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Cassini continues its exploration of the Saturn system with the 32-day Rev 211, which begins on December 24 at its farthest distance from the planet. This is also called the orbit's apoapse. At this point, Cassini is 3.33 million kilometers (2.07 million miles) from Saturn's cloud tops. Rev 211 occurs near the end of the first inclined phase, which lasts until March 2015, of the Cassini Solstice Mission. The inclined phase allows for polar views of Saturn and Titan as well as better vistas of Saturn's rings than those Cassini will have in the upcoming, second equatorial phase of the Solstice Mission. Fifty ISS observations are planned for Rev 211 with the majority focused on Saturn's atmosphere and on Titan during the T108 flyby.
ISS's first observation of Rev 211 is a lightcurve observation of the distant moon, Erriapus. This observation will begin an hour after apoapse and will last nearly 20 hours. 265 images will be acquired of Erriapus. The small moon's brightness will be measured in each frame, and the variations in how bright it is can be used to measure its rotation rate and the direction its rotation axis points. Erriapus will be 15.4 million kilometers (9.56 million miles) away at the time of the observation. On December 26, the Narrow-Angle Camera (NAC) will image Titan, looking for clouds across its northern sub-Saturn hemisphere, at a distance of 3.46 million kilometers (2.15 million miles). Three similar cloud monitoring observations will be acquired between December 27 and December 30. The closest of these will be taken on December 30 from a distance of 2.15 million kilometers (1.33 million miles) and will cover the northern leading hemisphere of Titan.
Immediately after the Titan observation on December 27, 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. Eight more storm watch observations will be taken between December 27 and January 7. Another six will be taken between January 15 and January 21. Finally, on December 27, 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. Similar sequences will be acquired on January 1, 2, and 6.
On January 2 and 3, ISS will acquire a 16-hour movie of the Encke Gap in Saturn's outer A ring. Afterward, ISS will acquire a movie of the F ring, observing its various channels and streamers created by the interaction between the ring material and the nearby moon, Prometheus. Between January 4 and 6, ISS will acquire a pair of distant observations of Iapetus, taken from a distance of around four million kilometers (2.49 million miles). These observations will be taken at very low phase angles (with images on January 6 taken when Iapetus is nearly full). These images can be used to study color variations within the dark terrain on the moon's sub-Saturn hemisphere and to better understand the moon's surface roughness. On January 7, ISS will observe Saturn's second largest outer satellite, Phoebe, from a distance of 12.7 million kilometers (7.92 million miles). While Phoebe will only be 3 pixels across in these images, the images can be used to better understand color variations across Phoebe by measuring how its brightness changes, in different filters, as it rotates.
On January 9 at 17:55 UTC, Cassini will reach periapse for Rev 211 at an altitude of 431,580 kilometers (268,170 miles) from Saturn's cloud tops. Most of the images acquired near periapse will cover Saturn's rings. On January 8, ISS will ride along with the Ultraviolet Imaging Spectrometer (UVIS) as it observes an occultation by Saturn's rings of the star Spica, followed by another UVIS observation, when ISS will acquire a series of images across Saturn's ring system. Next, ISS will acquire a set of high-resolution, color NAC images across the A and B rings. Afterward, ISS will monitor clouds across Titan's trailing hemisphere from a distance of 1.90 million kilometers (1.18 million miles). On January 9, ISS will acquire a high-resolution observation of Saturn's D ring and ride along with the Visual and Infrared Mapping Spectrometer (VIMS) as it observes a stellar occultation by the ring system of the red giant, Alpha Herculis. On January 10, the camera system will image the outer A ring, where it will be looking at propellers previously imaged by Cassini, followed by a high-resolution observation of the F ring. Propellers are small voids in Saturn's rings created by the gravitational interaction between large ring particles and the surrounding ring. Finally, ISS will ride along with another UVIS stellar occultation by the unlit side of Saturn's rings, this time involving Gamma Pegasi, one of the four stars of the Great Square.
Cassini encounters Titan on January 11 at 19:49 UTC for the 109th time. This is the first of seven Titan flybys planned for 2015, with the next encounter scheduled for February 12. T108 has a close-approach altitude of 970 kilometers (603 miles). Inbound, ISS will observe Titan's southern, sub-Saturn hemisphere. Outbound, ISS will be able to observe a crescent Titan over its northern, sub-Saturn hemisphere. Observations for this encounter will start with ISS acquiring a four-frame mosaic at a distance of 330,000 kilometers (205,000 miles). The mosaic, centered just south of Elba Facula, will cover most of the visible surface of Titan. Afterward, ISS will ride along with the Composite Infrared Spectrometer (CIRS) and VIMS as they observe Titan's surface and atmosphere.
From two hours before closest approach until five hours after, the Cassini RADAR team will control pointing. In addition to radiometry and scatterometry of Titan's surface, used to constrain composition and measure surface roughness, RADAR will acquire altimetry over Punga Mare. This altimetry data will be used to measure the depth of this methane/ethane sea by looking for a bottom echo. A bottom echo was measured earlier at Ligeia Mare, but not at Kraken Mare. An altimetry swath across Punga Mare will be used to help determine whether the lack of a bottom echo at Kraken is due to compositional differences (higher ethane content for example) or due to Kraken Mare being much deeper than Ligeia, as suggested by the steep cliffs along portions of the shoreline. Afterward, a RADAR SAR imaging swath will be acquired across Ligeia Mare, including over the "Magic Island", a transient feature near the middle of the sea. High-altitude SAR imaging will also be acquired across the Aaru dune field before closest approach and portions of the north polar region after closest approach.
After closest approach, ISS will ride along with CIRS as it acquires temperature map data of Titan's night side as well as compositional measurements along Titan's limb. In the day after closest approach, on January 13, ISS will acquire three cloud monitoring observations of Titan. ISS will also image a crescent Titan from a distance of 1.74 million kilometers (1.08 million miles) on January 15. On January 17 and 18, ISS will acquire a color photometric observation of the distant moon, Albiorix. The small moon will be 7.57 million kilometers (4.70 million miles) away at the start of the 37-hour observation. On January 20, ISS will ride along with UVIS as it observes a stellar occultation by the rings of the B-type star Beta Canis Majoris.
On January 25, Cassini will reach apoapse, bringing Rev 211 to a close and starting up the next orbit, Rev 212, which will include another targeted flyby of Titan.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).