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Cassini continues its exploration of the Saturn system with the 9.6-day Rev 190, which begins on May 16 at its farthest distance from the planet. This is also called the orbit's apoapse. At this point, Cassini is 1.30 million kilometers (0.81 million miles) from Saturn's cloud tops. Rev 190 occurs a year 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. Twenty-six ISS observations are planned for Rev 190 many of which take place during a targeted flyby of Titan later in the orbit.
On May 16, a couple of hours after apoapse, 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. Two more storm watch observations are planned for May 16, while three more will be taken the following day. Between the first two storm watch observations, ISS will acquire a distant Titan observation of the moon's sub-Saturn hemisphere. This observation will be used to look for clouds in Titan's atmosphere. These images will be taken from a distance of 2.53 million kilometers (1.57 million miles). Next, ISS will acquire a movie observation of the ring arc associated with the small moon Anthe, followed by 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 May 17, ISS will again take a look at Titan's sub-Saturn hemisphere, monitoring clouds that may appear in the region. This observation will be taken from a distance of 2.35 million kilometers (1.46 million miles). Next, 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 May 18, ISS will ride along with an Ultraviolet Imaging Spectrometer (UVIS) observation of Saturn's south polar aurora. In addition to making a movie of the planet's aurorae, the images will be used to independently measure the rotation period of Saturn's magnetic field.
On May 20 at 19:22 UTC, Cassini will reach periapse for Rev 190 at an altitude of 316,000 kilometers (196,350 miles) from Saturn. On May 19, ISS will observe Titan. This observation, taken from a distance of 1.80 million kilometers (1.12 million miles), will be used to look for clouds across the Fensal-Aztlan and Tsegihi regions of Titan. Later in the day, ISS will again ride along with UVIS as it observes Saturn's south polar aurorae. Shortly before periapse on May 20, ISS will ride along with the Composite Infrared Spectrometer (CIRS) as it observes the north polar region of Mimas along with the large crater Herschel, which will appear along with dawn terminator during the observation. On May 21, UVIS will observe an occultation of the B-type star, Theta Carinae, by Saturn's rings. It will also perform slewing observations of Saturn's aurorae.
Three days after periapse, Cassini encounters Titan on May 23 at 17:33 UTC for the 92nd time. This is the third Titan flyby planned for 2013, with the next encounter scheduled for July 10. T91 is a very low-altitude flyby with a close-approach altitude of 970 kilometers (603 miles). This flyby will allow for imaging of the southern anti-Saturn hemisphere of Titan outbound from the encounter. Before the encounter, 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 stare along the limb of Titan to measure aerosol and chemical abundances at different latitudes. ISS will ride along with these observations to observe the north polar lakes region. Several sets of wide-angle camera (WAC) and one set of narrow-angle camera (NAC) images will be taken of the region. The NACs will be centered near the north polar "Great Lakes" region, an area containing Bolsena Lacus, Neagh Lacus, and Ladoga Lacus. These three lakes, along with Jingpo Lacus, are the largest bodies of liquid methane/ethane outside of the three seas in Titan's north polar region.
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 southern and eastern portion 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. Afterwards, RADAR will acquire SAR imaging along a north-south swath running along 250 degrees West Longitude across the northern boundary of Belet, followed by a north-south altimetry swath west of Adiri. The altimetry swath also includes Concordia Regio, an area that experienced localized flooding in late 2010. The altimetry swath can be compared to the pattern of brightening following the storm to see if it matches with the idea that it was due to rain runoff that flowed into local basins and river flood plains. Finally, high-altitude SAR imaging will be acquired of southern Adiri, another area affected by 2010's "Arrow Storm".
After closest approach, ISS will acquire a mosaic that covers Titan's mid-latitudes south of Belet, including areas as far north as Concordia Regio. This observation will be used to improve the ISS surface map of Titan. Two frames will also be used for a final Cassini mission observation of the south polar vortex cloud, first observed in March 2012, before the sun sets on the feature in the south. Assuming that it is illuminated during the observation, this will be ISS' highest resolution imaging of the stratospheric cloud. ISS will also use the observation to track smaller, tropospheric clouds between 50 and 60 degrees South Latitude. Finally, ISS will ride along with CIRS again as the spectrometer performs several scans of Titan's southern hemisphere.
On May 25, ISS will acquire five observations designed to monitor clouds across Titan's southern and trailing hemispheres. The next day, on May 26, Cassini will reach apoapse on this orbit, bringing it to a close and starting Rev191.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).