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Cassini continues its series of seven-day-long orbits with Rev85, the spacecraft’s 86th orbit around the Ringed Planet. Cassini begins Rev85 on September 14 at its farthest distance from Saturn, called apoapsis. At this point, the spacecraft is 1.22 million km (761,000 mi) from Saturn. Like most of the orbits during 2008, Cassini revolves about Saturn at a high inclination, providing opportunities to view the ring system from high above the ring plane. During this orbit, Cassini will take the opportunity to view the rings in several observations. The first of these occurs on September 15, when Cassini will acquire a time-lapse movie of the F-ring over an entire F-ring “day.” Sequences such as these might reveal rapid changes within the ring. A second observation on September 15 involves several of Saturn’s small satellites as part of Cassini’s orbit determination campaign. On September 17, Cassini reaches periapse, its closest point to Saturn on Rev85. At that point, Cassini will be 237,537 km (147,599 mi) from Saturn’s cloud tops. Near periapse, the spacecraft will quickly pass high over the north polar region of Saturn before descending below the ring plane 38 minutes before closest approach. Two large azimuthal ring scans are planned for September 17. The first, acquired before periapse when Cassini is still above the ring plane, will cover most of the 1.470 Rs ringlet -- a narrow, eccentric ringlet in the outer C-ring. The second, acquired after periapse when Cassini is below the ring plane on the sun-lit side, will cover the outer A ring. Cassini imaging scientists are particularly interested in looking for “propellers,” voids created in the ring system by small moonlets embedded in the rings (see the "missing link" moonlets press release).
In addition to the two ring observations, two distant observations of Enceladus are also planned for September 17. The first, before periapse, is intended to provide additional phase angle coverage of the south polar plumes. It is not certain whether the plumes will be visible at the 140 degree phase angle of the observation (at high phase angles, Enceladus would appear as a narrow crescent, like our moon shortly before and shortly after New Moon). Shortly after periapse, Cassini ISS and the Composite Infrared Spectrometer (CIRS) will observe Enceladus as it enters the shadow of Saturn, similar to observations acquired of Rhea, Tethys, and Mimas in recent orbits (see PIA10469).
On September 19, Cassini will observe the sunlit side of the B ring, looking for forming spokes.
On September 20, Cassini will observe Rhea’s sub-Saturn hemisphere at a low phase angle, which is good for looking at color variations across a moon’s surface. Cassini will also observe Mimas’ anti-Saturn hemisphere, just as the moon starts a transit across Saturn. On September 20 and 21, as Cassini approaches apoapsis, ISS will perform two observations of Titan’s trailing hemisphere. These observations will be acquired from a distance of 1.55-1.81 million km (0.97-1.12 million mi). The observations are designed to look for cloud motions over an 18-hour period, as well as to look for changes at Kraken Mare (one of the large lakes in the northern polar region).
Rev85 finishes up with two small satellite orbit determination observations on September 20 and 21. On September 21, Cassini ISS and the Visual and Infrared Spectrometer (VIMS) will observe Saturn’s E ring as part of a phase function campaign.
Cassini begins Rev86 on September 21.
Image products created in Celestia. Enceladus and Mimas basemaps by Steve Albers. Saturn basemap by Björn Jónsson. All dates in Coordinated Universal Time (UTC).