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Cassini continues its exploration of the Saturn system with the 19-day Rev 217, which begins on June 7 at its farthest distance from the planet. This is also called the orbit's apoapse. At this point, Cassini is 2.43 million kilometers (1.51 million miles) from Saturn's cloud tops. Rev 217 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. Twenty-four ISS observations are planned for Rev 217 with the majority focused on Saturn's atmosphere and several of its icy satellites. This orbit includes a flyby of Dione on June 16.
For ISS's first observation of Rev 217, on June 9, the camera system will observe a crescent Titan from a distance of 1.88 million kilometers (1.17 million miles). Immediately afterward, ISS 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 the optical remote sensing (ORS) instruments back to Saturn as a waypoint between other experiments' observations. These sequences include blue, clear, two methane band, and one full-frame, continuum band filter images. Five more storm watch observations will be taken between June 10 and June 14. On June 10, ISS will observe the distant, outer moon Hyrrokkin from a distance of 14.7 million kilometers (9.12 million miles). This observation will be used to better constrain this eight-kilometer- or five-mile-wide moon's pole direction and shape. Four-color photometry will also be obtained. On June 12, 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. Another astrometric observation will be acquired on June 14.
On June 14, ISS will observe Enceladus as it passes in front of Mimas's northern hemisphere. Enceladus will be 1.19 million kilometers (0.74 million miles) away during the transit, while Mimas will be 1.46 million kilometers (0.91 million miles) from Cassini. On June 15, ISS will acquire a pair of observations of the distant moon, Tarvos, from a distance of 21.5 million kilometers (13.3 million miles). Like the earlier observation of Hyrrokkin, these observations will be used to constrain the small moon's shape and pole direction. Another Tarvos observation will be acquired on June 16. Between the two Tarvos observations on June 15, ISS will ride along with the Composite Infrared Spectrometer (CIRS) to observe Rhea's leading hemisphere from a distance of 220,000 kilometers (136,700 miles), obtaining global color images.
On June 16 at 12:44 UTC, Cassini will reach periapse for Rev 217 at an altitude of 188,520 kilometers (117,140 miles) from Saturn's cloud tops, near the orbit of Mimas. Early in the day, ISS will observe the small moon Polydeuces, which shares its orbit with Dione. The best images will be acquired near closest approach at a distance of 34,744 kilometers (21,589 miles), when the moon will appear as a tiny crescent, 14 pixels across. During periapsis, CIRS will observe the limb of Saturn, measuring the composition and density of its atmosphere at different altitudes. ISS will ride along to acquire images of the bright limb.
At 20:12 UTC on June 16, Cassini will flyby Dione at a distance of 516 kilometers (321 miles). This is the fourth flyby of Dione during the Cassini mission with one more coming on August 17 during Rev 220. The D5 flyby is mostly dedicated to gravity science in the five hours surrounding closest approach. Dione has the second highest density of Saturn's mid-sized icy satellites, implying a larger amount of rocky material in its interior, unlike Tethys, which is composed almost entirely of water ice. Researchers will be looking to understand how Dione's rock fraction is distributed in its interior: concentrated into a rocky core like Enceladus or undifferentiated like Rhea. The Ion and Neutral Mass Spectrometer (INMS) will also be looking to measure any gases that might be given off by Dione. ISS will acquire images before and after the gravity observation. On approach, ISS will acquire a nine-frame mosaic across the moon's sub-Saturn hemisphere. Leaving Dione, ISS will ride along with CIRS to observe a crescent Dione.
On June 17, ISS will acquire a pair of observations of Albiorix from a distance of 20.5 million kilometers (12.7 million miles). Again, these observations will be used to constrain its shape and pole direction, as well as its rotational period. On June 18 and 19, ISS will acquire images of a crescent Titan from a distance of 1.3 million kilometers (0.8 million miles). Also, on June 18, ISS will ride along with the Ultraviolet Imaging Spectrometer (UVIS) to image aurorae across Saturn's south polar region.
On June 25, Cassini will reach apoapse, bringing Rev 217 to a close and starting up the next orbit, Rev 218, which will include a targeted flyby of Titan.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).