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Cassini continues its exploration of the Saturn system with the 9.6-day Rev 247, which begins on October 27 at its farthest distance from the planet. This is also called the orbit’s apoapse. At this point, Cassini is 1.34 million kilometers (0.83 million miles) from Saturn’s cloud tops. Rev 247 occurs toward the end of the second inclined phase of the Cassini Solstice Mission. During this phase, Cassini will use encounters with Titan to gradually increase the inclination of its orbit. Twelve ISS observations are planned for Rev 247 with the majority focused on Saturn’s rings.
For its first observation for Rev 247, on October 28, ISS will acquire a quick observation of Saturn using the Wide-Angle Camera (WAC). This observation is part of a series of “Storm Watch” 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 observations. This observation will be taken at the very end of an F ring movie observation ISS will acquire at the end of Rev 246 and the start of Rev 247. They include blue, clear, two methane band, and one full-frame, continuum band filter images. Additional storm watch observation will be acquired on October 29 and November 4. Also on October 28, ISS will acquire a medium-resolution, color scan of the main rings using a string of Narrow-Angle Camera (NAC) pointings from the F ring to C ring. This observation will also include a seven-frame, Wide-Angle Camera (WAC) mosaic of Saturn and the main ring system.
On October 29, 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 as well as each other. Careful measurements of the positions of these moons are important for later imaging of them at much closer distances during the F ring orbits starting next month. Later that day and into October 30, ISS will observe Titan’s northern leading hemisphere for 11 hours. The NAC will acquire a set of images every 20 minutes in order to track cloud motion and cloud evolution, if they are present. Even if clouds are not present, the observation will help fill in a gap in the map of Titan near 60 degrees North Latitude, 10 degrees West Longitude.
On November 1 at 17:09 UTC, Cassini will reach periapse for Rev 247 at an altitude of 278,260 kilometers (172,900 miles) above Saturn's cloud tops, between the orbits of Tethys and Dione. During the periapse period, ISS and the rest of the remote sensing instruments will be focused on Saturn’s rings. Between October 31 and November 2, ISS will conduct three surveys of the propellers in the outer A ring. Propellers are voids in the ring created by the gravity of large, 100 – 1000 meter (328 – 3280 foot) ring particles. Due to the influence of the rings on their motion, these observations are needed to keep track of previously discovered propellers, like Earhart and Bleriot. On November 2, ISS will acquire another NAC mosaic strip spanning Saturn’s main ring system. On November 3, ISS will observe the G ring from near the ring plane and at high phase angles, useful for observing the vertical structure of this faint, dusty ring. On November 4, ISS will observe some of Saturn’s faint rings, including the E ring and ringlets in the D ring.
In addition to these ISS observations, Cassini’s other optical remote sensing instruments will be observing Saturn’s rings during the periapse period. Both the Ultraviolet Imaging Spectrometer (UVIS) and the Visual and Infrared Mapping Spectrometer (VIMS) will observe stellar occultations by Saturn’s rings. The UVIS occultations will use the blue-white giant stars Kappa Scorpii and Epsilon Sagittarii, while the VIMS occultation will use Alpha Centauri. The Radio Science Subsystem (RSS) will conduct a radio occultation of the A ring on November 2.
On November 6, Cassini will reach apoapse, bringing Rev 247 to a close, and starting up the next orbit, Rev 248, when Cassini will perform a targeted encounter with Titan.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).