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Cassini nears the end of its extended tour of the Saturn system with the 16-day-long Rev132, the spacecraft's 133rd orbit around the Ringed Planet. Cassini begins Rev132 on May 26 at its farthest distance from Saturn, called apoapse. At this point, Cassini is 2.22 million kilometers (1.38 million miles) from Saturn's cloud tops. Cassini is now in a slightly inclined orbit, providing Cassini's cameras their first good opportunity to observe the rings since January. Cassini will also perform its penultimate flyby of Titan of the first extended mission.
Cassini's ISS camera system starts its observations for Rev132 the day after apoapse by imaging Titan from a distance of 3.15 million kilometers (1.96 million miles). Cassini will be searching for clouds across the Senkyo and eastern Aztlan regions of Titan in this distant observation. Also on May 27, Cassini will acquire calibration images for its wide-angle camera. These images will use the camera's lamp to create new flat field images.
Between May 30 and June 3, in the run up to periapse on this orbit, Cassini will focus on Saturn's rings. The rings appear more "open" to Cassini now that the spacecraft is in a more inclined orbit. When Cassini was in Saturn's ring plane for the last few months, the rings appeared very narrow, but provided more opportunities for satellite mutual events and spacecraft encounters. ISS will be performing four observations of the rings during this time. On May 30, ISS will image the lit side of the main ring system, using a strip of eight, narrow-angle-camera (NAC) footprints (each using multiple color filter images) across the left ansa, and two wide-angle-camera (WAC) footprints covering Saturn and the right ansa (the side with Saturn's shadow). These images will be used as part of a photometry campaign to measure the brightness of different sections of the ring system as viewing and illumination angles change. On May 31, ISS will be riding along with an observation by the Visual and Infrared Mapping Spectrometer (VIMS) team to map structures within the F ring. The ISS team will use 96 NAC images to examine changes in these structures that result from the gravitational interaction between the ring particles and nearby moons such as Prometheus. On June 2, with distance to Saturn half of what it was during the May 30 photometry observation, ISS will use its narrow-angle camera to image the outer A ring along the left ansa. With 240 images acquired (one every 2.2 minutes), ISS will allow the rotation of the particles in the ring to bring much of this part of the ring system into view. Finally, during periapse on June 3, ISS will again ride along with VIMS, this time to observe an occultation of the red giant star Mira (Omicron Ceti). This occultation will cover most of the ring system as Cassini crosses below the ring plane, though ISS will acquire its images while Mira passes behind the F ring, again focusing on the irregular structure of the narrow ring.
On June 3 at 07:43 UTC, Cassini will reach the periapse of Rev132, its closest point to Saturn in the orbit, during the end of the VIMS observation of the Mira ring occultation. At periapse, the spacecraft will be 104,920 kilometers (65,194 miles) above Saturn's cloud tops. Immediately after the stellar occultation, Cassini will turn its focus to a series of satellite observations as the spacecraft performs a number of non-targeted encounters with Saturn's inner icy satellites. First up is Pandora as Cassini passes 97,222 kilometers (60,410 miles) from the F ring's shepherding moon at 07:16 UTC. Cassini's observations of the satellite begin a half hour after closest approach when the spacecraft is still a respectable 100,000 kilometers (62,000 miles) away. Cassini will observe the southern leading hemisphere of the small satellite, the side opposite the one seen during the last well observed encounter in September 2005. Next, ISS will take a number of images, including a nearly complete longitudinal scan of the outer A ring. This high-resolution observation, primarily using the narrow-angle camera, is designed to help search for many of the large propellers Cassini observed earlier in the mission, such as the small moonlet nicknamed "Earhart." Propellers are voids formed in the outer A ring by ring particles large enough to use their gravitational pull to carve airplane-propeller-shaped gaps but not large enough to create full gaps such as those created by Pan and Daphnis. Finally, Cassini will pass by the large icy moon Rhea at 20:31 UTC at a distance of 69,363 kilometers (43,100 miles). Cassini will image Rhea two hours after closest approach during an observation by the Composite Infrared Spectrometer (CIRS) team, acquiring three color filter images plus clear filter NAC and WAC images of the south polar region. Next, ISS will take a two-by-three-image mosaic of the southern anti-Saturn hemisphere which includes the impact basin Tirawa and a plethora of smaller craters like Awonawilona, Gborogboro, and Imberombera. Despite a fairly close encounter of 52,582 kilometers (32,672 miles), Tethys will not be imaged during this periapse pass.
