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Cassini continues its exploration of the Saturn system with the 35-day Rev 203, which begins on March 24 at its farthest distance from the planet. This is also called the orbit's apoapse. At this point, Cassini is 2.96 million kilometers (1.84 million miles) from Saturn's cloud tops. Rev 203 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. Forty-four ISS observations are planned for Rev 203 with the majority focused on Saturn's atmosphere and on Titan during the T100 flyby.
Two days into the orbit on March 26, ISS will image Titan, looking for clouds across its northern hemisphere, at a distance of 3.96 million kilometers (2.46 million miles). Cassini will take four additional Titan Monitoring Campaign observations between March 27 and April 2 at distances ranging from 3.86 million kilometers (2.40 million miles) on March 27 to 2.25 million kilometers (1.40 million miles) on April 2. The last observation on April 2 will show Titan at low phase angles over the northern leading hemisphere. Also on March 26, ISS will ride along with the Ultraviolet Imaging Spectrometer (UVIS) to image Saturn's atmosphere. UVIS will perform scans of Saturn, looking at the planet at extreme- and far-ultraviolet wavelengths. ISS will ride along with similar observations on March 31 and April 4. On March 27, ISS will ride along with the Visual and Infrared Mapping Spectrometer (VIMS) to image Saturn's north polar region. VIMS will focus on imaging Saturn's aurorae while ISS will take Wide-Angle Camera (WAC) images of the planet's north polar region. ISS will ride along with a similar observation on March 29. On March 28, ISS will ride along with VIMS as it maps the cloud features across Saturn's northern hemisphere. Like the auroral sequences, this observation will take 11 hours, a little longer than one Saturn day. A similar ride-along observation will be taken on April 3. On March 30 and 31, ISS will stare at Saturn's north polar vortex, acquiring Narrow- and Wide-Angle Camera movies of the vortex and the north polar hexagon which surrounds it. On April 5 and 6, ISS will acquire a pair of Saturn cloud-tracking observations. Researchers will use these images to measure wind speeds at various levels of Saturn's atmosphere by tracking the movement of clouds.
Cassini encounters Titan on April 7 at 13:41 UTC for the 101st time. This is the fourth of eleven Titan flybys planned for 2014, with the next encounter scheduled for May 17. T100 has a very low close-approach altitude of 963 kilometers (598 miles). This is the last in a series of flybys where Cassini approaches over Titan's sun-lit, north polar region and departs over the autumnal south pole. Inbound, ISS will observe Titan's north polar region as well as the northern, anti-Saturn hemisphere. Outbound, ISS will be able to observe a crescent Titan over its southern, sub-Saturn hemisphere. Observations for this encounter will start with ISS riding along with the Composite Infrared Spectrometer (CIRS) as it measures Titan's atmospheric and surface temperatures in the mid-infrared. The observation will include stares at the beginning and ending of the observation for ISS to acquire NAC images of Titan. Next, ISS will acquire a global mapping mosaic of Titan. This nine-frame mosaic will cover much of the visible face of Titan at a resolution of 1.52 kilometers (0.94 miles) per pixel. Afterward, ISS will ride along with two more CIRS observations, one focused on Titan's surface and the other on the limb of the moon, as well as two VIMS observations of Titan's surface.
The main focus of the flyby is the Ion and Neutral Mass Spectrometer (INMS) observation at closest approach. In the hour and a half before closest approach, VIMS will observe, with ISS riding along, a stellar occultation by Titan's atmosphere, of Alpha Scorpii (Antares). The observation will be used to better understand the structure of Titan's upper atmosphere. Afterward, VIMS and ISS will acquire a quick observation of Titan's surface at high-resolution, looking at an area in far-northeastern Dilmun. At closest approach, INMS will be directly measuring the composition of Titan's upper atmosphere. Cassini's trajectory will allow the instrument to measure the ionosphere's structure over local noon on Titan. In addition, Titan will be over Saturn local noon, so the spacecraft and the moon will be in the outer flank of the planet's magnetosphere. This is important for understanding the impact of solar input (both in terms of heat and the solar wind) on Titan's upper atmosphere. After closest approach, CIRS and UVIS will acquire more data about Titan's upper haze layers - its structure, temperature, and composition - as well as the now-cooling south polar vortex. ISS will ride along.
On April 9 at 16:06 UTC, Cassini will reach periapse for Rev 203 at an altitude of 686,220 kilometers (426,400 miles) from Saturn's cloud tops. Shortly afterward, ISS will ride along with VIMS to observe aurorae at Saturn's south pole. On April 11, ISS will observe Uranus as it appears near Saturn's F ring. Like during last year's observation of Earth while Saturn was in eclipse, Uranus will appear as a pale, blue dot. On April 12, 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. On April 13, 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 storm watch observations will be taken on April 14 and 18. On April 14, 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 April 14, ISS will also image a crescent Titan from a distance of 1.72 million kilometers (1.07 million miles). While the sequence is designed to observe Titan's upper haze layers, the phase angle is not so high that the surface around the moon's north polar region won't be visible. Similar observations will be acquired on April 16 and 18, though the April 16 sequence will be more focused on the moon's surface. Starting mid-day on April 14, ISS will acquire a day and a half-long observation of the small outer moon, Fornjot. While Fornjot will be too far away (19.5 million kilometers or 12.1 million miles) for Cassini to observe any surface details, this very long observation will be used, along with a similar observation during Rev 202, to measure its rotational period by observing how its brightness changes as it rotates. On April 18, 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. On April 19, ISS will observe the outer satellite, Kiviuq, from a distance of 10.8 million kilometers (6.69 million miles). This is one of the closest observations of this small satellite. Combined with earlier sequences, this observation will be used to determine where its rotational axis points and model its shape. Finally, on April 24, a series of WAC calibration images will be taken.
On April 27, Cassini will reach apoapse, bringing Rev 203 to a close and starting up the next orbit, Rev 204, which will include another targeted flyby of Titan.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).