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Cassini continues its exploration of the Saturn system with the 32-day Rev 208, which begins on August 3 at its farthest distance from the planet. This is also called the orbit's apoapse. At this point, Cassini is 3.01 million kilometers (1.87 million miles) from Saturn's cloud tops. Rev 208 occurs during the first inclined phase, which lasts until March 2015, of the Cassini Solstice Mission. The inclined phase allows 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-six ISS observations are planned for Rev 208 with the majority focused on Saturn's atmosphere and on Titan during the T105 flyby.
On September 5, ISS will image Titan, looking for clouds across its northern leading hemisphere, at a distance of 3.70 million kilometers (2.30 million miles). On September 6, ISS will ride along with the Ultraviolet Imaging Spectrometer (UVIS) as they perform a series of far- and extreme-ultraviolet scans across the face of Saturn. Similar observations will be acquired on September 9, 12, and 16. The next day, ISS will observe Saturn for an hour and a half, acquiring a set of Wide-Angle-Camera (WAC) images of the planet. On September 8 and 9, ISS will observe the distant moon, Kiviuq, for 24 hours, in order to determine its shape and 3-color spectrum. Kiviuq will be 15.5 million kilometers (9.63 million miles) away.
On September 10, ISS will again observe Titan's northern leading hemisphere, this time from a distance of 2.09 million kilometers (1.30 million miles). Additional cloud monitoring observations will be acquired on September 12, 14, and 16. Afterward on September 10, ISS will acquire WAC and NAC movies of Saturn's North Polar Vortex covering more than 24 hours. On September 12 and 14, ISS will ride along with the Visual and Infrared Mapping Spectrometer (VIMS) to look at Saturn's northern hemisphere. On September 16, ISS will acquire another northern hemisphere observation of Saturn.
On September 19 at 23:53 UTC, Cassini will reach periapse for Rev 208 at an altitude of 753,120 kilometers (467,970 miles) from Saturn's cloud tops. The ISS observations during the periapse period focus primarily on Saturn's rings. On September 18, ISS will ride along with UVIS as it observes the unlit side of the B ring. On September 19, ISS will ride along with VIMS to observe a stellar occultation by the Saturn's ring system by R Lyrae, a red giant star also in the constellation Lyra. Stellar occultations 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. ISS images will focus on the portions where the stars are occulted by the F ring. Later in the day, ISS will ride along with VIMS again, this time to observe the edge of Saturn's shadow on the rings. Afterward, the camera system 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 September 20, 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 September 22 at 05:23 UTC for the 106th time. This is the ninth of eleven Titan flybys planned for 2014, with the next encounter scheduled for October 24. T105 has a close-approach altitude of 1,400 kilometers (870 miles). Inbound, ISS will observe Titan's southern, sub-Saturn hemisphere. Outbound, ISS will be able to observe a crescent Titan over its northern, sub-Saturn hemisphere. Observations for this encounter will start with ISS riding along with CIRS as it acquires far-infrared vertical sounding observations. These can be used to measure the optical depth of Titan's aerosol and haze.
Afterward, ISS will ride along with UVIS as that instrument observes a stellar occultation when Titan passes in front of the star Eta Ursa Majoris. For UVIS, stellar occultations are used to provide high-resolution profiles of the hydrocarbons and haze in Titan's atmosphere as well as a temperature and pressure profile. While the Ion and Neutral Mass Spectrometer (INMS) can directly provide such information during close flybys, UVIS can probe deeper into Titan atmosphere, down to an altitude of 200 kilometers (124 miles), well below the minimum altitude Cassini can safely approach Titan. Ingress will be over Titan's northern mid-latitudes while egress will be over its southern mid-latitudes. Afterward, UVIS and ISS will observe Titan's night-side limb.
At closest approach, VIMS will control pointing, acquiring a mosaic across eastern Tsegihi and Aztlan as well as imaging Sinlap impact crater. Afterward, VIMS will look for specular reflection off Kraken Mare. Analysis of that data can be used to look for waves in Titan's largest hydrocarbon sea due to either winds or tidal currents. Afterward, ISS will ride along with a pair of CIRS, nadir-pointing and limb-pointing observations. These ISS observations are designed to monitor clouds that may be in the region (though few clouds have been seen in the last two years). UVIS will also have a far- and extreme-ultraviolet observation, where they scan across the visible disc to better understand Titan's upper atmosphere. On September 23, 24, and 25, as Cassini recedes from Titan, ISS will acquire a series of observations of Titan's visible crescent to monitor the north polar region for clouds.
On September 26, ISS will acquire a movie of Saturn's F ring covering more than 14 hours. On September 28, the camera system will observe Ijiraq for over 14 hours. Ijiraq will be 4.85 million kilometers (3.02 million miles) away during the observation. This is the closest Cassini will get to one of Saturn's small outer satellites during the Solstice Mission. Still, Ijiraq is too small for Cassini to see as more than a bright spot. However, various color filter images will be acquired in order to better understand the small moon's surface composition. On October 4, 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 October 4 and 5, ISS will acquire quick observations of Saturn using the Wide-Angle Camera (WAC). These two observations are 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. Finally, on October 5, ISS will observe Vega in order to calibrate the filters on the Wide- and Narrow-Angle Cameras.
On October 5, Cassini will reach apoapse, bringing Rev 208 to a close and starting up the next orbit, Rev 209, which will include another targeted flyby of Titan.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).