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Cassini continues its exploration of the Saturn system with the 24-day Rev168, which begins on June 17 at its farthest distance from the planet. This is also called the orbit's apoapse. At this point, Cassini is 2.83 million kilometers (1.76 million miles) from Saturn's cloud tops. This orbit includes non-targeted encounters with Titan and Tethys. Rev168 is near the beginning of 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 has viewed while in the earlier, equatorial phase of the Solstice Mission. Thirty-eight ISS observations are planned for Rev168, the vast majority dedicated to monitoring Saturn's storms and observing Saturn's rings.
ISS begins its observations for Rev168 on June 18, the day after Cassini passes apoapse, with a quick observation 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. Seven more such observations are planned between June 19 and June 25, while eight more will be taken between July 1 and July 10. After the first Storm Watch observation, ISS will take a look at Titan from a distance of 3.84 million kilometers (2.38 million miles). The observation is an effort to look for clouds in the moon's atmosphere as part of the "Titan Monitoring Campaign" (TMC). This observation of a half-phase Titan is designed to monitor clouds over the moon's Fensal-Aztlan dune field. ISS also will be taking shorter-wavelength images to study changes in Titan's upper haze layers. Between the second and third Storm Watch observations, on June 19, ISS will acquire an astrometric observation of Saturn's small, inner moons, including Atlas, Anthe, Prometheus, Janus, Pan, Pallene, Telesto, Helene, and Calypso. Astrometric observations are used to improve our understanding of the orbits of these small satellites, which can be influenced by Saturn's larger icy satellites.
On June 22 and 23, ISS will acquire a 37-hour light curve of the outer irregular satellite Ymir. Cassini will be at a distance of 15.3 million kilometers (9.49 million miles) from the 18-kilometer-wide (11.2-mile-wide) satellite. On June 24, ISS will use the WAC to observe spokes over the unlit side of the B ring as they enter and exit the shadow of Saturn on the rings. On June 25, ISS will acquire an astrometric sequence of Saturn's small inner satellites, including Prometheus, Pan, Daphnis, Janus, Methone, Polydeuces, Pallene, Telesto, Epimetheus, Atlas, Anthe, Calypso, Helene, and Pandora. After another Saturn storm watch observation, ISS will use its narrow-angle camera (NAC) to image the F ring of Saturn. The camera's field-of-view will be focused near the right ansa, and the various streamer-channels and moonlets associated with the narrow ring will rotate into view. This series of images will allow for the creation a movie of the current state of the F ring, which is quite dynamic due to impacts with small moonlets.
On June 27, Cassini will perform a non-targeted encounter with Titan at distance of 458,662 kilometers (284,999 miles). Cassini will pass over Titan's high southern latitudes with the trailing hemisphere illuminated. ISS and the Composite Infrared Spectrometer (CIRS) will perform a 15-hour observation of Titan during the flyby. At the beginning, ISS will perform a three-frame mosaic of the visible crescent, searching for clouds in the region. Over the next 13 hours, ISS will stare at the sunlit half of Titan, acquiring a set of images of the surface and upper haze layers every hour. CIRS will study the south polar atmospheric vortex and measure nitriles, or organic compounds containing nitrogen, that have been increasing in the region due to the change in season to autumn. At the end of the observation, ISS will acquire a two-frame mosaic covering the now half-phase Titan.
On June 29 at 00:27 UTC, Cassini will reach periapse for Rev168 at an altitude of 245,240 kilometers (152,390 miles) from Saturn. ISS observations during the periapse period will be taken during a non-targeted encounter with Tethys and include a set of rings observations, a limb scan of Saturn's atmosphere, and a distant observation of Titan. Before the Tethys encounter on the 29th, ISS will acquire late on June 27 another movie of the F ring at a much higher phase angle than the one taken two days earlier. ISS will then image the D ring, again at very high phase angles. A few hours later on June 28, ISS will acquire a few images of Saturn's limb while Cassini is in the planet's shadow. This will allow research to study the high-altitude haze layers in the planet's atmosphere. Late on June 28, ISS will ride along with CIRS to observe the non-targeted encounter with Tethys. At 21:09 UTC, Cassini will pass Tethys at a distance of 68,479 kilometers (42,718 miles). This encounter will allow for imaging of Tethys's northern leading hemisphere. Next, ISS will ride along with an Ultraviolet Imaging Spectrometer (UVIS) rings stellar occultation observation. ISS is hoping to catch a pair of F ring occultations of the star Kappa Canis Majoris. Afterward, ISS will search for moonlets in the Cassini Division. Late on June 29, ISS will acquire a TMC observation of Titan, covering the sub-Saturn hemisphere. The TMC observation will be taken from a distance of 2.00 million kilometers (1.24 million miles). Afterward, late on June 29 and into June 30, ISS will acquire a pair of movies of the outer B ring and the D ring.
On July 1, ISS will take a TMC observation of Titan, covering the sub-Saturn hemisphere of the large moon from a distance of 2.77 million kilometers (1.72 million miles). This observation will start out a bit differently than most, since Titan will be just emerging from behind Saturn. Initially, a set of red-green-blue images will be taken, followed by the standard set of surface and short-wavelength images a few minutes later when Saturn has moved out of the field of view. Another TMC will be taken the next day from a distance of 3.07 million kilometers (1.91 million miles), covering the Fensal-Aztlan region. A few hours later, ISS will image the A ring, searching for propellers, which are gravitationally-formed voids created by large ring particles. Of particular importance for this observation is to re-image previously observed propellers to better measure their sizes and orbits. On July 4, 6, and 10, ISS will take three more TMC observations of Titan. The closest of these comes on July 10 when Titan will be 1.75 million kilometers (1.09 million miles) away. This observation will cover the Shangri-La dune field which includes a very dark spot southeast of Mindanao Facula, recently presented by the Visual and Infrared Mapping Spectrometer (VIMS) team, that may or may not be a methane/ethane lake.
On July 10, Cassini will reach apoapse on this orbit, bringing it to a close and starting Rev169. Rev169 includes a targeted flyby of Titan.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).