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Cassini continues its new tour of the Saturn system with the 21-day-long Rev144, the spacecraft's 145th orbit around the Ringed Planet. Cassini begins Rev144 on January 20 at its farthest distance from Saturn, called apoapse. At this point, Cassini is 2.61 million kilometers (1.62 million miles) from Saturn's cloud tops. Cassini's orbit lays almost entirely within Saturn's ring plane and within the orbital plane of most its major satellites, affording an opportunity to encounter a few of its moons. During Rev144, Cassini will pass within 100,000 kilometers (62,000 miles) of several of Saturn's moons including Epimetheus, Calypso, Enceladus, and Helene.
ISS begins its observations for Rev144 on January 21 by imaging Saturn's moon Titan. This distant observation, taken from a distance of 3.44 million kilometers (2.14 million miles), will provide another opportunity to monitor surface changes seen across southern Senkyo in late 2010. Immediately afterward, ISS will perform astrometric observations of Saturn's small, inner moons. During this observation, the camera system will image Polydeuces, Telesto, Janus, Pallene, Atlas, Calypso, and Methone. Shortly after, the Wide-Angle Camera (WAC) will image Saturn. The tail of the large, bright storm that formed in December 2010 in the North Temperate Zone of Saturn should be visible during this observation. Finally on January 21, ISS will image the left ansa of the diffuse G ring. On January 22, ISS will acquire 359 images of distant Ijiraq, a 12-kilometer-wide (7-mile-wide) outer moon of Saturn. These images will be used to build up a light-curve of the satellite, which is useful for measuring the length of the moon's day and understanding how its brightness changes depending on lighting conditions. Ijiraq will be 9.65 million kilometers (6.00 million miles) away at the time.
On January 28, the Ultraviolet Imaging Spectrometer (UVIS) will perform a series of scans across the south polar region of Saturn designed to observe aurora over this pole. Afterward, UVIS will stare at the south pole while ISS rides along taking images. On January 29, UVIS and ISS will team up again, this time to image a crescent Dione. Using the data from this 12-hour observation (the equivalent of 4 hours of rotation on Dione), researchers will be looking for water plumes that maybe erupting from the trailing and anti-Saturn hemispheres of Dione. The moon's distance to Cassini will range from 440,000 to 980,000 kilometers (270,000 to 610,000 miles). On January 30, ISS will image Enceladus' south polar plume from a distance of 214,000 kilometers (133,000 miles), with many of the frames designed to have the body of Enceladus out of frame to limit light contamination from the highly reflective moon. Finally, the Composite Infrared Spectrometer (CIRS) will perform scans of Mimas as the spacecraft approaches the small, cratered moon for a non-targeted encounter, while ISS rides along. Closest approach to Mimas occurs at 01:55 UTC on January 31 and will occur at a distance of 138,570 kilometers (86,100 miles). Images taken during this time will show the large crater Herschel and the terrain to its east.
On January 31 at 02:50 UTC, during the Mimas CIRS observation, Cassini will reach the periapse of Rev144, its closest point to Saturn in the orbit. At periapse, the spacecraft will be 154,420 kilometers (95,950 miles) above Saturn's cloud tops. During this periapse passage, Cassini will focus on a series of non-targeted encounters with several of Saturn's icy satellites. First up was Mimas, whose closest approach occurred an hour before periapse. After the CIRS observation of Mimas, Cassini remote-sensing instruments will be pointed at Enceladus to record an encounter with that moon. Closest approach to Enceladus will occur at 05:52 UTC at a distance of 60,028 kilometers (37,300 miles). Two observations are planned to support this encounter. First, ISS will image a crescent Enceladus, focusing on the moon's south polar plume. More of Enceladus' surface will be visible in direct sunlight during the next observation, which will consist of four mosaics. Each is a four-panel mosaic covering the entire visible disk. The terrain visible in sunlight will include eastern Diyar Planitia, Harran Sulci, Bishangarh Fossae, and Khorasan Fossa. Next, ISS will take another look at Mimas, again focusing on the large impact crater Herschel. This observation will be taken from a distance of 168,000 kilometers (104,000 miles).
Next up in Cassini's mini-tour of Saturn's icy satellites is Helene, which Cassini will pass at a distance of 27,766 kilometers (17,253 miles) at 10:17 UTC. Imaging for this encounter will occur in two parts. First, during a period starting four minutes after closest approach, ISS will image Helene using eight color filters plus two clear filter images bracketing the color imaging. Eighteen minutes later, ISS will image Helene with three sets of polarized images. Polarized filters first will be combined with the UV3 filter, then the GRN filter, and then the MT2 filter. The phase angle of Helene (a measure of how much of the surface is illuminated by the sun as seen by Cassini) will improve as the sequence progresses. The phase angle will decrease from 83 degrees (a little more than half of Helene appearing to be illuminated by the sun) at the start of the observation to 63 degrees so that more of the surface is illuminated at the end of the observation. During this observation, the anti-Saturn hemisphere will be illuminated, similar to the lighting conditions during the closer flyby of Helene that occurred in March 2010. However, Cassini will be slightly above the ring plane at the time, so Helene's north polar region, which may be dominated by an impact crater, will be visible. Helene's south polar region was the focus of the March 2010 encounter.
Finishing up the periapse period, UVIS and the Visual and Infrared Mapping Spectrometer (VIMS) will take another look at Mimas following the Helene observations, and ISS will be riding along. The focus of this observation is the region west of Herschel crater, including the crater Arthur. The UVIS team is using its observation to continue to acquire a more complete dataset of longitude and phase angle coverage of Mimas. Afterward, the Optical Remote Sensing instruments will turn to a nearly full Enceladus, but ISS will not be taking images. The next day, February 1, ISS will image the Lagrangian points L4 and L5 along Mimas's orbit in order to search for possible co-orbital satellites. These observations should allow moons as small as 50 meters (164 feet) to be detected, if they exist. Co-orbital moons are known to exist for two of Saturn's satellites, Tethys (Telesto and Calypso) and Dione (Helene and Polydeuces).
Finishing up Rev144, on February 3 ISS will image Rhea as Dione passes behind it. Rhea will be 1.19 million kilometers (741,000 miles) away at the time, while Dione will be 1.94 million kilometers (1.21 million miles) away. On February 4, ISS will image a gibbous Titan at a distance of 2.39 million kilometers (1.49 million miles). This observation will allow researchers to monitor clouds and look for surface changes between southern Senkyo and "Okavango," a region that has been poorly imaged since the major arrow-shaped storm in late September and October 2010. Surface changes are possible in this region, though after four months they may have faded by this time. Afterward, ISS will take astrometric observations of Saturn's small, inner moons. This time, ISS will observe Atlas, Methone, Telesto, and Helene. Saturn will also be imaged using the WAC, and the tail of the northern storm will be visible. Finally, on February 5, ISS will image Ijiraq once again, this time as a support observation for the earlier light-curve images. Although a shorter observation, this one should improve the light-curve's signal-to-noise ratio.
On February 10, Cassini will reach apoapse on this orbit, bringing it to a close and starting Rev145. Rev145 will be the "Saturn and Titan Show" as all but one of the planned 15 ISS observations are dedicated to the two largest bodies in the Saturn system. This also includes a targeted flyby of Titan (T74).
Image products created in Celestia. Mimas and Enceladus basemap by Steve Albers. Helene shape model and basemap by "Fenerit". All dates in Coordinated Universal Time (UTC).