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Cassini continues its exploration of the Saturn system with the 7.2-day Rev 251, which begins on November 30 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 251 is one of 20 F-ring orbits that will take place between November 2016 and April 2017 where Cassini will orbit Saturn just outside the main ring system. Seven ISS observations are planned for Rev 251 with the majority focused on Saturn’s atmosphere.
For its first observation of Rev 251, on December 2, ISS will ride along with the Ultraviolet Imaging Spectrometer (UVIS) to image Saturn’s northern hemisphere using the Wide-angle Camera (WAC). On December 3, ISS will perform a similar ride-along observation, this time with a Visual and Infrared Mapping Spectrometer (VIMS) observation of Saturn’s north polar vortex and hexagon.
On December 4 at 13:31 UTC, Cassini will reach periapse for Rev 251 at an altitude of 89,799 kilometers (55,799 miles), near the orbits of Janus and Epimetheus. Early on December 4, ISS will ride along with UVIS to observe Saturn’s north polar region. UVIS will observe Saturn’s auroral activity while ISS will be observing its cloud systems. Shortly after periapse, ISS will ride along with the Composite Infrared Spectrometer (CIRS) to observe Enceladus’s south polar region as well as a quick observation of the southern, sub-Saturn hemisphere of Tethys to map out the red streaks seen earlier on that icy satellite. Tethys will be 352,000 kilometers (219,000 miles) from Cassini during this observation. On December 5, 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. A similar survey will be taken during Rev 253 on December 19.
On December 6, ISS will acquire a ten-hour, plume-monitoring observations of Enceladus from a distance of 1.09 million kilometers (675,000 miles). On December 7, ISS will acquire a distant observation of Erriapus, one of Saturn’s three dozen outer satellites. This observation will be taken from a distance of 11.8 million kilometers (7.31 million miles). By measuring how its apparent brightness changes over the course of this observation, Erriapus’s rotational period can be estimated. After this observation and one last downlink, Cassini will enter solar conjunction, when Cassini and Saturn are on the other side of the Sun from Earth. Between December 7 and December 13, Cassini will have very limited communications with Earth due to interference from the Sun’s atmosphere, so very few science observations will be acquired (mostly lower bitrate fields-and-particle observations). No ISS observations are planned for this period.
On December 8, Cassini will reach apoapse, bringing Rev 251 to a close and starting up the next orbit, Rev 252. Cassini will still be in solar conjunction during periapse on Rev 252. Cassini will reach its closest point in this orbit around Saturn on December 11 at 17:47 UTC at a distance of 89,739 kilometers (55,761 miles).
On December 15, Cassini will reach apoapse, bringing Rev252 to an end of starting Rev 253. Sixteen ISS observations are planned for Rev 253, as well as two more at the very end of Rev 252. Most of these will be focused on Titan and Saturn’s ring system. At 21:52 UTC, Cassini will perform at non-targeted encounter with Titan at a distance of 342,396 kilometers (212,755 miles). ISS and CIRS will perform a series of observations covering Titan’s leading hemisphere, north polar region, and its trailing hemisphere over a period of 41 hours starting early on December 15, near the end of Rev 252. The ISS observations are designed to monitor and track clouds across Titan’s northern hemisphere. A cloud tracking observation on October 29 revealed several clouds across Titan’s mid-northern latitudes and north polar region in the same area where ISS will be looking during these observations. The ISS observations planned for this segment will allow for several two- and four-frame mosaics to be acquired. CIRS observations planned for this segment will allow for mid-infrared thermal and composition sounding of Titan’s atmosphere.
On December 16, ISS will acquire a 19-hour, time-lapse movie of Saturn’s narrow F ring as it monitors changes in the structure of the rings caused by nearby moons and large ring particles. On December 18, ISS will observe Titan from a distance of 930,300 kilometers (578,100 miles) in order to monitor cloud activity across Titan’s north polar region and northern sub-Saturn hemisphere.
On December 18 at 21:50 UTC, Cassini will reach periapse for Rev 253 at an altitude of 88,654 kilometers (55,087 miles). Shortly before periapse, at 21:36 UTC, Cassini will perform a non-targeted encounter with Pandora, a small moon that orbits just outside Saturn’s F ring, at a distance of 22,082 kilometers (13,721 miles). ISS will acquire 63 images of this small moon from an hour and a half before closest approach until shortly afterward. These will be the highest resolution images planned of Pandora taken by Cassini. After the Pandora observation, ISS will acquire a series of very high-resolution radial scans of the unlit side of Saturn’s ring system. These will be the highest resolution, ISS ring observation to date. This observation will include images of the outer B ring and Cassini division, the middle A ring, and the outer A ring, including the Keeler Gap which is generated by the gravitational interaction between ring particles and the embedded small moon Daphnis.
On December 20, ISS will observe a crescent Titan from a distance of 1.77 million kilometers (1.10 million miles) in order to monitor Titan’s upper haze layers. On December 21, as Cassini starts to climb above the ring plane, ISS will observe Saturn’s faint, diffuse E ring.
On December 22, Cassini will reach apoapse, bringing Rev 253 to a close and starting up the next orbit, Rev 254.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).