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Cassini continues its exploration of the Saturn system with the 24-day Rev 233, which begins on February 27 at its farthest distance from the planet. This is also called the orbit’s apoapse. At this point, Cassini is 2.77 million kilometers (1.72 million miles) from Saturn’s cloud tops. Rev 233 occurs near the beginning of the second inclined phase of the Cassini Solstice Mission. Over the next several orbits, Cassini will use encounters with Titan to gradually increase the inclination of its orbit. Thirty-four ISS observations are planned for Rev 233 with the majority focused on Saturn’s atmosphere and its system of rings.
For its first observation for Rev 233, on February 28, ISS will observe Titan from a distance of 3.88 million kilometers (2.41 million miles). This observation is part of a campaign to monitor Titan for cloud activity. In the last couple of years, few clouds have been observed on Titan despite models predicting a surge in storms in the north polar region. More cloud monitoring observations will be acquired on March 3 and March 6. During the closest of these observations, on March 6, ISS will look for clouds across Titan’s northern leading hemisphere from a distance of 1.24 million kilometers (0.77 million miles). Immediately after the February 28 observation, ISS will acquire an astrometric observation of Saturn’s small, inner moons. Astrometric observations are used to improve our understanding of the orbits of these small satellites, which can be influenced by Saturn’s larger icy moons as well as each other. Careful measurements of the positions of these moons are important for later imaging of them at much closer distances during the F ring orbits later in 2016 and early next year. Additional astrometric observations will be taken on March 3 and 4.
Immediately after the astrometric observation on February 28, ISS will acquire a quick observation of Saturn using the Wide-Angle Camera (WAC). This observation is part of a series of “Storm Watch” sequences designed to take advantage of short, two-minute segments when the spacecraft turns to point the optical remote sensing (ORS) instruments back at Saturn, as a waypoint between observations. They include blue, clear, two methane band, and one full-frame, continuum band filter images. Four more Storm Watch observations will be taken between February 29 and March 5. On February 29 and March 1, ISS will observe the distant moon Ijiraq for nearly 30 hours. The observation will be taken from a distance of 8.70 million kilometers (5.40 million miles). While Ijiraq will only appear as a point of light, these two observations can be used to measure the length of its day as well the direction its north pole points.
On March 6, ISS will spend 14 hours acquiring a movie of the narrow F ring. On March 8, Cassini will perform a non-targeted encounter with Titan at a distance of 425,863 kilometers (264,619 miles). ISS will acquire a pair of Titan observations on March 7 and March 8. These observations will be used for mapping Titan’s northern leading hemisphere, searching for changes across the north polar region since it was last observed by ISS in early 2014. The observations will also observe Titan for clouds, which models suggest are overdue. The observation will swap between four-frame, mapping mosaics and longer cloud monitoring stares.
On March 9 at 23:00 UTC, Cassini will reach periapse for Rev 233 at an altitude of 306,170 kilometers (190,245 miles) above Saturn's cloud tops, just inside the orbit of Dione. On March 8 and 9, ISS will acquire a series of images of the A ring, focused on the region between the Encke Gap (carved by the small moon Pan) and the A ring’s outer edge. The movie is designed to recapture the giant propellers previously imaged by Cassini. Propellers are small voids in Saturn’s rings created by the gravitational interaction between large ring particles and the surrounding ring. A similar observation will be taken on March 10. Late on March 9, ISS will ride along with the Visual and Infrared Mapping Spectrometer (VIMS) as it observes a stellar occultation by the ring system of the supergiant star Epsilon Pegasi. Stellar occultations are used to measure the optical depth and fine-scale structure of the rings. Afterward, ISS will ride-along with the Composite Infrared Spectrometer (CIRS) to observe Mimas and its southern anti-Saturn hemisphere from a distance of 201,880 kilometers (125,440 miles). Early on March 10, ISS will acquire high-resolution images of the F ring.
Late on March 10, ISS will image the faint D ring, the innermost of the main rings. The observation will be acquired of the unlit side of the ring at high phase angles, making the dusty D ring easier to see. On March 11, ISS will follow that up with a movie observation of the D ring, near the edge of the shadow of Saturn cast on the rings. Afterward, ISS will ride along with the Ultraviolet Imaging Spectrometer (UVIS) to image Enceladus and its south polar plume from a distance of 979,130 kilometers (608,400 miles). During this observation, Enceladus and its plume will occult the star Epsilon Orionis, one of the stars of Orion’s Belt.
Between March 12 and March 19, ISS will acquire eight observations of Saturn’s atmosphere, including four ride along observations with CIRS and UVIS. The four prime observations on March 15, 16, and 17 will be used to track clouds within Saturn’s equatorial zone in order to measure wind speeds there. On March 17 and 21, ISS will acquire cloud monitoring observations of Titan. The observation on March 17 will cover Titan’s Fensal-Aztlan region from a distance of 2.74 million kilometers (1.70 million miles). The observation on March 21 will be taken from a distance of 1.79 million kilometers (1.11 million miles), and will cover Titan’s northern leading hemisphere. On March 21 and 22, ISS will observe the distant moon Surtur for 27 hours. The observation will be taken from a distance of 11.6 million kilometers (7.21 million miles).
On March 21, Cassini will reach apoapse, bringing Rev 233 to a close and starting up the next orbit, Rev 234. During the next orbit, Cassini will perform a targeted flyby of Titan.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).