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Cassini's journey at Saturn continues with Rev 46, its 47th orbit of the ringed planet, as Cassini observes Titan's trailing hemisphere, the small moon Atlas, Saturn's ring system, and Saturn itself. Cassini begins Rev46, on June 4 at its farthest distance from Saturn, called apoapsis. At this point, Cassini is 2.28 million km (1.4 million mi) from Saturn. The first week of Rev46 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 June 4 and 7, are part of a sequence of observations designed monitor changes in the F ring over the course of the Cassini mission.
A number of observations are dedicated to observing some of Saturn's small moons, in order to refine scientists' estimates of their orbital paths. These sequences include observations of some of Saturn's outer satellites, such as Paaliaq, Hati, and S/2004 S13 (the latter moon has yet to receive an official name from the IAU and still goes by its temporary designation). While these images will be taken too far away from the tiny moons to resolve surface detail, they are important for refining the satellites' orbits, and for obtaining what is known as "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 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 various times can provide information about the surface properties of an object. Several observations of Saturn's atmosphere are planned for the period from June 9 through June 12. Cassini reaches periapse, the closest point in its orbit around Saturn, on June 12, at a distance of 163,000 km (101,000 mi). The Saturn observations will focus on different aspects of the ringed planet, including searching for lightning, mapping and tracking cloud features, and observing Saturn near zero phase (when Cassini is almost directly between the Sun and Saturn). Cassini will also observe the rings at zero phase, which will provide information on the sizes of the particles in the rings and the sizes of the grains that make up those particles. Finally, Cassini will observe Mimas, the innermost of the mid-sized icy satellites of Saturn, again near zero phase.
Also on June 12, Cassini makes one of its closest passes of the small, inner satellite Atlas, at a distance of only 38,000 km (24,000 mi). With an average diameter of only 31 km (19 mi), Atlas is one of the smallest moons of Saturn. Even at this close distance, Atlas will only appear to be 120 pixels across (at the equator). However, these images may still provide important clues about the formation of one of the most distinguishing aspects of the tiny satellite: its equatorial bulge. The bulge is thought to have been created by material from Saturn's A ring being deposited preferentially along the equator of the satellite. Cassini encounters Titan for the 33rd time on June 13, with a close approach distance of only 975 km (605 mi) -- nearly the closest distance to which Cassini can safely approach Titan. Like the last few encounters with Titan, this flyby (known as T32) will allow for imaging of the northern portion of Titan's trailing hemisphere following closest approach. The cameras will observe the surface starting around five hours after closest approach, with the main ISS mosaic being obtained three hours later. With each encounter in this sequence (starting in February with T25), Cassini's imaging cameras 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 (1050-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, he Huygens probe landed just off the eastern tip of Adiri in January 2005. This observation will also allow imaging scientists to obtain additional views 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 Ion and Neutral Mass Spectrometer (INMS) team will be in the driver's seat during the 15 minutes before and after closest approach. This instrument will examine the chemical makeup of Titan's upper atmosphere. During previous encounters, the INMS team determined that large hydrocarbons, often with six or seven carbon atoms in each molecule, were found much higher in the atmosphere than expected. This pass will allow the INMS team to examine how the abundance of these large hydrocarbons varies with time and from one region on Titan to another. The other in situ instruments will also be controlling the spacecraft orientation, examining how Titan's atmosphere interacts with the magnetosphere of Saturn.
The last five days of Rev46 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 Mundilfari. Cassini will also acquire images of two of Saturn's faintest rings, the G and E rings.
Cassini begins the following orbit, number 48 (Rev47), on June 20, during which it will encounter Titan for the 34th time and perform a close flyby of Tethys. Distant observations are planned for Saturn, its ring system, and the geologically active moon Enceladus.