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Cassini continues its exploration of the Saturn system with the 16-day Rev 231, which begins on January 22 at its farthest distance from the planet. This is also called the orbit’s apoapse. At this point, Cassini is 2.17 million kilometers (1.35 million miles) from Saturn’s cloud tops. Rev 231 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. Twenty-five ISS observations are planned for Rev 231 with the majority focused on Titan and Saturn’s rings.
For the first observation of Rev 231, on January 23, 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. The observation includes blue, clear, two methane band, and one full-frame, continuum band filter images. Two more Storm Watch observations will be taken on January 26 and 27. Immediately after the storm watch observation on January 23, 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. A similar observation will be taken on January 26.
On January 26, ISS will image Titan from a distance of 2.94 million kilometers (1.83 million miles) in order to monitor clouds across Senkyo, a dark, sand-filled region on Titan’s sub-Saturn hemisphere. ISS will also take a set of red-green-blue, Wide-Angle-Camera (WAC) images of Titan and some images of Saturn’s rings. ISS will take more cloud monitoring observations on January 28, January 29, and January 30. The closest of these, taken on January 30 from a distance of 1.42 million kilometers (0.88 million miles), will be centered on Titan’s sub-Saturn hemisphere. Red-green-blue WACs will also be taken during the January 30 observation.
On January 30 at 04:56 UTC, Cassini will reach periapse for Rev 231 at an altitude of 151,170 kilometers (93,932 miles) above Saturn's cloud tops, just inside the orbit of Pallene. Many of the observations in the periapse period are focused on Saturn’s rings. On January 28, ISS will acquire an 11-hour movie of the Cassini Division, a low-density “gap” between the A and B rings in Saturn’s main ring system when Cassini is 1.13 million kilometers (0.70 million miles) from Saturn. On January 29, the camera system will image the outer A ring, where it will be looking at 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. Shortly afterward, ISS will ride along with the Composite Infrared Spectrometer (CIRS) to image the C ring. On January 30, ISS will observe tiny Anthe from a distance of 118,620 kilometers (73,710 miles). Despite the relatively close distance, Anthe will only appear 2 to 3 pixels across. A few hours later, ISS will acquire a high-resolution observation of Saturn’s innermost D ring.
On February 1 at 01:00 UTC, Cassini will perform a targeted encounter of Titan. This is Cassini’s 117th flyby of Titan, the second of eleven planned for 2016. The next encounter is planned for the next orbit on February 16. The T116 flyby has a close approach altitude of 1,400 kilometers (870 miles). This encounter will dramatically increase the inclination of Cassini’s orbit from 2.4 degrees to 16 degrees. On approach, CIRS will view the sub-Saturn hemisphere of Titan, acquiring nadir and limb sounding observations. ISS will ride-along with these sequences to look for clouds across Titan’s Fensal-Aztlan region.
Around closest approach, the Ultraviolet Imaging Spectrometer (UVIS) will be prime. First, UVIS and the Visual and Infrared Mapping Spectrometer (VIMS) will observe a solar occultation by Titan’s atmosphere, sampling Titan’s atmosphere near 63 degrees South latitude during ingress and 53 degrees North latitude during egress. At closest approach, UVIS will observe stellar occultations by Titan’s atmosphere of the stars Epsilon Orionis and Zeta Orionis, the center and right stars on Orion’s belt. These occultations will be used to provide composition and temperature profiles of Titan’s upper atmosphere. These profiles are of high value because they can fill a gap between where Cassini has directly sampled at altitudes down to 950 kilometers (590 miles) and where Titan’s atmosphere becomes optically thick around 200 to 350 kilometers (125 to 220 miles) and where CIRS and VIMS can acquire vertical profiles.
Outbound, ISS will acquire WAC images of a crescent Titan, looking to monitor clouds near Titan’s north pole and to look for changes in Titan’s upper haze layers. UVIS will then acquire a far- and extreme-ultraviolet scanning observation of Titan. Finally, CIRS will acquire a pair of nadir and limb sounding observations.
On February 2, ISS will observe the D ring at high phase angles. High phase angles are useful for observing dusty rings. On February 4, ISS will observe the distant moon Albiorix from a distance of 23.9 million kilometers (14.9 million miles). While Albiorix will only appear as a point of light, this observation can be used to measure the length of Albiorix’s rotation as well the direction the north pole points. On February 6, ISS will acquire a movie of the outer A ring.
On February 7, Cassini will reach apoapse, bringing Rev 231 to a close and starting up the next orbit, Rev 232. During the next orbit, Cassini will perform another targeted flyby of Titan.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).