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Cassini begins the 18-day Rev156 on October 28 at its farthest distance from Saturn, called apoapse. At this point, Cassini is 2.37 million kilometers (1.48 million miles) from Saturn's cloud tops. The spacecraft is in the middle of the first equatorial phase of the Cassini Solstice Mission, 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, to image the rings edge-on, and to look at Saturn's cloud tops without the rings obscuring the view. Eighteen ISS observations are planned for Rev156, many of which are designed to monitor cloud systems in Saturn's atmosphere. During this orbit, Cassini will also encounter the icy moon Enceladus.
ISS begins its observations for Rev156 two days after apoapse with a lengthy, 13-hour observation of Titan. This observation will be taken from a distance of 1.6 million kilometers (1.0 million miles) and is designed to observe clouds across Titan's trailing hemisphere. This observation is part of a series of near-apoapse Titan sequences known as TEAs (Titan Exploration at Apoapse). In order to keep integration of Cassini's observations as simple as possible in this extended-extended mission (thus reducing cost), Cassini will focus on one science target for much of the apoapse period, Titan in this case. While the observations at the end of the last orbit focused on Xanadu and eastern Shangri-La, the four sequences to be taken between October 30 and November 3 will focus on areas that saw intense rainfall during last year's "Arrow Storm." These images also will be used to assess the current state of surface changes seen in the wake of the storm. Of particular interest are bright spots that were seen in Adiri and south Senkyo as well as a pair of temporary lakes in south Belet. The spots in Adiri may have faded in recent months, while the spot in south Senkyo was seen as recently as October 19. The playas in south Belet unfortunately have not been observed since June, so this will be the imaging team's first opportunity in five months to see whether they have dried up. The areas where surface changes were seen in south Belet and Adiri will be observed up close by the Visual and Infrared Mapping Spectrometer (VIMS) during the T79 flyby in December.
On November 4, ISS will perform a quick observation of Saturn and its fading northern hemisphere storm. This observation is part of the "Storm Watch" campaign which is designed to take advantage of short, 2-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. These observations can be used to track the progress of the northern hemisphere storm that began in early December 2010 and now appears to be fading. Five more are planned between November 4 and 8. On November 4, ISS will acquire an astrometric observation of Saturn's small, inner moons.
On November 6 at 07:58 UTC, Cassini will reach periapse for Rev156 at a distance of 136,370 kilometers (84,740 miles). During this periapse, at 04:59 UTC, Cassini will perform a targeted encounter with Enceladus (E16). This is the last of three encounters planned during October and November. Cassini will pass the icy moon at a distance of 496 kilometers (308 miles). During the encounter, ISS will take three observations of the satellite. The first is taken on approach when only a thin crescent is visible. This observation is designed to monitor Enceladus' south polar plume. Composed of vapor and very fine particles, the plume is more visible at high phase angles than when the satellite appears more fully illuminated. This observation ends at 00:15 UTC. Next, CIRS will perform series a scans of the night side of Enceladus using its FP3 channel while ISS will acquire a few snapshots.
At closest approach, the RADAR instrument will be prime, acquiring radiometry, scatterometry, HiSAR, and SAR data over Enceladus, activities normally performed at Titan rather than one of the icy satellites (though HiSAR data was taken at Iapetus in September 2007). Inbound radiometry, scatterometry, and HiSAR data will focus on the anti-Saturn hemisphere. At closest approach, RADAR will acquire a SAR swath cutting across the southern trailing hemisphere of Enceladus. The swath starts at around 45 degrees south latitude, 195 degrees west longitude, continues south and west until RADAR gets its highest resolution data near 62 degrees south latitude, 292 degrees west longitude, and finishes around 45 degrees south latitude, 40 degrees west longitude. This swath will include portions of the ridges and troughs that bound the south polar terrain, but it is too far north to include any of the tiger stripes. The third ISS observation begins a couple of hours after closest approach, after Enceladus has left Saturn's shadow. This observation first uses a three-frame mosaic to cover the visible disk of Enceladus, then stares at the center of disk to acquire color photometry of the moon's leading hemisphere.
After the Enceladus flyby observations, Cassini ISS will turn its attention on Dione, passing by the moon at a distance of 132,000 kilometers (82,000 miles). ISS will first acquire a set of WACs (including RGB and CB3 frames) as Titan appears to pass beyond Dione's south pole. Titan will of course be much farther away at a distance of 1.10 million kilometers (0.68 million miles) from Cassini, making Titan look smaller than Dione. ISS will then follow up with as series of summed images designed for color photometry.
On November 7 and 8, Cassini will acquire a series of observations designed to look for small, previously unknown satellites in the L5 Lagrangian points of Enceladus and Rhea and the L4 Lagrangian point of Titan. Trojan moons are not uncommon in the Saturn system. For example, the small moon Telesto orbits near the Saturn-Tethys L4 point.
On November 15, Cassini will reach apoapse on this orbit, bringing it to a close and starting Rev157.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC). Enceladus and Dione basemaps by Steve Albers.