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Cassini continues its exploration of the Saturn system with the 14-day Rev 239, which begins on July 30 at its farthest distance from the planet. This is also called the orbit’s apoapse. At this point, Cassini is 1.81 million kilometers (1.12 million miles) from Saturn’s cloud tops. Rev 239 occurs during the second inclined phase of the Cassini Solstice Mission. Over the next several orbits, Cassini will use encounters with Titan to gradually increase the inclination of its orbit. Twenty-five ISS observations are planned for Rev 239 with the majority focused on Saturn’s rings and atmosphere.
For its first observation for Rev 239, on August 1, ISS will observe the distant outer moon Tarqeq. By measuring brightness changes over the course of this 14-hour observation, researchers can measure Tarqeq’s rotational period. Tarqeq will be 18.0 million kilometers (11.2 million miles) away during this observation. On August 2, ISS will acquire a 15-hour movie of the F ring, a narrow ring just outside the main ring system. The movie will be used to monitor the creation of clumps and channels, formed by the gravitational interaction between F ring material and nearby moons and moonlets. A similar movie will be taken during the outbound leg of this orbit on August 12.
On August 3, ISS will take a look at Titan’s northern trailing hemisphere from a distance of 2.24 million kilometers (1.39 million miles). ISS will be looking for clouds across Titan’s northern trailing hemisphere during this sequence. Two more cloud monitoring observations will be taken on August 4 and 5. The closer of the two, taken on August 5, will be taken from a distance of 1.92 million kilometers (1.19 million miles) and will cover the northern trailing and sub-Saturn hemispheres. Immediately after the Titan cloud observation on August 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 as well as each other. Careful measurements of the positions of these moons are important for later imaging of them at much closer distances during the F ring orbits later this year and early next year. A second astrometric observations will be acquired on August 4.
After the astrometric observation on August 3, ISS will acquire a quick observation of Saturn using the Wide-Angle Camera (WAC). This observation is part of a series of “Storm Watch” sequences designed to take advantage of short, two-minute segments when the spacecraft turns to point the optical remote sensing (ORS) instruments back at Saturn, as a waypoint between observations. They include blue, clear, two methane band, and one full-frame, continuum band filter images. Two more Storm Watch observations will be taken on August 4 and 5. On August 3, ISS will image the G ring arc.
On August 7 at 19:27 UTC, Cassini will reach periapse for Rev 239 at an altitude of 517,160 kilometers (321,350 miles) above Saturn's cloud tops, just inside the orbit of Rhea. ISS and the other remote sensing instruments will spend most of the periapse period observing Saturn’s rings. On August 5, ISS will observe several propellers in Saturn’s A ring. Propellers are voids in the ring created by the gravity of large, 100 – 1000 meter (328 – 3280 foot) ring particles. Due to the influence of the rings on the propeller’s motion, these observations are used to keep track of previously discovered propellers, like Earhart and Bleriot. A similar observation will be taken on August 6. On August 5, ISS will ride along with the Visual and Infrared Mapping Spectrometer (VIMS) as it observes a stellar occultation by Saturn’s rings of the star Antares (Alpha Scorpii). This is the first of three stellar occultations VIMS will observe (and ISS will ride along with) by the rings during this periapse period. On August 7, VIMS will observe an occultation of the red giant star R Cassiopeiae, and on August 8, it will observe an occultation of the near-red giant star Rho Persei.
On August 5, ISS will ride along with the Ultraviolet Imaging Spectrometer (UVIS) to observe Tethys at a distance of 1.25 million kilometers (0.78 million miles). Tethys will be nearly full from Cassini’s perspective, proving a good opportunity to map different color units across Tethys. On August 6, ISS will acquire a color scan of the lit face of the main ring system using the Narrow-Angle Camera (NAC). On August 7, ISS will observe the F ring for seven hours. On August 8, ISS will acquire a pair of high-phase observations of the unlit face of the rings. The second of these is a ride-along observation with VIMS. This will be used to look for meteor impacts into the C ring. On August 9, ISS will ride along with the Composite Infrared Spectrometer (CIRS) to look at the unlit side of the A ring. The last ring observation of Rev 239 occurs on August 13, when ISS will observe the Phoebe ring, a large, dusty, diffuse ring in the outer Saturn system. The ring is so faint that it is observed mostly by looking at the edge of Saturn’s shadow on the ring.
On August 10 at 08:37 UTC, Cassini will perform a targeted encounter of Titan. This is Cassini’s 123rd flyby of Titan, the eighth of eleven planned for 2016. The next encounter is planned for September 27 during Rev 243. T122 has a close approach flyby has a close approach altitude of 1,599 kilometers (994 miles). This encounter will increase the inclination of Cassini’s orbit from 48.7 degrees to 53.5 degrees and will shave four days from the spacecraft’s orbital period. The Radio Science Subsystem (RSS) will be prime for almost the entire encounter, with the exception of an eight-frame ISS mosaic of Titan’s crescent on the outbound leg. T122 is the last RSS gravity flyby (except for a ride-along sequence on T123) of the Cassini mission. Measurements from this pass will be used to detect a global subsurface ocean by measuring the short-period changes in Titan’s gravity field, to look for large-scale gravity anomalies, and determine how the icy crust responds to changes in Titan’s gravity field (induced by tides from Saturn). The Cosmic Dust Analyzer (CDA) will ride-along with RSS’s gravity pass, observing how dust grains respond to Titan’s upper atmosphere.
On August 13, Cassini will reach apoapse, bringing Rev 239 to a close and starting up the next orbit, Rev 240, which includes a flyby of Titan.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).