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Cassini continues its exploration of the Saturn system with the 30-day Rev 213, which begins on February 26 at its farthest distance from the planet. This is also called the orbit's apoapse. At this point, Cassini is 3.44 million kilometers (2.14 million miles) from Saturn's cloud tops. The Titan flyby during this orbit will complete Cassini's return to orbiting within Saturn's ring plane. This will start the second of two equatorial phases of the Cassini Solstice Mission which will last until January 2016. Thirty-one ISS observations are planned for Rev 213 with the majority focused on Saturn's atmosphere and on Titan during the T110 flyby.
ISS's first observation of Rev 213 is a continuation of the last observation of the previous orbit. ISS will be riding along with the Composite Infrared Spectrometer (CIRS) as it performs a calibration sequence. ISS will be able to acquire Narrow-Angle Camera (NAC) images of Saturn's bright limb. Immediately afterward, on February 26, 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. Four more storm watch observations will be taken between March 7 and March 12. Two more will be taken on March 24 and 25.
On March 9 and 10, ISS will acquire a pair of movies 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 12, the NAC will image Titan, looking for clouds across its sub-Saturn hemisphere, at a distance of 2.35 million kilometers (1.46 million miles). Immediately afterwards, early on March 13, ISS will ride along with the Visual and Infrared Mapping Spectrometer (VIMS) to perform an imaging scan across the illuminated side of Saturn's main ring system. After that observation, ISS will acquire a six-hour movie of the outer edge of Saturn's B ring with the Narrow-Angle Camera (NAC). Late on March 13, ISS will acquire another cloud monitoring observation of Titan's sub-Saturn hemisphere from a distance of 1.84 million kilometers (1.14 million miles).
On March 14 at 15:49 UTC, Cassini will reach periapse for Rev 213 at an altitude of 313,780 kilometers (194,970 miles) from Saturn's cloud tops. After periapse on March 14, ISS will ride along with VIMS as it observes a stellar occultation by Saturn's main ring system of X Ophiuchi, a double star with a Mira-type variable in the constellation Ophiuchus. On March 15, ISS will acquire a high resolution observation of Saturn's narrow F ring. Afterward, ISS will image Saturn's D ring, the innermost portion of the main ring system, at high resolution. This observation will help to constrain the particle density of the innermost portion of the D ring. Cassini will be passing between the inner D ring and Saturn's atmosphere during the proximal orbits of 2017.
Cassini encounters Titan on March 16 at 14:30 UTC for the 111th time. This is the third of seven Titan flybys planned for 2015, with the next encounter scheduled for May 7. T110 has a close-approach altitude of 2,275 kilometers (1,413 miles). Inbound, ISS will observe Titan's sub-Saturn hemisphere. Outbound, ISS will be able to observe a crescent Titan over its anti-Saturn hemisphere. In addition to the science that will be taken during the flyby, this encounter is important for shaping Cassini's orbit around Saturn, lowering its inclination to nearly 0 degrees (essentially within Saturn's ring plane). Observations for this encounter start with a pair of CIRS observations of Titan, which ISS will ride along with. These observations are designed to measure a temperature profile of Titan's atmosphere as well as monitor its composition across different parts of Titan. CIRS will also acquire a series of limb profiles of Titan's south polar region and southern mid-latitudes using its mid-infrared channel. Afterward, VIMS will acquire a medium-resolution mosaic of Titan's sub-Saturn hemisphere.
At closest approach, VIMS will control pointing with ISS acquiring ride-along images. VIMS will acquire a mosaic of the Quivira region of Titan as well as high-resolution images of Sinlap, the middle of Kraken Mare, and portions of Punga Mare and Ligeia Mare. As Cassini departs from Titan, CIRS will control pointing, acquiring a series of infrared observations of Titan. These include low latitude limb sounding where CIRS will be looking at gas and aerosol abundances. CIRS will also acquire temperature map data of Titan's night side as well as compositional measurements along Titan's limb. VIMS will look for specular reflections off Titan's lakes and seas. ISS will ride along, observing a crescent Titan.
On March 20 and 22, ISS will acquire high-phase observations of Saturn's diffuse E ring. The high phase angle will highlight the dust that is dominant in that ring. On March 25, ISS will observe a mutual event between Titan, Rhea, and Mimas, with all three moons fitting within the NAC field-of-view. From March 26 through 28, ISS will acquire a series of observations of Saturn's distant icy moon Iapetus. Due to its inclined and distant orbit, Cassini observations of Iapetus are rare. But on March 30, Cassini makes it closest pass of the moon during the Cassini Solstice Mission. These observations at the end of Rev 213 and several at the start of Rev 214 are in support of that distant encounter. Images acquired during the day-long observation on March 27 and 28 will be taken from a distance of 1.05 million kilometers (650,000 miles). These images will cover Iapetus' northern sub-Saturn hemisphere which includes the impact basins Turgis and Roland.
On March 28, Cassini will reach apoapse, bringing Rev 213 to a close and starting up the next orbit, Rev 214, which will include non-targeted encounters with Tethys and Dione.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).