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Cassini nears the end of its extended tour of the Saturn system with the 16-day-long Rev133, the spacecraft's 134th orbit around the Ringed Planet. This is the last full orbit of its first extended mission, which ends on July 1. Cassini begins Rev133 on June 11 at its farthest distance from Saturn, called apoapse. At this point, Cassini is 2.23 million kilometers (1.38 million miles) from Saturn's cloud tops. Cassini is now in a slightly inclined orbit, but will spend most of its camera observing time imaging Saturn's largest moon, Titan.
Cassini's ISS camera system starts its observations for Rev133 the day after apoapse by acquiring calibration images for its narrow-angle camera. These images will use stars in the Hyades open star cluster, and Theta Tauri in particular. On June 13, Cassini will image Titan from a distance of 3.26 million kilometers (2.02 million miles). Cassini will be searching for clouds across the eastern Aztlan, Quivira, and Fensal regions of Titan in this distant observation.
On June 19 at 07:02 UTC, Cassini will reach the periapse of Rev133, its closest point to Saturn in the orbit. At periapse, the spacecraft will be 97,949 kilometers (60,862 miles) above Saturn's cloud tops. A few minutes after periapse, Cassini will perform one of its closest encounters with the small satellite Pan to date, at a distance of 29,333 kilometers (18,226 miles). Unfortunately, no imaging is planned for this encounter.
Cassini encounters Titan on June 21 at 01:27 UTC for the 71st time. This is the second of two encounters in the month of June and is the last Titan encounter of Cassini's first extended mission, though 56 more are planned during the seven-year, second extended mission, known as the Cassini Solstice Mission. The close approach distance for the encounter (known as T70) is 880 kilometers (547 miles), close to the altitude limit for Cassini at Titan. 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 Composite Infrared Spectrometer (CIRS), Visual and Infrared Mapping Spectrometer (VIMS), and ISS teams will control spacecraft pointing, or be "prime." First up is CIRS, which will acquire a distant observation of the crescent of Titan, staring at the limb of Titan in order to acquire far-infrared spectra of Titan's haze layers. Next, ISS will spend an hour taking a photometric observation of Titan using the wide-angle camera. Afterward, CIRS will take a series of infrared spectral observations measuring the composition of Titan's atmosphere by performing the following: several limb scans in the mid-infrared at different northern latitudes around Titan's bright limb, far-infrared scans across the night side of the satellite, and far-infrared stares at the bright limb. At closest approach, the Radio Science Subsystem (RSS) team will have Cassini point its high-gain antenna at Earth in order to monitor how the spacecraft's signal is affected by the gravity of nearby Titan. The degree of this effect, called Doppler shift, and how it changes over the course of the flyby, can provide information on Titan's internal structure and any heterogeneity that might exist in Titan's lithosphere.
After the RSS closest approach observation, the Ultraviolet Imaging Spectrometer (UVIS) will take over, performing a quick north-to-south scan across the anti-Saturn hemisphere of Titan, just to the east of the Huygens landing site. Afterward, VIMS will acquire three mosaics, with ISS riding along. The first mosaic is a south-to-north scan across Ching-tu, Adiri, and Dilmun. The second REGMAP is a ten-frame global mosaic across the entire visible surface. This will be followed by a global, four-frame VIMS mosaic. Finally, ISS will acquire a wide-angle-camera photometric observation of Titan, followed by a VIMS low-resolution, global spectral observation of Titan's surface.
With play back completed early on June 22, ISS will image Titan on several occasions between June 22 and 25. These "caboose" observations are designed to track any clouds seen during the T70 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, and it is possible that clouds will not be visible in these observations (as was 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. Cloud observations only began in earnest in 2002 during the last southern summer solstice. The most extensive of these observations are CLOUD001 and CLOUD002, which will be taken on June 22 and 23. These 10- and 6.5-hour observations will allow Cassini scientists to track the evolution of any cloud features that might be visible across Titan's trailing hemisphere. Shorter, follow-up observations on June 23, 24, and 25 will provide additional cloud tracking information.
Finishing up Rev133, Cassini ISS will observe the north polar region of Saturn on June 26 in order to make a movie of the auroral oval created by the interaction between Saturn's magnetic field and upper atmosphere. A similar movie was created in October 2009. Finally, ISS will perform photometric calibration of both cameras using the solar-analog star Kappa1 Ceti, a 4.84-magnitude star in the constellation Cetus.
On June 27, Cassini will reach apoapse on this orbit, bringing this orbit to a close and starting Rev134. During Rev134, Cassini's next adventure at Saturn kicks off as the Cassini Solstice Mission begins on July 1. Cassini will also flyby Titan once again on July 7.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).