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Cassini continues its series of week-long orbits with Rev90, the spacecraftís 91st orbit around the Ringed Planet.
Cassini begins Rev90 on October 20 at its farthest distance from Saturn, called apoapsis. At this point, Cassini is 1.21 million km (756,000 mi) from Saturn. The spacecraft is in a high-inclination orbit here, as it is for most of 2008, providing opportunities to view the ring system from high above the ring plane. Such an orbit also provides an opportunity to study the polar regions of Saturn and its satellites. Cassini ISSís first few observations of Rev90 cover various parts of Saturnís ring system. On October 21, Cassini will acquire a medium-resolution, radial-scan strip that covers the dark side of the rings to the left of Saturn. Cassini will also acquire a series of images of the F ring designed to help monitor the various channels and clumps within that narrow ring.
On October 23, Cassini will acquire two rings observations. The first is a short, 4-hour movie of the outer A ring, including the Encke and Keeler Gaps. The second is a high-resolution, azimuthal scan of the unlit side of the F ring. On October 21 and 22, Cassini will acquire several small satellite observations, designed to study the orbital motions of Pan, Atlas, Pandora, Janus, Prometheus, and Epimetheus.
On October 24, Cassini reaches periapse, its closest distance to Saturn on Rev90. At that point, Cassini will be 240,000 km (150,000 mi) from Saturnís 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 periapse. Shortly after periapse, Cassini will perform its closest encounter with Mimas to date, passing only 57,000 km (36,000 mi) above the icy satellite. Unfortunately, no images of Mimas will be acquired near closest approach as only a thin crescent is visible from Cassini at this time. Mimas will be observed several hours earlier, when the satellite is 200,000 km (125,000 mi) away. ISS and the Ultraviolet Imaging Spectrometer (UVIS) will acquire a scan across the illuminated crescent of the satellite.
While Cassini is flying by Mimas, Cassiniís cameras will be pointed at the ring system, acquiring a high-resolution azimuthal scan of the Maxwell Ringlet, a narrow ring within the broader C ring. The next day, ISS will acquire a time-lapse movie of the Maxwell Gap where the ringlet resides. These observations are designed to help understand the origin of the ringletís wave-like structure.
On October 26, Cassini ISS will observe Rheaís leading hemisphere from a distance of 1.34 million km (830,000 mi). The Composite Infrared Spectrometer (CIRS) will perform a scan of the ring system, measuring the temperature of its sunlit side. Cassini will acquire orbit-determination observations of Saturnís small inner satellites, Janus, Helene, Anthe, Pan, Telesto, and Pandora.
On October 27, Cassini will image Titanís sub-Saturn hemisphere, Tethysís leading hemisphere, and the E and G rings.
Cassini begins Rev91 on October 28. Rev91 will be a particularly busy orbit, with not one, but two targeted encounters, this time with Enceladus and Titan. This is Cassiniís first multi-flyby orbit since October 2005 when Cassini encountered Dione and Telesto.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).