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Cassini continues its exploration of the Saturn system with the 7.2-day Rev 265, which begins on March 11 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 265 is the 15th of 20 F-ring orbits that will take place between November 2016 and April 2017 where Cassini will approach Saturn just outside the main ring system. Seven ISS observations are planned for Rev 265 with the majority focused on Saturn’s moons.
On March 11, ISS will perform its first observation of Rev 265 by acquiring a 13-hour, time-lapse movie of the narrow F ring to monitor features in the ring created by the gravity of nearby moons and ring clumps. On March 14, ISS will acquire an astrometric observation of several Saturn’s smaller moons. Astrometric observations are important for fine-tuning our understanding of the orbital motions of these moons, which are influenced by the gravitational pulls of Saturn’s larger icy satellites. Afterward, 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.
On March 14 at 22:26 UTC, Cassini will reach periapse for Rev 265 at an altitude of 86,605 kilometers (53,814 miles), near the orbits of Janus and Epimetheus. Most of the observations during the periapse period focus on Saturn’s moons and measuring the composition of gas and dust near the F ring. Inbound to Saturn, the Ultraviolet Imaging Spectrometer (UVIS) will perform repeated slews across Saturn’s north polar region to image the planet’s aurorae. Next, ISS will observe Tethys from 445,000 kilometers (277,000 miles) away. This observation will involve color imaging of Tethys’ sub-Saturn hemisphere to map the global pattern of red streaks on its surface. This is also the last targeted Tethys observation of the Cassini mission excepting a stellar occultation sequence in June. Afterward, the Ion and Neutral Mass Spectrometer (INMS) will measure the composition and density of the gasses near the F ring while Cassini is near periapse. The Cosmic Dust Analyzer (CDA) will then perform a similar observation of the dust environment.
On March 15, ISS will acquire a Wide-Angle-Camera (WAC) mosaic of Saturn’s ring system, while Cassini is in the shadow of Saturn. Mosaics like this highlight the distribution of dust in the ring system as well as provide a constraint on the density of particles in the inner D ring. Between March 16 and March 21, ISS will acquire four observations of Titan. The first two, to be taken on March 16 and March 18, will show a crescent to half-phase Titan and will be used to monitor changes in Titan’s upper haze layers due to the approaching northern summer solstice. On March 20, ISS will observe Titan from 971,500 kilometers (603,660 miles) away to monitor clouds across its northern sub-Saturn hemisphere and north polar region. Intense cloud streaks have recently been observed in this region, including during non-targeted encounters in Rev 247 and Rev 255. ISS will image Titan again on March 21, this time while Cassini is between Titan and the Sun, allowing for measurements of Titan’s opposition surge. This can be used to better understand the fine scale structure of Titan’s surface.
During this set of Titan observations, Cassini will reach apoapse, bringing Rev 265 to a close and starting up the next orbit, Rev 266. Five ISS observations are planned for this orbit, with most covering Titan and Saturn’s small satellites. Cassini will reach periapse on March 22 at 02:18 UTC at an altitude of 86,605 kilometers (53,814 miles), near the orbits of Janus and Epimetheus. On March 21, inbound to Saturn, the Ultraviolet Imaging Spectrometer (UVIS) will perform repeated slews across Saturn’s north polar region to image the planet’s aurorae. For an hour before periapse, ISS will acquire an observation of Earhart, a large ring particle that orbits just outside the Encke Gap in the outer A ring. The gravitational pull of ring particles like Earhart create propeller-shaped voids in the ring system. During periapse, CDA will measure the dust environment near the F ring. Afterward, the Visual and Infrared Spectrometer (VIMS) will observe Saturn’s south polar aurorae.
On March 22 and 23, after a radio occultation of the rings, ISS will observe the distant moon Hyrrokkin from 24.8 million kilometers (15.4 million miles) away. By measuring how Hyrrokkin’s brightness varies over the course of the 14-hour observation, and comparing the resulting lightcurves with previous measurements used to estimate its rotational period, researchers hope to estimate its north pole position and its approximate shape. On March 25, ISS will observe Titan from 1.66 million kilometers (1.03 million miles) away to monitor its upper haze layers.
On March 25, Cassini will reach apoapse, bringing Rev 266 to a close and starting up the next orbit, Rev 267.
Image products created in Celestia. Tethys map by Steve Albers. All dates in Coordinated Universal Time (UTC).