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Cassini begins the three-week Rev151 on July 21 at its farthest distance from Saturn, called apoapse. At this point, Cassini is 2.69 million kilometers (1.67 million miles) from Saturn's cloud tops. The spacecraft is in the middle of the first equatorial phase of the Cassini Solstice Mission, which lasts until May 2012. During this phase, the spacecraft's orbits lie within the equatorial plane of the planet, providing opportunities to encounter Saturn's numerous moons, to image the rings edge-on, and to look at Saturn's cloud tops without the rings obscuring the view. Thirty-eight ISS observations are planned for Rev151, the vast majority designed to monitor the large northern hemisphere storm first seen in December 2010.
ISS begins its observations for Rev151 less than an hour after apoapse with a Saturn storm watch observation. Twelve such observations are planned between July 21 and July 29. They are 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 summed images taken in blue, clear and two methane band filters. Each of these sequences also will include one full-frame (not summed), continuum band image taken with the CB2 filter. On July 23 and 24, ISS will acquire ride-along images with a unique Visual and Infrared Spectrometer (VIMS) experiment. VIMS will attempt to observe two transits by an exo-planet orbiting the primary star of HD 189733 in the constellation Vulpecula. The planet has a mass of 1.14 Jupiter masses and orbits its parent star every 2.22 days. The first observation is designed to observe the planet passing in front of the star. The second observation is designed to observe the planet passing behind the star. The star will appear as only a dim dot in the ISS images, and the exo-planet will not be visible. ISS may detect the star dim as the planet passes in front during the first observation. Earth-based spectroscopy revealed water vapor and carbon dioxide in the planet's atmosphere, and data from the Spitzer Space Telescope has been used to create a crude map of its cloud-top features. On July 24, ISS will acquire a Titan monitoring observation. Titan will be 1.38 million kilometers (0.86 million miles) away at the time. A half-phase Titan will be visible from Cassini, allowing surface features and clouds across Titan's Shangri-La region to be observed. On July 26 and 29, ISS will acquire astrometric observations of Saturn's small, inner moons. During these two observations, the camera system will image Atlas, Prometheus, Janus (both times), Calypso (both times), Polydeuces (both times), Epimetheus, Telesto, Methone, and Pandora. On July 30, ISS will ride along with the Ultraviolet Imaging Spectrometer (UVIS) during its observation of the large northern storm on Saturn. The scan will proceed slowly from west to east during the 16-hour observation.
On August 1 at 08:12 UTC, Cassini will reach periapse, its closest distance to Saturn for Rev151, at a distance of 183,740 kilometers (114,170 miles), during which three observations are planned. The first is a ride along observation with VIMS. The instrument will capture seven mosaics that cover the equatorial and northern mid-latitudes of Saturn. The mosaics will have two rows (one centered near the equator, the other near 15 degrees north) with three footprints in each row. Immediately afterward, UVIS, ISS and the other ORS instruments will focus their attention on an encounter with Rhea. While technically a non-targeted encounter, the flyby will actually take the spacecraft fairly close to the moon, 5,862 kilometers (3,642 miles) from its surface. The fields-of-view of these instruments will be trained on Epsilon Orionis, the middle star of Orion's Belt. UVIS will use this stellar occultation to measure Rhea's very thin atmosphere and any dust in its environment. By measuring the density of Rhea's atmosphere over both its day and night sides, the UVIS team hope to learn more about how the atmosphere is generated and how it responds to heating by the Sun. Wide- and narrow-angle images are planned during this observation, showing Rhea pass through the field-of-view as the cameras stay fixated on Epsilon Orionis. Finally, VIMS and ISS will turn back to Saturn to image the "string of pearls" belt in Saturn's northern hemisphere. The belt is located just north of the large northern storm. This belt predates the storm and was observed by VIMS as early as 2006. Each "pearl" is a cloud clearing that is spaced about 3.5 degrees of longitude apart in the belt. On August 3, ISS and UVIS will acquire another west-to-east scan across Saturn.
Beginning on August 5, ISS will acquire a series of wind tracking observations. These involve taking a series of images over the same region of Saturn in order to see how much cloud features are displaced from one another. The distance of displacement compared to the rotation of the planet can then be used to measure wind speeds in the various belts and zones of the planet. On August 5, two, two-hour observations are planned. Two, four-hour observations are planned for both August 7 and 8. On August 9 and 10, five-hour cloud tracking observations are on the schedule. On August 5, ISS will ride along with the Composite Infrared Spectrometer (CIRS) to collect a series of WAC color filter images (including RED, GRN, and BL1) of Saturn as CIRS acquires a mid-infrared map of planet. On August 7, a Titan cloud monitoring observation is planned, covering the Xanadu region of the satellite from a distance of 1.53 million kilometers (0.95 million miles). Two more Titan observations are planned for August 9 and 10, as Cassini images the anti-Saturn hemisphere of the satellite. The August 9 observation will be taken from a distance of 1.41 million kilometers (0.87 million miles) while the August 10 sequence will be acquired from a distance of 1.78 million kilometers (1.11 million miles).
On August 12, Cassini will reach apoapse on this orbit, bringing it to a close and starting Rev152. In the day or so before the end of the orbit, ISS will acquire two movie observations of Saturn's cloud tops, each lasting for a full Saturn rotation. Every 1.9 hours, the wide-angle camera will acquire a set of images using the CB2, MT2, MT3, RED, GRN, BL1, VIO, and UV filters. The field-of-view will favor the northern hemisphere in order to avoid the rings for the other instruments that will be riding along, particularly CIRS. ISS will also take a rotational light curve of the distant irregular satellite, Tarqeq. The observation is designed to measure the length of the small moon's day. Similar observations have been successfully taken of Albiorix, Siarnaq, Ymir, Kiviuq, and Bebhionn.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC). Rhea basemap by Steve Albers.