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Cassini continues its exploration of the Saturn system with the 7.2-day Rev 269, which begins on April 9 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 269 is the 19th 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. Nine ISS observations are planned for Rev 269 with the majority focused on Saturn’s rings.
On April 10, ISS will perform its first observation of Rev 269 by riding along with a stellar occultation observation by the Visual and Infrared Mapping Spectrometer (VIMS). The instruments will observe the red hypergiant star VY Canis Majoris pass behind Saturn’s ring system. Early on April 11, ISS will observe a crescent Titan from 1.60 million kilometers (1.00 million miles) away. This observation will be used to monitor changes to the moon’s outer atmospheric haze layers resulting from seasonal changes. Northern summer begins on Titan (and the rest of the Saturn system) on May 24 during Rev275.
On April 12 at 14:00 UTC, Cassini will reach periapse for Rev 269 at an altitude of 86,966 kilometers (54,038 miles), near the orbits of Janus and Epimetheus. VIMS and the Composite Infrared Spectrometer (CIRS) will observe a pair of stellar occultations by the rings of the stars Eta Carinae and Gamma Crucis. Early on April 12, ISS will acquire high-resolution color imaging of portions of the B and C rings. On April 12 at 13:41 UTC, Cassini will perform a close, non-targeted encounter with Atlas at a distance of 10,899 kilometers (6,772 miles). ISS will acquire 56 images during the hour and a half prior to the encounter. The closest of these images will be taken from 12,780 kilometers (7,940 miles) away. These would be the closest images yet acquired of one of Saturn’s “ring” moons. Atlas orbits in the Roche Division between the outer A ring and the F ring. These images will provide our highest resolution images to date of Atlas’s equatorial ridge, a feature thought to be produced by a build-up of impacting ring particles. While earlier, lower-resolution of Atlas showed this ridge to be largely featureless (compared to the richly texture “core” of Atlas), recent images of a similar ridge on Pan revealed grooves and small impact craters. After periapse, ISS will observe “Bleriot”, a large ring particle that orbits between the Encke and Keeler Gaps. The gravitational pull of ring particles like Earhart create propeller-shaped voids in the ring system. Later on April 12, ISS will conduct a survey of these propellers.
On April 13, ISS will acquire a Wide-Angle Camera (WAC) mosaic of Saturn’s ring system while the spacecraft is in the shadow of Saturn. Such high-phase angle observations are useful for mapping dust in the rings and for observing fainter rings like the E ring. In addition to observing all of Saturn’s rings with the WAC, ISS will also observe Tethys, Enceladus, Mimas, as well as Earth and the Moon just as they are about to be occulted by the A ring. If the Earth were resolvable, Cassini would be looking at South America, Africa, and the southern Atlantic Ocean. Between April 14 and 16, ISS will observe Saturn’s upper haze layers using a series of bright limb imaging sequences. On April 16, Cassini will reach apoapse, bringing Rev 269 to a close and starting up the next orbit, Rev 270, the last of Cassini’s ring-grazing orbits. Eighteen ISS observations are planned for this orbit, with most covering Titan and Saturn’s rings.
On April 18, ISS will observe a half-phase Titan from 1.51 million kilometers (0.94 million miles) away. This observation will be used to track clouds across Titan’s northern mid-latitudes on the moon’s sub-Saturn hemisphere. On April 18, ISS will acquire its final astrometric observation, taking a look at Epimetheus, Pan, Prometheus, Janus, Daphnis, Pandora, and Atlas. 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. For most of these moons, this is their last targeted observation for the Cassini mission. Afterwards, ISS will ride along with CIRS to observe the outer A ring. ISS will be trying to observe a clump of ring material on the very edge of the A ring known as “Peggy.” This observation will be used to see how its orbit has recently changed. This will help improve pointing for a planned observation of Peggy during Rev 293, which will be the final ISS observation of the Cassini mission.
On April 19 at 18:05 UTC, Cassini will reach periapse for Rev 270 at an altitude of 87,087 kilometers (54,113 miles), near the orbits of Janus and Epimetheus. ISS will observe the C ring plateaus followed by a super-high-resolution observation of the lit face of Saturn’s A ring (plus a couple of frames over the middle of the B ring). The radial resolution of this observation will range from 400 to 600 meters (437 to 656 yards) per pixel, useful for observing the fine-scale structure of the rings. A similar observation will be taken following periapse of the unlit face of the A ring, with some images having a resolution of 300 meters (328 yards) per pixel. During closest approach, INMS will be “prime”, performing in-situ measurements of the faint, primarily-oxygen atmosphere that surrounds the rings.
After the second A ring observation and a VIMS south polar auroral stare, ISS will conduct a survey of the propellers in the A ring, which is useful for improving our knowledge of the motion of these small moonlets. Early on April 20, ISS will ride along with a CIRS observation of the outer C ring and outer A ring. Later that day, ISS will observe Titan’s southern sub-Saturn hemisphere from 576,000 kilometers (358,000 miles) to monitor clouds. Afterwards, on April 21, ISS will observe Saturn’s D ring, the innermost of Saturn’s main rings.
On April 22 at 06:08 UTC, Cassini will perform a targeted encounter of Titan. This is Cassini’s 127th and final targeted flyby of Titan. T126 has a close approach flyby has a close approach altitude of 979 kilometers (608 miles). This encounter will lower Cassini’s periapse so that it lies between Saturn’s atmosphere and the innermost D ring, thus marking the transition into Cassini’s proximal or Grand Finale orbits. Inbound to the encounter, ISS will be able to observe the southern, sub-Saturn and leading hemispheres, while on the outbound leg, ISS will observe a crescent Titan and the north polar region of Titan while riding along with VIMS and CIRS. Inbound, ISS will acquire a pair of mosaics of Titan. The first, a nine-frame mosaic, will cover the visible surface at 1.5 kilometers (0.94 miles) per pixel. The second, a 13-frame mosaic, will cover Menrva, Fensal, Aztlan, and Hotei Arcus at 870 meters (955 yards) per pixel. CIRS will also acquire mid-infrared scans and a far-infrared nadir stare on the inbound segment.
RADAR will be “prime” during the 12 hours surrounding closest approach. RADAR will acquire radiometry and scatterometry on both the inbound and outbound legs of the encounter. Closer in, RADAR will acquire high-altitude Synthetic Aperture RADAR (SAR) imaging over northeastern Xanadu followed by altimetry over western Fensal. RADAR will then acquire a SAR swath over the northern leading hemisphere, running from 0 degrees North Latitude, 55 degrees West Longitude to 67 degrees North Latitude, 75 degrees West Longitude. At closest approach, RADAR will acquire an altimetry swath over several small lakes on Titan’s northern hemisphere, including Oneida Lacus. This swath will be used to measure the depth of these lakes. In prior altimetry observations of Titan’s north polar, methane seas, radar echos from the sea surface and the sea bottom were detected. The strength of the bottom echo can provide a constraint on lake composition. Next, RADAR will acquire another SAR swath, this time covering eastern Ligeia Mare and running south to 45 degrees North Latitude, 255 degress West Longitude. Finally, RADAR will acquire another altimetry swath followed by more high-altitude SAR covering Kraken Mare and the terrain south and east of it.
On April 23, Cassini will reach apoapse, bringing Rev 270 to a close and starting up the next orbit, Rev 271, which will include Cassini’s first plunge between the rings and Saturn itself.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).