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Cassini continues its exploration of the Saturn system with the 12-day Rev 191, which begins on May 26 at its farthest distance from the planet. This is also called the orbit's apoapse. At this point, Cassini is 1.34 million kilometers (0.83 million miles) from Saturn's cloud tops. Rev 191 occurs a year into the first inclined phase of the Cassini Solstice Mission, a phase which lasts until March 2015. 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. Twenty-six ISS observations are planned for Rev 190 with many observations focused on Saturn's atmosphere and rings.
On May 26, a few hours after apoapse, ISS will acquire an observation of the small, outer satellite, Siarnaq. These images will be used to measure the rotation period of this small moon. Given its small size and great distance from Saturn, it doesn't rotate synchronously like Saturn's closer and larger icy moons. Siarnaq is 40 kilometers (25 miles) across and will be 11.5 million kilometers (7.12 million miles) away. On May 27, 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 May 28, 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. The camera system will also ride along with a Visual and Infrared Mapping Spectrometer (VIMS) stellar occultation observation, as a red, super-giant star, Mu Cephei (also known as Herschel's Garnet Star), is occulted by Saturn's rings.
On May 29, 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. Later in the day, ISS will acquire a distant Titan observation of the moon's southern and sub-Saturn hemispheres. This observation will be used to look for clouds in Titan's atmosphere. These images will be taken from a distance of 1.20 million kilometers (0.75 million miles). Afterward, ISS will again ride along with VIMS as it observes an occultation by Saturn's rings of a red-giant star, R Cassiopeiae. Finally, ISS will acquire a movie observation of the Roche Gap between the A and F rings.
On May 31 at 13:54 UTC, Cassini will make a close, non-targeted flyby of Titan at a distance of 568,230 kilometers (353,080 miles). During this encounter, ISS will acquire a pair of observations of Titan's southern and sub-Saturn hemisphere. On May 30, the camera system will acquire an observation of Titan's southern hemisphere, looking for clouds across the Tsegihi and Mezzoramia regions. The stare portions of the observation will also be used to look at the south polar stratospheric cloud once an hour (assuming that it is still in sunlight). On May 31, a shorter observation will be acquired which includes a four-frame mosaic of Titan's sub-Saturn hemisphere.
On June 1 at 07:01 UTC, Cassini will reach periapse for Rev 191 at an altitude of 557,370 kilometers (346,330 miles) from Saturn. During periapse, ISS will acquire another movie of the F ring and ISS will again ride-along with VIMS as it observes an occultation by Saturn's rings of a red-giant star, R Carinae. On June 2, ISS will ride-along as the Ultraviolet Imaging Spectrometer (UVIS) observes an occultation of the B-type star, Delta Centauri, by Saturn's rings. Shortly afterward, ISS will image the outer A ring, where it will be looking at propellers previously imaged by Cassini. Finally, ISS will ride-along with VIMS as it acquires a radial scan of Saturn's rings.
On June 3, ISS will ride-along with VIMS as it maps the clouds across Saturn's northern hemisphere. On June 4, ISS will track individual cloud features in Saturn's northern hemisphere as they rotate around the planet, both to study cloud motions as well as to use those clouds to measure the optical depth of Saturn's upper haze layers. This is done by seeing how the haze obscures the cloud at different viewing angles. On June 7, Cassini will reach apoapse on this orbit, bringing it to a close and starting Rev192. During apoapse, ISS will be riding along with VIMS as it acquires a movie of Saturn's atmosphere.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).