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Cassini continues its extended tour of Saturn with Rev118, the spacecraft's 119th orbit around the Ringed Planet. Cassini begins Rev118 on September 8 at its farthest distance from Saturn, called apoapse. At this point, Cassini is 2.89 million kilometers (1.8 million miles) from Saturn's cloud tops. Although Cassini's orbits have grown to a length of 24 days, observations in Rev118 will be cut short by a week because solar conjunction will limit communications with the spacecraft. At solar conjunction, Saturn and Cassini will be on the opposite side of the sun from Earth. With radio signals passing so close to the sun, communications with Cassini will be limited.
Cassini ISS starts its observations for Rev118 the day after apoapse. On September 9, ISS will take its first observations of Iapetus during this orbit's non-targeted encounter. This first observation will occur when Cassini is 1.3 million kilometers (806,000 miles) from the two-toned satellite. Iapetus will show up as a crescent in this observation, making the observation more useful for photometry than mapping of the moon's surface features. Following the observation of Iapetus, Cassini will turn its wide-angle camera to Saturn's B ring, watching for spokes over the sunlit, northern side of the rings. This type of movie observation will later be repeated three more times between September 11 and 13. On September 10, ISS will look at a half-phase Titan over the satellite's trailing hemisphere from a distance of 1.97 million kilometers (1.23 million miles). This observation will provide an opportunity for Cassini to image the western parts of Belet, a dark, equatorial region which experienced a major storm back in April 2008.
On September 11, in addition to the B-ring spoke movie, ISS will image Iapetus, Titan, and a selection of propellers in Saturn's A ring. The Iapetus observation will be acquired from a distance of 1.18 million kilometers (732,000 miles) while the satellite still appears as a crescent, though the phase angle is less than it was on September 9. Titan also will be imaged while that moon is in eclipse, in the shadow of its parent planet. Imaging of the anti-Saturn side of Titan during eclipse will allow scientists to observe emission from excited nitrogen and hydrogen atoms and molecules in Titan's atmosphere. Finally, ISS will observe several propellers in Saturn's A ring. These voids are created by the gravitational interaction between large ring particles and the smaller particles that surround the large particles of the propellers. Following the day's B-ring spoke movie on September 12, ISS will observe Titan's trailing hemisphere from a distance of 2.8 million kilometers (1.74 million miles). The next day, September 13, ISS will observe a nearly half-phase Iapetus over its sub-Saturn hemisphere, this time from a distance of 1.24 million kilometers (773,000 miles). Later that day, ISS will track several propellers in the A ring. On September 14, ISS will observe two of Saturn's outer satellites: Iapetus from a distance of 1.34 million kilometers (834,000 miles) and Ijiraq from a distance of 6.67 million kilometers (4.15 million miles).
On September 15, Cassini begins solar conjunction. During this 5.5-day period that runs just past periapse on September 20, communications between Earth and the spacecraft will be limited, preventing the return of science data, so no imaging will be taken during this time. A few minutes before the end of solar conjunction, Cassini reaches periapse, its closest point to Saturn on Rev118. At this point, Cassini will be 197,300 kilometers (122,596 miles) from Saturn's cloud tops, between the orbits of Enceladus and Tethys. ISS's first observation following the end of solar conjunction is a series of images across the ring system where the team will be looking for shadows within the rings, such as those caused by eccentric ring structures or large particles. The next day, September 21, ISS will search for vertical features associated with propellers in Saturn's outer A ring.
On September 22, ISS will acquire a 10-frame, wide-angle-camera mosaic of Saturn and the entire ring system from above the ringplane. The view of Saturn that ISS will see at the time can be seen at right. In addition, ISS will take a look at Titan's north polar region, acquiring a two-frame mosaic over the north pole and parts of Titan's trailing hemisphere as well as later capturing single frames over the north pole to search for cloud motion. This observation of Titan coincides with a non-targeted encounter of the satellite when Cassini will approach within 293,616 kilometers (182,444 miles) of the moon. At the time of the observation, Titan will be 408,000 kilometers (254,000 miles) away and receding from Cassini. The next day, ISS will observe Titan again, this time from a distance of 792,000 kilometers (492,000 miles), looking at the trailing hemisphere, the dark region Belet, and the bright area Adiri. Comparisons between this observation and the one from the previous day would reveal longer term cloud evolution information. Also on September 23, ISS will acquire an astrometric observation of several of Saturn's small satellites including Prometheus, Telesto, Daphnis, Epimetheus, and Atlas. Astrometric observations are used to help provide better orbital calculations for some of these small rocks, which can be effected by gravitational interactions with the larger icy moons.
On September 24, ISS will acquire an eight-frame, color radial scan of Saturn's rings. This scan, combined with similar observations acquired over the last few months, will be used to better understand the particle size distribution in the system along the composition of different parts of the rings. Also, ISS will take another astrometric observation, this time covering Janus, Pandora, Calypso, Helene, and Telesto. Finally, Cassini ISS will turn its cameras back to Iapetus. This time, rather than observing Iapetus proper, the cameras will be pointed so that Iapetus is just out of the narrow-angle camera frame. The resulting images will be searched for possible ring-like structures or other dust sources near the satellite.
ISS closes out Rev118 with two more astrometric observations on September 28 and 30. During these two sequences, ISS will observe Methone, Pandora, Kari, Epimetheus, Telesto, Janus, Pan, Calypso, and Prometheus. Cassini reaches apoapse on October 2, bringing Rev118 to an end and starting Rev119. Rev119 includes a Titan flyby (T62) along with close, non-targeted encounters of Rhea, Tethys, and Mimas.
Image products created in Celestia. Iapetus basemap by Steve Albers. All dates in Coordinated Universal Time (UTC).