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Cassini continues its exploration of the Saturn system with the 15-day Rev 194, which begins on July 1 at its farthest distance from the planet. This is also called the orbit's apoapse. At this point, Cassini is 1.34 million kilometers (0.83 million miles) from Saturn's cloud tops. Rev 194 occurs during the first inclined phase, which lasts until March 2015, of the Cassini Solstice Mission. 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-three ISS observations are planned for Rev 194 with many focused on Titan, during a targeted flyby of that moon, and Saturn's atmosphere and rings.
On July 1, a few hours after apoapse, ISS will acquire a quick observation of Saturn using the 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. Two more observations are planned for July 2, and another will be taken on July 3. Immediately after the storm watching observation on July 1, ISS will track the faint ring arc that accompanies the small moon Anthe. On July 2, 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. Afterward, 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. On July 3, the camera system will ride along with a Visual and Infrared Mapping Spectrometer (VIMS) stellar occultation observation, as a red, super-giant star, Mu Cephei (also known as Herschel's Garnet Star), is occulted by Saturn's rings.
On July 7 at 04:09 UTC, Cassini will reach periapse for Rev 194 at an altitude of 557,030 kilometers (346,120 miles) from Saturn. On July 6, ISS will acquire a close-up observation of Saturn's faint, inner D-ring. On July 7, ISS will image the outer A ring, where it will be looking at propellers previously imaged by Cassini. Propellers are small voids in Saturn's rings created by the gravitational interaction between large ring particles and the surrounding ring. On July 7 and 8, ISS will ride along with the three other optical-remote-sensing instruments to observe the F ring occult three different stars. On July 7, ISS will ride along with the Composite Infrared Spectrometer (CIRS) to observe an occultation of the blue-giant variable star, Eta Carinae. Early on July 8, ISS will ride along with the Ultraviolet Imaging Spectrometer (UVIS) to observe the F-ring occult the B-type star Delta Centauri. A few hours later, ISS will ride along with VIMS as they observe the F-ring occult the red-giant star 2 Centauri. On July 9, ISS will ride along with UVIS as they observe Enceladus at very low phase angles from a distance of 1.22 million kilometers (0.76 million miles).
Three days after periapse, Cassini encounters Titan on July 10 at 13:22 UTC for the 93rd time. This is the fourth Titan flyby planned for 2013, with the next encounter scheduled for July 26. T92 is a low-altitude flyby with a close-approach altitude of 964 kilometers (599 miles). This flyby will allow for imaging of the north polar region inbound of the encounter and the southern anti-Saturn hemisphere of Titan on the outbound leg. Starting off the encounter, ISS will acquire a seven-frame mosaic of the north polar and northern leading hemisphere regions of Titan. This mosaic will provide more information of the distribution of lakes in the north polar region, fill-in the largest gap in ISS's map of Titan, and provide ISS's highest resolution observation of the impact basin Menrva to date. Afterward, the Composite Infrared Spectrometer (CIRS) will acquire spectral scans and other data of Titan's night side and sunlit crescent. CIRS will scan across Titan in order to map stratospheric temperatures. The instrument will also make scans along the limb of Titan to measure aerosol and chemical abundances at different altitudes above the moon's surface. 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 RADAR team will be prime. The team has planned several observations during this period. First, RADAR will acquire radiometry and scatterometry of the north polar region which can be used to measure the brightness temperature of the surface as well as its roughness. As Cassini approaches Titan, RADAR will acquire high-altitude SAR imaging of the terrain just east of Kraken Mare, Titan's largest methane/ethane sea. During closest approach, RADAR will acquire altimetry over Ligeia Mare. At this altitude and observing mode, sea surface roughness of several millimeters can be measured, giving researchers their most sensitive test for surface waves resulting from winds, drainage from rivers flowing into Ligeia, or tides. Afterward, RADAR will acquire three segments of SAR imaging: the first will cover small lakes on the leading hemisphere side of the north pole (providing stereo coverage when combined with data from T91; a north-south segment covering central Ligeia Mare and the terrain to its south; and a north-south segment running along 245 degrees West Longitude from 45 degrees North Latitude to 30 degrees South Latitude. This swath will also include portions of Concordia Regio, an area that saw heavy rainfall and localized flooding in late-2010. Outbound, RADAR will acquire high-altitude SAR data over the terrain between Concordia and Hetpet Regiones.
After RADAR's close-approach observations, ISS will ride along with VIMS during three of their observations in order to track any clouds that might be visible across Titan's southern, anti-Saturn hemisphere. Afterwards, ISS will acquire an eight-frame mosaic in order to map albedo variations across this part of Titan. Two additional frames will be acquired at the start and end of the mosaic over Titan's unlit south pole to observe the stratospheric vortex that current covers the pole. Sunlight continues to fade even at that high-altitude so this will likely be the last high resolution observation of the vortex. On July 11 and 12, ISS will acquire seven Titan cloud tracking observations that will cover its southern hemisphere.
On July 13 and 14, ISS will ride along with UVIS and VIMS to acquire wide-angle images of Saturn, allowing it monitor cloud motions across the planet's southern hemisphere. On July 14, ISS will take another look at Titan, looking for clouds across the moon's southern trailing hemisphere from a distance of 1.34 million kilometers (0.83 million miles).
On July 15, Cassini will reach apoapse, bringing Rev 194 to a close and starting up the next orbit, Rev 195, which includes another targeted flyby of Titan.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).