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Cassini continues its exploration of the Saturn system with the 20.5-day Rev 215, which begins on April 25 at its farthest distance from the planet. This is also called the orbit's apoapse. At this point, Cassini is 3.16 million kilometers (1.97 million miles) from Saturn's cloud tops. Rev 215 occurs shortly after the start of 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. Thirty-seven ISS observations are planned for Rev 215 with the majority focused on Saturn's atmosphere and icy satellites. ISS will also acquire several observations during the T111 flyby of Titan.
For ISS's first observation of Rev 215, starting just two hours after apoapsis, the camera system will observe the diffuse E-ring at a high phase angle. The E-ring is composed of fine, icy dust particles ejected from Enceladus' active, south polar region. The high-phase angle, meaning that the ring is generally back-lit, makes the dusty ring appear more visible than it normally would be when Cassini is between the ring and the Sun. Immediately afterward, on April 26, ISS will acquire a quick observation of Saturn using the Wide-Angle Camera (WAC). This observation is 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. Seven more storm watch observations will be taken between April 27 and May 6.
On May 2, ISS will perform a calibration observation using Titan to help improve calibration of images that use the polarization filters. The next day, ISS will observe Titan from a distance of 2.22 million kilometers (1.38 million miles), looking for clouds across the moon's Fensal-Aztlan region. ISS will look at Titan two more times before the T111 flyby, on May 4 and May 5. The closer of the two, taken on May 5, will be acquired from a distance of 1.07 million kilometers (0.67 million miles). That observation will cover the leading hemisphere, showing eastern Xanadu and western Fensal-Aztlan. On May 3, ISS will acquire an astrometric observation of Saturn's small, inner moons. Astrometric observations are used to improve our understanding of the orbits of these small satellites, which can be influenced by Saturn's larger icy moons. Two more such sequences will be acquired on May 4 and May 6. On May 5, ISS will ride along with the Visual and Infrared Spectrometer (VIMS) to observe the E ring.
Cassini encounters Titan on May 7 at 22:50 UTC for the 112th time. This is the fourth of seven Titan flybys planned for 2015, with the next encounter scheduled for July 7 during Rev 218. T111 has a close-approach altitude of 2,721 kilometers (1,691 miles). In addition to the science observation acquired during the encounter, the flyby will also shorten Cassini's orbit by nine days. Inbound, ISS will observe Titan's anti-Saturn hemisphere. Outbound, ISS will be able to observe a crescent Titan over its leading 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, VIMS will acquire a pair of mosaics of Titan's anti-Saturn and leading hemispheres, including Xanadu, which ISS will ride along with.
At closest approach, VIMS will control pointing with ISS acquiring ride-along images. VIMS will acquire a mosaic of eastern Shangri-La and Western Xanadu. VIMS will also acquire high-resolution images of Hotei Regio and an imaging swath across portions of eastern Xanadu to western Quivira. The swath will end around Bilbo Colles and Erebor Mons. As Cassini departs from Titan, CIRS will control pointing, acquiring a series of infrared observations of Titan. These include high-southern latitude limb sounding where CIRS will be looking at gas and aerosol abundances. This will help with understanding the development of the south polar vortex. CIRS will also 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.
On May 9 at 18:03 UTC, Cassini will reach periapse for Rev 215 at an altitude of 187,910 kilometers (116,760 miles) from Saturn's cloud tops, near the orbit of Mimas. During periapsis, ISS will ride along with a CIRS observation of Dione's sub-Saturn hemisphere from a distance of 525,000 kilometers (326,000 miles). Afterward, ISS will again ride along with CIRS, this time observing a crescent Tethys from a distance of 325,000 kilometers (202,000 miles), followed by another CIRS ride along observation covering the small moon, Polydeuces. The closest images of the 3-kilometer- or 1.86-mile-wide moon will be acquired from a distance of 45,000 kilometers (28,000 miles). Even at that close distance, Polydeuces will be only 11 pixels across. Afterward, early on May 10, ISS will monitor the south polar plume of Enceladus from a distance of 360,000 kilometers (224,000 miles).
On May 10, ISS will observe the small, distant moon Kiviuq from a distance of 9.85 million kilometers (6.12 million miles) for ten hours. This observation is design to measure the moon's orbital period by observing how its brightness changes as it rotates. A similar observation will be acquired of Paaliaq on May 18. Between those two observations, ISS will ride along with VIMS, CIRS, and the Ultraviolet Imaging Spectrometer (UVIS) to observe Saturn's atmosphere and aurorae. On May 12 and May 14, ISS will ride along with UVIS and VIMS to observe Saturn's south polar aurorae, or southern lights. Four other observations will be acquired during this period, with WAC images being acquired during scans of Saturn.
On May 19, Cassini will reach apoapse, bringing Rev 215 to a close and starting up the next orbit, Rev 216, which will include a non-targeted encounter with Hyperion.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).