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Cassini continues its exploration of the Saturn system with the 6.5-day Rev 289, which begins on August 17 at its farthest distance from the planet. This is also called the orbit’s apoapse. At this point, Cassini is 1.22 million kilometers (0.76 million miles) from Saturn’s cloud tops. Rev 289 is the 19th 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. Nine ISS observations are planned for Rev 289, with the majority covering Saturn’s atmosphere.
For its first observation of Rev 289, on August 17, ISS will observe the D ring at high-phase angles, which can emphasize the appearance of dust in Saturn’s innermost ring. On August 18 and 19, ISS will spend nearly 18 hours staring at Saturn’s narrow F ring. The time-lapse movie from this observation is designed to monitor features in the ring created by the gravity of nearby moons and ring clumps. On August 20, ISS will image a crescent Titan from 1.70 million kilometers (1.06 million miles) to continue monitoring seasonal changes to its upper haze layers.
On August 20 at 15:23 UTC, Cassini will reach periapse for Rev 289 at an altitude of 1,604 kilometers (996 miles) above Saturn’s cloud tops. This is the second of five final passes where Cassini dips into Saturn’s upper atmosphere. On the inbound leg, the Ultraviolet Imaging Spectrometer (UVIS) will acquire several scans of Saturn’s north polar region, observing the entire auroral oval. As Cassini approaches Saturn, the Composite Infrared Spectrometer (CIRS) will acquire a scan of the rings. At closest approach, ISS will acquire a Wide-Angle-Camera (WAC) movie of the rings as the spacecraft crosses the ring plane while the spacecraft is between the rings and Saturn. On the outbound leg, CIRS will observe Saturn’s south polar vortex. The Visual and Infrared Mapping Spectrometer (VIMS) will observe Saturn’s south polar auroral oval with ISS riding along.
On August 21, ISS will ride along with VIMS as it observes Saturn’s southern hemisphere. On August 23, ISS will acquire a WAC mosaic of the Large Magellanic Cloud, one of several satellite galaxies orbiting our Milky Way. Afterwards, ISS will observe the distant moon Thrymr from 15.0 million kilometers (9.32 million miles) away. While Thrymr won’t be resolved in these images, measurements of its brightness can provide information on its rotation speed and the orientation of its spin axis, as well as provide a rough estimate of its overall shape and determine whether it is a binary system, like many similar-sized objects orbiting the Sun. Finally, ISS will observe a crescent Titan from 1.51 million kilometers (0.94 million miles) away.
On August 23, shortly after the Titan observation, Cassini will reach apoapse, bringing Rev 289 to a close and starting up the next orbit, Rev 290. At this point, Cassini is 1.22 million kilometers (0.76 million miles) from Saturn’s cloud tops. Seven ISS observations are planned for this orbit, with the majority focused on Titan. On August 26, VIMS will acquire a mosaic of Saturn’s equatorial region. Afterward, CIRS will observe the limb of Saturn while ISS rides along. This observation is designed to observe the thermal structure of Saturn’s stratosphere. On August 27 at 02:20 UTC, Cassini will reach periapse for Rev 288 at an altitude of 1,570 kilometers (976 miles) above Saturn’s cloud tops. This is Cassini’s closest periapse for its entire mission (at least until the dive into the planet on September 15). RADAR will acquire a 2-cm, active-mode map of Saturn’s atmosphere, useful for understanding the ammonia concentration in Saturn’s atmosphere. INMS will ride along to conduct its second direct sampling of Saturn’s atmosphere. Outbound, UVIS will observe Saturn’s south polar auroral oval.
On August 28 at 01:07 UTC, Cassini will perform a non-targeted encounter with Titan at an altitude of 680,860 kilometers (423,070 miles). CIRS and ISS will acquire several observations over the course of a segment lasting 28 hours. The Titan observations include two stares covering Titan’s sub-Saturn hemisphere and another covering Titan’s full-disk. ISS will also observe Enceladus’s south polar plume from 1.03 million kilometers (0.64 million miles) away for more than 14 hours. On August 30, ISS will observe a crescent Titan from 1.59 million kilometers (0.99 million miles) away.
On August 30, Cassini will reach apoapse, bringing Rev 290 to a close and starting up the next orbit, Rev 291.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).