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Cassini continues its extended tour of Saturn with Rev117, the spacecraft's 118th orbit around the Ringed Planet. The spacecraft's first orbit following Saturn vernal equinox, Cassini begins Rev117 on August 19 at its farthest distance from Saturn, called apoapse. At this point, Cassini is 2.21 million kilometers (1.37 million miles) from Saturn's cloud tops. Although equinox occurred during the previous orbit, many observations during Rev117 focus on features only visible near equinox, including imaging of shadows of eccentric features within the ring system and faint lightning visible on Saturn's atmosphere without distracting ringshine.
Cassini ISS starts its observations for Rev117 a few minutes after apoapse. On August 19, ISS will acquire several north-to-south scans across Saturn's nightside. With ringshine on Saturn's atmosphere at a minimum, this observation will be used to search for lightning in Saturn's atmosphere. On August 20 to 22, ISS will take an observation each day of eccentric and other out-of-plane features in the main ring system. These features cast shadows on the ring system, making them easier to see than they would be otherwise. These shadow-casting structures include large moonlets imbedded within the rings, waves along the edge of ring gaps, and structures in the edges of the A and B rings. On August 22, ISS will look at a crescent Titan over the satellite's leading hemisphere from a distance of 1.63 million kilometers (1.01 million miles). While this observation would not be good for observing surface features on Titan due to the high phase angle, these images will help characterize any changes in the haze layers in the moon's upper atmosphere as the result of the approaching equinox or Titan's position in Saturn's magnetosphere. ISS will also acquire two radial scan mosaics of the two ring ansae as part of a photometric sequence of the sunlit, northern side of the ring system.
On August 23, ISS will take an astrometric observation of several of Saturn's small satellites including Helene, Atlas, Calypso, Methone, and Telesto. Astrometric observations are used to help provide better orbital calculations for some of these small rocks, which can be effected by gravitational interactions with the larger icy moons. Finally, on August 24, ISS will acquire a 10-frame, multi-color mosaic of the entire Saturn ring system, focusing on the photometry of the system near equinox.
Cassini encounters Titan on August 25 at 12:52 UTC for the 62nd time and the second encounter this month. This flyby is also the last in a series of ten flybys between April and August 2009 spaced 16 Earth days, or one Titan day, apart and occurring as Cassini is inbound toward Saturn. The close approach distance is only 970 kilometers (603 miles), close to the lowest safe altitude for a Titan flyby during the extended mission. This flyby, known as T61, will allow for imaging of the southern trailing hemisphere of Titan outbound to the encounter, similar to the area observed during the previous nine encounters in this series. On approach to Titan, the Visible and Infrared Mapping Spectrometer (VIMS), Composite Infrared Spectrometer (CIRS), and ISS teams will be controlling spacecraft pointing, or will be considered "prime." With Cassini approaching over the nightside, northern anti-Saturn hemisphere, CIRS will map atmospheric temperature and composition in the mid-infrared over Titan's south polar region looking for seasonal variations over this region now that fall has started over the southern hemisphere. VIMS and ISS will observe Titan's haze layers over a period of several observations, as Cassini will be approaching the satellite over Titan's nightside. This high-phase-angle geometry makes it difficult to observe the surface, but high-altitude, atmospheric structures become more accentuated.
Like during most of the ten encounters in the current series of Titan flybys, RADAR will be prime during closest approach on T61. During the approach to Titan, RADAR will run through some of its various observation modes, such as radiometry, which passively measures the 2.2-centimeter wavelength brightness of Titan's surface, and scatterometry, which measures the large-scale roughness of it. Following these observations, RADAR will acquire a HiSAR, low-resolution imaging swath in southeastern Shangri-la, just east of Shikoku Facula. This swath is designed to search for variations in the brightness of dunes in the region when comparing the swath with overlapping SAR swaths from T13 and T58. Such variations could be the result of transverse dunes similar to those that often run along the sides of and perpendicular to longitudinal dunes on Earth. At close approach, RADAR will acquire a SAR swath the runs roughly east to west across Adiri and Belet on Titan's trailing hemisphere. This will allow for stereo coverage over parts of Belet where the T8 and T61 swaths overlap. It also helps build up the map of longitudinal dunes within Belet.
