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Cassini continues its exploration of the Saturn system with the 7.6-day Rev 250, which begins on November 23 at its farthest distance from the planet. This is also called the orbit’s apoapse. At this point, Cassini is 1.26 million kilometers (0.78 million miles) from Saturn’s cloud tops. The T125 flyby of Titan during this orbit marks the end of the second inclined phase of the Cassini Solstice Mission and the beginning of the F-ring orbits. During 20-orbit phases, Cassini will encounter Saturn just outside its F ring, acquiring high resolution observations of the rings and the moons near them and measuring Saturn’s inner magnetic field. Eighteen ISS observations are planned for Rev 250 with the majority focused on Titan and Saturn’s icy satellites.
For its first observation of Rev 250, on November 24, ISS will acquire a distant observation of Suttungr, one of Saturn’s three dozen outer satellites. This observation will be taken from a distance of 17.4 million kilometers (10.8 million miles). By measuring how its apparent brightness changes over the course of this observation, as well as observation taken on November 26 and November 28, Suttungr’s rotational period can be estimated. On November 24 and 25, ISS will ride along with a Visual and Infrared Mapping Spectrometer (VIMS) and an Ultraviolet Imaging Spectrometer (UVIS) observation of Saturn’s northern hemisphere. ISS will use these observations to track clouds as VIMS acquires mosaics of the planet’s north polar region and UVIS observes Saturn’s north polar aurorae. By tracking clouds, researchers can measure wind speed and wind direction at different latitudes on Saturn.
On November 27 at 08:39 UTC, Cassini will reach periapse for Rev 250 at an altitude of 157,960 kilometers (98,150 miles) above Saturn's cloud tops, between the orbits of Mimas and Tethys. During the periapse period, ISS and the rest of Cassini’s instrument suite will be focused on Saturn’s icy satellites. On November 26, ISS will ride along with a pair of UVIS observations of Dione and Tethys. ISS will observe the sub-Saturn hemisphere of both satellites. On November 27 at 06:51 UTC, ISS will perform a close, non-targeted encounter of Enceladus at a distance of 20,306 kilometers (12,617 miles). During this encounter, ISS will acquire a mosaic across the icy moon’s heavily-cratered, north polar region. This will help to fill a gap in the global map that is currently filled with lower-resolution Voyager 2 data acquired in August 1981. During this mosaic, along with an observation after closest approach of the south polar region, ISS will use Saturnshine to view surface features on the night side of Enceladus. The Composite Infrared Spectrometer (CIRS) will observe the moon’s south polar region to look for changes in thermal output from the area that has now been in polar winter for several years.
On November 29 at 22:14 UTC, Cassini will perform a targeted encounter of Titan. This is Cassini’s 126th flyby of Titan, the final encounter planned for 2016. This is the second-to-last Titan encounter planned for the Cassini mission, with the final encounter (T126) planned for April 22, 2017. T125 flyby has a close approach altitude of 3,158 kilometers (1,963 miles). This encounter will increase the inclination of Cassini’s orbit from 61.4 degrees to 63.8 degrees and will shave 19 hours from the spacecraft’s orbital period. This encounter also marks the transition into Cassini’s F ring orbits, part of its Grand Finale tour at Saturn. With this encounter, the spacecraft’s periapse shifts to near the orbits of Janus and Epimetheus, or only 10,000 kilometers (6,200 miles) beyond Saturn’s narrow F ring.
Inbound, ISS will be able to observe the southern, sub-Saturn and leading hemispheres, while outbound, ISS will observe a crescent Titan and the north polar region of Titan. During the inbound segment, VIMS will be prime for most of the time, acquiring global mapping observations of the visible surface. As Cassini approaches, ISS will acquire a high-resolution mosaic of eastern Hotei Regio with the Narrow-Angle Camera (NAC) and a series of Wide-angle images over portions of eastern Xanadu.
At closest approach, VIMS will again be prime, acquiring high-resolution near-infrared spectral images of the area around Titan’s north pole including the northern parts of Punga Mare and Kivu Lacus. Outbound, CIRS will acquire several infrared spectral observations of Titan’s atmosphere. These include the last far-infrared limb observations and the last surface temperature map of the mission. CIRS will also take mid-infrared limb and nadir observations that will be used with others taken throughout the Solstice Mission to track seasonal changes in Titan’s stratosphere as solstice approaches in May 2017. ISS will ride along with some of these observations to track clouds across the north polar region of Titan. VIMS will also ride along to look for specular reflections off Kraken Mare. In particular, VIMS hopes to observe specular reflections near Seldon Fretum, a strait between the northern and southern Kraken Mare basins. Strong tides may produce large waves in that area.
On November 30, Cassini will reach apoapse, bringing Rev 250 to a close and starting up the next orbit, Rev 251, when Cassini will enter solar conjunction.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).