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Cassini continues its exploration of the Saturn system with the 32-day Rev 212, which begins on January 25 at its farthest distance from the planet. This is also called the orbit's apoapse. At this point, Cassini is 3.40 million kilometers (2.11 million miles) from Saturn's cloud tops. Rev 212 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. Forty-four ISS observations are planned for Rev 212 with the majority focused on Saturn's atmosphere and on Titan during the T109 flyby.
For ISS's first observation of Rev 212 a couple hours after apoapse on January 25, the camera system will ride along with the Composite Infrared Spectrometer (CIRS) as it performs a calibration sequence. ISS will be able to acquire Narrow-Angle Camera (NAC) images of Saturn's bright limb. A similar observation will be taken on February 26. Immediately afterward, on January 26, 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 January 26 and February 8. Another seven will be taken between February 19 and 26. Later on January 26, 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 February 4. Finally, on January 26, ISS will acquire a lightcurve observation of the distant moon, Siarnaq. This observation will last nearly 13 hours. 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. Siarnaq will be 23.1 million kilometers (14.3 million miles) away at the time of the observation. A similar observation of Siarnaq will be taken on February 3.
On February 7, 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. On February 8, 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. On February 8, the NAC will image Titan, looking for clouds across its northern trailing hemisphere, at a distance of 2.46 million kilometers (1.53 million miles). A similar observation will be taken on February 10 from a distance of 1.77 million kilometers (1.10 million miles).
On February 10 at 17:17 UTC, Cassini will reach periapse for Rev 212 at an altitude of 355,200 kilometers (220,700 miles) from Saturn's cloud tops. On February 9, ISS will ride along with the Visual and Infrared Mapping Spectrometer (VIMS) to image the northern hemisphere of Saturn which is increasingly in the shadow of the planet's ring system as it approaches winter solstice in May 2017. The WAC will perform a similar ride-along observation on February 10 covering the southern hemisphere. Early on February 10, at 06:46 UTC, Cassini will perform a non-targeted encounter with Saturn's second largest moon, Rhea. The spacecraft will fly by the moon at a distance of 46,943 kilometer (29,169 miles). ISS will acquire a pair of mosaics of Rhea. The first, an eleven-frame mosaic, will be taken at a distance of 78,400 kilometers (48,700 miles), and will cover the northern, anti-Saturn hemisphere. The second, a 16-frame mosaic, will be taken at a distance of 53,700 kilometers (33,400 miles). The highest resolution images will have a scale of 305 meters (1,000 feet) per pixel. This mosaic will cover a half phase Rhea. This is the first of 20 targeted and non-targeted flybys of icy satellites (moons other than Titan) this year where ISS will be acquiring images and the only one of Rhea. On February 11, ISS will image Saturn's D ring, the innermost portion of the main ring system, at high resolution. This observation will help to constrain the particle density of the innermost portion of the D ring. Cassini will be passing between the inner D ring and Saturn's atmosphere during the proximal orbits of 2017.
Cassini encounters Titan on February 12 at 17:08 UTC for the 110th time. This is the second of seven Titan flybys planned for 2015, with the next encounter scheduled for March 16. T109 has a close-approach altitude of 1,200 kilometers (746 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 a pair of VIMS observations of Titan, which ISS will ride along with. These observations will be used to monitor any clouds that might be visible across the sub-Saturn hemisphere. After an observation by the Ultraviolet Imaging Spectrometer (UVIS), VIMS will be prime instrument during closest approach. The instrument will observe the Sinlap impact crater in eastern Fensal followed by an imaging swath across parts of the north polar region which will include Bolsena Lacus and Punga Mare. UVIS and the Composite Infrared Spectrometer (CIRS) will acquire a series of observations after closest approach. CIRS will acquire temperature map data of Titan's night side as well as compositional measurements along Titan's limb. VIMS will look for specular reflections off Titan's lakes and seas. ISS will ride along, observing a crescent Titan.
On February 15, ISS will acquire a time-lapse movie of the Cassini Division between the A and B rings. On February 18 and February 26, ISS will ride along with CIRS calibration observations. Also on February 18, ISS will ride along with UVIS to observe a stellar occultation by Saturn's ring system of the B-type star, Kappa Orionis, the "left knee" of the constellation Orion. On February 22 and 23, ISS will observe the distant, outer moon Loge from a distance of 18.6 million kilometers (11.5 million miles) away. The brightness of Loge will be measured using a series of image taken over a period of 1.5 days. How its brightness changes over time will be used to measure the length of its day.
On February 26, Cassini will reach apoapse, bringing Rev 212 to a close and starting up the next orbit, Rev 213, which will include another targeted flyby of Titan. Rev 213 will also bring the first inclined phase of the Cassini Solstice Mission to an end, beginning the second equatorial phase.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).