Following RADAR's prime observation, VIMS will be prime during most of the outbound leg of the encounter, with the exception of a 30-minute ISS photometry observation. VIMS will map much of the visible surface of Titan, including the dark terrain region Senkyo and several bright, northwest-to-southeast trending streaks south of Senkyo. Based on earlier VIMS data of the region, these bright structures were initially interpreted as mountains. VIMS will be focusing on the equatorial region during their mosaic sequence, an area that was not well observed by the VIMS team earlier in the mission.
On August 27, Cassini reaches periapse, its closest point to Saturn on Rev117. At this point, Cassini will be 197,810 kilometers (122,913 miles) from Saturn's cloud tops, between the orbits of Enceladus and Tethys. During this close approach period, Cassini will observe Janus from a distance of 265,000 kilometers (165,000 miles) as the satellite moves through the shadow of Saturn's main ring system. Parts of Janus will be partially darkened as the shadow of the various parts of the ring, with different opacities, obscure sunlight on the satellite's surface. Next, ISS will turn its cameras to Titan's sub-Saturn hemisphere, 1.43 million kilometers (889,000 miles) away, searching for clouds. Of particular interest for this and similar observations over the next few years is to see how Titan's atmosphere responds to seasonal changes by observing the intensity and distribution of storm systems across Titan. Recent, ground-based observations of a storm system over southern Belet in April 2008 further suggest that the spring and fall seasons may bring more equatorial storms to Titan. ISS also will observe the small Trojan satellite, Telesto, from a distance of 35,000 kilometers (22,000 miles), shortly after a non-targeted encounter with the moon. Such images will help further refine our shape model of the moon, which, after several close encounters during the nominal mission, is already more developed than many of the other "rocks." Finally, ISS will take a look at the shadow of structures within the ring system.
A couple days later, on August 29, ISS again will observe Titan's sub-Saturn hemisphere from a distance of 2.61 million kilometers (1.62 million miles) looking for any changes in cloud structures that might be visible in the region when compared to the sequence taken two days earlier. Next, ISS will acquire an astrometric observation of several Saturn's small satellites, including Methone, Calypso, Pallene, Daphnis, and Prometheus. Finally, ISS will take another radial scan mosaic of the left ansa of the lit face of the main ring system. During this observation, it might be possible to see spokes on Saturn's B ring. These spokes will be the focus of nine time-lapse movies to be acquired between August 30 and September 8. Cassini ISS will close out August with an observation of Titan's Fensal-Aztlan region from a distance of 3.19 million kilometers (1.98 million miles), again monitoring possible clouds over this region. In addition, ISS will take another astrometric observation, this time of Pallene, Atlas, Calypso, Epimetheus, and Telesto.
During the last week of Rev117, in addition to the spoke formation movies, ISS will be focused on astrometric observations of Saturn's small satellites on September 3, 4, 6, and 7. The moons that will be imaged during these sequences include Bestla, Janus, Daphnis, Epimetheus, Pallene, Polydeuces, Methone, Telesto, Helene, Kari, Kiviuq, Pandora, Calypso, and Prometheus, as well as several giant propellers within Saturn's A ring. On September 3, ISS will take several narrow-angle and wide-angle observations of Saturn's atmosphere, monitoring cloud formations across Saturn's northern hemisphere. Finally, ISS will observe a crescent Titan from a distance of 1.93 million kilometers (1.2 million miles) in order to observe haze structures in Titan's upper atmosphere.
Cassini reaches apoapse on September 8, bringing Rev117 to an end and starting Rev118.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).