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Cassini continues its exploration of the Saturn system with the 17-day Rev166, which begins on May 11 at its farthest distance from the planet. This is also called the orbit's apoapse. At this point, Cassini is 2.37 million kilometers (1.47 million miles) from Saturn's cloud tops. This orbit includes a flyby of Saturn's largest moon, Titan. The flyby will push the spacecraft from its current equatorial orbit into a tilted one, beginning the first of two inclined orbit phases during the Cassini Solstice Mission. This phase of the mission, which lasts until March 2015, will allow for polar views of Saturn and Titan as well as better vistas of Saturn's rings than those Cassini has viewed while in the earlier, equatorial phase of the Solstice Mission. Twenty-seven ISS observations are planned for Rev166, the vast majority dedicated to Saturn and Titan storm cloud monitoring, as well as to the Titan flyby.
ISS begins its observations for Rev166 an hour after Cassini passes apoapse on May 11 with an eight-hour light curve of the outer irregular satellite Erriapus. Cassini will be making a relatively close pass of the small, 10-kilometer-wide (6-mile-wide) satellite, at a distance of 7.3 million kilometers (4.5 million miles). On May 12, ISS will take a look at Titan from a distance of 2.90 million kilometers (1.80 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 of a gibbous Titan is designed to monitor clouds over the moon's Senkyo dune field. ISS also will be taking shorter-wavelength images to study changes in Titan's upper haze layers. On May 13, ISS will take another TMC observation of Titan that will allow for monitoring of cloud features across the Senkyo dune field from a distance of 3.13 million kilometers (1.95 million miles). Each day between May 14 and 17, ISS will ride along with the Ultraviolet Imaging Spectrometer (UVIS) in order to take movies, three to six hours in length, of Saturn's aurora australis (southern lights), using the narrow-angle camera (NAC). On May 16 and 17, ISS will take two more TMC observations of Titan. The first, taken from a distance of 3.08 million kilometers (1.91 million miles), will have Saturn's rings in the way, so a set of red, green, and blue filter images will be taken rather than the typical set of infrared surface images. The second, taken from a distance of 2.60 million kilometers (1.61 million miles), will cover the Fensal-Aztlan region of Titan.
On May 20 at 06:26 UTC, Cassini will reach periapse for Rev166 at an altitude of 134,090 kilometers (83,319 miles) from Saturn. ISS observations during the periapse period will be taken during non-targeted encounters with Tethys and the tiny moon Methone. First up is Tethys, as Cassini will fly by the icy satellite at a distance of 53,806 kilometers (33,433 miles). ISS will acquire a global, eight-frame mosaic covering the trailing hemisphere, including a string of unrelated craters: Phemius, Polyphemus, and Ajax. This mosaic will be followed by a long series of clear filter images covering Tethys' sub-Saturn hemisphere, including Ithaca Chasma. After the Tethys encounter, ISS will turn its attention on Methone, a small, 3.2-kilometer-wide (2-mile-wide) moon. At 06:57 UTC, Cassini will pass by the tiny moon at a distance of 1,861 kilometers (1,156 miles), the closest the spacecraft has passed by this moon, or any of the small moons Cassini discovered in 2004 orbiting between Mimas and Enceladus. The best images will be taken 12 minutes after closest approach, from a distance of 4,500 kilometers (2,800 miles). At that distance, the pixel scale will be 26 meters (85 feet) per pixel and Methone will appear 120 pixels across. Two sets of color filter images will be taken, with the first set starting at 07:10 UTC and the second set starting 30 minutes later.
Two days after periapse, Cassini encounters Titan on May 22 at 01:10 UTC for the 84th time. This is the fourth of nine Titan flybys planned for 2012, with the next encounter scheduled for June 7. T83 is a low-altitude flyby with a close-approach distance of 955 kilometers (593 miles). This flyby will allow for imaging of the Adiri region and the anti-Saturn hemisphere of Titan outbound from the encounter. Before the encounter, the Composite Infrared Spectrometer (CIRS) and the Visual and Infrared Mapping Spectrometer (VIMS) will acquire spectral scans and other data of Titan's night side. VIMS will search for specular, or mirror-like, reflections off the northern lakes. CIRS will scan across Titan using its far-infrared channel as well as perform a limb integration. ISS will ride along to acquire images of Titan's upper haze layers, which are more easily visible at high phase angles.
At closest approach, control of spacecraft pointing will switch to the RADAR and the Ion and Neutral Mass Spectrometer (INMS) instruments. RADAR will acquire a SAR swath that will stretch from north of Adiri near 20 degrees north latitude, 210 degrees west longitude; north through a region of small lakes near 80 degrees north latitude, 150 degrees west longitude; across the far northern leading hemisphere; then back south to around 40 degrees north latitude, 20 degrees west longitude. This swath will be used to look for changes in the northern lakes seen during the T16 and T19 flybys in 2006. INMS also will be looking for seasonal changes in atmospheric composition between the northern flybys in 2006 and 2007 and now, as spring progresses. Before closest approach, RADAR will acquire a HiSAR swath around 20 degrees north latitude, 165 degrees west longitude. After the flyby, the instrument will acquire a HiSAR swath to the east of the Menrva impact basin. Inbound and outbound scatterometry, radiometry and altimetry will also be taken by RADAR. Afterward, CIRS will map surface temperatures across the visible disk to look for diurnal and albedo-related differences, while VIMS will monitor the anti-Saturn hemisphere for clouds (with ISS riding along during both observations).
On May 23, ISS will acquire 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. Three more such observations are planned for May 27. Between the first two storm watch observations, ISS will acquire an astrometric observation of Saturn's small, inner moons, including Calypso, Pandora, Atlas, Anthe, Pan, Janus, and Daphnis. Astrometric observations are used to improve our understanding of the orbits of these small satellites, which can be influenced by Saturn's larger icy satellites. Between the second and third storm watch sequence on May 23, ISS will acquire a pair of movies of Saturn's ring system, its first such movie of the first inclined phase. The first movie, six hours in length, will cover the innermost D ring, while the second, more than 8 hours in length, will cover the outer portion of the A ring, between the Keeler and Encke Gaps. On May 25, ISS will acquire a large set of photometry calibration images of the star Vega, in the constellation Lyra. On May 27, ISS will take a TMC observation of Titan, covering the trailing hemisphere of the large moon from a distance of 2.54 million kilometers (1.58 million miles). Finally, ISS will acquire another astrometric sequence, this time imaging Calypso, Epimetheus, Telesto, and Prometheus.
On May 11, Cassini will reach apoapse on this orbit, bringing it to a close and starting Rev167. Rev167 includes a targeted flyby of Titan and a non-targeted encounter with Mimas.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC). Mimas basemap by Steve Albers.