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Cassini continues its exploration of the Saturn system with the 18-day Rev163, which begins on March 18 at its farthest distance from the planet. This is also called the orbit's apoapse. At this point, Cassini is 2.37 million kilometers (1.47 million miles) from Saturn's cloud tops. The spacecraft is nearing the end of the first equatorial phase of the Cassini Solstice Mission, a phase which lasts until May 2012. During this phase, the spacecraft's orbits lie within the equatorial plane of the planet, providing opportunities to encounter Saturn's numerous moons, image the rings edge-on, and look at Saturn's cloud tops without the rings obscuring the view. Thirty-eight ISS observations are planned for Rev163, the vast majority dedicated to Saturn storm monitoring and to encounters with Enceladus, Janus, and Dione.
ISS begins its observations for Rev163 the day after apoapse on March 19 with three, quick observations of Saturn and its faded northern hemisphere storm. These "Storm Watch" observation sequences are 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. Ten more such observations are planned between March 20 and 25, with another five planned between April 1 and 3. Also on March 19, ISS will acquire an astrometric observation of Saturn's small, inner moons, including Calypso, Helene, Methone, Anthe, Epimetheus, and Pandora. Astrometric observations are used to improve our understanding of the orbits of these small satellites, which can be influenced by Saturn's larger icy satellites. Additional astrometric observations will be taken on March 23, 24, and April 3.
On March 20, ISS will take a look at Titan from a distance of 1.32 million kilometers (0.82 million miles). The observation is an effort to look for clouds in the moon's atmosphere as part of the "Titan Monitoring Campaign" (TMC). This observation, of a half-phase Titan, is designed to monitor clouds over the moon's Shangri-La dune field. On March 21, ISS will search for possible satellites at Titan's L4 point, a Lagrangian point that lies 60 degrees ahead of the large moon on its orbit. Similar moons have been found at Dione's and Tethys' L4 points (Helene and Telesto, respectively). An observation covering Titan's L5 point, which lies 60 degrees behind Titan, will be taken on March 26. On March 25, ISS will preform a unique observation of Saturn's shadow on the Phoebe ring. The ring lies along the orbit of the outer moon Phoebe and is likely composed of particles ejected from micrometeorite impacts on the small moon, not unlike the faint dust rings associated with Saturn's inner small moons, such as Anthe or Atlas. The camera system will be pointed at the edge of Saturn's shadow on the ring. On March 26, ISS will perform a TMC observation of Titan that will allow for monitoring of cloud features across the Senkyo dune field from a distance of 1.44 million kilometers (0.89 million miles).
On March 27 at 21:32 UTC, Cassini will reach periapse for Rev163 at an altitude of 135,950 kilometers (84,460 miles) from Saturn. ISS observations during the periapse period will be taken during a targeted encounter of Enceladus and later during non-targeted encounters of Janus and Dione.
Cassini will fly by Enceladus (E17) at an altitude of 74 kilometers (46 miles) at 18:30 UTC on March 27. ISS will image the icy satellite's south polar plume from distances of 335,000 kilometers (208,000 miles) down to 113,500 kilometers (70,500 miles) while the satellite is just a thin crescent. Next, the Composite Infrared Spectrometer (CIRS) will acquire a series of mid-infrared scans across the night side of Enceladus, as well as a pair of scans across the south polar terrain (found in earlier flybys of Enceladus to be a thermal hotspot) as well as a far-infrared raster scan using CIRS's FP1 channel. During the two hours surrounding closest approach, the Ion and Neutral Mass Spectrometer (INMS) will be prime, analyzing the composition of Enceladus' south polar plume as the spacecraft flies through it. The spacecraft's path will take it along the length of Baghdad Sulcus, allowing INMS to resolve individual jets from this "tiger stripe" fracture. Finally, ISS will acquire a nine-frame, clear and UV3 filter mosaic of Enceladus' leading hemisphere terrain as the spacecraft recedes from the moon.
At 21:34 UTC, Cassini will fly by one of Saturn's smaller satellites, Janus, at a close-approach distance of 43,851 kilometers (27,247 miles). ISS will acquire a series of images using many of the narrow-angle camera's filters. All of these images save the final one will be taken with Saturn as a backdrop, though only the second set of images will be against Saturn's day side. A total of 29 images are planned for this observation. At 05:07 UTC on March 28, Cassini will perform a non-targeted encounter of Dione at a distance of 43,997 kilometers (27,338 miles). ISS will acquire a nine-frame mosaic of Dione a couple of hours later, covering the moon's anti-Saturn hemisphere. At the end of the mosaic, the narrow-angle camera will be used to watch as Mimas and Tethys pass behind the limb of Dione.
Later on March 28, ISS will perform a TMC observation of Titan from a distance of 1.88 million kilometers (1.17 million miles). This will allow for monitoring of cloud features across the Fensal-Aztlan region of the moon. Afterward, ISS will search for moons at Rhea's L5 Lagrangian point. Next, ISS will ride along with a CIRS observation of Rhea. ISS will image several mutual events of various moons, including an occultation of Mimas. Finally, ISS will acquire a long series of images of Rhea. These images are designed to improve the photometric calibration of ISS images as well as the camera's shutter offset.
On April 3, ISS will take another look at Titan, this time from a distance of 1.65 million kilometers (1.02 million miles) looking at western Xanadu and eastern Shangri-La. Later that day, ISS will acquire a lengthy, nine-hour observation of Thrymr, one of Saturn's distant, irregular moons. This observation will be taken from a distance of 10.3 million kilometers (6.41 million miles), and when combined with additional observations of this irregular satellite, is designed to measure Thrymr's rotational period. On April 5, ISS will ride along with the Visual and Infrared Mapping Spectrometer to image the G ring.
On April 5, Cassini will reach apoapse on this orbit, bringing it to a close and starting Rev164. Rev164 includes a targeted flyby of Enceladus and close, non-targeted flyby of Tethys.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC). Rhea and Enceladus basemaps by Steve Albers.