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Cassini continues its new tour of the Saturn system with the 24-day-long Rev139, the spacecraft's 140th orbit around the Ringed Planet, the fifth full orbit of the new tour and the first orbit under the new program, the Cassini Solstice Mission. Cassini begins Rev139 on October 4 at its farthest distance from Saturn, called apoapse. At this point, Cassini is 2.88 million kilometers (1.79 million miles) from Saturn's cloud tops. Cassini is in a slightly inclined orbit, 4.7 degrees out of the ring plane. This is still close enough to provide Cassini an opportunity to examine some of Saturn's moons. Cassini will perform several interesting non-targeted flybys (encounters with satellites at distances of less than 100,000 kilometers, or 62,000 miles, that do not require engine burns to reach) of some of Saturn's icy satellites.
Cassini's ISS camera system starts its observations for Rev139 on October 6 with 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 Janus, Epimetheus, Pallene, Methone, and Anthe. Another astrometric observation will be acquired on September 17, covering Calypso, Polydeuces, Pallene, Anthe, Epimetheus, Pandora, Janus, and Atlas. Cassini ISS will perform similar astrometric observations on October 12 and 22, imaging many of the same moons. In those two cases, additional wide-angle-camera images (WACs) of Saturn will be taken at the end of these SATELLORB observations. On October 8, ISS will take a four-hour observation of the small irregular satellite, Kiviuq. This observation is part of a campaign of observations of this and other small satellites that are taken determine their rotational periods, study their surface properties, and to determine if they are binary objects. Earlier images allowed researchers to pin down the length of Kiviuq's day to 21.82 hours, a remarkably long rotational period for an object this size. This 16-kilometer-diameter (10-mile-diameter), outer satellite will be 10.19 million kilometers (6.33 million miles) away. ISS will acquire a similar observation of Hyrrokkin on October 23. On October 12, Cassini will image Titan while the satellite appears as a narrow crescent. This will permit Cassini to monitor changes in the moon's upper haze layers. This observation will be acquired from a distance of 1.20 million kilometers (746,000 miles).
On October 13, Cassini ISS will image Saturn's faint G ring, a narrow ring between the orbit of Mimas and the main ring system. The next day the spacecraft will perform a non-targeted encounter with Titan at a distance of 172,370 kilometers (107,105 miles). The Composite Infrared Spectrometer (CIRS) will acquire composition data of Titan's atmosphere during this encounter as ISS rides along taking images. In some of the later WACs, the area covered by a large storm on Titan's trailing hemisphere (seen during the last orbit) may become visible. No major surfaces changes are expected, though localized flooding may be possible depending on topography. The last great storm of this magnitude seen on Titan by Cassini, back in October 2004, led to major flooding within the lowland plain Arrakis Planitia near Titan's south pole. On October 14, the Visual and Infrared Mapping Spectrometer (VIMS) and ISS will image Dione during a 14-hour observation of its thin crescent. The purpose of this sequence is to search for possible Enceladus-like plumes on this icy body, though to date none have been found despite earlier extensive searches.
On October 16 at 17:56 UTC, Cassini will reach the periapse of Rev139, its closest point to Saturn in the orbit. At periapse, the spacecraft will be 187,080 kilometers (116,246 miles) above Saturn's cloud tops. During this periapse period, Cassini ISS and the other remote-sensing instruments will focus on Saturn's icy satellites and stellar occultations of Saturn's atmosphere. First up is Enceladus, as Cassini images the moon's south polar plumes from a distance of 665,000 kilometers (413,000 miles), looking for changes within the plumes. Next, Cassini will image a stellar occultation of Saturn's atmosphere, both as the star passes behind Saturn (ingress) and out from behind it (egress). These observations are designed to improve our understanding of Saturn's upper haze layers by observing how the star's light is blocked by Saturn's atmosphere. This first observation probes Saturn's southern temperate atmosphere using Alpha Ceti, a variable red giant star also known as Mira. VIMS and ISS will perform a similar occultation later in the day, this time using the star Alpha Hydrae to examine the northern hemisphere.
