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Cassini begins the 18-day Rev155 on October 10 at its farthest distance from Saturn, called apoapse. At this point, Cassini is 2.37 million kilometers (1.48 million miles) from Saturn's cloud tops. The spacecraft is in the middle of the first equatorial phase of the Cassini Solstice Mission, which lasts until May 2012. During this phase, the spacecraft's orbits lie within the equatorial plane of the planet, providing opportunities to encounter Saturn's numerous moons, image the rings edge-on, and look at Saturn's cloud tops without the rings obscuring the view. Twenty-six ISS observations are planned for Rev155, many of which are designed to monitor cloud systems in Saturn's atmosphere. During this orbit, Cassini will also encounter the icy moon Enceladus.
ISS begins its observations for Rev155 four hours after apoapse with a Saturn storm watch observation. The storm watch campaigns are 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. These observations can be used to track the progress of the northern hemisphere storm that began in early December 2010. Eleven more are planned between October 17 and 23. On October 11, Cassini will enter solar conjunction, a five-day period when the sun sits almost directly between the spacecraft and its controllers on Earth. The solar corona during this time disrupts communications, so scientific observations and downlinks are much more limited. However, the Radio Science Subsystem (RSS) will use this time to study the properties of the sun's outer atmosphere by sending X-, Ka-, or S-band radio signals to Earth. By seeing how the signal is affected, the RSS team hopes to learn more about the electron content in the corona. On October 18, ISS will monitor Titan during a pair of observations that are designed to search for clouds across the moon's sub-Saturn hemisphere. The first, M60R2CLD291 will be taken from a distance of 1.33 million kilometers (0.83 million miles) while the other, CLOUD001, will be acquired from a distance of 1.17 million kilometers (0.73 million kilometers).
On October 19 at 12:23 UTC, Cassini will reach periapse for Rev155 at a distance of 136,330 kilometers (84,710 miles) from Saturn. During this periapse, Cassini will perform a targeted encounter with Enceladus. This encounter, called E15, will have a closest approach at 09:22 UTC at a distance of 1,231 kilometers (765 miles). This is the second of three encounters planned for October and November. During the encounter, ISS will take four observations of the satellite. The first will be taken on approach when only a thin crescent is visible. This observation is designed to monitor Enceladus' south polar plume. Composed of vapor and very fine particles, the plume is more visible at high phase angles than when the satellite appears more fully illuminated at low phase angles. This observation ends at 06:40 UTC, before closest approach. Next, the Composite Infrared Spectrometer (CIRS) will perform a scan of the night side of Enceladus, while ISS will acquire a mosaic of the narrow crescent. Around closest approach, the Ultraviolet Imaging Spectrometer (UVIS) will observe a stellar occultation of Enceladus' plume using Epsilon Orionis, the middle star of Orion's Belt. Stellar occultations provide high-resolution information of the density and location of the jets that combine to form the moon's south polar plume. ISS will ride along taking context imaging of this occultation. The fourth ISS observation is a ride along observation with CIRS. CIRS will measure the temperature of Enceladus' equatorial region using its FP1 channel while the moon is in eclipse. ISS will image the moon as it leaves Saturn's shadow, taking wide-angle-camera images during egress.
On October 20, ISS will image the G ring arc, tracking its location as it is maintained by a 7:6 orbital resonance with Mimas and resupplied by micrometeorite impacts on the embedded moonlet, Aegeaon. On October 21, ISS will acquire a 15-hour light curve observation of the outer, irregular satellite, Loge. This observation will be used to pin down the orbital period of that small moon, as Cassini ISS has done for several other outer satellites such as Albiorix, Siarnaq, Ymir, Bebhionn, and Kiviuq. On October 22, ISS will image Titan's Fensal-Aztlan region from a distance of 2.41 million kilometers (1.50 million miles) in an effort to look for clouds in the moon's atmosphere. On October 22 and 25, ISS will acquire a pair of astrometric observations of Saturn's small, inner moons. To finish up Rev155, Cassini will spend the bulk of three days, October 24 to 26, observing Titan in a trio of long observations of the moon. These observations will allow ISS to track clouds on Titan for long periods and allow researchers to measure wind speeds in the troposphere, assuming that clouds are visible during this period. These observations will be taken from distances ranging from 2.28 million kilometers (1.42 million miles) for the October 24 observation to 1.59 million kilometers (0.99 million miles) during the October 26 one.
On October 28, Cassini will reach apoapse on this orbit, bringing it to a close and starting Rev156. The next orbit will see another targeted encounter of Enceladus.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC). Enceladus basemaps by Steve Albers.