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Cassini continues its exploration of the Saturn system with the 6.5-day Rev 285, which begins on July 22 at its farthest distance from the planet. This is also called the orbit’s apoapse. At this point, Cassini is 1.21 million kilometers (0.75 million miles) from Saturn’s cloud tops. Rev 285 is the 15th of 22 proximal orbits that will take place between April 2017 and the end of the mission in September. During these orbits, Cassini’s closest approach to Saturn occurs between the ring system and Saturn’s atmosphere. Ten ISS observations are planned for Rev 285, with most occurring during a non-targeted encounter with Saturn’s largest moon, Titan.
On July 25 at 18:59 UTC, Cassini will reach periapse for Rev 285 at an altitude of 2,756 kilometers (1,712 miles) above Saturn’s cloud tops. On July 24, ISS will image Titan from 970,300 kilometers (602,900 miles) away to monitor clouds across Titan’s northern sub-Saturn hemisphere. Afterwards, the Ultraviolet Imaging Spectrometer (UVIS) will observe portions of Saturn’s northern auroral zone. During most of July 25, including through periapse, the spacecraft will roll to enable the magnetometer to perform calibration observations and to observe Saturn’s inner magnetic field. The Radio and Plasma Wave Science (RPWS) instrument will ride along to listen for Saturnian lightning. Outbound, early on July 26, ISS will ride along with UVIS as it observes Saturn’s southern auroral zone.
On July 26 at 23:20 UTC, Cassini will perform a non-targeted encounter with Titan at an altitude of 494,302 kilometers (307,145 miles). ISS will acquire three mosaics covering the Titan’s sub-Saturn hemisphere. These will be used to track the evolution and motion of Titan’s mid-northern latitude cloud streaks. These mosaics will also include stares at Titan’s northern mid-latitudes to monitor cloud motion at a faster cadence as well as stares at Titan’s south pole to support CIRS mid-infrared spectroscopy. On the outbound leg of the flyby, ISS will acquire two more long stares of a crescent Titan, again to track clouds across Titan’s northern mid-latitudes, though the higher phase angle may limit the visibility of fainter cloud streaks. On July 28, ISS will acquire a pair of observations of the distant, irregular moon Kiviuq from 11.1 million kilometers (6.90 million miles) away. These observations, covering a total of 21 hours, are designed to measure the small moon’s lightcurve, which can be used to measure the length of its day, estimate the orientation of its rotational axis, and calculate its approximate shape.
On July 29, Cassini will reach apoapse, bringing Rev 285 to a close and starting up the next orbit, Rev 286. At this point, Cassini is 1.21 million kilometers (0.75 million miles) from Saturn’s cloud tops. Nine ISS observations are planned for this orbit, with the majority focused on Saturn’s icy satellites. On July 29, ISS will image a crescent Titan from 1.48 million kilometers (0.92 million miles) to continue monitoring seasonal changes to its upper haze layers. On July 30, ISS will ride along with the Composite Infrared Spectrometer (CIRS) to image the lit face of Saturn’s A and B rings using multiple color filters on the Wide-Angle Camera (WAC). On July 31, ISS will conduct a 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 needed to keep track of previously discovered propellers, like Earhart and Bleriot.
On August 1 at 06:09 UTC, Cassini will reach periapse for Rev 286 at an altitude of 2,840 kilometers (1,765 miles) above Saturn’s cloud tops. On the inbound leg, UVIS will acquire their highest-resolution imaging swaths of Saturn’s northern aurorae. During closest approach, the Visual and Infrared Mapping Spectrometer (VIMS) will be prime, acquiring its highest resolution imaging of both Saturn’s polar aurorae. On the outbound leg, ISS will ride along with a pair of CIRS scans of Enceladus’s southern hemisphere. In the middle of polar night, much of Enceladus’s south polar region, including almost all the tiger stripes, are on the moon’s nightside. Afterward, ISS will observe a crescent Dione from 762,500 kilometers (473,800 miles) away to search for plumes of gas and dust emanating from its surface. A similar plume search will be taken on August 2. These are the last targeted Dione images planned for the Cassini mission. Also, on August 2, ISS will observe a plume known to exist, at Enceladus. On August 3, ISS will observe several of Saturn’s faint rings, like the D, G, E, and Pallene rings, from near the ring plane and at high phase angles.
On August 4, Cassini will reach apoapse, bringing Rev 286 to a close and starting up the next orbit, Rev 287, when Cassini will perform a non-targeted encounter with Titan and image Triton.
Image products created in Celestia. Enceladus basemap by Steve Albers. All dates in Coordinated Universal Time (UTC).