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Cassini continues its exploration of the Saturn system with the 11-day Rev 243, which begins on September 18 at its farthest distance from the planet. This is also called the orbit’s apoapse. At this point, Cassini is 1.49 million kilometers (0.93 million miles) from Saturn’s cloud tops. Rev 243 occurs during the second inclined phase of the Cassini Solstice Mission. Over the next several orbits, Cassini will use encounters with Titan to gradually increase the inclination of its orbit. Twenty-six ISS observations are planned for Rev 243 with the majority focused on Saturn’s rings and a Titan flyby.
For its first observation of Rev 243, on September 20, 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 later this year and early next year. A second astrometric observations will be acquired on September 21. Immediately after both astrometric observations, ISS will acquire quick observations of Saturn using the Wide-Angle Camera (WAC). These observations are 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. They include blue, clear, two methane band, and one full-frame, continuum band filter images.
On September 21, ISS will acquire an observation of Titan from a distance of 2.26 million kilometers (1.40 million miles). Images from this observation will be used to monitor clouds in Titan’s atmosphere across its northern, trailing hemisphere. A similar cloud monitoring observation will be taken on September 23 from a distance of 1.72 million kilometers (1.07 million miles) and cover the sub-Saturn hemisphere. On September 22, ISS will acquire a 15-hour movie of the F ring, a narrow ring just outside the main ring system. The movie will be used to monitor the creation of clumps and channels, formed by the gravitational interaction between F ring material and nearby moons and moonlets.
On September 24 at 10:56 UTC, Cassini will reach periapse for Rev 243 at an altitude of 399,700 kilometers (248,360 miles) above Saturn's cloud tops, between the orbits of Dione and Rhea. On September 23, right after the Titan cloud observation, ISS will ride along with the Visual and Infrared Mapping Spectrometer (VIMS) to observe a stellar occultation of the red supergiant star Antares by the rings. Stellar occultations can be used to probe the fine-scale structure of Saturn’s rings. Two more stellar occultation by the rings will be observed by VIMS and ISS during the periapse period on September 24. These occultations will use the Mira-variables X Ophiuchi and R Cassiopeiae. After the Antares occultation, ISS will ride-along with VIMS to observe the rings at very low phase angles, when Cassini will be directly between the Sun and the rings. This will allow the instruments to observe the opposition effect on the rings. Early on September 24, ISS will observe several of Saturn’s small, inner moons. This includes Polydeuces, a Trojan moon orbiting at the same distance as Dione, as well as Atlas and Prometheus. The closest images will be of tiny Polydeuces from a distances of 127,000 kilometers (79,000 miles). During periapse, ISS will image the outer edge of the A ring, tracking a transient clump of ring material nicknamed “Peggy”. Afterward, ISS will be conducting a 100-minute-long survey 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 used to keep track of previously discovered propellers, like Earhart and Bleriot. On September 25, ISS will monitor Enceladus’s south polar plume for five hours.
On September 27 at 04:23 UTC, Cassini will perform a targeted encounter of Titan. This is Cassini’s 124th flyby of Titan, the ninth of eleven planned for 2016. The next encounter is planned for November 13 during Rev 248. The T123 flyby has a close approach altitude of 1,736 kilometers (1,079 miles). This encounter will increase the inclination of Cassini’s orbit from 53.7 degrees to 57.9 degrees and will shave 2.4 days from the spacecraft’s orbital period. Inbound, ISS will be able to observe the southern, sub-Saturn hemisphere, while outbound, ISS will observe a crescent Titan and the north polar region of Titan.
During the T123 encounter, ISS will ride-along with observations by the Composite Infrared Spectrometer (CIRS), the Ultraviolet Imaging Spectrometer (UVIS), and VIMS. ISS will acquire WAC images during mid-infrared spectral scans performed by CIRS at the start of the encounter as well as during UVIS spectral scans on both the inbound and outbound legs. ISS will use the Narrow-Angle Camera (NAC) during the VIMS observations on the inbound and outbound legs in order to monitor Titan for clouds. VIMS will be prime during closest approach on T123. The instrument will observe a stellar occultation by Titan’s atmosphere of the star Arcturus, the fourth-brightest star in the night sky. The occultation will provide information on Titan’s atmospheric structure and composition. At closest approach, VIMS will acquire a high-resolution “noodle-strip” cutting across Xanadu. ISS will acquire flat-field calibration images. This, along with images planned for T125, will be used to help calibrate images of Saturn’s small, outer satellites.
Outbound, in addition to the UVIS and VIMS observations, CIRS will perform a series of limb observations. These will be used to look at the structure of Titan’s atmosphere, looking for differences in composition, temperature, and aerosols with altitudes at different latitudes on Titan. In the day after the encounter, on September 28, ISS will acquire a set of cloud monitoring observations of Titan’s north polar region.
On September 29, Cassini will reach apoapse, bringing Rev 243 to a close and starting up the next orbit, Rev 244.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).