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Cassini continues its exploration of the Saturn system with the 28-day Rev 214, which begins on March 28 at its farthest distance from the planet. This is also called the orbit's apoapse. At this point, Cassini is 3.16 million kilometers (1.97 million miles) from Saturn's cloud tops. Rev 214 occurs shortly after the start of the second Equatorial phase of Cassini's extended-extended mission. During this phase, Cassini will orbit within the orbital plane of Saturn's rings and most of its major satellites, allowing for frequent encounters with Saturn's icy satellites. Fifty-three ISS observations are planned for Rev 214 with most focused on Saturn's atmosphere and icy satellites.
For ISS's first observation of Rev 214, on March 29 and 30, the camera system will observe the northern sub-Saturn hemisphere of the distant icy satellite Iapetus from a distance of 979,000 kilometers (609,000 miles). ISS will observe Iapetus again on March 31 and April 1 from similar distances. While distant compared to the September 2007 flyby, this is one of the closest approaches Cassini will make to Iapetus during the Solstice Mission and provides improved coverage of the moon's northern hemisphere. Starting early on April 2, ISS will observe the small, distant moon Paaliaq from a distance of 9.63 million kilometers (5.98 million miles) for more than a day and a half. This observation is design to measure the moon's orbital period by observing how its brightness changes as it rotates. Combined with observations on April 10, April 12, and April 23, changes in its brightness can be used to estimate the orientations of its pole axis and its rough shape. The April 12 observation will be acquired from a relatively close distance of 6.78 million kilometers (4.21 million miles).
On April 4, ISS will observe a crescent Titan from a distance of 1.52 million kilometers (0.95 million miles). For the first orbit since Rev 196 in August 2013, Cassini will not be flying by Titan during Rev 214. Instead, ISS will acquire eight more haze monitoring observations, between April 5 and April 19, showing a narrow, hazy crescent. The closest of these will be acquired on April 17 from a distance of 1.42 million kilometers (0.88 million miles). Later on April 4, 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. Three more astrometric observation sequences will be acquired between April 5 and April 9. Immediately afterward the April 4 astrometric sequence, ISS will acquire a quick observation of Saturn using the Wide-Angle Camera (WAC). This observation is part of a series of "Storm Watch" observation sequences designed to take advantage of short, two-minute segments when the spacecraft turns the optical remote sensing (ORS) instruments back to Saturn as a waypoint between other experiments' observations. These sequences include blue, clear, two methane band, and one full-frame, continuum band filter images. Seven more storm watch observations will be taken between April 5 and April 8. Eleven more will be taken between April 13 and April 20.
On April 4, ISS will also observe Tethys pass behind the south pole of Dione. Dione will be 2.37 million kilometers (1.47 million miles) away, while Tethys will be a little farther away at a distance of 2.85 million kilometers (1.77 million miles). Afterward, ISS will spend 24 hours looking at the faint E ring, a diffuse ring generated by material erupting from Enceladus' south polar region. On April 9, ISS will ride along with the Ultraviolet Imaging Spectrometer (UVIS) to observe Saturn's south polar aurorae, or southern lights.
On April 11 at 17:09 UTC, Cassini will reach periapse for Rev 214 at an altitude of 276,500 kilometers (171,800 miles) from Saturn's cloud tops, between the orbits of Tethys and Dione. At 17:58 UTC, Cassini will perform a non-targeted encounter with Tethys, passing over its anti-Saturn and trailing hemispheres at a distance of 52,866 kilometers (32,849 miles). ISS will acquire five observations during this encounter, including two stares, a nine-frame mosaic, and two, fourteen-frame mosaics, one acquired shortly before closest approach and another after. The two largest, global mosaics will be centered around the large, ancient crater Penelope. After the Tethys encounter, ISS will observe Dione as Cassini passes it at a distance of 110,134 kilometer (68,434 miles). ISS will acquire a mosaic of Dione's trailing hemisphere. It will also watch as Rhea appears to pass above Dione's north pole. Rhea will be 500,000 kilometers (311,000 miles) away during the mutual event.
On April 16, ISS will acquire high-phase observations of Saturn's diffuse E ring. The high phase angle will highlight the dust that is dominant in that ring. Observations near the ring plane will also allow ISS to observe the vertical structure of dust in the E ring. The E ring is generated by dust from Enceladus' south polar plume, and that will be observed by ISS on April 13 and 14. On April 14, ISS will observe the main ring system edge-on as the spacecraft crosses below the ring plane. On April 24, immediately after this orbit's fourth ISS observation of Paaliaq, ISS will spend 24 hours observing the small, distant moon Bebhionn. Researchers are hoping to observe brightness variations as the satellite rotates so they can measure its rotation speed. Bebhionn will be 10.0 million kilometers (6.24 million miles) away during the observation.
On April 25, Cassini will reach apoapse, bringing Rev 214 to a close and starting up the next orbit, Rev 215, which will include a targeted flyby of Titan and a non-targeted encounter with tiny Polydeuces.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).