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Rev 91: October 28 - November 4, 2008
Cassini continues its series of week-long orbits with Rev91, the spacecraft's 92nd orbit around the Ringed Planet.
Cassini begins Rev91 on October 28 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. On October 28, Cassini will acquire two observations of Cassini's small, inner satellites. The first of these observations is designed to search for additional satellites between Mimas and Enceladus. Prior searches in this region by Cassini ISS revealed the existence of the small moons Anthe, Pallene, and Methone. The second observation is designed to further refine the orbital elements of some of Saturn's small, inner moons: Prometheus, Pan, Calypso, and Anthe. A third observation planned for the first day of Rev91 covers streamers in the F ring. Ring scientists hope to study changes in these dust streamers, generated by Prometheus as it pulls in dust from the narrow ring.
On October 29, Cassini will acquire a time-lapse movie of the F ring. The observation is designed to study the same part of this ring under different lighting conditions and possible short-term changes. On October 30, the Ultraviolet Imaging Spectrometer and ISS will observe Saturn's north polar region. UVIS will acquire several spectral scans of Saturn's aurorae, while ISS will search for lightning on Saturn's night side using the Wide-Angle Camera's H-alpha filter.
On October 31, Cassini will perform a targeted encounter of Enceladus, its 4th of the year and the 7th performed during Cassini's orbital mission at Saturn. During the encounter, Cassini reaches periapse, its closest point to Saturn on Rev91, at 235,000 km (146,000 mi) from the center of Saturn. Near periapse, Cassini will quickly pass high over the north polar region of Saturn before descending below the ring plane 35 minutes before periapse. Before the encounter begins, Cassini will turn its cameras for two distant observations of Enceladus. The first, designed by the ISS team, will looks at Enceladus' south polar plume in an attempt to measure the current flux of material from the plume. This will allow the team to compare these measurements to the direct measurements by Cassini fields-and-particles instruments during the flyby. Shortly afterward, UVIS will perform a similar measurement from high above Enceladus's north polar region.
After the encounter, on November 1, Cassini will turn its cameras back to Enceladus for one more UVIS observations of the south polar region. Next, Cassini will observe the southern anti-Saturn side of Rhea from a distance of 475,000 km (295,000 mi). Finally, Cassini will observe both ansae of the rings with the Wide-angle Camera, looking for spokes in the B ring.
Cassini encounters Titan on November 3 for the 47th time and the first in more than three months. The close approach distance is only 1100 km (683 mi). This flyby (known as T46) will allow for imaging of the leading hemisphere of Titan. Inbound to the encounter, when Cassini observes Titan at moderate phase angles over southwestern Xanadu, the CIRS and ISS teams trade off control spacecraft pointing (or are "prime") until one hour before the encounter when the Radio Science (RSS) Team will take over. During this approach period, ISS will acquire two mosaics of Titan. The first, GLOBMAP001, will cover the entire sunlit area shown at left and will be acquired between 105,000 and 187,000 km (65,000 and 116,000 mi) from Titan. The second, REGMAP001, is an 11-frame mosaic centered around the Hotei Arcus region of Titan, containing a feature thought to be cryovolcanic. One thing scientists will be looking for are changes at Hotei Arcus between this observation and a similar sequence acquired during an encounter in July. Changes could be due to fresh water-ammonia cryolava flows or cryoclastic deposits.
T46 is a gravity pass for Cassini. During closest approach, Cassini will point its antenna at Earth. The RSS team will use the Doppler effect on Cassini's signal to refine our knowledge of Titan's gravity field, measure Titan's Love number (a measure of the general distribution of mass within Titan and is affected by the presence of an internal ocean), and look for irregular mass distributions (like mass concentrations, or "mascons", below the surface). At closest approach, CAPS and RPWS will also focus on the magnetic environment around Titan. On the outbound leg of the encounter, CIRS, UVIS, and ISS trade off prime coverage.
Cassini begins Rev92 on November 4.
Image products created in Celestia. Enceladus basemap by Steve Albers. All dates in Coordinated Universal Time (UTC).