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Cassini continues its exploration of the Saturn system with the 6.5-day Rev 283, which begins on July 9 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 283 is the 13th of 22 proximal orbits that will take place between April 2017 and the end of the mission in September. During these orbits, Cassini’s closest approach to Saturn occurs between the ring system and Saturn’s atmosphere. Twenty-one ISS observations are planned for Rev 283, most occurring during a non-targeted encounter with Saturn’s largest moon, Titan.
On July 10 at 13:51 UTC, Cassini will perform a non-targeted encounter with Titan at an altitude of 264,310 kilometers (164,230 miles). ISS will acquire a series of mosaics covering the northern sub-Saturn hemisphere and the north polar region. These will be used to track the evolution and motion of Titan’s mid-northern latitude cloud streaks. In recent months, these streaks have become increasingly prominent both in brightness (suggesting greater cloud top heights) and in areal extent. Between these mosaics, the Composite Infrared Spectrometer (CIRS) will acquire mid-infrared temperature maps of Titan’s atmosphere. ISS will ride along to capture more images of Titan’s clouds. The ISS and CIRS observations will cover a period of 30 hours. During this observation period, the Ultraviolet Imaging Spectrometer (UVIS) will observe a stellar occultation as Beta Canis Majoris passes behind Saturn.
On July 12 at 20:48 UTC, Cassini will reach periapse for Rev 283 at an altitude of 2,805 kilometers (1,743 miles) above Saturn’s cloud tops. Inbound, CIRS will observe the lit face of the A ring to measure its temperature and structure and the boundary of Saturn’s shadow on the B ring to measure how the temperature of ring particles change as they cross into the shadow. This provides information about the fine-scale structure of particles in the B ring. VIMS will perform a similar scan of the edge of Saturn’s shadow on the rings. ISS will conduct a survey of the propellers in the outer 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 needed to keep track of previously discovered propellers, like Earhart and Bleriot.
As Cassini approaches Saturn, CIRS will observe plateau regions in Saturn’s C ring with ISS riding along. Afterwards, ISS will perform a scan of the lit face of the C and inner B rings. ISS will perform a similar scan after closest approach of the unlit face of these rings. At closest approach to Saturn and during the ring plane crossing, the Ion and Neutral Mass Spectrometer (INMS) will directly measure the composition of Saturn’s ionosphere and exosphere as well as the composition of the ionosphere of Saturn’s rings. Outbound, ISS will observe the outer edge of the A ring. On July 14, ISS will acquire a distant observation of Enceladus’s south polar plume to measure how its brightness changes as Enceladus orbits Saturn. UVIS will observe several stellar occultations by Saturn’s atmosphere of stars in Orion’s belt. The Visual and Infrared Mapping Spectrometer (VIMS) will acquire several near-infrared mosaics of Saturn’s nightside. On July 15, CIRS will acquire a mid-infrared thermal map of Saturn’s nightside.
On July 16, Cassini will reach apoapse, bringing Rev 283 to a close and starting up the next orbit, Rev 284. At this point, Cassini will be 1.21 million kilometers (0.75 million miles) from Saturn’s cloud tops. Ten ISS observations are planned for this orbit, with the majority focused on Saturn’s atmosphere. On July 16 and 17, ISS will acquire a pair of observations of the limb of a crescent Saturn to monitor Saturn’s upper haze layers and to observe cloud shadows. On July 17, CIRS will observe Saturn near 58 degrees North latitude in order to measure the effects of ring rain on Saturn’s atmosphere. Ring rain results from molecules of water from Saturn’s rings following magnetic field lines and falling into Saturn’s atmosphere. On July 18, UVIS will observe Saturn’s north polar auroral ring.
On July 19 at 07:55 UTC, Cassini will reach periapse for Rev 284 at an altitude of 2,711 kilometers (1,684 miles) above Saturn’s cloud tops. During this periapse pass, the Radio Science Sub-System (RSS) will conduct a radio occultation of Saturn’s rings as well as conduct another gravity experiment to measure the planet and the ring’s gravitational field. The radio occultation will be used to observe the fine-scale structure of the rings. This pass, when combined with earlier experiments during Cassini’s Grand Finale, will be used to learn how mass is distributed within Saturn. For example, scientists will be looking to see if Saturn has a compact, rocky and/or metallic core, or if it’s larger and made of less dense material like currently suspected at Jupiter based on Juno data. This is RSS’s last observation of the Cassini mission. The Magnetometer will be riding along to measure variations in Saturn’s magnetic field as Cassini gets very close to the planet.
On July 20, ISS will image a crescent Titan from 1.92 million kilometers (1.20 million miles) away to continue monitoring seasonal changes to its upper haze layers. Similar haze monitoring observations will be acquired on July 21 and 22. Also on July 20, ISS will ride along with UVIS to observe Saturn’s south polar aurorae. UVIS will observe several stellar occultations of stars in Orion’s belt by Saturn’s atmosphere. On July 21, ISS will observe the limb of Saturn to monitor its upper haze layers. On July 21 and 22, VIMS will acquire near-infrared observations of Saturn’s nightside.
On July 22, Cassini will reach apoapse, bringing Rev 284 to a close and starting up the next orbit, Rev 285, when Cassini will perform a non-targeted encounter with Titan.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).