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Cassini continues its exploration of the Saturn system with the 6.5-day Rev 275, 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 1.21 million kilometers (0.75 million miles) from Saturn’s cloud tops. Rev 275 is the 5th of 22 proximal orbits that will take place between April 2017 and the end of the mission in September, where Cassini will approach Saturn between the ring system and Saturn’s atmosphere. Nine ISS observations are planned for Rev 275 with the majority focused on Titan.
On May 21, ISS will ride along with the Visual and Infrared Mapping Spectrometer (VIMS) to image Saturn’s nightside using multiple color and near-infrared filters. While this region won’t be directly sunlit, it will be illuminated by ringshine. On May 22 at 03:14 UTC, Cassini will reach periapse for Rev 275 at an altitude of 2,659 kilometers (1,652 miles) above Saturn’s cloud tops, the fifth of 22 periapses where Cassini will pass between the rings and Saturn to provide unique observations of Saturn’s internal structure, the composition of its upper atmosphere, and the structure and mass of its rings. On this pass, Cassini will transmit a signal from the High-Gain Antenna (HGA) to Earth to provide higher resolution information about Saturn’s internal structure by measuring variations in Saturn’s gravity field and to conduct a radio occultation of Saturn’s rings. Outbound on May 22, ISS will ride along with the Ultraviolet Imaging Spectrometer (UVIS) to image Saturn’s south polar aurorae. Between the UVIS observations, VIMS will observe a stellar occultation of the red supergiant star Betelgeuse by Saturn.
On May 24 at 00:18 UTC, Cassini will perform a non-targeted encounter with Saturn’s moon Titan at an altitude of 117,950 kilometers (73,290 miles). ISS will acquire three mosaics as well as a time-lapse observation to look for clouds south of Kraken Mare. The mosaics will be focused on Titan’s sub-Saturn hemisphere. The highest resolution of these is a four-frame mosaic covering the Senkyo dune field from 169,000 kilometers (105,000 miles) away on the outbound leg. The Composite Infrared Spectrometer (CIRS) will also acquire a mid-infrared limb-scan on the inbound leg of the encounter, as well as a mid-infrared temperature map on the outbound leg. CIRS limb scans are useful for measuring variations in atmospheric composition, density, and temperature with altitude and latitude. Before the CIRS limb-scan, VIMS and ISS will observe Dione at high phase angles from 658,000 kilometers (409,000 miles). VIMS will be looking for possible plumes over Dione’s “wispy terrain” terrain fractures. At closest approach to Titan, RADAR will acquire an altimetry swath near a set of bright streaks near 25 degrees South Latitude, 330 degrees West Longitude. While the distance to Titan might make interpreting altitudes difficult, this scan will be used to look for specular reflections that might come from a wet or “RADAR” smooth surface. Similar reflections were seen from Earth at this latitude range prior to the Cassini mission.
On May 24 at 21:31 UTC, the Saturn system will reach northern summer solstice, beginning northern summer and southern winter on Saturn and across most of Saturn’s major satellites. Seasons on Saturn last around 7.5 years, with vernal equinox occurring back in August 2009. The practical effect of this for the Cassini mission is that the greatest amount of the north polar regions of Saturn and Titan are illuminated, and conversely, the greatest amount of their southern hemispheres are not. On May 25, Cassini will reach apoapse, bringing Rev 275 to a close and starting up the next orbit, Rev 276. At this point, Cassini is 1.21 million kilometers (0.75 million miles) from Saturn’s cloud tops. Eleven ISS observations are planned for this orbit, with most covering Saturn’s rings and icy satellites. Later on May 25, ISS will acquire a high-phase observation of the Cassini Division between Saturn’s A and B rings.
On May 28 at 14:26 UTC, Cassini will reach periapse for Rev 276 at an altitude of 3,854 kilometers (2,395 miles) above Saturn’s cloud tops. On the inbound leg, on May 27, UVIS will observe a stellar occultation by the rings of the B-type star Epsilon Canis Majoris. On May 28, ISS will observe the F ring and will acquire a short, color radial scan across the lit face of the B and C rings. VIMS will acquire a scan along the edge of Saturn’s shadow on Saturn’s rings. The instrument will also observe a stellar occultation by Saturn’s ring system of the red giant star Gamma Crucis. Nearing closest approach, RADAR will acquire radiometry and scatterometry scans across Saturn’s ring system. At closest approach, the High-Gain Antenna (HGA) will be pointed to protect the spacecraft from possible D ring particles, but will then immediately turn to allow ISS to acquire high-resolution images of the unlit face of the C, D, and inner B rings.
On May 29, ISS will acquire a pair of observations of Enceladus and its south polar plume from 922,000 kilometers (573,000 miles) and 1.18 million kilometers (734,000 miles) away. These will be used to measure the intensity of the plume at different points in the moon’s orbit. ISS will acquire a similar observation while riding along with UVIS on May 30. On May 29, ISS will observe a crescent Titan from 2.01 million kilometers (1.25 million miles). This will be used to monitor changes in Titan’s upper haze layers. On May 30, ISS will image the sub-Saturn hemisphere of Iapetus from 2.47 million kilometers (1.54 million miles) away. Also on May 30, after the UVIS observation of Enceladus, ISS will acquire a lightcurve observation of the small, distant moon Bebhionn from 9.33 million kilometers (5.80 million miles) away. By measuring variations in its brightness in this sequence and in other observations taken from different angles, the direction its north pole points and Bebhionn’s approximate shape can be determined. On May 31, during the end of the orbit ISS will perform a photometric calibration observation using the A-type star Vega.
On May 31, Cassini will reach apoapse, bringing Rev 276 to a close and starting up the next orbit, Rev 277.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).