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Cassini begins the 20-day Rev153 on September 3 at its farthest distance from Saturn, called apoapse. At this point, Cassini is 2.71 million kilometers (1.68 million miles) from Saturn's cloud tops. The spacecraft is in the middle of the first equatorial phase of the Cassini Solstice Mission. This phase lasts until May 2012. During this phase, the spacecraft's orbits lie within the equatorial plane of the planet, providing opportunities to encounter Saturn's numerous moons, image the rings edge-on, and to look at Saturn's cloud tops without the rings obscuring the view. Sixty-one ISS observations are planned for Rev153, the majority designed to monitor cloud systems in Saturn's atmosphere. The spacecraft also will encounter a number of Saturn's moons, including Titan, Pallene, Tethys, Enceladus, and Hyperion, for which ISS will acquiring imaging.
ISS begins its observations for Rev153 two days after apoapse with a satellite search observation. ISS will image the L5 Lagrange point region of the moon Iapetus, about 60 degrees behind of the icy satellite in its orbit. This type of Lagrange point has been found to host Trojan moons before in the Saturn system. Cassini discovered the L5 Trojan moon of Dione now named Polydeuces in 2004. Another, Calypso, shares the same orbit as Tethys but lies 60 degrees behind it. This satellite search observation could detect objects as small as 90 meters near Iapetus' L5 point. Similar observations will be acquired of the L5 regions for Rhea and Dione on September 21 and 22, respectively. On September 6, ISS will take four, quick Saturn storm watch observations. Twenty-two more such observations are planned between September 15 and 22. These are designed to take advantage of short, 2-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. These can be used to track the progress of the northern hemisphere storm that began in early December 2010. Later, ISS will acquire a Titan monitoring observation. Titan will be 3.19 million kilometers (1.98 million miles) away at the time. Visible surface features during this observation include eastern Xanadu and western Fensal. Finally, on September 6, ISS will acquire an astrometric observation of Saturn's small, inner moons. During this observation, the camera system will image Pallene, Anthe, Methone, Epimetheus, and Atlas. Additional astrometric observations are planned for September 7, 9, 16, 19, and 21. On September 7, ISS will image the Xanadu region of Titan again, this time from a distance of 2.72 million kilometers (1.69 million miles). Another will be taken on September 9, revealing a crescent Titan where western Xanadu should be visible. That observation will be acquired from a distance of 1.47 million kilometer (0.92 million miles). On September 8, ISS will acquire a 21-hour light curve of the outer, irregular satellite, Paaliaq. This will be used to pin down the orbital period of that small moon, as Cassini ISS has done for several other outer satellites like Albiorix, Siarnaq, Ymir, Bebhionn, and Kiviuq.
Two days before periapse, Cassini encounters Titan on September 12 at 02:50 UTC for the 79th time. This is the fifth of six Titan flybys planned for 2011 with the next encounter scheduled for December 13. T78 is a relatively high-altitude flyby with a close-approach distance of 5,821 kilometers (3,617 miles). This flyby will allow for imaging of the Senkyo region of Titan outbound from the encounter. The Composite Infrared Spectrometer (CIRS) will be the primary pointing instrument during the inbound leg of this flyby while Titan is visible as a narrow crescent. The instrument will perform a variety of nadir-pointing, temperature map scans as well as limb scans measuring aerosols over Titan's south pole. Shortly before closest approach, the Visual and Infrared Mapping Spectrometer (VIMS) will observe a stellar occultation of Chi Aquarii, a 4.93-magnitude, red-giant star in the constellation Aquarius, by Titan's atmosphere. At closest approach, the Ultraviolet Imaging Spectrometer (UVIS) will obtain data during a solar occultation by Titan. This will provide a detailed vertical profile of nitrogen in Titan's atmosphere at altitudes between 900 and 2,300 kilometers (560 and 1,430 miles). These results can be compared to in situ vertical profiles taken by the Ion and Neutral Mass Spectrometer (INMS) during closer encounters with Titan. The rate of change in nitrogen density also provides a measure of the temperature in the atmosphere. Thirty minutes after closest approach the Cassini Plasma Spectrometer (CAPS) will take over, though only ride-along observations by other instruments will be taken as CAPS is not currently functional. Afterward, UVIS will use its Hydrogen Deuterium Absorption Cell to measure the atomic Hydrogen-to-Deuterium ratio in Titan's atmosphere. This can be used to help constrain the origin of methane in Titan's atmosphere.
