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Cassini continues its exploration of the Saturn system with the 9.6-day Rev 189, which begins on May 6 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 189 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-four ISS observations are planned for Rev 189 with the majority focused on Saturn's atmosphere.
A little over an hour after apoapse, 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. Thirteen more storm watch observations are planned for this orbit. Immediately afterward, 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 7, in addition to four Saturn storm watch observations, 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 that can be influenced by Saturn's larger icy moons. On May 9, ISS will acquire a 12-hour movie with the narrow-angle camera (NAC) of the unlit side of the outer B ring.
On May 11 at 05:37 UTC, Cassini will reach periapse for Rev 189 at an altitude of 316,230 kilometers (196,500 miles) from Saturn. During the periapse period, Cassini's various optical-remote sensing instruments will focus on Saturn's ring system. First though, late on May 10, the Radio Sub-system (RSS) will acquire a pair of radio occultation observations of Saturn's atmosphere. These are designed to measure the density, pressure, and temperature of Saturn's upper atmosphere along with measuring the vertical profile of the electron density in the ionosphere. Earlier in the day, the Composite Infrared Spectrometer (CIRS) will measure the helium abundance near the RSS ingress occultation point. On May 11, the Visual and Infrared Mapping Spectrometer (VIMS) will map the composition of the lit face of Saturn's rings before ISS searches for propellers in the outer A ring. Propellers are small voids in Saturn's rings created by the gravitational interaction between large ring particles and the surrounding ring. On May 12, ISS will ride-along with VIMS as it observes an occultation by Saturn's rings of the red-giant star W Hydrae.
On May 14, ISS will image several of Saturn's faint rings at low phase and low elevation. ISS will take a look at the E ring and the Anthe and Methone ringlets. Next, ISS will look for clouds across Titan's sub-Saturn hemisphere from a distance of 2.37 million kilometers (1.47 million miles). Afterward, ISS will acquire another astrometric observation of Saturn's small, inner moons. Finally on May 14, ISS will watch as the small moon Pandora appears to pass under Mimas's south pole. On May 15, ISS will again look for clouds across Titan's sub-Saturn hemisphere, this time from a distance of 2.49 million kilometers (1.55 million miles).
On May 16, Cassini will reach apoapse for this orbit, bringing it to a close and starting Rev190. Cassini will encounter Titan during the next orbit on May 23.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).