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Cassini continues its exploration of the Saturn system with the 13-day Rev179, which begins on January 12 at its farthest distance from the planet. This is also called the orbit's apoapse. At this point, Cassini is 1.66 million kilometers (1.03 million miles) from Saturn's cloud tops. Rev 179 occurs eight 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. Twenty-two ISS observations are planned for Rev 179 with the majority focused on Saturn's atmosphere and rings.
On January 13, ISS begins its observations for Rev 179 a day 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. Seven more observations are planned between January 14 and 17, while another three will be taken after periapse on January 24 and 25. Immediately after the first observation, 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 January 15, 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. Another astrometric observation will be taken on January 25. On January 16, 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 18 at 22:16 UTC, Cassini will reach periapse for Rev 179 at an altitude of 388,620 kilometers (241,480 miles) from Saturn. Late in the day on January 17, ISS will acquire a high-resolution movie of the inner D ring at high phase angles. This movie will be used to assess the density of dust in the inner portion of the ring system, a region Cassini will pass through in its final orbits in 2017. Next, ISS will ride along with a Visual and Infrared Mapping Spectrometer (VIMS) observation of the F ring. Early on January 18, ISS will monitor the south polar plume of Enceladus from a distance of 775,000 kilometers (481,000 miles). These images will be at a very high phase angle, achievable because the sun will be eclipsed by Saturn during the observation. Later that day, a few minutes after periapse, ISS will image the C ring of Saturn in search of small, embedded moonlets.
To round out the periapse period on January 19 and early on the 20th, ISS will ride along with a VIMS stellar occultation by the ring system of the Mira-like, red giant star HD120285 (one of the brightest stars in the night sky, in the near-infrared). Next, ISS will image the outer A ring, looking at the propellers previously seen in this part of the ring system. Propellers are small voids in Saturn's rings created by the gravitational interaction between large ring particles and the surrounding ring. Finally, ISS will acquire a high-resolution, low-phase, color scan of Saturn's ring system.
On January 25, Cassini will reach apoapse on this orbit, bringing it to a close and starting Rev180.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).