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Cassini continues its exploration of the Saturn system with the 24-day Rev173, which begins on October 6 at its farthest distance from the planet. This is also called the orbit's apoapse. At this point, Cassini is 2.66 million kilometers (1.66 million miles) from Saturn's cloud tops. Rev173 is near the start of 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-eight ISS observations are planned for Rev173, most focused on Saturn's atmosphere and rings. During the last week of the orbit, from October 22 to 29, Cassini will enter solar conjunction, when the spacecraft and Saturn will be on the other side of the Sun from Earth. Communications with the spacecraft are limited during this period due to interference from the sun's outer atmosphere. Thus, no ISS observations are performed during this orbit after October 19.
ISS begins its observations for Rev173 a few hours after apoapse on October 6 with three quick observations 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 such observations are planned between October 12 and 16.
Also on October 6, ISS will image a half-phase Titan from a distance of 3.76 million kilometers (2.34 million miles). The observation is an effort to look for clouds in the moon's atmosphere as part of the "Titan Monitoring Campaign" (TMC). This observation is designed to monitor clouds over portions of the moon's sub-Saturn hemisphere. ISS also will be taking shorter-wavelength images to study changes in Titan's upper haze layers. After another Saturn storm watch observation, 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 such observation will be taken on October 12. Also on October 6, 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. Additional spoke movies will be taken on October 10 and 12. Movies will also be taken of the faint, inner D ring and the narrow F ring on October 15 using the narrow-angle camera (NAC).
On October 18 at 09:02 UTC, Cassini will reach periapse for Rev173 at an altitude of 414,480 kilometers (257,550 miles) from Saturn. During the periapse period on October 17, ISS will acquire a 41-frame mosaic using the WAC of the inner Saturn system while Cassini is in eclipse. This observation will allow the dusty E and G rings to be imaged in detail. Afterward, ISS will monitor the south polar plume of Enceladus from a distance of 940,000 kilometers (580,000 miles). Next, ISS will acquire several short mosaics of the faint D ring at close range using the NAC. On October 18, ISS will ride along with the Ultraviolet Imaging Spectrometer (UVIS) to observe an occultation of the blue subgiant star Gamma Columbae by the F ring followed by a Visual and Infrared Mapping Spectrometer (VIMS) observation of an occultation of the red-orange giant star Lambda Velorum. Afterward, ISS will acquire high-resolution NAC images of the F ring.
Early on October 19, ISS will re-image several propellers in the A ring in order to improve our knowledge of their orbits. These propeller-shaped features are small voids formed by large particles or objects in Saturn's main rings. Afterward, ISS will ride along with a UVIS observation of Enceladus at a distance of 966,000 kilometers (600,000 miles). Finally, ISS will ride along again with UVIS to observe an occultation by Saturn's main ring system of the blue-giant star Spica, brightest star in the constellation Virgo.
On October 30, Cassini will reach apoapse on this orbit, bringing it to a close and starting Rev174. Rev174 includes the penultimate Titan flyby for 2012.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).