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Cassini begins the 20-day Rev145 on February 10 at its farthest distance from Saturn, called apoapse. At this point, Cassini is 2.61 million kilometers (1.62 million miles) from Saturn's cloud tops. The spacecraft is in the middle of the first equatorial phase of the Cassini Solstice Mission, a phase which lasts until May 2012. During this phase, the spacecraft's orbits lie within the equatorial plane of the planet, within its ring plane, and in the orbital plane of most of Saturn's major moons. For this orbit, the equatorial orbit allows Cassini to pass over Titan's equator and image Saturn's atmosphere unobstructed by its rings. In fact, all the ISS observations for Rev145 will be targeted at Titan and Saturn, with the exception of a distant Siarnaq observation on February 22 designed to study how the small moon's brightness changes with phase angle.
Inbound on Rev145, Cassini encounters Titan on February 18 at 19:04 UTC for the 75th time. This is the first of six Titan flybys planned for 2011 with the next encounter scheduled for April 19. This encounter, named T74, is a relatively high altitude flyby with a close-approach distance of 3,650 kilometers (2,268 miles). This flyby will allow for imaging of the sub-Saturn hemisphere of Titan outbound to the encounter. For most of this flyby, the Cassini Plasma Spectrometer (CAPS) will be prime with Cassini's High Gain Antenna pointed at Earth to allow the Radio Science Subsystem (RSS) to acquire gravity data during this pass. CAPS will be measuring the interaction between Titan and Saturn's magnetosphere near the dusk terminator of Titan. RSS will be using this observation to improve its mass determination for Titan as well as too look for higher-order gravity coefficients that can be used to identify mass concentrations (or mascons) in Titan's lithosphere.
ISS has no prime observations during this flyby, but will be used during observations by the Composite Infrared Spectrometer (CIRS) that bookend the CAPS/RSS period. Inbound, this ride-along observation will allow ISS to monitor high-altitude hazes in Titan's atmosphere, while outbound the camera systems will monitor clouds in the region. Scientists are looking for changes in the distribution of clouds on Titan as the moon's weather system moves further into northern spring. For comparison's sake, each Saturn calendar "day" lasts 29 Earth days, and February 18, 2011, on Earth will be the equivalent of April 9 in Saturn's year. Coincidentally, this day will be one Saturnian calendar "day" after the first Saturnian anniversary of the Voyager 2 encounter 30 Earth years earlier. This will also be Cassini's first opportunity to observe up-close the region that was underneath the massive, "arrow" storm that was seen in September 2010. While this area has been seen at low resolution on at least two occasions since the storm, higher resolution observations are more useful for mapping the distribution of bright, intermediate, and dark albedo terrains in the south Senkyo region.
On February 20 at 13:44 UTC, Cassini will reach the periapse of Rev145. At a distance of 223,930 kilometers (139,140 miles), this is closest point to Saturn in this orbit. The day after the February 22 Siarnaq observation, Cassini ISS will begin a series of cloud tracking observations on Saturn. Each observation will last five hours. There are eight observations planned: one on February 23rd, one on the 24th, two on the 25th, two on the 26th, one on March 5th, and one on March 6th. Each sequence will start out with a set of 13 wide-angle-camera frames using various color filters including RED, GRN, and BL1 for true color composites, followed by sequences of narrow-angle-camera frames in BL1, CB2, MT2, and MT3 taken every 10 minutes in order to track cloud speeds. While the WACs will certainly show some portions of the large, northern storm or its long tail, the NACs are more focused. For example, those taken on February 23 and 24 will be focused on clouds between 20 and 30 degrees north (for reference, the large storm is centered between 34 and 44 degrees north). While they may miss much of the main storm system, they should capture portions of its tail. Those taken on February 25 will focus on cloud structures in the equatorial zone between the equator and 10 degrees north, allowing ISS to also image a passing moon or two, such as Rhea. It is the images that will be taken during the two, five-hour observations on February 26 that could be the most enlightening, at least with respect to the northern storm, as those images will look at cloud structures on Saturn between 30 and 50 degrees north, where the storm is centered. The two March observations will focus on clouds between 20 and 30 degrees south.
Interspersed with these Saturn cloud tracking observations, Cassini ISS will also be looking for clouds on Titan. Titan observations are planned for February 25, February 27, March 3, and March 5. The first observation will focus on the Fensal-Aztlan region of Titan and will be taken from a distance of 3.25 million kilometers (2.02 million miles), followed two days later by an observation of the eastern Xanadu region (including Menrva and Hotei Regio) from a distance of 3.14 million kilometers (1.95 million miles). The next pair of Titan observations will focus on the moon's anti-Saturn side, with the the March 3 observation targeting the Shangri-la dark region of Titan from distance of 2.00 million kilometers (1.24 million miles). The observation on March 5, taken from a distance of 2.2 million kilometers (1.37 million miles), will be the most interesting of these four sequences, with a view centered near Adiri, eastern Belet, and "Okavango."
On March 6, Cassini will reach apoapse on this orbit, bringing it to a close and starting Rev146. The Saturn and Titan show continues since all but two of the planned 21 observations are dedicated to the two largest bodies in the Saturnian system.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).