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Cassiniís orbits around Saturn come at a fast clip now, with the eight-day-long Rev69, the spacecraftís 70th orbit of the ringed planet. Despite the short orbit, Cassiniís observations include targeted flyby of Titan--the last of the primary mission--as well as sequences involving Saturnís atmosphere, Tethys, Enceladus, and Janus. Cassini begins Rev69 on May 21 at its farthest distance from Saturn, called apoapsis. At this point, Cassini is 1.34 million km (835,000 mi) from Saturn. The high inclination of this orbit allows for detailed study of Saturnís ring system and northern hemisphere from high above the ring plane. On May 22, Cassiniís ISS and Visual and Infrared Mapping Spectrometer (VIMS) will acquire numerous observations of Saturnís northern hemisphere, looking at cloud dynamics in this region. During this period, ISS will acquire four, 2x1 wide-angle-camera mosaics of the region. These mosaics can be used to measure wind speeds in the northern hemisphere, then compare these measurements to those previously acquired. It is possible that wind speeds might change as the seasons shift on Saturn. The following day, the Ultraviolet Imaging Spectrometer (UVIS) and ISS will acquire observations of Saturnís north polar region, looking for auroras and examining cloud structures in this region. ISS will use many of the filter combinations available on the wide-angle camera as part of a photometry and polarimetry study. On May 25, Cassini reaches periapse, its closest point to Saturn on Rev69. At that point, Cassini will be 193,000 km (120,000 mi) from Saturnís cloud tops. Near periapse, Cassini will acquire several non-targeted observations of Saturnís icy satellites, including Enceladus, Prometheus, Janus, and Tethys. Prior to periapse, Cassini will observe Tethys from a distance of 739,000 km (459,000 mi) above the moonís north polar region and northern sub-Saturn hemisphere. Three hours later, Cassini will observe the small, inner moon, Prometheus. While this observation will be from a distance of 545,000 km (339,000 mi), this should be among the best observations yet acquired of this small shepherd moon. As Cassini nears periapse, the spacecraft will pass into Saturnís shadow for several hours. This provides an opportunity to observe high haze layers in Saturnís atmosphere without worrying about exposing the camera to the sun.
Following periapse, Cassini will turn to look at Janus, another small, inner moon of Saturn and the co-orbital moon of Epimetheus. The three-hour long observation will start while the satellite is in eclipse, and most of the data will be acquired by the Composite Infrared Spectrometer (CIRS). Near the end of the observation, ISS will look at Janusí south polar region through several filters. Finally, ISS will observe the southern trailing hemisphere of Enceladus from a distance of 498,000 km (309,000 mi).
Cassiniís closest approach to Saturn on Rev69 coincides with the landing of the Phoenix spacecraft on Marsí northern lowlands. It is possible that many of the Cassini images scheduled to be acquired around May 25 could be lost as the antennas of the Deep Space Network provide focused support for the entry, descent, and landing phases of the Phoenix mission. Cassini encounters Titan for the 45th and final time of the primary mission on May 28 with a closest approach distance of only 1,400 km (870 mi). This flyby (known as T44) will allow for imaging of the anti-Saturnian hemisphere of Titan. Inbound to the encounter, when Cassini will observe Titan at moderate phase angles over southwestern Xanadu, the ISS, VIMS, UVIS, and CIRS teams trade off control spacecraft pointing until 30 minutes before the encounter, when RADAR will take over. ISS will acquire a high-resolution mosaic of the bright oddity named Hotei Arcus, a suspected cryovolcanic feature in southwestern Xanadu. Scientists will be interested to see how ISS observations compare with those acquired this year by RADAR. CIRS will acquire a mid-IR temperature map of the southern leading hemisphere of Titan.
The RADAR team will be in control of spacecraft pointing during Cassiniís closest approach to Titan. RADAR will acquire a full SAR swath that runs from southeast to northwest, starting in southern Xanadu, across southwest Xanadu, eastern Tui Regio, and Eir Macula, through north-central Shangri-la (including Nicobar Faculae and northern Oahu Facula), across southern Dilmun, and finally ending north of western Adiri. Among the highlights from this swath include Tui Regio, a possible cryovolcanic feature. Earlier radiometry and scatterometry data over southwestern Xanadu suggested a sharp difference in the surface properties in this area. The SAR swath will follow the boundary between higher and lower dielectric properties, measured by the RADAR polarimetry mode earlier in the mission. RADAR will also acquire an altimetry swath across parts of the T43 SAR swath, in southern Xanadu west of Hotei Arcus.
Following closest approach, ISS, CIRS, and VIMS will trade-off control of spacecraft pointing. ISS will acquire two mosaics of Titanís surface in the northern part of the trailing hemisphere. Both will be full-disk mosaics at differing resolutions. RADAR will acquire HiSAR, radiometry, and scatterometry observations during the 4.5 hours after closest approach; however, some of this data will not be received on Earth due to the Deep Space Network support for the Phoenix mission. CIRS will perform a mid-IR integration over the trailing hemisphere.
Cassini begins the following orbit, Rev70, on May 29. Rev70 includes distant observations of Enceladus, Saturnís atmosphere, and rings.