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Cassini continues its new tour of the Saturn system with the 20-day-long Rev136, the spacecraft's 137th orbit around the Ringed Planet and the second full orbit of the Cassini Solstice Mission. Cassini begins Rev137 on August 4 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 in a slightly inclined orbit, 4.5 degrees out of the ring plane. This is still close enough to provide Cassini an opportunity to focus on the planet's icy satellites this orbit.
Cassini's ISS camera system starts its observations for Rev136 two days after apoapse by taking 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 Helene, Telesto, Pandora, Janus, and Pallene. On August 7, Cassini 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 1.83 million kilometers (1.14 million miles). ISS will repeat this observation exactly one day later when the spacecraft is 1.32 million kilometers (0.82 million miles) away from Titan. Immediately afterward, another astrometric observation will be acquired, this time covering Polydeuces, Methone, Pandora (in eclipse), Anthe, and Epimetheus. ISS will also obtain a light-curve observation of the irregular satellite, Kiviuq. This outer satellite will be 9.24 million kilometers (5.74 million miles) away. This observation is part of a campaign of observations of this small moon that are taken at different phase angles. This provides information about the surface properties of the satellite, such as surface roughness, even when the spacecraft never comes very close to Kiviuq.
On August 11, Cassini will perform a non-targeted encounter of Titan at a distance of 417,010 kilometers (259,118 miles). Two observations are planned for this encounter. The first will be early on August 12. This four-frame mosaic will cover the trailing hemisphere, providing some of Cassini's best coverage of the region between Senkyo and Belet. The second will be taken the next day and again consists of a four-frame mosaic. This mosaic will be acquired from a distance of 700,000 kilometers (440,000 miles). By this point, Cassini's viewpoint will have rotated around to cover the sub-Saturn hemisphere and will be centered over far western Senkyo. Between these two Titan observations, Cassini will acquire its best-ever observations of the small, outer, irregular satellite Albiorix, taken at a distance of 5.63 million kilometers (3.5 million miles). While still too far away to resolve surface features on this moon, this 16-hour light-curve observation should allow for the calculation of its rotational period and determine if the satellite is actually a binary, like a number of similar-sized bodies in the Asteroid and Kuiper belts.
On August 13, Cassini will perform two encounters of icy satellites as the spacecraft approaches periapse, a non-targeted flyby of Dione at a distance of 108,621 kilometers (67,493 miles) and a targeted one of Enceladus. Two ISS observations are planned for the Dione encounter. The first is a high-phase observation that will be acquired at distances ranging from 223,800 to 314,500 kilometers (139,100 to 195,400 miles). This sequence is designed to look for possible plumes like those that emanate from the south polar region of Enceladus. The second observation will be acquired during closest approach. This four-frame mosaic, taken during Composite Infrared Spectrometer (CIRS) scans of Dione's surface, will cover portions of Dione's leading hemisphere.
The second satellite flyby of the day occurs at 22:29 UTC when Cassini will pass at an altitude of 2,651 kilometers (1,647 miles) over Enceladus's south polar region. This encounter, also known as E11, is the 12th flyby Cassini has preformed of this dynamic satellite (the first encounter in February 2005 was only later designed a targeted flyby and is known as E0). ISS early observations for this flyby focus on the south polar jets, with the satellite itself often near the edge of field-of-view to reduce light contamination. The first two plume observations occur between 12:50 and 16:00 UTC, between the two Dione sequences. These will have a top resolution of 1.3 kilometers (0.8 miles) per pixel. After the second Dione observation, Cassini will again observe the jets right up until shortly before closest approach. For all of these images, Enceladus will be visible as a crescent, the perfect viewing geometry for viewing the satellite's faint, dusty plumes. While similar observations have been made during prior flybys, as we get further from equinox, more of the lower portion of the jets will be in Enceladus' shadow, invisible to visible light cameras until the next equinox in 2024. At closest approach, CIRS will acquire several mid-infrared temperature scans of the south polar region, including a high-resolution scan running down the axis of Damascus Sulcus. These are designed to pin down the highest temperatures on Enceladus as the trench will be on the night side of Enceladus at the time.
After closest approach, ISS will take a several mosaics of Enceladus's surface, centered over the sub-Saturn and leading hemispheres. The first is a 23-frame clear-filter mosaic, followed by a 10-frame, three-color mosaic. Both will cover nearly the entire surface. This region is slightly to the west of the area covered during encounters performed in November 2009 and May 2010.
On August 14 at 01:03 UTC, Cassini will reach the periapse of Rev136, its closest point to Saturn in the orbit. At periapse, the spacecraft will be 147,600 kilometers (91,700 miles) above Saturn's cloud tops. During periapse, the Ultraviolet Imaging Spectrometer (UVIS) and ISS will observe a stellar occultation by the rings of the bright star Rigel in the constellation Orion. This will improve our knowledge of the structure of the main ring system by observing the star "blink" as it passes behind denser ringlets. By measuring the depth and timing of these blinks, the UVIS team can calculate the opacity and position of different structures in Saturn's rings. Shortly after Visual and Infrared Mapping Spectrometer (VIMS) and ISS observations of Enceladus from a distance of 95,000 kilometers (59,000 miles), Cassini will perform a non-targeted flyby of another of Saturn's moons, Tethys. This encounter will occur at a distance of 36,965 kilometers (22,968 miles). ISS will acquire four mosaics of the icy moon as it flies by. The first two are large, clear-filter mosaics covering portions of the anti-Saturn and trailing hemispheres taken near closest approach. These two mosaics include our best images to date of the degraded impact basin, Penelope. These mosaics also provide our best views so far of Tethys's north polar region. The other two mosaics will be taken shortly afterward, providing color information of the moon's anti-Saturn hemisphere.
ISS has two more observations for Rev137 following its satellite encounters. On August 16, another astrometric observation will be acquired, this time covering Pallene, Epimetheus, Helene, Prometheus, and Janus. On August 17, the camera will image Titan's sub-Saturn hemisphere at a distance of 2.95 million kilometers (1.83 million miles).
On August 24, Cassini will reach apoapse on this orbit, bringing it to a close and starting Rev137. Rev137 includes two non-targeted flybys of Dione and Titan.
Image products created in Celestia. Enceladus, Dione, and Tethys basemaps by Steve Albers. All dates in Coordinated Universal Time (UTC).