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Cassini continues its new tour of the Saturn system with the 20-day-long Rev141, the spacecraft's 142nd orbit around the Ringed Planet. Cassini begins Rev141 on November 20 at its farthest distance from Saturn, called apoapse. At this point, Cassini is 2.63 million kilometers (1.63 million miles) from Saturn's cloud tops. Cassini's orbit lays nearly exactly within Saturn's ring plane and within the orbital plane of most its major satellites, affording an opportunity to encounter a few of its moons. During Rev141, Cassini will fly by several of Saturn's moons, including Enceladus, Hyperion, and Janus.
Cassini's ISS camera system starts its observations for Rev141 on November 26 with its first observation in several weeks. The spacecraft entered safe mode on November 2, and subsequent observations were lost. ISS will begin its restarted observations by imaging Titan from a distance of 1.94 million kilometers (1.21 million miles). While the high phase angle of this observation will prevent surface imaging, these images can be used to assess the changes that have been ongoing within the upper haze layers of Titan's atmosphere as a result of the change over from northern winter to spring. Also on November 26, the Ultraviolet Imaging Spectrometer (UVIS) will stare at the south pole of Saturn in order to image its aurora australis.
On November 28 at 03:29 UTC, Cassini will fly by Saturn's moon Hyperion at a distance of 71,702 kilometers (44,553 miles). Hyperion is a large, irregular moon that lies beyond the orbit of Titan. Three major imaging sequences are planned. The first, lasting seven hours, will include seven frames covering the approach to Hyperion, each separated by about one hour. Each frame will consist of five to six images: two to three clear filter, narrow-angle-camera (NAC) images and one image each using the UV3, GRN, and IR1 filters. The second sequence is similar, covering closest approach and the start of the outbound leg. Like the first, the beginning of this sequence will include several frames, separated by 1.5 hours, each using five to six images of different filters. At then end of this sequence, ISS will image Hyperion using an expanded set of filters, including polarized ones. The final sequence covers the outbound leg, again using multiple frames separated in time in order for a time-lapse movie to be created on the ground and consisting of six images (three color plus three bracketed clear filter images).
On November 30 at 09:33 UTC, Cassini will reach the periapse of Rev141, its closest point to Saturn in the orbit. At periapse, the spacecraft will be 154,920 kilometers (96,263 miles) above Saturn's cloud tops. During this periapse passage, Cassini will perform a targeted encounter of the active icy satellite, Enceladus. This encounter is also known as E12 as it is the 12th targeted flyby of this moon. Closest approach takes place at 11:53 UTC. Inbound, ISS will image the jets coming off the south polar region, while the Composite Infrared Spectrometer (CIRS) performs several scans across Enceladus in order to obtain a nighttime temperature map, useful for measuring the thermal inertia of the moon's surface. At closest approach, Cassini will fly just 50 kilometers (31 miles) above terrain located at 62 degrees north latitude, 54 degrees west longitude on Enceladus. The high-gain antenna will be pointed at Earth at closest approach, allowing telescopes to track Cassini's signal as its shifts due to the influence of Enceladus' gravity. The degree to which the signal shifts can be used as a measure of the varying gravitational pull of Enceladus across its surface. Combined with a topographic map of Enceladus, this will allow scientists to map the moon's subsurface structure. During the gravity measurement, ISS will take 10 five-millisecond NAC and WAC images as the moon passes through the cameras' fields-of-view at closest approach. These images will cover small portions of Enceladus's leading hemisphere terrain, a region of ropy terrain similar to south polar region in texture but a bit older in surface age.
Following closest approach, CIRS, the Visual and Infrared Mapping Spectrometer (VIMS), and UVIS will trade off being the primary instrument, while ISS rides along. These sequences will cover the anti-Saturn hemisphere of Enceladus as the moon transits across Saturn. Of particular interest will be the boundary between the youthful, leading hemisphere terrain and the comparatively ancient, though still tectonically reworked, anti-Saturn terrain. The first ride-along observation, with CIRS, will include two, four-frame, clear-filter mosaics of Enceladus. The ride along observation with VIMS will have ISS run through a variety of color filter views of a full-disk Enceladus. Finally, during the more distant UVIS ride-along observations, ISS will take six clear-filter images, including one where Dione passes behind Enceladus. Afterward, ISS will image the area around Rhea's L4 Lagrangian region, an area that leads Rhea on its orbit by 60 degrees. These points have provided islands of orbital stability for some of Saturn's satellites, like Telesto, which lies within the L4 region for Tethys, and Polydeuces, which lies within the L5 region of Dione. To date, no co-orbital satellites have been discovered that share Rhea's orbit. This will help to make up for a similar observation that was to have been taken during Rev140 but was lost due to the safing event.
On Dec. 4, ISS will take a look at Saturn's faint E ring, a diffuse torus of small icy particles originating from Enceladus' south polar plume. Since Cassini is located within Saturn's ring plane, the spacecraft can get a better idea of the vertical structure of this ring. Unlike particles in the other rings of Saturn, the icy particles in the E ring have a slightly inclined orbit (a result of gravitational interactions with Enceladus and the other icy satellites and from the initial southward impulse from erupting from Enceladus' south polar region). This inclination gives the ring a double appearance when viewed edge-on, as in this case. Finishing up Rev141, Cassini will perform astrometric observations of Saturn's small, inner moons on December 4 and 5. During these two observations, ISS will observe Polydeuces, Telesto, Epimetheus, Anthe, Calypso, Atlas, Prometheus, and Pandora. At the end of both observations, ISS will clean up with WAC images of Saturn. On December 5, ISS will image a half-phase Titan at a distance of 3.42 million kilometers (2.13 million miles). The camera will image the sub-Saturn hemisphere of the satellite, allowing researchers to continue to study the recent weather changes that have been occurring across Titan's equatorial region and that have brought methane rain to select regions of previously dry terrain. On December 6, ISS will image Dione as it passes in front of Tethys's north polar region. Dione will be 2.05 million kilometers (1.28 million miles) away at the time, while Tethys will be slightly farther away at a distance of 2.19 million kilometers (1.36 million miles).
On December 10, Cassini will reach apoapse on this orbit, bringing this orbit to a close and starting Rev142. Rev142 includes non-targeted encounters with Daphnis, Pan, Pandora, and Dione. Cassini will also perform a close, targeted flyby of Enceladus on December 21.
Image products created in Celestia. Enceladus and Dione base maps by Steve Albers. All dates in Coordinated Universal Time (UTC).