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Cassini's journey at Saturn continues with Rev 45, its 46th orbit of the ringed planet, as the intrepid spacecraft observes Titan's trailing hemisphere, Saturn's ring system, and several of the planet's icy satellites. Cassini begins Rev45 on May 19 at its farthest distance from Saturn, called apoapsis. At this point, Cassini is 2.25 million km (1.4 million mi) from Saturn. The first week of Rev45 is filled with observations of Saturn's small satellites (to refine their orbits), the F ring and the rest of the ring system, and Saturn's atmosphere. The F ring observations, planned for May 19 and 21, are part of a sequence of observations designed to monitor changes in the F ring over the course of the Cassini mission.
A number of observations are dedicated to observing several of Saturn's small moons to refine our estimates of their orbital paths. One 9.5 hour-long observation follows Pan for one complete orbit of Saturn. This will allow scientists to determine Pan's then-current orbit, rather than use images taken on multiple orbits to determine an average orbit for the ring-embedded moon. Several observations of Saturn's atmosphere are planned for May 23-25. These are designed to monitor Saturn's cloud features, search for lightning, and examine Saturn's Aurora Borealis. Cassini reaches periapse, the closest point in its orbit around Saturn, on May 27 at a distance of 193,000 km (120,000 mi). During Cassini's periapse passage, the cameras will be turned to many of Saturn's icy satellites. First up is Tethys, which Cassini encounters on May 26 with a closest approach distance of 102,000 km (63,000 mi). Image sequences acquired during this encounter will provide observations of Tethys' sub-Saturnian hemisphere, or the side of Tethys that always faces toward Saturn. One of the most prominent features to be imaged will be Ithaca Chasma, a large canyon that cuts north to south across this face of Tethys. Stereo observations and shadow measurements will aid in measuring the depth of the canyon. Several observations will use many of the filters available on the imaging cameras. These images, along with spectral measurements taken by the Visual and Infrared Mapping Spectrometer (VIMS), will help identify the non-ice constituents on Tethys' surface, particularly in the streak of dark material running west from Ithaca Chasma.
On May 26, Cassini will turn its sights on the innermost of Saturn's mid-sized icy satellites, Mimas. Cassini passes within 127,000 km (79,000 mi) of the heavily-cratered moon on this date. However, when Cassini makes its closest approach, Mimas will be within the shadow of its parent planet. The eclipse will provide an opportunity for the Composite Infrared Spectrometer (CIRS) to measure Mimas' thermal inertia -- a surface property related to how "fluffy" or hard a surface is (imagine the difference between walking barefoot on sand and on bare rock) -- by measuring how quickly the surface of Mimas cools during the eclipse. Following the eclipse, Cassini will capture observations of Mimas' surface, including the prominent 130-km (80-mi) wide impact basin named Herschel.
On May 27, Cassini's cameras will be trained on Enceladus and Dione. The Enceladus observations will be taken from a distance of 620,000 km (385,000 mi) when Cassini is almost directly between the Sun and Enceladus. While this geometry won't provide detailed views of Enceladus' strange surface features, it will allow for observations of Enceladus' opposition surge. The opposition surge is a spike in the brightness of a planetary body that is seen at very low phase angles; it is caused by a lack of topographic shading and the backscattering of photons due to their interaction with the surface material. Scientists can use measurements of Enceladus' brightness in this opposition surge observation to make a number of conclusions about the materials that coat the surface. Later that day, Cassini's cameras will acquire color observations of Dione's sub-Saturnian hemisphere from a distance of 850,000 km (530,000 mi). At the same time, the Ultraviolet Imaging Spectrometer (UVIS) will observe a stellar occultation. Such measurements can be used to determine if Dione has a very thin atmosphere and if it is currently outgassing, as has been suggested by some of Cassini's other instruments. Rounding out Cassini's tour of the mid-sized icy satellites on this orbit will be Rhea. Late in the day on May 27, Cassini will pass within 177,000 km (110,000 mi) of the large, cratered moon. Several observation sequences are planned, focusing on the southern part of the leading hemisphere (the side of Rhea that always faces toward the direction of motion along its orbit). Two large impact basins on Rhea's anti-Saturnian hemisphere will be visible along the limb. These observations provide an opportunity for stereo imaging, which is useful in creating topographic maps of the surface. Cassini encounters Titan for the 32nd time on May 28, with a close approach distance of 2,300 km (1,430 mi), making it a relatively distant encounter compared to some of the recent, sub-1,000 km approaches. Like the last few encounters with Titan, this flyby (known as T31) will allow for imaging of the northern portion of Titan's trailing hemisphere following closest approach. The cameras will observe the surface, starting around 5 hours after closest approach, with the main ISS mosaic. With each encounter in this sequence (starting in February with T25), we observe terrain farther and farther south within the trailing hemisphere (the side of Titan that always faces away from its direction of motion along its orbit). At the center of the planned mosaic is the 1,700-km (1,050-mi) wide bright region known as Adiri. This region contains several dark patches known to consist of dune fields 100-200 km (60-120 mi) across. In addition, the Huygens probe landed just off the eastern tip of Adiri in January 2005. This observation will also allow us to obtain additional images of several intriguing features observed during the last few flybys, such as a 90-km (55-mi) wide dark spot, thought to be an impact crater, to the north of Adiri.
The Cassini radio science (RSS) team will be in the driver's seat during the hour before and after closest approach. On approach, the RSS team will use Cassini's high-gain antenna to perform a radio occultation by transmitting a signal that passes through the various layers of Titan's atmosphere on its way to Deep Space Network radio telescopes on Earth. Such measurements can be used to understand how Titan's atmospheric density varies with altitude and how that density profile changes over time (by comparing this encounter's results with previous radio occultations). Following closest approach, the Cassini bistatic experiment will use the high-gain antenna to search for specular reflections on Titan's surface and to study the electrical properties of the surface. During this pass, this method will be used to examine the surface properties of features near a bright-dark boundary on Titan's trailing hemisphere. (The region the bistatic experiment will study is near the center of the Titan image above.) Such a swath will provide information about both bright and dark terrain, and about several bright spots visible north of Belet, a large expanse of dark terrain to the west and north of Adiri. The last 5 days of Rev45 continue the ring and small satellite observations that characterized the first week of this orbit. Several observations are planned for imaging Saturn's small satellites and refining the orbits of these little worlds. Like some of the observations acquired during the previous orbit, these include several of Saturn's tiny outer satellites - this time Siarnaq and Fornjot. Cassini will also acquire color images of the main ring system, the G ring, and the E ring.
On May 31, Cassini will observe Titan from a distance of 1.29 million kilometers (800,000 miles), allowing for images of Titan's trailing hemisphere at image scales better than 7.6 kilometers (4.7 miles) per pixel. This observation was designed to study the light scattering behavior of Titan's atmosphere and surface, and also to provide some of the best images acquired so far of the western part of Belet.
Cassini begins the following orbit, number 47 (Rev46), on June 4, during which it will closely approach Titan for the 33rd time. In addition, high-resolution scans of Saturn's rings and distant observations of the moons Paaliaq, Hati, and S/2004 S13 are also planned.
Image products created in Celestia. Rhea and Tethys basemaps by Steve Albers.