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Cassini continues its exploration of the Saturn system with the 9.5-day Rev188, which begins on April 26 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 188 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. Sixteen ISS observations are planned for Rev 188 with the majority focused on Saturn's atmosphere.
Two days after apoapse, 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. Also on April 28, ISS will acquire a pair of quick observations of Saturn using the wide-angle camera (WAC). These 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. Two more storm watch observations are planned for this orbit. After the first storm watch observation, ISS will ride along with an Ultraviolet Imaging Spectrometer (UVIS) observation of a stellar occultation of the A-type star Vega by Dione. Finally, 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 29, ISS will ride along with a UVIS observation of Saturn's south polar aurora. In addition to making a movie of the planet's aurorae, the images will be used to independently measure the rotation period of Saturn's magnetic field.
On May 1 at 15:59 UTC, Cassini will reach periapse for Rev 188 at an altitude of 316,450 kilometers (196,630 miles) from Saturn. During the periapse period, ISS will acquire several observations of Saturn's atmosphere. First, ISS will look for high-altitude haze layers above Saturn's south polar region by acquiring images of Saturn's limb while Cassini is in the planet's shadow. ISS will then ride along with the Composite Infrared Spectrometer (CIRS), taking a 30-minute movie of Saturn's nightside in order to search for lightning in the planet's southern high- to mid-latitudes. Next, ISS will monitor the south polar plume of Enceladus from a distance of 546,000 kilometers (280,000 miles). Late on the 1st, ISS will ride along with the Visual and Infrared Mapping Spectrometer (VIMS) to acquire several mosaics of Saturn's north polar region using the Wide-Angle Camera (WAC). Spring has progressed far enough that the entirety of the hexagonal jet stream that lies near 77 degrees North latitude will be in sunlight. The hexagon would fill the WAC images from this observation, but ISS will be covering it with two-by-two mosaics.
On May 3, ISS will ride along with VIMS again, this time acquiring several two-frame mosaics of Saturn's northern hemisphere. ISS will then track various cloud features as they move across the face of Saturn as the planet rotates. Afterward, ISS will acquire another astrometric observation of Saturn's small, inner moons. On May 5, between a pair of Saturn storm watch observations, ISS will acquire a movie of the G ring arc that is associated with the small moon Aegaeon.
On May 6, Cassini will reach apoapse on this orbit, bringing it to a close and starting Rev189.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).