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Cassini continues its exploration of the Saturn system with the 24-day Rev 237, which begins on June 17 at its farthest distance from the planet. This is also called the orbit’s apoapse. At this point, Cassini is 2.49 million kilometers (1.55 million miles) from Saturn’s cloud tops. Rev 237 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. Thirty-five ISS observations are planned for Rev 237 with the majority focused on Saturn’s rings and atmosphere.
For its first observation of Rev 237, on June 18, ISS will look at Titan’s northern trailing hemisphere from a distance of 3.54 million kilometers (2.20 million miles). ISS will be searching for clouds across Titan’s north polar region and at northern mid-latitudes during this sequence. Between June 20 and June 24, ISS will acquire similar cloud monitoring observations every couple of days. The closest of these will be the second cloud monitoring observation on June 24, acquired from a distance of 2.30 million kilometers (1.43 million miles) and will be focused on Fensal and the mid-latitude blandlands north of it. Immediately after the Titan cloud observation on June 18, 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. Three more astrometric observations will be acquired on June 20, June 22, and July 9.
After the astrometric observation on June 18, 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. Seven more Storm Watch observations will be taken between June 19 and June 25. Another six will be taken between July 1 and July 10. On June 22, ISS will track the Methone arc and the D72 ringlet for 22 hours. On June 25, ISS will ride along with the Visual and Infrared Spectrometer (VIMS) to track the zero phase point on Saturn’s rings, where Cassini is directly between the Sun and Saturn’s rings.
On June 29 at 04:56 UTC, Cassini will reach periapse for Rev 237 at an altitude of 586,780 kilometers (364,610 miles) above Saturn's cloud tops, outside the orbit of Rhea. ISS and the other remote sensing instruments will spend most of the periapse period observing Saturn’s rings. On June 26, ISS will acquire a color scan of the A and B rings. On June 27, ISS will acquire a pair of D ring observations. The D ring, which includes the D27 ringlet, is the innermost of Saturn’s rings. On June 28, ISS will ride along with VIMS as it observes a stellar occultation by Saturn’s rings of the star Beta Pegasi. On June 29, ISS will acquire a pair of observations of 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 their motion, these observations are used to keep track of previously discovered propellers, like Earhart and Bleriot. ISS will also ride along with the Ultraviolet Imaging Spectrometer (UVIS) to observe Saturn’s south polar aurora. On June 30, ISS will ride along with the Composite Infrared Spectrometer (CIRS) to observe the unlit side of the A ring.
On July 1, ISS will acquire movies of Saturn’s faint rings and the Encke Gap in the outer A ring. On July 4, Cassini’s time as the only active giant planet orbiter finally comes to an end with the arrival of the Juno spacecraft at Jupiter. To accommodate Juno’s Jupiter Orbit Insertion maneuver, Cassini will be more reliant on the Deep Space Network’s 34-meter radio dishes, limiting available data bandwidth during this time. For the three days following orbit insertion, UVIS will be conducting several system scans. On July 9, ISS will acquire a cloud monitoring observation of Titan at a distance of 2.37 million kilometers (1.47 million miles).
On July 11, Cassini will reach apoapse, bringing Rev 237 to a close and starting up the next orbit, Rev 238, which includes a flyby of Titan.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).