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Cassini continues its exploration of the Saturn system with the 12-day Rev183, which begins on March 4 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 183 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-nine ISS observations are planned for Rev 183 with the majority focused on Saturn's atmosphere and rings. Cassini will also perform a targeted flyby of Saturn's second largest moon, Rhea.
On March 4, ISS begins its observations for Rev 183 with a pair of 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 March 5 and 7, while seven more will be acquired between March 10 and 14, after periapse. Between the two "Storm Watch" observations on March 4, 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 March 6 and 13. After the second "Storm Watch" observation, 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 March 6, ISS will acquire a series of images of Saturn rings using the WAC. These images will be tracking spokes -- a ring phenomenon Cassini has monitored throughout the mission -- 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 March 7, ISS will image the outer A ring, looking at the propellers previously seen in this part of the ring system. Propellers are small voids in Saturn's rings created by the gravitational interaction between large ring particles and the surrounding ring. On March 8 and 9, ISS will acquire a pair of observations of the small, distant, outer moon, Hyrrokkin. These observations will be used to measure the rotation period of the small moon. Given its small size and great distance from Saturn, it likely does not rotate synchronously like Saturn's closer and larger icy moons. Hyrrokkin is eight kilometers (5 miles) across and will be 10 million kilometers (6.22 million miles) away.
On March 10 at 03:40 UTC, Cassini will reach periapse for Rev 183 at an altitude of 424,310 kilometers (263,650 miles) from Saturn. A few hours earlier on March 9 at 18:17 UTC, ISS will perform a targeted encounter with Saturn's second largest moon, Rhea. This is Cassini's fourth targeted encounter with Rhea. The spacecraft will pass within 997 kilometers (620 miles) of the icy moon. Prior to closest approach, ISS will ride along with an observation by the Composite Infrared Spectrometer (CIRS). CIRS will be acquiring a temperature map of Rhea's south polar region, while ISS will acquire a pair of narrow-angle (NAC) and wide-angle (WAC) images that covers a crescent Rhea. As Cassini makes its pass - going from over the moon's south polar region, north over the equator just before closest approach, to over its north polar region - the Radio Sub-System (RSS) will be used to measure Rhea's gravity field. The High-gain antenna will be pointed at Earth during the encounter, and the effect of Rhea's gravity on the spacecraft will be measured by looking at the Doppler Effect on Cassini's signal. This will be used to refine our understanding of Rhea's internal structure. Shortly after closest approach during the RSS observation, Rhea will pass through the NAC field-of-view and the camera system will acquire 10 sets of images, with the first and last including a WAC context frame. These images, with a top resolution of 18 meters (59 feet) per pixel, will cover small portions of Rhea's north polar and sub-Saturn regions. Afterward, ISS will acquire a 12-frame mosaic covering the moon's northern, sub-Saturn hemisphere. Images from this mosaic will have a resolution of between 370 and 520 meters (1,210 and 1,710 feet) per pixel. Finally, ISS will acquire a set of global color images from a distance of 269,000 kilometers (167,000 miles).
As the spacecraft departs from Rhea and nears periapse, ISS will acquire a pair of distant Titan observations on March 13 and 14. These observations will cover the moon's sub-Saturn hemisphere and will be used to monitor cloud activity in the moon's dense atmosphere. These two observations will be taken from a distance of 2.57 and 2.60 million kilometers (1.60 and 1.62 million miles). Also on March 14, ISS will acquire a movie of the Anthe ring arc.
On March 16, Cassini will reach apoapse on this orbit, bringing it to a close and starting Rev184.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).Rhea basemap by Steve Albers.