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Cassini continues its exploration of the Saturn system with the 40-day Rev 199, which begins on November 7 at its farthest distance from the planet. This is also called the orbit's apoapse. At this point, Cassini is 3.81 million kilometers (2.37 million miles) from Saturn's cloud tops. Rev 199 occurs during the first inclined phase, which lasts until March 2015, of the Cassini Solstice Mission. The inclined phase will allow for polar views of Saturn and Titan as well as better vistas of Saturn's rings than those Cassini had while in the earlier, equatorial phase of the Solstice Mission. Thirty-seven ISS observations are planned for Rev 199, most observations are focused on Saturn's atmosphere and rings and on Titan during the T96 flyby.
Rev199 starts with Cassini still in solar conjunction, which continues until late on November 8. During this period, communications between Earth and Cassini are limited and few scientific investigations are performed. The first observation for the camera system, ISS, will be acquired on November 9. ISS will observe the small, outer moon Skoll from a distance of 11.5 million kilometers (7.13 million miles) in order to measure its rotational period. While most moons of Saturn have days that match their orbital period, Skoll and other small outer moons typically have days lasting a few hours to one Earth day (compared to orbital periods exceeding one Earth year). Skoll will be too far away to observe any surface features, so its day will be calculated by observing slight variations in its apparent brightness as it rotates. Between November 11 and 18, ISS will take four movies of Saturn's B ring, looking for dust spokes. On November 22 and 23, ISS will acquire a cloud and haze monitoring observation of Titan. Observations like this one are designed to track changes in the distribution of cloud across Titan as well as to monitor changes to its haze layers. The November 22 observation will be taken from a distance of 2.66 million kilometers (1.66 million miles). A similar observation will be taken on November 23 from a distance of 2.30 million kilometers (1.43 million miles). Both observations will cover Titan's north polar region, though the later one will have a better phase angle for observing the surface.
On November 23, 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. Two more astrometric observations will be acquired on November 28 and December 3. Immediately afterward, ISS will acquire a quick observation of Saturn using the 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. Two more will be taken later on December 6 and 7. Also on November 23, ISS will ride along with the Visual and Infrared Mapping Spectrometer (VIMS) as it observes an occultation of the red giant star L2 Puppis by the ring system.
On November 25, ISS will acquire a movie of the Encke Gap in the outer A ring. This gap contains and is maintained by the small moon, Pan. On November 27, ISS will ride along with VIMS to acquire a mosaic of Saturn's north polar region using the WAC. Spring has progressed far enough that the entirety of the hexagonal jet stream that lies near 77 degrees North latitude will be in sunlight. ISS will be imaging the hexagon with a two-by-two mosaic rather than centering the field-of-view on the north pole. On November 28, ISS will ride-along with VIMS as it maps the clouds across Saturn's northern hemisphere. Later that day, ISS will image the outer A ring, where it will be looking at propellers previously imaged by Cassini. Propellers are small voids in Saturn's rings created by the gravitational interaction between large ring particles and the surrounding ring. A similar observation will be acquired of the A ring's unlit side on December 3.
Cassini encounters Titan on December 1 at 00:41 UTC for the 97th time. This is the eighth and final Titan flyby planned for 2013, with the next encounter scheduled for January 1, 2014. T96 has a close-approach altitude of 1,400 kilometers (870 miles). This flyby will allow for imaging of the north polar region inbound to the encounter and the southern anti-Saturn hemisphere of Titan on the outbound leg. Before the encounter on November 29, ISS will acquire three cloud monitoring observations. Early on November 30, ISS will ride-along with the Composite Infrared Spectrometer (CIRS) as it stares at Titan in order to acquire mid-infrared spectral and temperature information about Titan's atmosphere. Afterward, ISS will acquire three mosaics that will cover the northern hemisphere and north polar region of Titan. These mosaics will allow researchers to study the distribution of lakes across the north polar region, compare them to their appearance at other wavelengths, and to search for small clouds that might be in the area. In addition, these observations will help fill in the largest gap in ISS's map of Titan. The final mosaic will contain four frames covering some of the small lakes on the leading hemisphere side of the north pole as well as Mackay Lacus.
During closest approach, VIMS will be the prime instrument with ISS riding along. VIMS will acquire several images of an area of small lakes on the leading hemisphere side of the north pole. VIMS will be mapping the distribution of both filled lakes and empty lakes coated with evaporites. These evaporites appear bright at VIMS' longer wavelengths. VIMS will also acquire a noodle-like image strip covering the border between dark, dune-filled Shangri-La and bright and rough Xanadu near Titan's equator, crossing Tui Regio in southwestern Xanadu. After the closest approach period, CIRS will acquire more data about Titan's upper haze layers - its structure, temperature, and composition - as well as the now-cooling south polar vortex. VIMS will also acquire a global mapping observation of Titan's southern hemisphere in order to search for clouds.
During the playback of T96 data following the encounter, late on December 1 at 22:43 UTC, Cassini will reach periapse for Rev 199 at an altitude of 1.12 million kilometers (0.69 million miles) from Saturn's cloud tops. Cassini will be 420,500 kilometers (261,290 miles) from Titan at the time.
On December 2, ISS will ride along with VIMS to observe cloud systems across the southern hemisphere of Saturn. On December 2 and 3, ISS will ride along with a VIMS observation of an occultation of the star R Lyrae by Saturn's F ring. On December 6, ISS will acquire a movie of the F ring, observing its various channels and streamers created by the interaction between the ring material and the nearby moon, Prometheus. On December 7, a similar ring movie will be taken of the D ring.
On December 17, Cassini will reach apoapse, bringing Rev 199 to a close and starting up the next orbit, Rev 200, which will include another targeted flyby of Titan.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).