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Cassini continues its exploration of the Saturn system with the 12-day Rev185, which begins on March 28 at its farthest distance from the planet. This is also called the orbit's apoapse. At this point, Cassini is 1.47 million kilometers (0.92 million miles) from Saturn's cloud tops. Rev 185 occurs nearly 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 185 with many observations planned during a targeted flyby of Titan later in the orbit.
On March 28, a few hours after apoapse, ISS will acquire an observation of the small, outer satellite, Hyrrokkin. These images will be used to measure the rotation period of the small moon. Given its small size and great distance from Saturn, it doesn't rotate synchronously like Saturn's closer and larger icy moons. Hyrrokkin is eight kilometers (5 miles) across and will be 9.49 million kilometers (5.90 million miles) away. Later that day, ISS will acquire a distant Titan observation. This observation will be used to look for clouds in Titan's atmosphere. These images will be taken from a distance of 2.23 million kilometers (1.39 million miles). 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 March 30 and 31, ISS will take two 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. On April 1, ISS will ride along with a stellar occultation using the Visual and Infrared Mapping Spectrometer (VIMS) by the ring system of the Mira-like, red giant star R Cassiopeiae. Later that day, it will acquire a 12-hour movie of the gap between the A and F rings, also known as the Roche Division.
On April 3 at 01:46 UTC, Cassini will reach periapse for Rev 185 at an altitude of 423,880 kilometers (263,390 miles) from Saturn. On April 2, ISS will monitor the south polar plume of Enceladus from a distance of 830,000 kilometers (515,000 miles). Next, ISS will image a crescent Mimas at a distance of 590,000 kilometers (366,000 miles). ISS will finish April 2 with three ring observations: first covering the inner D ring (in a region Cassini will fly through numerous times in mid-2017); then a high-resolution, F ring observation; and finally, an observation of the outer A ring, where ISS 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 April 3 and 4, ISS will acquire a pair of ride along observations of stellar occultations by Saturn's F ring. The first involves the B-type star Delta Centauri while the April 4 observation uses the red giant star, R Hydra.
Two days after periapse, Cassini encounters Titan on April 5 at 21:44 UTC for the 91st time. This is the second Titan flyby planned for 2013, with the next encounter scheduled for May 23. T90 is a low-altitude flyby with a close-approach altitude of 1400 kilometers (870 miles). This flyby will allow for imaging of the southern anti-Saturn hemisphere of Titan outbound from the encounter. Before the encounter, 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 VIMS team will be prime. VIMS will acquire image spectral cubes of several surface features as Cassini makes it pass over Titan. It will acquire a noodle image that runs from the small dune fields to the west of the Menrva impact basin, across portions of western Xanadu, across Tui Regio, then south to Hobal Virga. Next, VIMS will acquire several mosaics of the southern anti-Saturn hemisphere as well as the region around the bright area, Adiri. Afterward, CIRS will acquire another pair of compositional and temperature maps. ISS will ride along with VIMS and CIRS to monitor cloud activity on the moon.
On April 7, Cassini will reach apoapse on this orbit, bringing it to a close and starting Rev186. During apoapse, ISS will ride along with CIRS to acquire some parting shots of Titan's trailing hemisphere.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).