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Cassini continues its series of seven-day-long orbits with Rev80, the spacecraft's 81st orbit around the Ringed Planet. This orbit includes the second targeted flyby of Cassini's extended mission: an exciting, ultra-close encounter with the icy moon Enceladus. (See the special Enceladus Rev80 Looking Ahead feature for more details about the encounter and the exciting observations to be acquired during the flyby.) Cassini will also observe Saturn's ring system and various satellites.
Cassini begins Rev80 on August 8 at its farthest distance from Saturn, called apoapsis. At this point, the spacecraft is 1.22 million km (761,000 mi) from Saturn. Cassini's first observations of the orbit involve Saturn's small inner satellites and rings. On August 8, Cassini will observe various small moons in the inner part of the Saturn system in order to better constrain their orbital motions, which can be affected by nearby larger satellites. On August 9, Cassini will take a look at the F ring in order to monitor channels and streamers generated in the ring by the close passage of Prometheus, one of the F ring's two shepherd moons. On August 11, Cassini will perform the much-anticipated flyby of Enceladus, passing within 52 km (32 mi) of the surface. This is the first of seven targeted encounters that are slated for the next two years. The flyby will provide an opportunity for the Cosmic Dust Analyzer (CDA) to measure small particles in Enceladus' south polar plume that might have been missed during a similar encounter back in March. ISS will attempt to acquire several high-resolution images of the plume hot spot/vent locations within the south polar region. Prior to the encounter (17 hours before closest approach), Cassini makes a distant observation with its wide-angle camera in an attempt to observe the outer portion of the Enceladus plume, where it starts to feed the E ring with fresh icy particles. The "tendrils" here were previously observed at a very high phase angle, when Cassini was in the shadow of Saturn, so the cameras may not be able to observe the tendrils here.
Shortly after the Enceladus encounter, on August 11, Cassini reaches periapse, its closest point to Saturn on Rev80. At that point, Cassini will be 237,537 km (147,599 mi) from the planet's cloud tops. Near periapse, Cassini will quickly pass high over the north polar region of Saturn before descending below the ring plane 40 minutes before closest approach. Most of the observations at periapse are centered on Enceladus. The first non-Enceladus ISS observation after periapse takes place on August 14 when ISS acquires a movie sequence of the F ring, looking for subtle changes in the various clumps and channels previously seen in the narrow ring. Two icy satellite observations are planned for August 14 and 15. The first covers Rhea's trailing hemisphere from a distance of 1.64 million km (1.02 million mi). The second covers Tethys's leading hemisphere from a distance of 1.12 million km (698,000 mi). Three observations are also planned that will focus on Saturn's numerous small, inner satellites as part of the ISS team's orbit determination campaign. Finally, on August 15, ISS and the Visual and Infrared Mapping Spectrometer (VIMS) will observe Saturn's E and G rings.
The final observation of the orbit is a mosaic of the northern leading hemisphere of Titan, to be acquired during a non-targeted encounter that continues into Rev81. These observations will provide the ISS cameras' best views to date of the northwestern part of Xanadu and of Menrva, a large impact basin. Combined with observations acquired during the T45 flyby in July of this year, these observations will help fill a significant gap in the ISS map of Titan near 100 degrees west longitude. The current best data there was acquired very early in Cassini's tour of Saturn.
Cassini begins Rev81 on August 15. During the upcoming orbit, the spacecraft will perform non-targeted encounters with Titan and the small moon Pallene.
Image products created in Celestia. Enceladus basemap by Steve Albers.