On September 13 at 23:01 UTC, Cassini will reach periapse for Rev153 at a distance of 134,710 kilometers (83,700 miles). During this periapse, Cassini will pass within 100,000 kilometers of seven of Saturn's moons, of which three will be imaged by ISS. Although Rhea is first up with an encounter distance of 25,409 kilometers (15,788 miles), the moon will not be imaged. Neither will any imaging occur during the Pandora non-targeted encounter. During an encounter with Tethys, there won't be any observations until Cassini is 300,000 kilometers (186,000 miles) distant (described below). The next moon, Enceladus, will be imaged during two observations. The first covers the moon's south polar plume while the spacecraft is approaching the satellite's night side and only a slim crescent will be visible. During periapse and the encounter with Enceladus at an altitude of 42,224 kilometers (26,236 miles), ISS will acquire two mosaics of Enceladus: first a nine-frame mosaic taken during closest approach to the moon, and then a four-frame mosaic an hour later. Both mosaics will focus on the sub-Saturn hemisphere of Enceladus.
After the Enceladus encounter, ISS will image Pallene as Cassini passes at a distance of 25,960 kilometers (16,130 miles) of this small moon which orbits between Mimas and Enceladus. At that distance, Pallene will appear 38-by-26 pixels across as the moon is only 5.8-by-4.0 kilometers (3.6-by-2.5 miles) in size, though it will appear initially as only a small, slim crescent. The observation continues until Cassini is 40,000 kilometers (25,000 miles) away in order to image the small moon at a slightly better phase angle. Immediately afterward, ISS and CIRS will turn their attention to Tethys, acquiring an observation of the moon's leading hemisphere from a distance of 300,000 kilometers (186,000 miles). The observation is intended so that CIRS can examine a possible thermal inertia anomaly similar to the "Pac-man" high-thermal inertia region on Mimas. A similar bluish swath is seen on Tethys to the one that coincides with "Pac-man" on Mimas. The color and temperature features on both moons may be related to the effects of electron bombardments from Saturn's magnetosphere on these moons' icy surfaces. Finally, on September 14, ISS will monitor clouds over the sub-Saturn hemisphere of Titan from a distance of 1.95 million kilometers (1.21 million miles).
On September 15 and 16, ISS will images parts of Saturn's outer ring system while in the ring plane. The first observation will target the E ring as the camera maps the vertical structure of the ring as seen at low phase angles, which is useful for mapping the distribution of larger particles in the diffuse ring. The second observation focuses on the G ring arc. Both the E ring and G ring arc are generated from material from a couple of Saturn's icy satellites: Enceladus in the case of the E ring and tiny Aegeaon in the case of the G ring.
On September 16 at 13:23 UTC, Cassini will perform a non-targeted encounter with the irregularly shaped moon, Hyperion, the second time in a month. Cassini will pass the icy moon at a distance of 58,015 kilometers (36,048 miles). During the encounter, ISS will take two observations of the satellite. The first, acquired as part of the Scientist for a Day (SFAD) program, will allow for true-color imaging of the moon. The second observation will use a more extensive set of filters. These observations will be taken several hours after closest approach, with the SFAD observation occurring when Hyperion is 91,000 kilometers (57,000 miles) away. Due to Hyperion's nearly chaotic rotation, it can't be predicted with much certainty which side of Hyperion will be visible during this encounter. During last month's flyby, the Bond-Lassell impact basin, the basin's tall central peak (resembling a more ancient version of the one currently being studied by Dawn on the asteroid Vesta), and the Helios impact crater were imaged, not unlike the preview image at right. Immediately afterward, ISS will again image Titan's sub-Saturn hemisphere from a distance of 2.84 million kilometers (1.76 million miles). Shortly afterward, ISS will acquire another Scientist for a Day observation, this time looking at Tethys pass in front of Titan's south polar hazes. During this true-color observation, Tethys will be 1.46 million kilometers (0.91 million miles) away from Cassini.
On August 17, ISS will image two more mutual events between two or more of Saturn moons. During the first, Enceladus will pass in front of Titan's south polar region. Enceladus will be 1.59 million kilometers (0.99 million miles) away, while Titan will be 3.00 million kilometers (1.87 million miles) away. An hour and a half later, ISS will watch as Dione passes in front of Titan, along with the outer parts of Saturn's main rings and Pandora.
Between September 19 and 22, two more observations are planned in addition to the nine Saturn storm watch observations, two astrometric sequences, and two satellite search observations at the L5 points for Rhea and Dione described earlier. On September 19, ISS will image Titan's sub-Saturn hemisphere from a distance of 3.31 million kilometers (2.06 million miles). On September 22, ISS will acquire a true-color wide-angle-camera sequence of Saturn, followed by astrometric images of some of Saturn's small satellites.
On September 22, Cassini will reach apoapse on this orbit, bringing it to a close and starting Rev154. The next orbit will see a targeted encounter of Enceladus and non-targeted observations of Dione.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC). Dione, Tethys, and Enceladus basemaps by Steve Albers.