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Cassini continues its exploration of the Saturn system with the 10-day Rev186, which begins on April 7 at its farthest distance from the planet. This is also called the orbit's apoapse. At this point, Cassini is 1.30 million kilometers (0.81 million miles) from Saturn's cloud tops. Rev 186 occurs nearly 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 ISS observations are planned for Rev 186 with most focused on Saturn's atmosphere and rings.
On April 8 and 9, ISS will ride along with a pair of long Titan observations by the Composite Infrared Spectrometer (CIRS). Each observation will allow ISS to track clouds on Titan's southern trailing hemisphere for 15 hours. The April 8 observation will be taken from a distance of 1.05 million kilometers (0.65 million miles), while the observation the next day will be taken from a distance of 1.27 million kilometers (0.79 million miles). On April 10, ISS will acquire a NAC movie of the Encke Gap, which includes the small moon Pan.
On April 12 at 12:35 UTC, Cassini will reach periapse for Rev 186 at an altitude of 316,440 kilometers (196,630 miles) from Saturn. Late on April 11, ISS will acquire an observation of 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. Shortly before periapse on April 12, ISS will monitor the south polar plume of Enceladus from a distance of 450,000 kilometers (280,000 miles). Afterward, ISS will ride along as CIRS observes a stellar occultation by Saturn's rings of the Mira-like red giant star R Doradus. While R Doradus is barely visible to the naked eye at visible wavelengths, at 1.25 microns in the near-infrared, it is the second-brightest star in the night sky (only Betelgeuse in the constellation Orion is brighter). A similar occultation will be observed by the Visual and Infrared Mapping Spectrometer on April 13, this time involving W Hydrae, another Mira-like red giant star. Before the W Hydrae occultation, ISS will acquire an observation of Titan, looking for clouds across the moon's sub-Saturn hemisphere. It will be taken from a distance of 1.79 million kilometers (1.11 million miles). Finally on April 13, ISS will acquire a high-resolution color mosaic of the lit face of Saturn's rings. The mosaic won't cover Saturn's entire ring system, but will be a radial scan from the D ring out to the F ring.
On April 14, ISS will take a quick observation of Saturn using the wide-angle camera (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. Five more storm watch observations are planned for April 15, 16, and 17. Immediately after the Storm Watch observation on April 14, ISS will look for storms on Titan. ISS will observe the moon's sub-Saturn hemisphere from a distance of 2.26 million kilometers (1.40 million miles). It will take two more Titan observations on April 15 and 17. On April 15, 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 16, ISS will acquire an observation of the small, outer satellite, Siarnaq. The observation is designed to allow researchers to measure the length of its day. Given its small size and great distance from Saturn, Siarnaq doesn't rotate synchronously like Saturn's closer and larger icy moons. Earlier measurements from 2009 and 2010 yielded inconsistent results, and this observation and other observations this summer will be used to improve our understanding of its rotational state. Siarnaq is 40 kilometers (25 miles) across and will be 10.9 million kilometers (6.76 million miles) away.
On April 17, Cassini will reach apoapse on this orbit, bringing it to a close and starting Rev187.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).