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Cassini continues its exploration of the Saturn system with the 7.2-day Rev 261, which begins on February 10 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 261 is the 11th 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. Eleven ISS observations are planned for Rev 261 with the majority focused on Titan and Saturn’s atmosphere.
On February 14 at 06:18 UTC, Cassini will reach periapse for Rev 261 at an altitude of 89,498 kilometers (55,611 miles), near the orbits of Janus and Epimetheus. Most of the observations during the periapse period focus on Saturn’s atmosphere. During ISS’ first observation for Rev 261, late on February 13, the camera system will ride along with a Visual and Infrared Mapping Spectrometer (VIMS) observations of Saturn’s north polar region. Early on February 14, ISS will acquire a pair of observations of Saturn’s bright limb to examine the planet’s upper haze layers, both before and after periapse. Similar observations will be acquired late on February 15 as well as toward the end of Rev 262 on February 24. Early on February 15, ISS will ride along with the Ultraviolet Imaging Spectrometer (UVIS) to image Saturn’s south polar aurorae.
On February 17, Cassini will perform a non-targeted encounter with Titan at a distance of 186,798 kilometers (116,070 miles). The Composite Infrared Spectrometer (CIRS) and ISS will acquire a series of observations throughout February 17. This and similar sequences of observations taken during close Titan encounters throughout the Cassini Grand Finale will be used to map Titan’s northern sub-Saturn hemisphere, to monitor cloud development and compare their frequency and location to climate models, and to track changes in Titan’s north polar atmosphere resulting from seasonal activity. Titan and the rest of the Saturn system are approaching northern summer solstice in May. ISS will acquire a series of mosaics during this encounter of Titan’s north polar region and its northern sub-Saturn hemisphere.
During the encounter period, on February 17, Cassini will reach apoapse, bringing Rev 261 to a close and starting up the next orbit, Rev 262. Fifteen ISS observations are planned for this orbit, with most covering Saturn’s rings. On February 19, ISS will ride along with a stellar occultation by the rings by VY Canis Majoris, one of the largest known stars in the night sky. By measuring how the star’s light dims as the rings pass in front of it, the fine-scale structure of the rings can be observed. ISS images will be focused on the period when the F ring will occult the star. ISS will acquire images during similar stellar occultations of Beta Crucis on February 20 and of Betelgeuse on February 22. Next, on February 19, ISS will acquire an astrometric observation of several of 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 acquire a nine-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 February 21 at 10:24 UTC, Cassini will reach periapse for Rev 262 at an altitude of 87,389 kilometers (54,301 miles), near the orbits of Janus and Epimetheus. Most of the observations during the periapse period focus on Saturn’s rings. For an hour before and after closest approach, ISS will acquire a pair of observations of Santos-Dumont, a large ring particle that orbits between the Keeler and Encke Gaps in the outer A ring. The gravitational pull of ring particles like Santos-Dumont create propeller-shaped voids in the ring system. A lower-resolution, survey observation of these propellers will be acquired on a few hours later.
At 10:01 UTC, Cassini will perform a non-targeted encounter with Epimetheus, an irregularly-shaped moon of Saturn orbiting between Mimas and Saturn’s main rings, at a distance of 8,265 kilometers (5,136 miles). ISS will acquire 26 images during the 30 minutes before the encounter, hopefully including Epimetheus’s northern hemisphere. After the second Santos-Dumont observation, ISS will observe the middle of the unlit C ring. On February 22, ISS will ride along with VIMS to observe the unlit side of the rings while Cassini is in Saturn’s shadow. On February 23, ISS will acquire another observation of the D ring and inner C ring.
On February 25, Cassini will reach apoapse, bringing Rev 262 to a close and starting up the next orbit, Rev 263.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).