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Cassini continues its exploration of the Saturn system with the 14-day Rev 223, which begins on October 7 at its farthest distance from the planet. This is also called the orbit's apoapse. At this point, Cassini is 2.00 million kilometers (1.24 million miles) from Saturn's cloud tops. Rev 223 occurs during the second Equatorial phase of Cassini's extended-extended mission. During this 10-month phase, Cassini will orbit within the orbital plane of Saturn's rings, allowing for frequent encounters with Saturn's icy satellites. This orbit includes the first of three targeted flybys with Enceladus planned for October and December 2015. Twenty-seven ISS observations are planned for Rev 223 with the majority focused on Enceladus and Saturn's atmosphere.
For ISS's first observations of Rev 223, on October 8, the Narrow-Angle Camera (NAC) will monitor clouds across Titan's Fensal-Aztlan region from a distance of 2.89 million kilometers (1.79 million miles). Immediately afterward, the camera system will acquire a quick observation of Saturn using the Wide-Angle Camera (WAC). This observation is part of a series of "Storm Watch" observation sequences designed to take advantage of short, two-minute segments when the spacecraft turns to point the optical remote sensing (ORS) instruments back at Saturn, as a waypoint between other experiments' observations. These sequences include blue, clear, two methane band, and one full-frame, continuum band filter images. Eight more Storm Watch observations will be acquired between October 8 and 13, while another three will be taken on October 17 and 18. ISS will observe Titan on October 9, looking for clouds across its sub-Saturn hemisphere, from a distance of 2.56 million kilometers (1.59 million miles). The camera system will do so again on October 11, this time from a distance of 1.69 million kilometers (1.04 million miles). On October 12, ISS will ride along with a pair of Visual and Infrared Mapping Spectrometer (VIMS) observations of Saturn's E ring, a large, diffuse ring made up of icy particles emitted from the south polar region of Enceladus.
After the Titan observation on October 11, ISS will acquire an astrometric observation of Saturn's small, inner moons. Astrometric observations are used to improve our understanding of the orbits of these small satellites, which can be influenced by Saturn's larger icy moons. A similar astrometric observation will be acquired on October 12. Another way to improve our understanding of the orbits of Saturn's moons is through mutual events, where one moon passes in front of or near another moon, as seen from Cassini. ISS will observe one such mutual event on October 11, observing Tethys pass behind Rhea. Rhea will be 1.14 million kilometers (711,000 miles) from Cassini, while Tethys will be 1.49 million kilometers (928,000 miles) away. Another mutual event will be observed on October 18. During that observation, Rhea will pass behind Tethys' north pole. Tethys will be 1.68 million kilometers (1.04 million miles) from Cassini, while Rhea will be 2.11 million kilometers (1.31 million miles) away.
On October 14 at 14:06 UTC, Cassini will reach periapse for Rev 223 at an altitude of 114,870 kilometers (71,380 miles) from Saturn's cloud tops, between the G ring and the orbit of Mimas. On October 13, ISS will acquire seven-hour observation of the small, distant, outer moon Hati. By measuring the brightness of Hati in the 89 images planned for this observation, researchers hope to calculate how long its day is, where its north pole points, and estimate its shape. Hati will be 12.8 million kilometers (7.98 million miles) away. A follow-up observation of Hati will also be acquired on October 15. Early on October 14, ISS will ride along with the Composite Infrared Spectrometer (CIRS) to observe Dione's trailing hemisphere. CIRS will perform several temperature scans across the face of Dione over a five-hour period. ISS will acquire images of a crescent Dione.
At 10:42 UTC on October 14, Cassini will perform a targeted flyby of Enceladus for the twentieth time. The close approach altitude is 1,838 kilometers (1,142 miles) over the moon's high northern latitudes. This is the first of three Enceladus flybys planned for 2015, with another coming up during the next orbit on October 28. ISS is prime during most of this encounter, allowing it and the other optical-remote-sensing instruments to observe Enceladus's anti-Saturn hemisphere on the inbound leg of the encounter, and a narrow, sunlit crescent outbound. ISS will acquire a six-hour long observation, covering closest approach of Enceladus. Much of this time will be reserved for scans by CIRS and UVIS. However, ISS will acquire a seven-frame mosaic shortly before closest approach centered around al-Yaman Sulci and the "snowman" chain of craters to its north (al-Haddar, Shahrazad, and Dunyazad). After closest approach, ISS will acquire images of a crescent Enceladus and areas illuminated by Saturn-shine. After a Cosmic Dust Analyzer (CDA) observation during the ring-plane crossing, ISS will acquire an observation of Enceladus's south polar plume designed, along with a companion observation on October 15, to see how activity varies depending on Enceladus's position along its orbit.
On October 21, Cassini will reach apoapse, bringing Rev 223 to a close and starting up the next orbit, Rev 224, which will include another targeted flyby of Enceladus.
Image products created in Celestia. Enceladus map by Paul Schenk. All dates in Coordinated Universal Time (UTC).