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Cassini's journey at Saturn continues with Rev 47, its 48th orbit of the ringed planet, as Cassini observes Titan's trailing hemisphere; the icy satellites Tethys, Enceladus, and Mimas; Saturn's ring system; and Saturn itself. Cassini begins Rev47, on June 20 at its farthest distance from Saturn, called apoapsis. At this point, Cassini is 2.3 million km (1.4 million mi) from Saturn. The first week of Rev47 is filled with observations of Saturn's small satellites (to refine their orbits), the F-ring, and Saturn's atmosphere. The F-ring observations, planned for June 22 and 24, are part of a series designed to monitor changes in the F ring over the course of the Cassini mission.
Cassini's orbit currently lies within Saturn's ring plane, which is not ideal for observing the ring system. However, these orbits do allow for close encounters with Saturn's icy satellites (as in late 2005) and mutual event observations. Mutual events occur when two or more satellites appear to approach one another from Cassini's perspective. One such mutual event observation will occur on June 21 when Tethys nearly occults Enceladus. At this time, Enceladus and Tethys are actually separated by 260,000 km (162,000 mi).
A number of observations are dedicated to observing several of Saturn's small moons to refine our estimates of their orbital paths. These sequences include observations of some of Saturn's outer satellites, such as Tarvos and Kiviuq. While these images are taken too far away from the tiny moons to resolve surface detail, they are important for refining the orbits of these satellites and for obtaining phase coverage. From Earth, we see these satellites almost fully illuminated by the Sun. During these observations, Cassini will be able to observe these satellites at near half-phase (like the First or Last Quarter Moon from Earth). Viewing a planetary surface at different phase angles and measuring its brightness at these times can provide information on the surface properties of an object. Multiple filters will also be used to better determine the surface composition of these moons. Cassini reaches periapsis, the closest point in its orbit, on June 28 when Cassini is 145,000 km (90,000 mi) above Saturn's cloud-tops. During this orbit's periapsis passage, Cassini will encounter three of Saturn's icy satellites: Tethys, Enceladus, and Mimas.
On June 27, Cassini will approach Tethys at a distance of 16,500 km (10,250 mi). This will be Cassini's second-closest encounter with Tethys after the September 2005 encounter when Cassini came within 1,500 km (930 mi) of the icy satellite. Like the more distant encounter two orbits ago, this flyby will allow for observations of Tethys' Ithaca Chasma canyon system, a large fracture that cuts across the moon's Saturn-facing hemisphere. High-resolution observations are planned for the canyon system, the terrain to the west of Ithaca, and the relatively fresh impact crater Telemachus at the north end of the canyon. These observations will provide a useful comparison to similar resolution observations taken in September 2005 when the Sun was more or less directly behind the spacecraft. In this case, the Sun will be to the right, accentuating topography.
Also on June 27, Cassini will fly past Mimas at a distance of 98,000 km (61,000 mi). The imaging cameras will observe Mimas from a distance of 180,000 km (112,000 mi), examining the topography of the large impact crater Herschel. On June 28, Cassini encounters Enceladus at a distance of 89,150 km (55,400 mi). Cassini will image the side of Enceladus that faces away from Saturn about 11 hours after the encounter, from 275,000 km (171,000 mi) away. Saturn and the shadows of its rings will provide a backdrop for these observations. Cassini encounters Titan for the 34th time on June 29, with a close approach distance of 1,932 km (1,200 mi) -- making it one of Cassini's more distant encounters with Titan. Like the last few Titan flybys, this approach (known as T33) will allow for imaging of Titan's trailing hemisphere following closest approach. The cameras will observe the surface starting around one hour after closest approach, with observations continuing until four hours after closest approach. 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). ISS will cover a small region over Adiri, a 1,700-km (1,050-mi) wide bright region at the center of the image at left. These observations are intended to provide coverage over a portion of the Synthetic Aperture Radar swath obtained in October 2005. The RADAR team observed patches of sand dune-covered terrain, large plains regions, and linear mountain chains in this area. By observing these areas up-close, the imaging team hopes to better understand how these region types can be differentiated in ISS images and also how to determine their presence in areas where complimentary RADAR coverage does not exist. Particularly, the imaging team is interested in being able to differentiate between rough or mountainous terrain and bright plains. This mosaic will also cover a portion of the dark terrain north of Adiri. Several chains of bright spots have previously been observed in this area. This higher resolution coverage will hopefully reveal some of the smaller members of these chains.
The Radio Science Subsystem team will be in the driver's seat during the two hours before and one hour after closest approach. The team will use this encounter, along with two others in the primary mission, to provide a better estimate of Titan's mass and determine how that mass is distributed in the interior. Such estimates will provide a better understanding of the moon's interior structure and determine whether Titan possesses an internal water ocean, like Ganymede, Callisto, and Europa -- three Galilean satellites of Jupiter.
The last 5 days of Rev47 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, including the small outer satellite Skathi. Cassini will also acquire images of two of Saturn's faintest rings: the G and E rings.
Cassini begins the following orbit, number 49 (Rev48), on July 9, during which it will encounter Titan for the 35th time. Now that Cassini is once again orbiting in Saturn's ring plane, numerous observations of the satellites are planned. Cassini will encounter Helene, one of two small moons sharing Dione's orbit around Saturn, for its closest flyby of the primary mission (though an even more exciting one is planned for the possible extended mission). Cassini will also observe Enceladus, Mimas, and Hyperion. Stay tuned!
Image products created in Celestia. Tethys and Enceladus maps by Steve Albers.