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Cassini continues its extended tour of the Saturn system with the 16-day-long Rev125, the spacecraft's 126th orbit around the Ringed Planet. Cassini begins Rev125 on January 19 at its farthest distance from Saturn, called apoapse. At this point, Cassini is 2.23 million kilometers (1.39 million miles) from Saturn's cloud tops.
Cassini's ISS cameras start their observations for Rev125 a day after apoapse by imaging Saturn's largest moon, Titan, from a distance of 3.16 million kilometers (1.96 million miles). This observation will provide an opportunity to monitor clouds in Titan's troposphere on the satellite's sub-Saturn hemisphere. Also on January 20, ISS will acquire an astrometric observation of several of Saturn's small satellites including Prometheus, Methone, Anthe, and Telesto. Astrometric observations are used to help provide better orbital calculations for some of these small rocks which can be affected by gravitational interactions with the larger icy moons. On January 23, ISS will take a photometry observation of the unlit side of Saturn's rings. The view of Saturn from Cassini during the observation is shown above left. This observation includes a wide-angle-camera frame centered on Saturn and two narrow-angle-camera mosaics over both ansae of the rings. On January 24, the Visual and Infrared Mapping Spectrometer (VIMS) and ISS will image the left ansa of the faint G ring, a narrow ring between the main ring system and the orbit of Mimas. During this 11-hour observation, a bright arc of material, generated by micrometeorite impacts on the surface of the 500-meter-wide (1,640-foot-wide) moon Aegaeon, will pass through the field of view. On January 26, as Cassini nears the inner part of the Saturn system, ISS will image Enceladus's south polar plume from a distance of 945,000 kilometers (587,000 miles). To limit contamination of this observation by sunlight reflecting off the surface of Enceladus, most of the images will be targeted so that the moon is just off the edge of the frame.
On January 27 at 04:06 UTC, Cassini will reach the periapse of Rev125, its closest point to Saturn in the orbit. At periapse, the spacecraft will be 94,852 kilometers (58,938 miles) above Saturn's cloud tops. ISS and the other Cassini investigations have a complex schedule planned for the periapse period. Starting on January 26 at 18:05 UTC and running for four of the next seven hours, the Radio Science Subsystem (RSS) team will conduct an ansa-to-ansa radio occultation of Saturn's ring system and Saturn's atmosphere. During this observation, Cassini will send a radio signal back to Earth as the rings and Saturn pass between the spacecraft and Earth. This occultation is unique as it is a rare example of an ansa-to-ansa occultation during a single observation, making it an excellent opportunity to look for eccentric structures in the rings, such as ringlets that don't have a uniform thickness all the way around Saturn. A prime example of such structures will be sampled by this radio occultation as RSS observes the ringlet in the middle of the Encke Gap in the outer A ring shortly after a passage by the small moon Pan.
Following the occultation, Cassini will continue to approach not only Saturn, but the small moons Prometheus and Aegaeon as well. Both of these moons will be imaged beginning shortly before periapse and after Cassini crosses above the ring plane. First, Cassini will observe portions of the G ring and its bright arc. Next, Cassini will turn its cameras to Prometheus, with images acquired from as close as 33,452 kilometers (20,786 miles) away. This will allow for higher resolution images than those acquired during a non-targeted encounter in December 2009. However, these observations will be at higher phase angles (mitigated a bit by Saturnshine), so less of Prometheus's surface will be illuminated by the Sun. Cassini will acquire three sets of multi-spectral imaging of Prometheus during this encounter, providing color and stereo data over Prometheus's northern and trailing hemispheres. Cassini's view for the third set is shown above right. Finally, ISS will image from as close as 14,507 kilometers (9,014 miles) away Aegaeon and the bright G ring arc the moon helps generate. While this will provide the highest resolution images of this moon to date (85 meters or 280 feet per pixel), Aegaeon is so small that it will appear only five or six pixels across. Regardless, data from this encounter should provide a better estimate of Aegaeon's size.
