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Cassiniís journey at Saturn continues with Rev 49, its 50th orbit of the ringed planet. Cassini has a full plate during this more than one-month-long orbit, as the spacecraft observes Saturn, its ring system, and four of its satellites: Titan, Tethys, Rhea, and Iapetus. Cassini begins Rev49, on August 9 at its farthest distance from Saturn, called apoapsis. At this point, Cassini is 4.2 million km (2.6 million mi) from Saturn. This is the most distant from Saturn Cassini has been since late November 2004. The first week of Rev49 is filled with observations of Saturnís atmosphere. Each 10-hour-long observation is designed to examine cloud motions over a full Saturn day. The observations will allow for better understanding of circulation in the planetís atmosphere.
Following observations on August 16, Cassini will suspend science operations for solar conjunction. During this annual two-week period, Cassini and Saturn will be behind the Sun from the Earthís perspective. This orientation limits communications, as the Sunís upper atmosphere, or corona, increases noise in radio communications between the spacecraft and its operators on Earth. Communication is possible during some parts of solar conjunction, but bandwidth is limited to engineering data during much of this time period. Science operations resume shortly before Cassini reaches periapse, the closest point to Saturn in its orbit, on August 29. Cassiniís closest approach distance to Saturn during Rev49 will be 320,000 km (200,000 km). The first observation after solar conjunction will be a movie of the F ring as part of a ride-along observation with the Composite Infrared Spectrometer (CIRS). Also on August 29, Cassini encounters the icy moon Tethys at distance of 52,600 km (32,700 mi). This encounter will provide an opportunity to image the Odysseus impact basin at resolutions better than 500 meters (1,640 feet) per pixel. Researchers can use these observations to calculate the age of the impact from the number of smaller impact craters that lie within the basin. Additional images will be obtained of Tethysí leading hemisphere and the area around Odysseus, which is chock-full of secondary impacts (craters formed by falling material excavated by the Odysseus impact). The observation plan includes three mosaics, providing an opportunity for stereo coverage over Odysseus basin. Cassini performs its second targeted Rhea flyby of the nominal mission on August 30. The encounter occurs at distance of 5,109 km (3,175 mi). While inbound, Cassiniís cameras will observe Rheaís Saturn-facing hemisphere. Even though only a small portion of this side will be illuminated by the Sun, longer exposure times will be used to observe the night side, which will be illuminated by Saturn. At closest approach, Cassini will acquire a mosaic surrounding a fresh, 75-kilometer-wide crater on Rheaís leading hemisphere. This impact crater is distinctive in distant observations of Rhea thanks to its extensive ray pattern, which is similar to those seen on the Moon at young craters like Copernicus and Tycho. A high-resolution observation of this crater was planned for the previous targeted encounter in 2005, but the images just barely missed the crater, revealing its eastern rim. The new observations will be taken from slightly farther away, hopefully capturing this relatively young impact feature. Another high-resolution mosaic is planned to observe tectonic structures and a small ray crater to the west of the aforementioned crater. Following this observation, a full-disk, 24-image mosaic is planned for the anti-Saturn hemisphere of Rhea, including two large and ancient impact basins. For part of this observation, Saturn will provide a backdrop, as shown in the image at left. Cassini encounters Titan for the 36th time on August 31, with a closest approach distance of 3,302 km (2,051 mi). Unlike the previous Titan encounter, this flyby (known as T35) will allow for imaging of the anti-Saturn hemisphere, centered near the bright equatorial region named Adiri. ISS and the Visual and Infrared Mapping Spectrometer (VIMS) will be observing Titan from around the time of closest approach until nine hours later. Prior to the encounter, Cassini will be observing Titan at high phase angles using the CIRS instrument. These observations are intended to measure the composition of Titanís thick atmosphere at different altitudes. Shortly before closest approach, the Ultraviolet Imaging Spectrometer (UVIS) and VIMS will perform a stellar occultation, using Antares (Alpha Scorpii) to probe the structure of Titanís atmosphere.
In the two hours following closest approach, the VIMS instrument will observe Titan at high resolution. VIMSí highest resolution observation will occur over Titanís north polar region. It remains to be seen if VIMS will be able to observe the surface, as the last several observations of this area by that instrument revealed high-altitude, ethane-rich clouds that will likely obscure the surface at 2-5 micron wavelengths. VIMS will also acquire several high-resolution observations across Titanís anti-Saturn hemisphere--particularly over the Dilmun, Shangri-la, and Adiri regions, as well as over Nicobar Facula and Mindanao Facula in central Shangri-la. Following the VIMS observations, ISS will obtain two mosaics of Titan over its anti-Saturn hemisphere. A high-resolution mosaic (with pixel scales between 550-890 meters or 1,800-2,900 ft) will cover the dark terrain north of Adiri, including a 90-kilometer-wide candidate impact crater. Another global-scale mosaic is also planned. In the 10 days following the T35 flyby, Cassini will observe Saturnís atmosphere, its ring system, Titanís trailing hemisphere, and Iapetus. Like the observations at the beginning of this orbit, Saturnís atmosphere will be in full view, allowing for cloud tracking over a full Saturn day. Unlike during those earlier observations, Cassini will be orbiting out of the ring plane, allowing for observations of the ring system.
Fast approaching is Iapetus. Cassini will perform its only targeted encounter with the two-toned moon on September 10. Several observations are planned for Iapetus as Cassini approaches, during the first 10 days of September. Check back on September 6 for more details on this exciting encounter.