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Cassini continues its exploration of the Saturn system with the 13-day Rev178, which begins on December 29 at its farthest distance from the planet. This is also called the orbit's apoapse. At this point, Cassini is 1.65 million kilometers (1.03 million miles) from Saturn's cloud tops. Rev 178 occurs seven months into the first inclined phase of the Cassini Solstice Mission, a phase which lasts until March 2015. The inclined phase will allow for polar views of Saturn and Titan as well as better vistas of Saturn's rings than those Cassini had while in the earlier, equatorial phase of the Solstice Mission. Fifteen ISS observations are planned for Rev 178 with the majority focused on Saturn's atmosphere.
On December 30, ISS begins its observations for Rev 178 an hour and a half after apoapse with a quick observation of Saturn using the wide-angle camera (WAC). These observations are 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. Another Storm Watch observation is planned 12 hours later. Immediately after the second observation, ISS will acquire an 11-hour movie of the Encke Gap. This 325-kilometer (202-mile) gap in the outer A ring is carved by the gravitational effects of the small moon Pan, which orbits in the middle of the gap. The movie will focus on observing the crenulations that are visible along the edge of the gap as well as the narrow dust ring that accompanies Pan.
On January 2, ISS will acquire a series of images of Saturn rings using the WAC. These images will be tracking spokes -- a ring phenomenon Cassini has monitored throughout the mission -- over the B ring. With Cassini over the unlit side of the rings and with Saturn at a high phase angle, the spokes, if visible, will be brighter than the dark B ring. On January 3, ISS will ride along with a pair of Ultraviolet Imaging Spectrometer (UVIS) observations of Saturn's south polar aurora. In addition to making a movie of the planet's aurorae, the images will be used to independently measure the rotation period of Saturn's magnetic field. ISS will also look at Saturn's aurora during a Visual and Infrared Mapping Spectrometer (VIMS) observation of Saturn's south pole on January 4.
On January 5 at 15:10 UTC, Cassini will reach periapse for Rev 178 at an altitude of 388,080 kilometers (241,140 miles) from Saturn. Late in the day on January 4, ISS will image the limb of Saturn while the Sun is behind the planet. These images will provide an excellent opportunity for observing the various haze layers in the planet's upper atmosphere. Early on January 5, ISS will observe a half-phase Titan from a distance of 1.16 million kilometers (0.72 million miles). This observation is designed to look for clouds in the moon's atmosphere as part of the "Titan Monitoring Campaign" (TMC). The observation on January 5 is designed to monitor clouds over the moon's sub-Saturn hemisphere. ISS will also be taking shorter-wavelength images to study changes in Titan's upper haze layers. Afterward, ISS will acquire a pair of WAC images during two VIMS stellar occultation observations. These stellar occultations involve the stars Epsilon Eridiani and L2 Puppis passing behind the night side limb of Saturn. Finally, late in the day on the 5th, ISS will ride along with VIMS to acquire a mosaic of Saturn's north polar region using the Wide-Angle Camera (WAC). Spring has progressed far enough that the entirety of the hexagonal jet stream that lies near 77 degrees North latitude will be in sunlight. ISS will be imaging the hexagon with a two-by-two mosaic rather than centering the field-of-view on the north pole like it did in November.
On January 6, ISS will acquire a set of color images using the Narrow-Angle Camera (NAC) while pointed at Saturn's limb, near the equator. Combined with the early observation taken when Cassini was in Saturn's shadow, images from this observation will be used to better understand the structure of Saturn's upper atmosphere. Immediately afterward, ISS will ride along with VIMS to acquire several, WAC mosaics of Saturn's atmosphere. Finally, on January 12, just a few hours before the end of the orbit, ISS will search for clouds across Titan's Fensal-Aztlan region from a distance of 2.54 million kilometers (1.58 million miles).
On January 12, Cassini will reach apoapse on this orbit, bringing it to a close and starting Rev179.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).