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Cassini continues its series of seven-day-long orbits with Rev84, the spacecraft's 85th orbit around the Ringed Planet. Cassini begins Rev84 on September 6 at its farthest distance from Saturn, called apoapsis. At this point, the spacecraft is 1.22 million km (761,000 mi) from Saturn. Cassini starts the orbit in its annual solar conjunction period, when science observations are limited to the fields-and-particles instruments and communications between Earth and the spacecraft are also limited. This year, solar conjunction lasts between September 1 and September 7. The first remote sensing observation following solar conjunction covers some of Saturn's small inner satellites. This sequence, planned for September 7, is intended to help constrain the orbital motions of these moons. A similar observation is planned for September 8. On that date Cassini will also 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.
On September 10, Cassini will observe Tethys as it passes into the shadow of Saturn. The Composite Infrared Spectrometer (CIRS) will measure Tethys' surface temperature as it responds to the sudden darkness. ISS will observe Tethys' surface under reflected light from other moons and reflected ringshine from Saturn. On September 10, Cassini reaches periapse, its closest point to Saturn on Rev84. 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. A few hours after periapse, Cassini will turn its sights on the south polar region, focusing on cloud dynamics and auroras.
On September 12, Cassini will observe Enceladus at a low phase angle, which is good for looking at color variations across its surface. On September 13, as Cassini approaches apoapsis, ISS will perform several observations. The first involves several of Saturn's small satellites as part of Cassini's orbit determination campaign. The second is a medium-resolution observation of Rhea's anti-Saturn hemisphere, acquired from a distance of 630,000 km (390,000 mi). Cassini will then observe an arc of material in the G ring that was recently observed to have a denser clump of material within it. Next, Cassini will observe a transit of Enceladus by Dione. Finally, Cassini will once again search for moonlets orbiting in the space between Mimas and Enceladus, a fruitful region for Cassini after the discoveries of Anthe, Methone, and Pallene.
Cassini begins Rev84 on September 14.
Image products created in Celestia. Rhea and Tethys basemaps by Steve Albers. Saturn basemap by Björn Jónsson. All dates in Coordinated Universal Time (UTC).