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Cassini continues its series of seven-day long orbits with Rev81, the spacecraft's 82nd orbit around the Ringed Planet. This orbit includes two non-targeted encounters with Titan and the small moon Pallene. Cassini will also observe Saturn's ring system and various satellites. Cassini begins Rev81 on August 15 at its farthest distance from Saturn, called apoapsis. At this point, Cassini is 1.22 million km (761,000 mi) from Saturn. The spacecraft begins this orbit in the middle of a non-targeted encounter with Saturn's largest moon, Titan. Six hours after apoapsis, Cassini makes its closest approach to Titan, at a distance of 302,305 km (187,843 mi). Early on August 16, Cassini will acquire its second of two observations for this encounter; the first was acquired near the end of the previous orbit, the day before. In this case, ISS will take a five-frame mosaic over the north polar region of Titan. This will complement a similar observation acquired in April when numerous clouds were seen over the polar region (see PIA10434).
On August 16, Cassini will take a look at the F ring as part of a monitoring campaign to look for changes in the various clumps, knots, and gores in the ring caused by gravitational interactions with Prometheus, Pandora, and large chunks within the ring. In addition, on August 16 and 17, Cassini will observe various small satellites in the inner part of the Saturn system in order to better constrain their orbital motions, which can be affected by nearby large satellites. On August 18, Cassini will turn its focus to the rings as it acquires two images of the D ring and a partial azimuthal scan along the Encke Gap in the outer A ring.
On August 19, Cassini reaches periapse, its closest point to Saturn during Rev81. At that point, Cassini will be 237,537 km (147,599 mi) from Saturn's cloud tops. Near periapse, Cassini will quickly pass high over the north polar region of Saturn before descending below the ring plane 38 minutes before closest approach. Shortly after periapse, Cassini will perform its closest encounter to date with 4.4-km (2.7-mi) wide Pallene, at a distance of 61,000 km (38,000 mi). Pallene is a small moon (discovered by Cassini in 2004) that orbits between Mimas and Enceladus. Despite the close encounter, Pallene will only appear 12 pixels across (at best) in ISS narrow-angle-camera images planned for the encounter. Following the Pallene observation, Cassini will turn its sights on Rhea shortly before the moon enters the shadow of Saturn. The ISS cameras will then acquire frames every 33 seconds as Saturn's shadow marches across the surface of Rhea. The simulated frame at right shows an expected view from one of these frames. CIRS will observe Rhea during the eclipse to watch how the surface responds to the sudden loss of sunlight. Such responses in temperature can provide a measure of small-scale surface textures (powder versus gravel, for example). On August 20 and 21, Cassini will look back toward the rings. Two ring observations are planned for these days: a movie sequence of the F ring, designed to look at a single point on the narrow ring over several hours; and a movie sequence of the B ring, designed to look at spokes on the ring's sunlit side. Then, on August 22, Cassini will observe the leading hemisphere of Dione from a distance of 1.2 million km (743,000 mi), as part of the ISS team's satellite photometry campaign. Photometry sequences examine the brightness of a target, be it a ring or a satellite, in order to learn about a host of properties, such as particle sizes and surface roughness.
A busy slate of observations is planned for ISS on August 22, the last day of Rev81. A ten-frame, wide-angle-camera color mosaic covering Saturn and the main ring system is planned for the first ISS observation of the day. Two additional ring observations are planned, this time of the faint outer rings. These include an observation of the arc in the G ring and a photometry sequence covering the E ring. Two satellite photometry observations are planned, first covering Tethys' sub-Saturn hemisphere, then covering Mimas' sub-Saturn hemisphere. Finally, Cassini will observe various small satellites in the inner part of the Saturn system in order to better constrain their orbital motions.
Cassini begins Rev82 on August 22.
Image products created in Celestia. Enceladus basemap by Steve Albers. Saturn basemap by Björn Jónsson. All dates in Coordinated Universal Time (UTC).