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Cassini starts its new tour of the Saturn system with the 20-day-long Rev135, the spacecraft's 136th orbit around the Ringed Planet and the first full orbit of the Cassini Solstice Mission, the second mission extension. Cassini begins Rev135 on July 15 at its farthest distance from Saturn, called apoapse. At this point, Cassini is 2.56 million kilometers (1.59 million miles) from Saturn's cloud tops. Cassini is now closer to Saturn's ring plane and will use this orbit to observe mutual events between two or more of Saturn's moons and Saturn itself.
Cassini's ISS camera system starts its observations for Rev135 two hours after apoapse by observing while riding along with an Ultraviolet Imaging Spectrometer (UVIS) observation of Saturn's atmosphere on July 15. UVIS will scan east-to-west across the disk of Saturn, and ISS will image the planet, first with a series of narrow-angle-camera (NAC) filters then later with a series of wide-angle-camera (WAC) filters, when the field-of-view crosses the center of the planet. This observation will be acquired from a distance of 2.6 million kilometers (1.6 million miles). This observation will be repeated at increasing phase angles on July 17, July 21, and July 23 from distances of 2.5 million kilometers (1.5 million miles), 1.7 million kilometers (1.1 million miles), and 1.1 million kilometers (700,000 miles), respectively. But first, also on July 15, Cassini will image Titan from a distance of 3.75 million kilometers (2.33 million miles). Cassini will be searching for clouds across the eastern Aztlan, Quivira, and Fensal regions of Titan in this distant observation. A month earlier, a similar observation over this region revealed clouds over southeastern Aztlan around Elba Facula, though the clouds were gone a Titan day later. ISS also will obtain a light-curve observation of the irregular satellite, Kiviuq. This outer satellite will be 9.3 million kilometers (5.8 million miles) away, but the light curve observation will help scientists understand the moon's rotation and determine if it might be a binary object. On July 16, ISS will acquire an astrometric observation of several of Saturn's small, inner satellites in order to improve our knowledge of the motions of these moons. This observation will include images of Calypso, Atlas, Pandora, Telesto, and Polydeuces.
On July 18, ISS will acquire a pair of observations of Saturn's atmosphere designed to measure wind speeds on Saturn. At the start of both, the WAC will take 13 images using a variety of filters for photometry and polarimetry studies. Then ISS will image a region between 10 and 20 degrees south latitude on Saturn 49 times over a period of 1 hour, 40 minutes. Because these two observations will be separated by one Saturn day, by comparing the positions of various cloud features in this region, researchers will be able to calculate wind speeds at different latitudes and within individual storms. On July 20, another astrometric observation will be acquired, this time covering Anthe, Helene, Telesto, Epimetheus, Janus, and Calypso. On July 22, ISS will image Titan while the satellite appears as a very thin crescent. While this is a poor opportunity to look for clouds, Titan's haze layers will be easier to see and will be visible all around the satellite's limb. This observation will be acquired from a distance of 1.84 million kilometers (1.1 million miles). Afterward, Cassini will turn its attention to the dark limb of Saturn. ISS will take more than 2,000 NAC images of the night side of Saturn, one taken every 24 seconds. Researchers hope to image lightning within storms on Saturn's southern hemisphere. During a brief period a few hours into the observation, Rhea will be in the NAC field-of-view just off the limb of Saturn before being occulted by the planet.
On July 25 at 03:13 UTC, Cassini will reach the periapse of Rev135, its closest point to Saturn in the orbit. At periapse, the spacecraft will be 147,210 kilometers (91,472 miles) above Saturn's cloud tops. During this periapse period, the Visual and Infrared Mapping Spectrometer (VIMS), with ISS riding along, will acquire a number of full-disk mosaics of Saturn spread out over three observations. These are designed to study the dynamics of Saturn's cloud structures. The first two observations will be taken on July 24, first when Saturn appears as a thin crescent, then when Cassini is in the shadow of the gas giant. While ISS will be able to image ring shine on Saturn atmosphere, VIMS can image cloud features backlit by Saturn's internal heat. The third observation will occur after periapse on July 25 while over Saturn's day side.
Early on July 27, Cassini will cross below the ring plane. During the brief window when the spacecraft is near the orbital plane of most of Saturn's major moons, ISS can image mutual events between two or more satellites. Late on July 26, the NAC will image Dione as it cross in front of Rhea's north polar region. While Rhea is actually larger than Dione, they will appear approximately the same size because Dione will be much closer to Cassini at a distance of 1.11 million kilometers (690,000 miles) while Rhea will be 1.59 million kilometers (990,000 miles) away. Cassini also will image a mutual event on July 27 between Prometheus, Janus, and Epimetheus. Janus will first pass over the north pole of Prometheus and will then be occulted by Epimetheus. Between these two observations, ISS will take another astrometric sequence of images, this time covering Pallene, Anthe, Polydeuces, Epimetheus, Atlas, Prometheus, and Methone.
On July 28 and July 30, ISS will acquire a pair of observations of Titan in order to monitor weather on the moon's sub-Saturn hemisphere. During these observations, Titan will be 2.6 million kilometers (1.62 million miles) and 3.39 million kilometers (2.11 million miles) away, respectively. On July 31, the NAC will take a 5-frame mosaic of the region of space around Iapetus. The observation is designed to detect dust that maybe in the moon's large Hill sphere, the region of space where the gravitational pull of Iapetus dominates over that of Saturn or the sun. Images of Iapetus will cover the moon's trailing and sub-Saturn hemispheres from a distance of 1.97 million kilometers (1.22 million miles). Also on July 31, ISS will image the small, outer satellite Albiorix as part of a light-curve campaign. These observations are designed to measure the length of the moon's day and to search for binary moons. Finally on August 2, another astrometric observation will be acquired, this time covering Polydeuces, Anthe, Methone, Pandora, Prometheus, Epimetheus, and Telesto.
On August 4, Cassini will reach apoapse on this orbit, bringing it to a close and starting Rev136. This next, busy orbit will be focused around a targeted encounter with Enceladus as well as close non-targeted encounters with Titan, Dione, and Tethys.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).