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Cassini's journey at Saturn continues with its 40th orbit of the ringed planet, revealing new territory on Titan and taking spectacular mosaics of Saturnís ring system.
Cassini began its 40th orbit, Rev39, on February 10 at its farthest distance from Saturn, called apoapse. (Remember that Rev01 and Rev02 where replaced by three orbits, RevA through RevC, earlier in the mission. Hence, the numbering scheme for orbits is offset.)
For the first week of this revolution, Cassini will continue with its campaign of observing Saturnís atmosphere, a campaign that started shortly before periapse (Cassini's closest point to Saturn) on the previous orbit. Cassini will also observe Iapetus' Saturn-facing hemisphere from a Cassini-Iapetus distance of a little more than 2 million kilometers. Like Earth's moon, Iapetus is tidally locked to its parent planet, with one side always facing Saturn and the other side always facing away. And like Earth's moon, Iapetus experiences lunar eclipses, in this case, Saturn getting between Iapetus and the Sun. Cassini observes one of these Iapetian lunar eclipses on February 13 and 14. Later in the week, on February 15 and 16, Cassini turns its instruments on Hyperion, coming within 200,000 km of the oddly textured moon. Because of Hyperion's semi-chaotic rotation, it can not be predicted with any certainty what region of the satelliteís surface will be observed during this flyby.
During Rev39's second week, Cassini reaches its closest point to Saturn (585,000 km) on February 19 and obtains several mosaics of Saturn's ring system. These narrow- and wide-angle camera mosaics are designed to take advantage of Cassini's position high above the sunlit side of the rings, near periapse, to study variations in brightness across the rings, both radially out from Saturn, and azimuthally around the rings. The mosaics produced from these observations should be breathtaking when pieced together.
Cassini encounters Titan on February 22, in a flyby called T25, from a distance of 1,000 km (620 miles). Prior to closest approach on T25, Cassini will observe the south polar region of Titan. Cassini images taken in June 2005 revealed a number of dark spots near the South Pole, including Ontario Lacus, a 220-km wide dark region with rounded margins that the Imaging Team noted at that time as the best example of a lake on Titan. These dark features are thought to be equivalent to the lakes found by the RADAR instrument in the North Polar region. At closest approach, the RADAR instrument will obtain a Synthetic Aperture RADAR (SAR) swath across the sub-Saturn hemisphere of Titan. The swath will cover a wide range of latitudes, from 30 degrees South to 85 degrees North. Expected coverage includes portions of the dark regions Fensal and Aztlan (areas expected to filled with longitudinal dunes), the bright regions western Quivira (an area ISS observed to be filled with apparent channels) and north-central Tsegihi (along with Shiwanni Virgae, dark lineaments thought to be sediment-filled tectonic fractures), and the North Polar lake region.
After closest approach, the Cassini instruments will observe the northern portion of the trailing hemisphere (the hemisphere of Titan that faces away from the direction of motion). This region has not been observed before by the Cassini cameras at higher image scales than 35 kilometers per pixel, though a small portion was observed by the RADAR instrument during T21 (December 12, 2006). From the scant data that are available, this area appears similar to other northern, mid-latitude "bright" regions seen elsewhere on Titan. These areas generally have lower albedo contrasts than the equatorial region, with some dark regions with southwest-northeast trends. Thus, in addition to filling in a major gap in our global map of Titan, observations during T25 will help us better understand the geology of these northern mid-latitude regions. Cassini will also observe the equatorial dark region named Belet. This dark region was shown by RADAR during T8 (October 28, 2005) and T21 to be filled with dark, longitudinal sand dunes. Cassini camera observations of the bright features within Belet, shaped by the dunes diverting around them, will allow scientists to better understand the orientation of these long dunes through the rest of Belet.
Cassini wraps up Rev39 on February 27. Following the Titan encounter, Cassini speeds away from Saturn observing the ring system, particularly the F ring, Saturn's smaller satellites, and Titan. On February 25, Cassini observes the northern Saturn-facing hemisphere of Titan, once again looking at a region only obliquely observed before.
Cassini begins orbit 41, Rev40, on February 27, during which Cassini will obtain a rotation movie of Saturn and encounter Titan for T26.