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Cassini continues its exploration of the Saturn system with the 32-day Rev 201, which begins on January 19 at its farthest distance from the planet. This is also called the orbit's apoapse. At this point, Cassini is 2.72 million kilometers (1.69 million miles) from Saturn's cloud tops. Rev 201 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. Fifty-one ISS observations are planned for Rev 201, the majority focused on Saturn's atmosphere and rings and on Titan during the T98 flyby.
One day into the orbit on January 20, 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. Eleven more sequences will be taken between January 21 and January 29, while another eight will be taken between February 7 and February 19. Immediately after the first storm watch observation, ISS will acquire a movie of Saturn's B ring, looking for dust spokes. Four more spoke movies will be acquired during Rev 201, on February 11, 14, 16, and 19. On January 22, 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. Occultations like this one are used to understand the structure of Saturn's ring system and to look for any changes that might occur due to meteor impacts or slight changes in Saturn's gravity field. On January 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. Four more astrometric observations will be acquired between January 26 and February 18.
On January 26, ISS will image Titan, looking for clouds across its northern sub-Saturn hemisphere, at a distance of 2.91 million kilometers (1.81 million miles). Cassini will take another look at Titan on January 26 from a distance of 2.40 million kilometers (1.49 million miles). On January 27, ISS will observe the tiny moon, Aegaeon, and its accompanying ring arc in the G ring as they orbit Saturn for 13 hours. On January 29, 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.
Cassini encounters Titan on February 2 at 19:13 UTC for the 99th time. This is the second of eleven Titan flybys planned for 2014, with the next encounter scheduled for March 6. T98 has a close-approach altitude of 1,236 kilometers (767 miles). This flyby will allow for imaging of the north polar region and northern anti-Saturn hemisphere inbound to the encounter and the southern anti-Saturn hemisphere of Titan on the outbound leg. Before the encounter on February 1, ISS will acquire four cloud monitoring observations. Early on February 2, 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 a global mapping mosaic of Titan. This twelve-frame mosaic will cover much of the visible face of Titan at a resolution of 870 meters (2,850 feet) per pixel. Next, CIRS will stare at Titan, acquiring compositional information about the moon's atmosphere.
During closest approach, RADAR will be the prime instrument, acquiring observations from six hours before closest approach to six hours after. On both the inbound and outbound legs, RADAR will acquire scatterometry and radiometry data, which are used to measure the roughness, composition, temperature, and near-surface structure of Titan's surface. As the spacecraft approaches Titan, it will acquire high-altitude SAR imaging swaths across western Dilmun reaching as far south as the crater Selk. During closest approach, RADAR will acquire an altimetry swath across eastern Dilmun into the central Shangri-La dune field. RADAR will then acquire a RADAR SAR swath across the southern anti-Saturn hemisphere, running south from Hobal Virga to Ontario Lacus. The observation will be used to look for changes in the lake level at Ontario now that the sun has set there. The observation will also be used as part of a stereo pair with the swath taken during the next flyby, which will be useful for understanding the topography around the lake. On the outbound leg, RADAR will acquire another high-altitude SAR swath, this time over the south polar region.
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.
On February 4 at 18:12 UTC, Cassini will reach periapse for Rev 201 at an altitude of 919,690 kilometers (571,470 miles) from Saturn's cloud tops. Four hours after periapse, 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. Afterward, ISS will ride along with VIMS as they take a series of images along the boundary of Saturn's shadow on the unlit side of the main ring system. Later on February 5, ISS will ride along with VIMS again to acquire a movie of the F ring. On February 9 and 10, ISS will acquire a pair of movies covering portions of the unlit side of the rings side at high-phase angles. This viewing geometry makes rocky portions of the ring appear dark and more dust-filled areas bright. The February 9 movie will cover the D ring, while the movie taken the next day will cover the Roche Division between the outer edge of the A ring and the F ring. A few hours after periapse, ISS will ride along with VIMS as it observes the F ring and the main ring system occult of the M-type, red-giant star Gamma Eridani.
On February 20, Cassini will reach apoapse, bringing Rev 201 to a close and starting up the next orbit, Rev 202, which will include another targeted flyby of Titan.