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Cassini continues its exploration of the Saturn system with the 12-day Rev 240, which begins on August 13 at its farthest distance from the planet. This is also called the orbit’s apoapse. At this point, Cassini is 1.50 million kilometers (0.93 million miles) from Saturn’s cloud tops. Rev 240 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-two ISS observations are planned for Rev 240 with the majority focused on Saturn’s rings and atmosphere.
For its first observation for Rev 240, on August 14, ISS will take a look at crescent Titan from a distance of 1.57 million kilometers (0.98 million miles). ISS will be using this observation to monitor changes in Titan’s upper haze layers. Three more cloud monitoring observations will be taken on August 15, 17, and 20. The closest of these, taken on August 17, will be taken from a distance of 1.42 million kilometers (0.08 million miles) and will cover the northern trailing hemisphere. Immediately after the Titan cloud observation on August 14, 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. Additional astrometric observations will be acquired on August 15 and 23.
After the astrometric observation on August 14, 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. Five more Storm Watch observations will be taken on August 15 and 23. On August 15, ISS will acquire an observation of the star HD 71334, a near-twin of the Sun in the constellation Puppis. This will be used to help calibrate Cassini’s observations of Saturn’s distant, outer moons. On August 15, 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 their motion, these observations are used to keep track of previously discovered propellers, like Earhart and Bleriot.
On August 19 at 14:26 UTC, Cassini will reach periapse for Rev 240 at an altitude of 400,660 kilometers (273,810 miles) above Saturn's cloud tops, between the orbits of Dione and Rhea. In addition to a pair of Titan haze monitoring observations, ISS will also ride along with an Ultraviolet Imaging Spectrometer (UVIS) observation of Saturn’s south polar aurorae on August 19.
On August 21, ISS will ride along with a Visual and Infrared Mapping Spectrometer (VIMS) stellar occultation observation. During this sequence, Saturn’s rings will pass in front of the red supergiant star, Betelgeuse. This and other stellar occultations can be used to monitor changes in the structure of the rings. Later on August 21, ISS will observe the distant outer moon Skathi. By measuring brightness changes over the course of this 14-hour observation, research can measure Skathi’s pole direction and estimate its shape. Skathi will be 17.8 million kilometers (11.1 million miles) away during this observation. A similar observation of the distant moon Erriapus will be acquired on August 23 from a distance of 18.6 million kilometers (11.5 million miles). On August 22, ISS will observe Hyperion from a distance of 325,150 kilometers (202,040 miles). The low phase of this observation will allow for mapping of color variations across this chaotic moon’s surface. On August 23, ISS will acquire a seven-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.
On August 25, Cassini will reach apoapse, bringing Rev 240 to a close and starting up the next orbit, Rev 241.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).