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Cassini continues its exploration of the Saturn system with the 6.5-day Rev 277, which begins on May 31 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 277 is the 7th 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. Eleven ISS observations are planned for Rev 277 with the majority focused on Saturn’s rings.
On June 1, after a similar observation carrying over from the end of Rev 276, ISS will perform its first observation of Rev 277, a photometric calibration observation using the double star Theta Tauri. On June 3, ISS will observe the F ring and will acquire a short, color radial scan across the lit face of the D, C, and inner B rings. ISS will also 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. The Composite Infrared Spectrometer (CIRS) will acquire a thermal scan across the main rings while it is looking nearly straight down on the rings. This reduces foreshortening and improves the radial resolution of the scan.
On June 4 at 01:42 UTC, Cassini will reach periapse for Rev 277 at an altitude of 3,825 kilometers (2,377 miles) above Saturn’s cloud tops. During the ring plane crossing right before periapse, Cassini’s high-gain antenna will be pointed in the direction the spacecraft is moving to shield it from ring particles. It will then turn to acquire a RADAR scatterometry scan across the main ring system. This map of the roughness of the rings will have a resolution of 4 kilometers (2.5 miles) in the outer A ring down to 100 meters (330 feet) in the inner C ring. Afterward, CIRS will perform a high-resolution thermal scan of the unlit face of Saturn’s main rings. Finally, the Visual and Infrared Mapping Spectrometer (VIMS) will observe the Sun as it passes behind the rings. Observing the amount of sunlight that reaches VIMS as the Sun passes behind different portions of the rings provides information about the structure of the main rings and its optical opacity.
Later in the day on June 4, ISS will ride along with VIMS as it observes a stellar occultation as the red supergiant star Betelgeuse passes behind Saturn’s rings. VIMS will also watch as Betelgeuse passes behind Saturn. On June 5, ISS will ride along as CIRS observes the unlit side of the C ring in area known as the C ring plateau. Later in the day, ISS will ride along with stellar occultation observations by the Ultraviolet Imaging Spectrometer (UVIS). During these observations, two of the stars of Orion’s Belt, Epsilon Orionis and Zeta Orionis, will be tracked as they pass behind Saturn’s main rings. Afterwards, ISS will again ride along with UVIS as they observe Epsilon Orionis, the middle star of Orion’s Belt, pass behind Tethys. This is the last observation targeting Tethys by ISS of the Cassini mission. Afterwards, on June 6, ISS will observe Saturn’s G ring at high phase angles as the spacecraft approaches the ring plane. During the observation, the G ring arc, with the tiny moon Aegaeon at its heart, will pass through the field-of-view of the Narrow-Angle Camera (NAC). Later on June 6 and into June 7, ISS will ride along with CIRS as it observe the outer A ring.
On June 7, Cassini will reach apoapse, bringing Rev 277 to a close and starting up the next orbit, Rev 278. At this point, Cassini is 1.21 million kilometers (0.75 million miles) from Saturn’s cloud tops. Eighteen ISS observations are planned for this orbit, with most taking place during a non-targeted encounter with Titan. That encounter takes place on June 8 at 18:45 UTC at an altitude of 367,300 kilometers (228,230 miles). ISS will acquire a series of four-frame mosaics covering the northern sub-Saturn hemisphere. 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 one-hour mosaics, 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 27 hours.
On June 10 at 12:53 UTC, Cassini will reach periapse for Rev 278 at an altitude of 3,342 kilometers (2,077 miles) above Saturn’s cloud tops. 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 measure the mass of Saturn’s rings. RSS will then conduct a radio occultation of Saturn’s rings. The Cosmic Dust Analyzer (CDA) will also be used during this pass to measure the composition of dust in Saturn’s inner D ring. On June 11, VIMS will map Saturn’s southern hemisphere in the infrared, all it to track clouds in polar and ring-shadow darkness. VIMS will focus on how changing seasons at Saturn’s winter pole affect chemistry, cloud structures, and wind speeds. On June 11 and 12, ISS will observe Enceladus’s south polar plume for more than 12 hours from 1.11 million kilometers (0.69 million miles) away. On June 12 and 13, ISS will ride along with UVIS to acquire polarimetry data of a crescent Saturn using the Wide-Angle Camera (WAC).
On June 13, Cassini will reach apoapse, bringing Rev 278 to a close and starting up the next orbit, Rev 279.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).