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Cassini continues its extended tour of Saturn with Rev121, the spacecraft's 122nd orbit around the Ringed Planet. Cassini begins Rev121 on November 11 at its farthest distance from Saturn, called apoapse. At this point, Cassini is 2.49 million kilometers (1.55 million miles) from Saturn's cloud tops. During Rev121, Cassini orbits in the ring plane and orbital plane of Saturn's main satellites, providing an opportunity for Cassini to encounter some of Saturn's moons as well as observe mutual events between the various satellites.
Cassini ISS starts its observations for Rev121 a few hours after apoapse with two observations of mutual events involving a few of Saturn's moons. This includes an observation of a near-conjunction between Dione and Rhea and another showing Tethys partially occulted by Enceladus. In both cases, these moons are between 2.3 and 2.9 million kilometers (1.4 and 1.8 million miles) from Cassini. Another observation on November 11 will focus on determining the positions of propellers in Saturn's A ring (propeller features are voids in the ring created by the gravity of large ring particles). Cassini also will look at other large ring particles, such as the B-ring moonlet S/ 2009 S 1. Starting the next day, November 12, and running through November 19, the Cassini ISS camera will acquire one- and two-frame, wide-angle-camera mosaics of a crescent Saturn each day (including two of these observations on November 12). The mosaics are designed to study how the photometry and polarimetry (which focuses on the high altitude hazes over the poles) of Saturn's atmosphere has changed since earlier in the Cassini mission. Also starting on November 12 and running through the 19th, except for November 15 and 18, Cassini ISS will acquire 6-frame, north-south strips across Saturn's nightside using the narrow-angle camera. These mosaics are designed to search for lightning within storms in the planet's atmosphere. These observations will also include a wide-angle-camera frame over Saturn's dayside and a narrow-angle frame at different spots on the nightside (usually near the southeast dark limb). In addition to these two observation campaigns, on November 12, ISS will acquire a sequence of images of the distant moon Kiviuq (Saturn XXIV), a 16-kilometer (10-mile) wide moon 13.7 million kilometers (8.5 million miles) away.
On November 13, ISS will observe a half-phase Titan, 1.7 million kilometers (1.1 million miles) distant, as part of a campaign monitoring clouds on Titan, this time focusing on the western Belet region of that satellite. A similar observation will be acquired on November 15 when Titan is 2.1 million kilometers (1.3 million miles) from Cassini. ISS will also image an occultation of Hyperion by Titan when Hyperion is 2.8 million kilometers (1.8 million miles) from Cassini and Titan is 1.7 million kilometers (1.1 million miles) away. On November 15, ISS will image a mutual event as Enceladus transits the disk of Rhea. During this sequence, Enceladus is 2.3 million kilometers (1.4 million miles) from Cassini and Rhea 2.7 million kilometers (1.7 million miles) away. On November 17, ISS will image M48, an open star cluster in the constellation Hydra, using the NAC and the Pleiades using the WAC as part of a calibration of the charge transfer function of the CCDs of each camera. On November 18, ISS and the Visual and Infrared Mapping Spectrometer (VIMS) will observe Saturn's E and G rings to better fill in information on phase curves and color of these rings. On November 19, ISS will observe a transit of Rhea across the disk of Titan. In its final observation before the Enceladus flyby sequence, the Visual and Infrared Mapping Spectrometer (VIMS) and ISS will image the narrow crescent and nightside of Saturn as part of a global cloud dynamics campaign. While ISS will be search for lightning in its images of the nightside, VIMS can directly observe cloud formations by looking for Saturn's internal heat filtered through its clouds at five microns.
On November 21, at 2:10 UTC, Cassini will perform its eighth targeted encounter of Saturn's moon Enceladus and the second such encounter this month. During this flyby, Cassini will approach from Enceladus's nightside, pass almost directly over its south pole at a distance of 1,607 kilometers (999 miles), then depart the satellite over the moon's dayside, leading hemisphere. On approach, ISS and VIMS will acquire several observations of the crescent of Enceladus. In addition to providing information on particle grain sizes and macro-scale roughness of the surface of Enceladus, high-phase-angle observations such as these are also a great opportunity to image the plumes of Enceladus. The plume sources will remain near the limb, providing a great chance to monitor these features at high resolution. During these observations, Enceladus will be eclipsed by Saturn. Following the eclipse and a transit of Enceladus across the disk of Saturn, ISS will observe the plumes and their sources on the ground using increasingly high resolution, culminating in a mosaic of the plume over the limb of Enceladus. This is designed to look for fine scale structure in the jets emanating from Enceladus's south polar region as well as search for additional sources of the jets.
With seven minutes to go until closest approach, Cassini will turn so that the Composite Infrared Spectrometer (CIRS) can measure thermal emission from Enceladus's surface. During this observation, FP3HIRES001, CIRS will be considered "prime" and will control spacecraft pointing, with ISS riding along. This multi-part sequence will allow CIRS to use different scan modes that will provide ISS an opportunity to generate mosaics of Enceladus' leading hemisphere and south polar region. In some cases, the CIRS instrument will use a mode in which it stares at different points on the surface, allowing ISS to take an image at each stop. The spacing and timing of these stops is designed to allow ISS to create mosaics. In other cases, CIRS will scan across the surface, and carefully timed ISS images can also be used for mosaics. The leading hemisphere of Enceladus has to this point been the most poorly imaged region on this satellite, but low-resolution observations suggest that it has some of the youngest terrain on Enceladus. The first part of FP3HIRES001 will consist of a stare at plume source VII along Baghdad Sulcus. ISS will image the terrain around this plume source, searching for changes since it was last imaged during an October 2008 encounter. CIRS will look for warm material at the source site.
