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Cassini continues its exploration of the Saturn system with the 32-day Rev 207, which begins on August 3 at its farthest distance from the planet. This is also called the orbit's apoapse. At this point, Cassini is 2.85 million kilometers (1.77 million miles) from Saturn's cloud tops. Rev 207 occurs during the first inclined phase, which lasts until March 2015, of the Cassini Solstice Mission. The inclined phase allows 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. Forty-eight ISS observations are planned for Rev 207 with majority focused on Saturn's atmosphere and on Titan during the T104 flyby.
On August 3, ISS 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 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. Ten more storm watch observations will be taken between August 4 and August 15. On August 4, ISS will image Titan, looking for clouds across its northern sub-Saturn hemisphere, at a distance of 3.81 million kilometers (2.37 million miles). On August 5, ISS will ride along with a calibration observation for the Ultraviolet Imaging Spectrometer (UVIS). ISS will acquire images of the blue-giant star Spica (Alpha Virginis) as UVIS slews. On August 12, 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. Additional astrometric observations will be taken on August 13 and August 15.
On August 13, ISS will perform a non-targeted encounter with Titan at a distance of 959,146 kilometers (595,986 miles). ISS will perform a pair of cloud monitoring observations in conjunction with this encounter on August 13 and August 15 at a distance of 965,500 kilometers (600,000 miles) and 1.15 million kilometers (0.72 million miles). The August 13 observation will cover the northern, anti-Saturn hemisphere, while the August 15 sequence will focus on the trailing hemisphere. On August 13 and 14, ISS will spend thirteen hours observing the outer satellite Tarvos, from a distance of 30.9 million kilometers (19.2 million miles). Combined with earlier sequences, this observation will be used to determine where its rotational axis points and to model its shape. On August 14, ISS will observe several faint rings from a vantage point near the ring plane, including the E ring, the G ring arc, and Methone and Pallene ringlets. On August 16, ISS will acquire a movie of the F ring, observing its various channels and streamers created by the interaction between the ring material and the nearby moon, Prometheus.
On August 19 at 05:26 UTC, Cassini will reach periapse for Rev 207 at an altitude of 901,430 kilometers (560,120 miles) from Saturn's cloud tops. The ISS observations during the periapse period focus primarily on Saturn's atmosphere. On August 17, ISS will ride along with UVIS as they perform a series of far- and extreme-ultraviolet scans across the face of Saturn. Immediately afterwards, ISS will take a series of images over a period of six hours designed to track clouds across Saturn's equatorial region. On August 18, ISS will ride along with VIMS to observe aurorae at Saturn's south pole. On August 19, ISS will acquire three scans of Saturn's bright limb. The camera system will also ride along with the Composite Infrared Spectrometer (CIRS) as they observe Saturn's unlit south polar region.
Cassini encounters Titan on August 21 at 08:09 UTC for the 105th time. This is the eighth of eleven Titan flybys planned for 2014, with the next encounter scheduled for September 22. T104 has a low, close-approach altitude of 964 kilometers (599 miles). Inbound, ISS will observe Titan's southern, sub-Saturn hemisphere. Outbound, ISS will be able to observe a crescent Titan over its northern, sub-Saturn hemisphere. Observations for this encounter will start with ISS riding along with CIRS as it measures Titan's atmospheric and surface temperatures in the mid-infrared. The observation will include stares at the beginning and ending of the observation for ISS to acquire NAC images of Titan. Next, ISS will acquire a global mapping mosaic of Titan. This eight-frame mosaic will cover the visible face of Titan and will be centered just south of eastern Aztlan near Nath. The images from this mosaic will have a resolution around 1.50 kilometers (0.93 miles) per pixel. Afterward, ISS will ride along with a CIRS observation focused on Titan's surface as well as a Visual and Infrared Mapping Spectrometer (VIMS) cloud monitoring observation.
The Ion and Neutral Mass Spectrometer (INMS) and RADAR will be prime during closest approach. The spacecraft will be flying by Titan while it over Saturn's local midnight longitude. Measuring Titan's ion environment during this period will compliment a similar data set acquired when Titan was over local Saturn noon during T100 back in April. Before closest approach, RADAR will acquire a number of observations of Titan during this period, including scatterometry and radiometry of Titan's southern, sub-Saturn hemisphere, HiSAR mapping of Garotman Terra, and ride-along SAR right before closest approach covering a swath from northwestern Garotman to around 50 degrees North latitude, 335 degrees West longitude. During their prime time, RADAR will acquire an altimetry swath across the middle of Kraken Mare and the "estuary" that links Kraken and Ligeia Mares. The depth of the sea could be determined from this dataset. Next, RADAR will acquire a very short, low-incidence SAR imaging swath across portions of Ligeia Mare, including the "magic" island seen during T92. The feature has only been seen in one SAR swath and has been observed by neither VIMS nor ISS. Afterward, RADAR will acquire another altimetry swath southeast of Ligeia Mare which will include a few, small, "empty" lakes.
After closest approach, VIMS will acquire a pair of global mapping observations. These sequences will also be used to look for specular reflection off Kraken Mare as well as produce maps of the north polar region. ISS will also have an eight-frame, global-mapping mosaic, covering much of the visible crescent. ISS will also ride along with a pair of CIRS, nadir-pointing observations. These ISS observations are designed to monitor clouds that maybe in the region (though few clouds have been seen in the last two years).
During a week and a half of Rev 207, ISS will primarily focus on observations of Saturn's atmosphere. These include ride along observations with VIMS on August 22 and September 1, where the two instruments will be mapping Saturn's northern hemisphere, paying particular attention to the hexagon that surrounds the north pole. On August 23, August 27, and September 3, ISS will ride along with UVIS as the spectrometer scans across Saturn. ISS will acquire polarimetry images with the Wide-Angle Camera during these sequences. ISS will also ride along with CIRS on August 25, August 26, and August 28. The camera system will be used to track cloud motion in Saturn's atmosphere. Three non-Saturn observations will also be taken. On August 24 and September 3, ISS will observe Titan's north polar region and northern, sub-Saturn hemisphere from a distance of 1.47 million kilometers (0.91 million miles) and 4.00 million kilometers (2.49 million miles), respectively. On August 30 and 31, ISS will observe the distant moon, Thrymr, for more than a day and a half, in order to measure its pole position, the direction the north pole points to in the sky, and its rotational period. Thrymr will be 17.4 million kilometers (10.8 million miles) away.
On September 4, Cassini will reach apoapse, bringing Rev 207 to a close and starting up the next orbit, Rev 208, which will include another targeted flyby of Titan.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).