.5 months when Cassini is out of communications with Earth.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).