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Cassini continues its new tour of the Saturn system with the 24-day-long Rev140, the spacecraft's 141st orbit around the Ringed Planet and the sixth full orbit of the Cassini Solstice Mission tour that began at the start of July (programmatically, the new mission began on October 1). Cassini begins Rev140 on October 28 at its farthest distance from Saturn, called apoapse. At this point, Cassini is 2.9 million kilometers (1.8 million miles) from Saturn's cloud tops. Cassini is in a slightly inclined orbit, 3 degrees out of the ring plane. This is still close enough to provide Cassini an opportunity to examine some of Saturn's moons, such as Dione and Enceladus. Cassini will also perform a targeted flyby of Saturn's largest moon, Titan.
Cassini's ISS camera system starts its observations for Rev140 on October 29 with an astrometric observation of several of Saturn's small, inner satellites in order to improve our knowledge of the motions of these moons. This observation will include images of Pandora, Helene, Polydeuces, Methone, Telesto, and Anthe. The set will also include a few wide-angle-camera images (WACs) of Saturn. Also on October 29, Cassini will image Titan while the satellite appears half-illuminated. This will permit Cassini to search for clouds across Titan's trailing hemisphere. This observation will be acquired from a distance of 1.85 million kilometers (1.15 million miles). An important goal of this observation will be to see if the band of clouds near Titan's equator, thought to be related to the large, arrow-shaped storm seen in late September, is still active 10 days after Titan was last observed and if this band of clouds is visible on this side of the moon. Finally, ISS will take a 16-hour observation of the small irregular satellite, Kari. During this sequence, Kari will be the brightest it will ever be during the Cassini Solstice Mission, making this a great opportunity to acquire a light curve of the moon. This observation is part of a campaign of observations of this and other small satellites taken to determine the moons' rotational periods, study their surface properties, and to determine if they are binary objects. This 7-kilometer (4-mile) diameter, outer satellite will be 10.2 million kilometers (6.34 million miles) away. The next day, Cassini ISS will acquire a similar rotational light-curve observation of Bergelmir, a 6-kilometer-diameter (4-mile-diamter) satellite 17.6 million kilometers (10.9 million miles) away.
In the four-day run up to periapse during Rev140, Cassini's remote sensing instruments will be focused primarily on studying Saturn's atmosphere and rings. For example, on November 5, ISS will acquire its first observation dedicated to a ring of dust that shares the orbit of the outer satellite Phoebe. The camera will be focused on Saturn's shadow cast on the ring. On November 6, the camera system will image Saturn's faint D ring, the innermost of Saturn's main rings, at high phase angles. On November 7 and 8, ISS will ride along with the Visual and Infrared Mapping Spectrometer (VIMS) to search for lightning on Saturn's night side.
On November 9 at 18:16 UTC, Cassini will reach the periapse of Rev140, its closest point to Saturn in the orbit. At periapse, the spacecraft will be 187,000 kilometers (116,196 miles) above Saturn's cloud tops. Inbound, ISS will take a 15-frame mosaic of the limb of Saturn while the sun is behind the planet. This very-high phase observation will permit Cassini scientists to study high-altitude haze structures in Saturn's atmosphere. Each narrow-angle-camera (NAC) frame will cycle through a variety of filters that probe different altitude clouds and hazes, such as UV3, MT1, and CB1. Next, Cassini will turn its attention to Dione as it passes by the moon at a distance of 98,927 kilometers (61,470 miles). The Composite Infrared Spectrometer (CIRS) will first take a look while the moon is in Saturn's shadow. Afterward, ISS will capture a four-frame mosaic of Dione's leading hemisphere. Afterward VIMS and ISS will observe an occultation by Saturn of the star Alpha Ceti. Next, ISS and CIRS will image Enceladus as Cassini passes the geologically-active world at a distance of 43,927 kilometers (27,294 miles). These images will focus on the leading hemisphere of the satellite. Of particular interest is how the brightness of the region being imaged changes at different polarization angles. Finally, ISS will ride along with a CIRS limb scan of Saturn's atmosphere, a low-phase-angle complement to the observation taken earlier in the day when Cassini was in Saturn's shadow. On November 10, ISS will ride along with VIMS to take a movie of Saturn's clouds and with CIRS to image a crescent Titan.
Cassini encounters Titan on November 11 at 13:37 UTC for the 74th time. This is the third of 56 Titan flybys planned for the new extended mission with the next encounter scheduled for February 18. The close approach distance for the encounter (known as T73) is 7,611 kilometers (4,729 miles). This flyby will allow for imaging of the anti-Saturn hemisphere of Titan outbound to the encounter. For much of the inbound leg of T73, when Titan will appear as a thin crescent from Cassini, the CIRS team will be controlling pointing in order to measure the composition of Titan's atmosphere and haze layer aerosols. These observations include two limb scans to measure how the atmosphere's composition changes with seasons. CIRS will also perform stares at the center of the night side of Titan. At closest approach, CIRS will focus on Titan's limb over the northern hemisphere. Outbound, VIMS will control pointing, mapping Titan's surface and cloud features. ISS will take images during this flyby by riding along with other instruments' observations, so no large mosaics are planned. The ride-along images should be useful for cloud monitoring, and if present, the clouds' motions and development can be tracked. The area will have been covered by the October 29 distant observation for comparison. Follow-up observations on November 13 and 14 will allow researchers to track clouds in the two days following the encounter. These will also cover the area that was under the arrow storm a month and a half earlier. An important goal will be to detect signs of surface changes that resulted from flooding caused by the storm's torrential rains.
Finishing up Rev140, Cassini will perform astrometric observations of Saturn's small, inner moons on November 14 and 18. During these two observations, ISS will observe Helene, Calypso, Pallene, Epimetheus, Prometheus, Anthe, Janus, Pandora, Telesto, Methone, and Pandora. At the end of the second observation, ISS will clean up with WAC images of Saturn. On November 14, ISS will also image the area around Rhea's L4 Lagrangian region, an area that leads Rhea on its orbit by 60 degrees. These points have provided islands of orbital stability for some of Saturn's satellites, like Telesto, which lies within the L4 region for Tethys, and Polydeuces, which lies within the L5 region of Dione. To date, no co-orbital satellites have been discovered that share Rhea's orbit. On November 15, ISS will image the distant irregular satellite Tarvos at a distance of 5.7 million kilometers (3.54 million miles), one of Cassini's closest encounters with one of Saturn's irregular satellites during the Cassini Solstice Mission. Cassini will still be too far away to resolve Tarvos as anything more than a point of light.
On November 20, Cassini will reach apoapse on this orbit, bringing it to a close and starting Rev141. Rev141 includes non-targeted encounters with Hyperion, Titan, Atlas, and Janus. Cassini will also perform a close, targeted flyby of Enceladus on November 30.
Image products created in Celestia. Enceladus and Dione basemaps by Steve Albers. All dates in Coordinated Universal Time (UTC).