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Cassini continues its exploration of the Saturn system with the 20-day Rev 232, which begins on February 7 at its farthest distance from the planet. This is also called the orbit’s apoapse. At this point, Cassini is 2.15 million kilometers (1.34 million miles) from Saturn’s cloud tops. Rev 232 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. Forty ISS observations are planned for Rev 232 with the majority focused on Titan, Saturn’s atmosphere, and its rings.
For the first observation of Rev 232, on February 10, ISS will acquire a movie observation of the narrow F ring, lasting more than 13 hours. On February 11, ISS will observe Titan during a pair of observations from a distance of 2.97 million kilometers (1.85 million miles). These observations are 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 February 12 and February 14. All four observations will be used to look for clouds across Titan’s sub-Saturn hemisphere. Afterward, 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 subsequent imaging of them at much closer distances during the F ring orbits later this year and early next year. A similar observation will be taken later on the 11th. Additional astrometric observations will be taken on February 12, 19, 21, and 25.
Immediately after the first astrometric observation on February 11, 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. Two more Storm Watch observations will be taken on February 11. Four storm watch observations will be taken on February 12 and 13, and seven more will be taken between February 19 and 26. On February 13, 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.
On February 15 at 02:57 UTC, Cassini will reach periapse for Rev 232 at an altitude of 173,640 kilometers (107,890 miles) above Saturn's cloud tops, just inside the orbit of Enceladus. The only ISS observation planned for the periapse period is a ride-along observation with the Visual and Infrared Mapping Spectrometer (VIMS) during a non-targeted encounter with Enceladus. Cassini will encounter Enceladus at a distance of 82,650 kilometers (51,360 miles). This encounter will allow imaging of Enceladus’s southern trailing hemisphere. During the periapse period, the Ultraviolet Imaging Spectrometer (UVIS) will observe several stellar occulations by Saturn’s atmosphere. The magnetometer will also directly measure the magnetic flux tube between Enceladus and Saturn’s atmosphere.
On February 16 at 23:52 UTC, Cassini will perform a targeted encounter of Titan. This is Cassini’s 118th flyby of Titan and the third of eleven planned for 2016. The next encounter is planned for April 4 during Rev 234. T117 flyby has a close approach altitude of 1,018 kilometers (633 miles). This encounter will increase the inclination of Cassini’s orbit from 16 degrees to 20.6 degrees. On approach, CIRS will view the sub-Saturn hemisphere of Titan, acquiring nadir and limb sounding observations. VIMS will then acquire a pair of mapping observations. ISS will ride-along with these sequences to look for clouds across Titan’s Fensal-Aztlan region.
Around closest approach, the Radio Science Sub-system (RSS) will be prime. The primary focus of its observation is a grazing radio occultation of Titan’s atmosphere. RSS will measure the refractivity of Titan’s atmosphere, which allows composition, temperature, and pressure to be derived. These measurements can also be used to measure wind speeds in Titan’s troposphere, the lowest part of its atmosphere. On egress from the occultation, RSS will acquire a radio bistatic swath across several of the small lakes on the anti-Saturn side of Titan’s north polar region. This can provide estimates of surface reflectivity, roughness, and dielectric constant (which is often related to composition). Outbound, CIRS will acquire a series of nadir and limb sounding observations of a crescent Titan. The day after the encounter, ISS will acquire several cloud monitoring observations. The high phase angle may make it difficult to observe clouds, but would make observing Titan’s upper haze layers easier.
On February 21, ISS will acquire a movie of the Encke Gap, in the outer A ring, that will last 19 hours. On February 22 and 23, ISS will observe the distant moon Skoll for more than 30 hours. The observation will be taken from a distance of 18.1 million kilometers (11.2 million miles). While Skoll 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 the north pole points. On February 26, ISS will image the outer edge of the A ring for calibration purposes.
On February 27, Cassini will reach apoapse, bringing Rev 232 to a close and starting up the next orbit, Rev 233. During the next orbit, Cassini will perform a non-targeted flyby of Titan.
Image products created in Celestia. Enceladus basemap by Steve Albers. All dates in Coordinated Universal Time (UTC).