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Cassini continues its exploration of the Saturn system with the 19-day Rev 216, which begins on May 19 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 216 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. Twenty-one ISS observations are planned for Rev 216 with the majority focused on Saturn's atmosphere.
For ISS's first observation of Rev 216, occurring on May 19, the camera system will ride along with the Visual and Infrared Mapping Spectrometer (VIMS) to observe Saturn's south polar aurorae. ISS and VIMS will acquire similar observations on May 23 and May 30. On May 20, ISS will observe a half-phase Titan from a distance of 1.66 million kilometers (1.03 million miles). Surface features from across eastern Xanadu, like Hotei Arcus and the impact basin Menrva, will be visible. Also on May 20, ISS will ride along with the Ultraviolet Imaging Spectrometer (UVIS) as it scans across the disc of Saturn, observing the disc at extreme- and far-ultraviolet wavelengths. ISS and UVIS will acquire a similar observation on May 24. On May 21 and 22, ISS will ride along with the Composite Infrared Spectrometer (CIRS) to again observe Saturn's atmosphere. ISS images acquired during the May 22 observation using the Wide-Angle Camera (WAC) will also show Titan pass in front of Saturn, and later, early on May 23, a string of Saturn's smaller moons, including Mimas, Enceladus, and Epimetheus.
On May 25, ISS will acquire a 16-hour, Saturn cloud tracking observation. In addition to measuring wind-speed, this observation will be used to better understand the effect of Saturn's upper haze layers on the lower-lying cloud's appearance, by observing the clouds at different emission angles. Emission angles range from 0 degrees when looking straight down to 90 degrees when observing a feature at the horizon or limb. On May 26, ISS will ride along with a 19-hour, VIMS observation to acquire a global map of Saturn.
On May 28 at 14:54 UTC, Cassini will reach periapse for Rev 216 at an altitude of 188,590 kilometers (117,184 miles) from Saturn's cloud tops, near the orbit of Mimas. During periapsis, RADAR will acquire an equatorial map of Saturn's turbulent deep atmosphere by observing the planet at microwave wavelengths. This is a follow-up to observations acquired in 2005, 2009, 2010, and 2011. Similar microwave observations will be acquired by the Juno spacecraft at Jupiter in 2016 and 2017. VIMS will acquire similar deep atmosphere observations at near-infrared wavelengths both before and after the RADAR observation. On May 29, ISS will acquire images of a crescent Titan from a distance of 1.95 million kilometers (1.21 million miles).
On May 31, ISS will monitor the south polar plume of Enceladus from a distance of 1.35 million kilometers (0.84 million miles). Later that day, at 13:36 UTC, Cassini will perform a non-targeted encounter with the icy moon Hyperion at a distance of 34,286 kilometers (21,304 miles). Hyperion orbits just beyond that of Titan and is only 270 kilometers (168 miles) on average. This is the best encounter with Hyperion since August 2011 and is the last close encounter with the satellite during the Cassini mission. ISS will acquire a series of imaging observations, along with CIRS scans of Hyperion's surface, including a pair of high-resolution mosaics. The best images will have a resolution of 205 meters (0.13 miles) per pixel.
On June 2, ISS will acquire a pair of 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. Two more storm watch observations will be taken June 4. Also on June 2, ISS will ride along with VIMS to 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. A similar ISS observation will be acquired on June 4. Also on June 4, ISS will observe a crescent Titan from a distance of 1.87 million kilometers (1.16 million miles).
On June 7, Cassini will reach apoapse, bringing Rev 216 to a close and starting up the next orbit, Rev 217, which will include a targeted flyby of Dione and a non-targeted encounter with Polydeuces.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).