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Cassini begins the three-week Rev152 on August 12 at its farthest distance from Saturn, called apoapse. At this point, Cassini is 2.71 million kilometers (1.68 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, image the rings edge-on, and look at Saturn's cloud tops without the rings obscuring the view. Thirty-seven ISS observations are planned for Rev152, the vast majority designed to monitor cloud systems in Saturn's atmosphere.
ISS begins its observations for Rev152 a day after apoapse with a Titan monitoring observation. Titan will be 2.81 million kilometers (1.74 million miles) away at the time. Visible surface features during this observation include eastern Senkyo and western Belet. This also includes regions most affected by last fall's large arrow-shaped storm, which produced extensive surface changes as a result of methane rainfall. This observation will provide a useful data point for measuring how those changes fade with time. On August 13 and 14, ISS will acquire four, five-hour movie observations of Saturn's clouds. These are designed to measure wind speeds on Saturn. A similar quartet of observations will be taken on August 17 and 18, though each of those is four hours long. Within each set, the start times of the first two observations are separated by one Saturn day (approximately 11 hours), the third observation starts immediately after the end of the second, and the fourth starts one Saturn day after the beginning of the third. By measuring the displacement of a cloud seen in the first set, for example, compared to its position in the second set, scientists can calculate wind speeds in the jet where the cloud resides. On August 15, the Ultraviolet Imaging Spectrometer (UVIS) will scan across the disk of Saturn, mapping its spectra in the extreme- and far-ultraviolet (EUVFUV) while ISS rides along. The scan will proceed slowly from west to east during the 16-hour observation. On August 17, ISS will image Titan again, this time from a distance of 3.53 million kilometers (2.19 million miles) and covering the sub-Saturn hemisphere. A similar observation from a distance of 3.07 million kilometers (1.91 million miles) will be taken on August 19, covering Titan's Fensal-Aztlan region. On August 19, 21 and 22, ISS will take three more Saturn observations, first riding along with another UVIS scan, then taking a cloud monitoring observation, and finally riding along with UVIS again.
On August 23 at 03:58 UTC, Cassini will reach periapse for Rev152 at a distance of 183,730 kilometers (114,160 miles), during which one observation is planned. ISS will ride along with a Visual and Infrared Mapping Spectrometer (VIMS) observation of the "String of Pearls," a belt of cloud clearings first announced by VIMS in 2006. The "String of Pearls" is located north of the current northern hemisphere storm. On August 25, ISS will take a quick Saturn storm watch observation. Thirteen such observations are planned between August 25 and September 1. 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 blue, clear, two methane band, and one full-frame, continuum band filter images.
On August 25 at 16:48 UTC, Cassini will perform a non-targeted encounter with the irregularly shaped moon, Hyperion. Cassini will pass the icy moon at a distance of 24,978 kilometers (15,520 miles). During the encounter, ISS will take several, multi-filter mosaics of the satellite. The first, three-frame mosaic covers the crescent of Hyperion as Cassini approaches. After a few CIRS scans at closest approach, ISS will acquire a multi-spectral frame over the center of the sunlit crescent followed by a four-frame mosaic. Finally, ISS will acquire another three-frame mosaic of a now gibbous Hyperion, followed by a CIRS scan, and then one last ISS multi-spectral frame consisting of 30 images using various filter combinations.
On August 26, ISS and VIMS will monitor the right ansa of the diffuse E ring, followed by a long observation looking at the G ring arc from more than 2 million kilometers (1.24 million miles). Both the E ring and G ring arc are generated from material from one of Saturn's icy satellites: Enceladus in the case of the E ring and tiny Aegeaon in the case of the G ring. Cassini will observe Titan twice more on August 29 and September 1. The narrow-angle camera will take images of the moon's trailing and sub-Saturn hemispheres. Also on both days, ISS will acquire astrometric observations of Saturn's small, inner moons. During these two observations, the camera system will image Anthe, Telesto, Methone, Janus (both times), Atlas (both times), Pandora, Polydeuces, and Prometheus.
On September 3, Cassini will reach apoapse on this orbit, bringing it to a close and starting Rev153. The next orbit will see another Hyperion non-targeted encounter, a Titan encounter, and a non-targeted encounter with Pallene.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC). Rhea basemap by Steve Albers.