Cassini encounters Titan on June 5 at 02:26 UTC for the 70th time. This is the first of two encounters in the month of June and is the second-to-last encounter of Cassini's first extended mission, though 56 more flybys are planned during the seven-year, second extended mission, the Cassini Solstice Mission. The close approach distance for the encounter (known as T69) is 2,044 kilometers (1,270 miles). This flyby will allow for imaging of the anti-Saturn hemisphere of Titan outbound to the encounter. For much of the inbound segment of the encounter, when only a thin crescent will be visible, the CIRS, VIMS, and RADAR teams will control spacecraft pointing, or be "prime." First up is VIMS, which will acquire a distant observation of the crescent of Titan. Next, CIRS will focus on measuring the composition of Titan's atmosphere by performing several limb scans in the mid-infrared at different latitudes around Titan. One focus for these teams' observations will be the atmosphere over Titan's south pole. Titan started its 7.5-year-long southern autumn last August. CIRS will examine how Titan's atmosphere has responded to the change in seasons. RADAR will perform radiometry and scatterometry scans across Titan's sub-Saturn hemisphere.
The RADAR observation continues until 15 minutes before closest approach, when VIMS takes over to acquire close up observations (REGMAP001) of one of Titan's large northern seas, Ligeia Mare. This VIMS observation, which runs until two hours after closest approach, will include nine footprints across the north polar region, including three footprints covering portions of Ligeia Mare. The first frame, taken when Cassini is 2,500 kilometers (1,500 miles) and closing from Titan, is centered on the region between the western end of the Ligeia and the eastern "arm" of Kraken Mare, at center left in the image shown at left. Unlike ISS images which have two axes (horizontal and vertical pixel data), VIMS observations have a third axis for data related to wavelength. So, VIMS observations are referred to as cubes. This spectral cube will hopefully detect methane, ethane, or both within the tributaries that feed into Kraken Mare in this region. The VIMS cube taken right at closest approach will cover the southern shoreline of the methane sea, just left of bottom center in the image at left. The third of the close approach cubes will focus on the center of the sea and is designed to confirm the presence of liquid methane and ethane within this (presumably) deepest part of the Ligeia. The rest of the REGMAP001 observation will be used to create a mosaic of the north polar region. ISS will ride along with the close approach segment of the observation with four WAC frames. This assumes that the weather on Titan will co-operate as sporadic clouds have been observed over Ligeia Mare by ISS since vernal equinox last year.
After the VIMS closest approach observation, the Ultraviolet Imaging Spectrometer (UVIS) will take over, performing a slow, seven-hour scan across Titan's equatorial region and its upper haze layers. CIRS will then stare at the center of Titan in order to perform a far-infrared compositional observation. Next, ISS has its only prime observation in the T69 sequence, acquiring a nine-frame mosaic across Titan's anti-Saturn hemisphere, including the dark regions Belet, western Shangri-La and the bright region Adiri. MONITORNA001 will be taken from distances ranging from 239,000 to 275,000 kilometers (149,000 to 171,000 miles) from Titan. Finally, VIMS will take a low-resolution, global spectral cube of Titan's surface.
With playback completed early on June 6, ISS will then image Titan on several occasions on June 6 and 7, with a follow-up observation on June 10. These "caboose" observations are designed to track any clouds seen during the T69 encounter over several days in order to measure their speed and track their evolution. Knowing the speed and direction of Titan's methane clouds can allow researchers to infer wind speeds in the mid-to-upper troposphere (the lowest part of a planetary atmosphere), which is important for understanding global circulation. Monitoring the evolution of clouds can be helpful in understanding the processes that allow these clouds to form and whether they produce rain (or at least virga, rain that doesn't reach the ground). However, weather on Titan is quite unpredictable at this stage of our understanding of Titan's weather, and it is possible that clouds will not be visible in these observations, as is currently the case after T68 in late May 2010. Understanding the pattern of when clouds are visible and where they are is also important as we have yet to observe this alien climate for an entire Titan year because cloud observations only began in earnest in 2002 during the last southern summer solstice. The last observation on June 10 will also include additional filters for looking at structures in Titan's haze layers, including the north polar dark collar that is now clearly visible and was last seen by Voyager 2.
On June 11, Cassini will reach apoapse on this orbit, bringing it to a close and starting Rev133. During Rev133, the last full orbit of the first extended mission, Cassini will fly by Saturn's moon Titan once again. In fact, most of the observations for Rev133 will be dedicated to Saturn's largest moon.
Image products created in Celestia. Rhea basemap by Steve Albers. All dates in Coordinated Universal Time (UTC).