On October 16, Cassini will perform two non-targeted encounters for which there will be imaging by ISS. The first is an encounter with the battered icy moon Mimas. Cassini will pass this satellite at a distance of 69,947 kilometers (43,463 miles). ISS will view Mimas as it appears as a crescent, imaging the moon as Cassini approaches. As Mimas rotates, ISS will image the region around Herschel crater. The high phase angle will highlight the topography in the region. During the latter parts of this observation, Mimas will pass in front of Saturn. Immediately following its Mimas observation, ISS will turn its cameras to Pallene as Cassini makes a close pass of that moon. At a distance of 36,125 kilometers (22,447 miles), ISS will be able to take our best images to date of this small moon. However, Pallene is a very small moon with dimensions of 5.8 kilometers (3.6 miles) by 5.6 kilometers (3.5 miles) by 4 kilometers (2.5 miles). At its closest during this observation, Pallene will only appear 27 by 18 pixels across in ISS narrow-angle-camera images (NACs). Saturn also will be in the background during this sequence, making this a great opportunity to measure the shape of Pallene as even the moon's dark side will be silhouetted against the atmosphere of Saturn.
On October 17, Cassini will perform two more non-targeted encounters of Saturn's icy satellites. These encounters involve first Dione and later Rhea. Cassini will pass by Dione at a distance of 31,708 kilometers (19,702 miles). ISS will acquire several mosaics of Dione as Cassini flies by: a three-color WAC sequence while Dione is at half-phase; a 1x6 NAC mosaic of the terminator region along 90 degrees west longitude; an 11-frame NAC mosaic across Dione's northern hemisphere, including portions of Janiculum Dorsa; a frame covering the north pole of Dione that will be paired with one of the images from the 11-frame mosaic to generate stereo coverage of the area; a 1x2 NAC mosaic covering the Evander impact basin on Dione's southern hemisphere; and a seven-frame mosaic to fill in gaps between the Evander mosaic and the earlier northern hemisphere mosaic. The best resolution images will have a scale of 190 meters (623 feet) per pixel. Each mosaic frame consists of a full-size, clear filter image and SUM2 (512 pixels on a side as opposed to 1024) images using the UV3, GRN, and IR1 filters. A short CIRS observation will precede these mosaics, and ISS will ride along during that observation to take more images when Dione appears as a thin crescent, though much of the area outside the crescent of sunlit terrain also will be illuminated by light reflected off Saturn. A similar sequence will be conducted at Rhea immediately afterward, as Cassini flies by at a distance of 38,750 kilometers (24,078 miles). CIRS will view a thin crescent as ISS rides along, again with much of surface illuminated by Saturn. The ISS mosaic of Rhea is more simplified compared to the one of Dione, with 30 frames that will cover much of Rhea's anti-Saturn and northern hemispheres. Prominent among the surface features to be imaged are the two neighboring impact basins, Tirawa and Mamaldi. Like the Dione mosaics, each frame will consist of clear, UV3, GRN, and IR1 images, though the equatorial and north-polar footprints will include extra IR3 images for compositional and color studies of these regions. The best resolution images will have a scale of 233 meters (764 feet) per pixel.
Finishing up Rev138, on October 18, Cassini will acquire three observations as part of JPL's Cassini Scientist for a Day program, which provides school children with an opportunity to propose targets for imaging. Last year, they picked color sequences of Tethys with the ring system, a crescent Titan image, and a wide-angle color image of Saturn. This year, they will acquire true-color images of Rhea's trailing hemisphere from a distance of 717,000 kilometers (445,000 miles), a color movie of a transit of Titan's south pole by Tethys as Enceladus passes nearby, and a color movie of Dione and Titan as they pass behind Saturn's dark limb. Between these observations, ISS will also perform a cloud monitoring sequence at Titan while the moon is 2.57 million kilometers (1.6 million miles). This observation will focus on the moon's sub-Saturn hemisphere and the area west of September's great arrow/dove-shaped storm. Another Scientist for a Day observation is planned for October 22 as ISS acquires a true-color movie of Saturn's atmospheric clouds.
On October 28, Cassini will reach apoapse on this orbit, bringing it to a close and starting Rev140. Rev140 includes a targeted flyby of Titan (T73) and non-targeted flybys of Enceladus and Dione.
Image products created in Celestia. Mimas and Dione basemaps by Steve Albers. All dates in Coordinated Universal Time (UTC).