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Cassini continues its exploration of the Saturn system with the 7.2-day Rev 256, which begins on January 5 at its farthest distance from the planet. This is also called the orbit’s apoapse. At this point, Cassini is 1.22 million kilometers (0.76 million miles) from Saturn’s cloud tops. Rev 256 is one of 20 F-ring orbits that will take place between November 2016 and April 2017 where Cassini will approach Saturn just outside the main ring system. Eleven ISS observations are planned for Rev 256 with the majority focused on Saturn’s rings.
For its first observation of Rev 256, on January 7, ISS will acquire a 15-hour, time-lapse movie of the narrow F ring to monitor features in the ring created by the gravity of nearby moons and ring clumps. On January 8, ISS will acquire an astrometric observation of several of Saturn’s smaller moons. Astrometric observations are important for fine-tuning our understanding of the orbital motions of these moons, which are influenced by the gravitational pulls of Saturn’s larger icy satellites.
On January 9 at 9:41 UTC, Cassini will reach periapse for Rev 256 at an altitude of 88,172 kilometers (54,788 miles), near the orbits of Janus and Epimetheus. Most of the observations during the periapse period focus on Saturn’s rings. On January 8, 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. Early on January 9, ISS will acquire short, high-resolution, color scans of the B and C rings as well as a longer, clear-filter scan across the A ring and outer B ring. At closest approach, the Ion and Neutral Mass Spectrometer (INMS) will directly measure the gas and ion environment near the F ring. Later on January 9, ISS will acquire a high-resolution observation of the unlit side of the outer A ring and the F ring. On January 10, ISS will observe the Pallene ring arc.
On January 11, ISS will acquire a pair of observations of a crescent Titan from 1.96 million kilometers (1.22 million miles) away. Both observations will be used to monitor changes in Titan’s upper haze layers. Similar observations will be acquired during Rev 257 on January 13 and during Rev 258 on January 21 and 22. After the first Titan observation on January 11, ISS will observe the D ring at high phase angles. Finally, ISS will ride along with the Ultraviolet Imaging Spectrometer (UVIS) to observe Saturn’s atmosphere as it passes above the ring plane.
On January 12, Cassini will reach apoapse, bringing Rev 256 to a close and starting up the next orbit, Rev 257. Eight ISS observations are planned for this orbit, with the majority covering Saturn’s atmosphere. On January 13, ISS will ride along with a UVIS calibration observation of the star Spica. ISS will use the observation to calibrate the UV filters and to check the point-spread function of all of the filters on the Wide-Angle Camera (WAC). On January 14, ISS will observe a half-phase Titan from 1.38 million kilometers (0.86 million miles) away. Later that day, ISS will ride along with UVIS to track clouds across Saturn’s north polar region.
On January 16 at 13:33 UTC, Cassini will reach periapse for Rev 257 at an altitude of 88,232 kilometers (54,825 miles), near the orbits of Janus and Epimetheus. Most of the observations during the periapse period focus on Saturn’s atmosphere. Early on January 16, ISS will ride along with the Visual and Infrared Mapping Spectrometer (VIMS) and the Composite Infrared Spectrometer (CIRS) to observe Saturn’s atmosphere. During the VIMS observation, ISS will create a mosaic of Saturn’s northern hemisphere, while during the CIRS observation, ISS will observe Saturn’s limb hazes. 14 minutes before periapse, Cassini will perform a non-targeted encounter with Daphnis at a distance of 22,655 kilometers (14,077 miles). Daphnis is an 8-kilometer (5 mile) wide moon orbiting within the Keeler Gap in the outer A ring. This will be the closest images of Daphnis to date though the small moon will only be 60 pixels across at closest approach. Images will also be taken during the two hours prior to closest approach for improved shape modeling.
On January 18, ISS will ride along with UVIS to observe Saturn’s southern hemisphere. Similar ride along observation will be acquired during Rev 258 on January 21 and 22. On January 20, Cassini will reach apoapse, bringing Rev 257 to a close and starting up the next orbit, Rev 258. Nine ISS observations are planned for this orbit, with most covering Saturn’s satellites. On January 20, ISS will acquire images of the bright limb of Saturn, used to monitor Saturn’s upper haze layers.
On January 23 at 17:26 UTC, Cassini will reach periapse for Rev 258 at an altitude of 88,232 kilometers (54,825 miles), near the orbits of Janus and Epimetheus. Most of the observations during the periapse period focus on Saturn’s magnetosphere. UVIS will observe Saturn’s north polar aurorae on January 23 with an observation of the south polar aurorae after periapse. At closest approach, the Ion and Neutral Mass Spectrometer (INMS) will directly measure the gas and ion environment near the F ring.
On January 25, ISS will acquire a distant observation of Tarqeq, one of Saturn’s three dozen outer satellites. This observation will be taken from 18.9 million kilometers (11.7 million miles) away. By measuring how its apparent brightness changes over the course of this observation, Tarqeq’s rotational period and rotational axis can be estimated. A similar observation covering around six hours will be acquired on January 27 into Rev 259. A distant observation of Paaliaq will also be acquired on January 26 from 10.4 million kilometers (6.43 million miles) away. On January 25 and 26, ISS will observe Enceladus’s south polar plume from 1.11 million kilometers (0.69 million miles) away, monitoring changes in the amount of dust in the plume as the moon orbits Saturn.
On January 27, Cassini will reach apoapse, bringing Rev 258 to a close and starting up the next orbit, Rev 259.