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Cassini continues its exploration of the Saturn system with the 20-day Rev 218, which begins on June 25 at its farthest distance from the planet. This is also called the orbit's apoapse. At this point, Cassini is 2.43 million kilometers (1.51 million miles) from Saturn's cloud tops. Rev 218 occurs during the second Equatorial phase of Cassini's extended-extended mission. During this 10-month phase, Cassini will orbit within the orbital plane of Saturn's rings, allowing for frequent encounters with Saturn's icy satellites. Twenty-seven ISS observations are planned for Rev 218 with the majority focused on Saturn's atmosphere and the Titan flyby on July 7.
For ISS's first observation of Rev 218, on June 29 and June 30, the camera system will observe the small, outer moon Kiviuq for more than 30 hours. Images from this long observation will be used to better measure Kiviuq's shape and determine its spin state. These are determined by measuring how the moon's apparent brightness varies with time. Similar observations by the Hubble Space Telescope were recently used to determine that Pluto's moon Nix had an oblong shape and rotates chaotically. Kiviuq will be 11.4 million kilometers (7.07 million miles) away from Cassini during this observation. Two similar sequences will be acquired on July 1 and July 2. On July 3, ISS will observe Saturn's largest moon, Titan, from a distance of 2.43 million kilometers (1.51 million miles). This observation will be used to monitor clouds across the parts of Titan's sub-Saturn hemisphere.
On July 5 at 11:14 UTC, Cassini will reach periapse for Rev 218 at an altitude of 187,830 kilometers (116,712 miles) from Saturn's cloud tops, near the orbit of Mimas. On July 3, ISS will ride along with the Ultraviolet Imaging Spectrometer (UVIS) to image aurorae across Saturn's south polar region. A similar aurora observation will be taken on July 13. On July 4, ISS will image Titan as part of a calibration observation of the polarizer filters. Pandora and Saturn's main ring system will enter the field of view over the course of the observation. On July 4 and July 5, during the periapse period, Cassini will focus on magnetometer and the Magnetic Imaging Instrument (MIMI) observations of Saturn's inner magnetic field.
Cassini encounters Titan on July 7 at 08:10 UTC for the 113th time. This is the fifth of seven Titan flybys planned for 2015, with the next encounter scheduled for September 28 during Rev 222. T112 has a fairly high close-approach altitude of 10,953 kilometers (6,806 miles). In addition to the science observation acquired during the encounter, the flyby will also increase the length of Cassini's orbit by three days. Inbound, ISS will observe Titan's sub-Saturn hemisphere. Outbound, ISS will be able to observe a crescent Titan over its trailing hemisphere. Observations for this encounter start with a pair of Composite Infrared Spectrometer (CIRS) observations of Titan, which ISS will ride along with. These observations are designed to measure a temperature profile of Titan's atmosphere as well as monitor its composition across different parts of Titan. Afterward, ISS will acquire a pair of mosaics of Titan's sub-Saturn hemisphere, including Xanadu. The first mosaic, acquired from a range of 174,440 to 99,880 kilometers (108,390 to 62,060 miles), will use 18 footprints cover most of the visible disk. The second mosaic, acquired from a range of 94,830 to 45,250 kilometers (58,920 to 28,740 miles), will use 12 footprints. It will cover portions of Fensal and Aztlan, as well as most of the bright region Quivira and the Sinlap impact crater. Afterward, CIRS will observe Titan's limb over Titan's north pole.
At closest approach, CIRS will control pointing, acquiring observations of Titan's atmosphere. These observations will involve limb sounding at both high northern and southern latitudes, allowing temperature and compositional differences between the winter and summer poles to be measured. The flyby will permit the highest northern latitude limb sounding of the Solstice Mission for CIRS. After closest approach, CIRS will acquire temperature map data of Titan's night side as well as compositional measurements along Titan's limb. ISS will ride along, observing a crescent Titan. Afterward, VIMS and CIRS will acquire several observations of Titan's nightside and illuminated crescent.
On July 8, ISS will acquire a pair of observations of Saturn's faint rings, including the D ring, E ring, and the Methone ring at high phase angles. Each of these rings are faint and dusty, making them easier to observe at high phase angles. Immediately after both ring observations, ISS will acquire quick observations of Saturn using the Wide-Angle Camera (WAC). These observations are part of a series of "Storm Watch" observation sequences designed to take advantage of short, two-minute segments when the spacecraft turns the optical remote sensing (ORS) instruments back to Saturn as a waypoint between other experiments' observations. These sequences include blue, clear, two methane band, and one full-frame, continuum band filter images. Four more storm watch observations will be taken between July 10 and June 13. Also on July 8, ISS will image Enceladus, monitoring plume activity at its south pole.
On July 10, the camera system will observe a crescent Titan from a distance of 1.72 million kilometers (1.07 million miles). On July 11, ISS will observe the distant, outer moon, Ymir, from a distance of 15.7 million kilometers (9.74 million miles). This observation will be used to better constrain this 18-kilometer or 11-mile-wide moon's pole direction and shape. Five-color photometry will also be obtained. A similar observation will be acquired of small and distant Bestla on July 16.
On July 16, Cassini will reach apoapse, bringing Rev 218 to a close and starting up the next orbit, Rev 219, which will include non-targeted flybys of Dione, Titan, and Enceladus.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).