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Cassini continues its new tour of the Saturn system with the 20-day-long Rev137, the spacecraft's 138th orbit around the Ringed Planet and the third full orbit of the Cassini Solstice Mission. Cassini begins Rev138 on August 24 at its farthest distance from Saturn, called apoapse. At this point, Cassini is 2.58 million kilometers (1.6 million miles) from Saturn's cloud tops. Cassini is in a slightly inclined orbit, 4.6 degrees out of the ring plane. This is still close enough to provide Cassini an opportunity to examine some of Saturn's moons, though the camera and other remote sensing instrument teams will focus on Saturn during orbit.
Cassini's ISS camera system starts its observations for Rev137 two days after apoapse by obtaining a light-curve observation of the irregular satellite, Hyrrokkin. This 8-kilometer-diameter (5-mile-diameter), outer satellite will be 9.19 million kilometers (5.71 million miles) away. This observation is part of a campaign of observations of this and other small satellites that are taken to determine their rotational periods and to determine if they are binary objects. This first observation of Hyrrokkin on August 26 will use 258 images and last 15 hours. Similar observations will be taken of Greip and Kiviuq on August 27 and 30, respectively. A shorter, follow-up observation on September 12 will help measure the brightness of Hyrrokkin at a different phase angle than the earlier observation and to improve the quality of the light curve. This will provide information about the surface properties of the satellite, such as surface roughness, even when the spacecraft never comes very close to Hyrrokkin.
On August 28, Cassini will image Titan while the satellite appears half-illuminated. This will permit Cassini to search for clouds across Titan's trailing hemisphere. This observation will be acquired from a distance of 1.74 million kilometers (1.08 million miles). ISS will repeat this observation later in the orbit on September 8 when the spacecraft is 1.28 million kilometers (0.79 million miles) away from Titan and on September 10 when Cassini is 1.36 million kilometers (0.85 million miles) away from the haze-shrouded moon. Titan may be at too high a phase angle during the September 8 opportunity to allow cloud monitoring. The September 10 observation covers Titan's anti-Saturn hemisphere, and may be a better opportunity for cloud monitoring because it will be taken at lower phase angle. On August 29, Cassini will take an astrometric observation of several of Saturn's small, inner satellites in order to improve our knowledge of the motions of these moons. This observation will include images of Polydeuces, Calypso, Methone, Atlas, and Pallene. Two more of these astrometric observations will be acquired on September 7 and 10, covering Calypso, Helene, Pallene, Methone, Janus, Pandora, Polydeuces, and Prometheus. On August 31, Cassini will acquire long-range movies of the faint dust rings associated with Pallene, Methone, and Anthe, small satellites that orbit between Mimas and Enceladus. These rings are created when micrometeorites blast fine particles off the surface of these moons, whose gravity is not strong enough to keep the particles from going into orbit around Saturn. On September 2, the Visual and Infrared Mapping Spectrometer (VIMS) and ISS will take two long observations of Saturn's main ring system. The first is a movie of clumps and channels within the narrow F ring. The second will be taken while Cassini is in Saturn's shadow, permitting very high-phase-angle observations of the ring system, similar to the observation taken back in September 2006.
On September 3 at 01:39 UTC, Cassini will reach the periapse of Rev137, its closest point to Saturn in the orbit. At periapse, the spacecraft will be 148,590 kilometers (92,330 miles) above Saturn's cloud tops. A few hours after periapse, Cassini will perform a non-targeted encounter with the icy moon Dione at a distance of 39,310 kilometers (24,426 miles). ISS will acquire a 24-frame, 3-color (plus clear filter) mosaic of Dione's northern, anti-Saturn hemisphere, providing some of our best coverage to date of the north polar region. This complements the data taken during the previous orbit which focused on the other side of Dione. Immediately afterward, ISS will turn its attention to the left ansa of the B ring, the densest of the main rings of Saturn. Using the narrow-angle camera, ISS will image the outer edge of the ring, where the B ring ends and the Cassini Division begins. This region of the ring system will be covered during three, 35-frame, time-lapse movies that are interrupted by Composite Infrared Spectrometer (CIRS) scans of the ring system. On September 4, ISS will ride along with the VIMS and the Ultraviolet Imaging Spectrometer (UVIS) to observe Saturn's atmosphere. In the first observation, with UVIS, ISS will acquire photometric and polarimetric data of Saturn's clouds and hazes. This observation will be repeated on September 8 from a slightly greater distance as Cassini recedes from Saturn. In the second, with VIMS, ISS will take a two-by-two mosaic of Saturn's day side and several frames that cover its night side to monitor lightning. ISS and VIMS will take a similar set of data the next day. With Cassini near the ring plane on September 5, the NAC camera will image the small moon Telesto, 1.61 million kilometers (1.00 million miles) away from Cassini, as it passes in front of Rhea, another 240,200 kilometers (149,300 miles) distant.
Between September 7 and 10, ISS will acquire five Saturn cloud tracking observations. These observations are designed to search for and track cloud structures at different latitudes on Saturn's southern hemisphere: 10 degrees south (near the edge of the ring shadow) for the observations on September 7 and 8 and 30 degrees south for those on September 9 and 10. These will also be combined with images taken with the wide-angle camera to provide full-disk context for the narrow-angle cloud tracking images.
On September 13, Cassini will reach apoapse on this orbit, bringing it to a close and starting Rev138. Rev138 includes a targeted flyby of Titan.
Image products created in Celestia. Dione basemaps by Steve Albers. Saturn basemap by Bjorn Jonsson. All dates in Coordinated Universal Time (UTC).