During the next part of FP3HIRES001, CIRS will acquire measurements across the southern half of Enceladus's leading hemisphere. During these slews, ISS will acquire a 24-frame, clear-filter mosaic of this region, providing the team with their highest resolution observations of the young terrain on the leading hemisphere. The high resolution observations from this sequence will cover the south polar terrain near the bottom limb. The next sequences will provide lower-resolution clear-filter mosaics across Enceladus including a four-frame mosaic across the entire visible disk; a single, multi-color frame near the center of the disk at the beginning of a VIMS point-and-stare observation; a six-frame mosaic during a CIRS scan; and a four-frame mosaic across Enceladus's equator. Following this lengthy CIRS observation and a few images acquired during an RCS/RWA transition, CIRS will acquire a dayside temperature map, while ISS acquires a number of color filter frames covering the entire visible surface of Enceladus. During this CIRS observation, Cassini reaches periapse, its closest point to Saturn on Rev121. At this point, Cassini will be 132,920 kilometers (82,592 miles) from Saturn's cloud tops, between the orbits of Mimas and Enceladus. Finally, the Ultraviolet Imaging Spectrometer (UVIS) will conduct a point-and-stare observation while ISS acquires another sequence of color filter images.
Immediately after downlinking its data from the Enceladus encounter, Cassini will begin acquiring observations for its second encounter of Rev121, a 24,455-kilometer (15,195-mile) non-targeted encounter with Saturn's second largest moon, Rhea. This encounter will provide an opportunity to acquire 12-frame, three-color and clear-filter mosaics across Rhea's trailing hemisphere. Highlights from this region include Rhea's wispy terrain, marked by sets of tectonic faults and a large impact basin near the center of the hemisphere. Additional multi-color imaging will also be acquired across Rhea's equator to examine a series of faint streaks along the equator that may be the remains of a transient ring around Rhea as well as WAC color imaging during a VIMS observation.
After the playback, on November 22, Cassini will turn its cameras to Saturn, examining the dynamics of clouds and jets in Saturn's northern hemisphere. Cassini ISS will also image Titan at high-phase angles (126 degrees), as part of a campaign to monitor Titan's high-altitude haze layers. Titan will be 1.37 million kilometers (849,000 miles) away during this observation. With Cassini receding from Saturn, the camera team will restart their campaign observations in which the wide-angle camera will acquire one- and two-frame mosaics of a crescent Saturn. These observations will be taken each day between November 23 and November 30. Also on November 23, ISS will begin another imaging campaign with observations each day until November 30 (except November 25), this time looking for lightning on Saturn's nightside. Just like the similar campaign earlier in the orbit, this search will utilize six-frame, north-south NAC strips across the nightside, a single WAC frame over the dayside for context, and a single NAC frame over the nightside, each of which usually will be taken near the southeast dark limb. Also on November 23, ISS will image the small moon Pandora as it partially occults Epimetheus. Next, ISS and UVIS observe a transit of Enceladus across the disk of Saturn. Finally on November 23, ISS will acquire an astrometric observation of several of Saturn's small satellites including Polydeuces, Atlas, and Calypso. Astrometric observations are used to help provide better orbital calculations for some of these small rocks, which can be effected by gravitational interactions with the larger icy moons.
On November 24, ISS will image two transits by the co-orbital moons Janus and Epimetheus across the disk of Rhea. ISS and the Visual and Infrared Mapping Spectrometer (VIMS) again will take a look Saturn's E and G rings to better fill in information on their phase curves and color. On November 25, and over the next three days, ISS will acquire four astrometric observations known as SATELLORBs of the following small, Saturnian moons: Pallene, Epimetheus, Anthe, Methone, Prometheus, Pandora, Helene, Daphnis, Janus, and Telesto. On November 26, ISS will take the first of three imaging sequences (the others on November 29 and 30) of Saturn's outermost icy moon, Iapetus. Although these images will be acquired from 1.39 to 1.69 million kilometers (0.87 to 1.05 million miles) away from Iapetus, these images will provide additional information on the satellite's shape and phase curve. The impact basin Naimon should be visible within the thin crescent of Iapetus during these observations. In addition, ISS will observe two mutual events, a Rhea occultation of Tethys's north polar region and a Titan occultation of Tethys, There will also be a photo op sequence of Epimetheus behind Saturn's A ring. On November 27, ISS will image Titan twice. The first sequence is a mutual event observation as Titan occults Mimas. The second observation is the first of three observations during this latter part of Rev121 to monitor clouds on Titan. The other observations are planned for November 29 and 30. In this first observation, with Titan 1.1 million kilometers (681,000 miles) from Cassini, ISS will observe the western Belet region of Titan's equator. This observation might also catch an occultation of Rhea by Titan.
Closing out the orbit, ISS will observe an occultation of Tethys by Dione on November 28. On November 29, in addition to the previously mentioned Iapetus and Titan observations, ISS will acquire a seven-frame, wide-angle-camera mosaic of Saturn's E ring, a faint ring associated with the plume at Enceladus's south pole. This mosaic will cover much of the inner part of the Saturn system, out to the orbit of Tethys.
Cassini reaches apoapse on November 30, bringing Rev121 to an end and starting Rev122. Rev122 includes a flyby of Saturn's largest moon, Titan (T63).
Image products created in Celestia. Rhea and Enceladus basemaps by Steve Albers. All dates in Coordinated Universal Time (UTC).