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Cassini continues its exploration of the Saturn system with the 17-day Rev164, which begins on April 5 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. Forty-eight ISS observations are planned for Rev164, the vast majority dedicated to Saturn storm monitoring and to encounters with Enceladus and Tethys.
ISS begins its observations for Rev164 three days after apoapse on April 8 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. Three more such observations are planned for April 11, with another eighteen planned between April 16 and 23. Also on April 8, ISS will take a look at Titan from a distance of 1.81 million kilometers (1.12 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 Belet dune field. Later that day, ISS will acquire an astrometric observation of Saturn's small, inner moons, including Daphnis, Pallene, Helene, Janus, Methone, and Calypso. 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 April 11, 16, 19, 20, and 23.
On April 11, 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 2.31 million kilometers (1.44 million miles). Another TMC sequence will be acquired on April 13, covering Titan's Saturn-facing hemisphere from a distance of 1.79 million kilometers (1.11 million miles). Later that day, ISS will search for possible satellites at Rhea's L5 point, a Lagrangian point that lies 60 degrees behind the icy moon on its orbit. Similar moons have been found at Dione's and Tethys' L5 points (Polydeuces and Calypso, respectively). A similar sequence covering Rhea's L4 point (60 degrees ahead of it along its orbit) will be acquired on April 15.
On April 14 at 17:03 UTC, Cassini will reach periapse for Rev164 at an altitude of 135,940 kilometers (84,470 miles) from Saturn. ISS observations during the periapse period will be taken during a targeted encounter of Enceladus and later during a close, non-targeted encounter of Tethys.
Cassini will fly by Enceladus (E18) at an altitude of 74 kilometers (46 miles) at 14:02 UTC on April 14. ISS will image the icy satellite's south polar plume from distances of 340,500 kilometers (211,600 miles) down to 118,540 kilometers (73,660 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 scan across the south polar terrain (found in earlier flybys of Enceladus to be a thermal hotspot). 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. The sulcus will be in darkness, but ISS will acquire a single narrow-angle camera/wide-angle camera image pair during this observation just as the ISS field-of-view crosses onto Enceladus' day side. The frame will be centered near 74 degrees south latitude, 18 degrees west longitude and taken from an altitude of 74 kilometers (46 miles). The high velocity of the spacecraft will smear the images and leave the NAC image with an useful pixel scale of about 18 meters (59 feet) per pixel instead of what ideally would be a pixel scale of 44 centimeters per pixel (17 inches per pixel) at that range. Because of the smearing, the WAC image will have about the same useful resolution as the NAC, only it will cover a much larger area Afterward, the Composite Infrared Spectrometer (CIRS) will obtain a map of Enceladus sun-lit side using its FP3 channel, focusing particularly on the sub-solar point where the CIRS expects the highest temperatures and best signal-to-noise.
At 22:06 UTC, Cassini will fly by another of Saturn's icy satellites, Tethys, at a close-approach distance of 9,053 kilometers (5,625 miles). This is Cassini's best encounter with the moon since a targeted encounter in September 2005. After a CIRS observation of Tethys' night side, ISS will capture a frame over Tethys' night side, imaging the moon's surface features in Saturn-shine. After closest approach, it will acquire a 22-frame mosaic of Tethys' anti-Saturn hemisphere, including some frames late in the mosaic of the large impact basin Odysseus. Afterward, ISS will ride along with the Visual and Infrared Mapping Spectrometer (VIMS), imaging Tethys through a series of color filters as the moon just about fits the NAC field-of-view.
Later on April 15, ISS will perform a TMC observation of Titan from a distance of 1.51 million kilometers (0.94 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 L4 Lagrangian point. On April 19, ISS will watch as Dione occults the southern hemisphere of Rhea. Dione will be 1.82 million kilometers (1.13 million miles) away while Rhea will be 2.44 million kilometers (1.51 million miles) away. The difference in distance will make Dione appear to be about the same size as Rhea. Later that day, ISS will acquire another TMC observation of Titan from a distance of 1.20 million kilometers (0.75 million miles), covering the boundary between Xanadu and Shangri-La. ISS will take another look at the area the next day, this time from a distance of 1.12 million kilometers (0.69 million miles). Later on the 20th, ISS will search for possible satellites at Titan's L5 point. Finally, the camera system will watch as Rhea, 1.79 million kilometers (1.12 million miles) away, occults the southern hemisphere of Tethys, 2.11 million kilometers (1.31 million miles) from Cassini.
On April 23, ISS will take another look at Titan, this time from a distance of 1.69 million kilometers (1.05 million miles) looking at the Belet dune field. Later that day, ISS will ride along with the Visual and Infrared Mapping Spectrometer to image the G ring.
On April 23, Cassini will reach apoapse on this orbit, bringing it to a close and starting Rev165. Rev165 includes a targeted flyby of Enceladus and close, non-targeted flyby of Dione.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC). Dione and Enceladus basemaps by Steve Albers.