Next, Cassini's optical remote sensing instruments will turn their attention to Dione for a non-targeted encounter of that much larger moon (compared to Prometheus and Aegaeon). Two observations will be performed. The first will take place during closest approach (45,081 kilometers, or 28,012 miles), with imaging beginning with Dione appearing as a crescent. The phase angle will increase as the observation progresses. This sequence will provide the highest resolution imaging of the north polar region to date. The second observation will begin 2.5 hours after closest approach. ISS will acquire a four-frame mosaic of Dione's trailing hemisphere. Cassini's view of Dione during this observation is shown above left. Also on January 27, ISS and VIMS will acquire several mosaics of Saturn and the lit face of Saturn's ring system. The primary ISS observation will include 19 WAC frames covering Saturn and its main ring system for a large, multi-spectral mosaic.
Cassini encounters Titan on January 28 at 22:29 UTC for the 67th time. This is also the last of four Titan encounters spaced 16 days apart that have been used to raise the inclination of the spacecraft's orbit to 20 degrees above the ring plane then back down to an equatorial orbit. This brief inclination increase was designed to set up the ansa-to-ansa radio occultation of Saturn's rings two days earlier. Now that this occultation is complete, this encounter will be used to return Cassini to an equatorial orbit. The close approach distance for the encounter (known as T66) is 7,490 kilometers (4,654 miles) -- one of the more distant Titan flybys. Cassini will approach over Titan's night side, passing over its southern mid-latitudes over the trailing hemisphere, and Cassini will be able to observe Titan's sun-lit, anti-Saturn hemisphere on the outbound leg of this flyby. During the 14.5 hours prior to closest approach, the Composite Infrared Spectrometer (CIRS), Ultraviolet Imaging Spectrometer (UVIS), and ISS teams will control spacecraft pointing, or be "prime." CIRS will measure the composition of Titan's atmosphere and haze layers on the dark limb of the moon over its sub-Saturn hemisphere using far-infrared spectroscopy. UVIS will conduct a west-to-east scan across the equator of the moon to perform similar compositional measurements, this time in the far- and extreme-ultraviolet. ISS will observe Titan's crescent to monitor changes in Titan's north polar haze layers.
During closest approach, ISS will have prime pointing during the encounter, allowing the instrument to gather high resolution imaging of Titan's southern mid-latitudes. This is the first of two flybys in a row where ISS will be prime at closest approach. In this case, ISS will acquire a 14-frame, narrow-angle-camera mosaic of a large dark feature located at 40 degrees south latitude, 280 degrees west longitude (visible at the center of the graphic shown at right). This feature is one of the most distinct albedo features at this latitude range on Titan. The origin of this feature is not yet known, but a better understanding should come from this ISS high-resolution imaging and VIMS ride-along data. One possibility is that this feature is a dry lake that is occasionally filled by methane from the mid-latitude cloud bands, but RADAR imaging of a similar feature further to the west seems to make this less likely. The best quality imaging from this observation will likely come from the wide-angle camera, but the narrow-angle-camera images should provide updated flatfield calibration files at the very least.
Following closest approach, UVIS, CIRS, and VIMS will have prime pointing times. CIRS and UVIS will acquire similar observations to those taken prior to closest approach -- this time over Titan's dayside. VIMS will monitor clouds structures on Titan's anti-Saturn hemisphere.
On January 30, ISS will observe Titan over a period of 9.5 hours. These sequences are designed to monitor clouds on Titan and changes in Titan's north polar haze layers. These 10 frames (each consisting of 11 or 12 images) will be taken from distances ranging from 691,000 to 960,000 kilometers (430,000 and 596,000 miles). Similar to imaging taken after the Titan encounter during the previous two orbits, these frames will be centered north of Titan's equator to ensure coverage of clouds over Titan's northern mid-latitudes, but might cause some clouds in the southern hemisphere to be missed, particularly in the earlier frames. These frames will be compared to five more cloud monitoring observations to be acquired on each of the next five days from 1.2 to 3.2 million kilometers (0.75 and 2 million miles) away, allowing for more extended cloud tracking. On January 31, ISS will observe a transit of Prometheus across Enceladus. Prometheus will be 1.96 million kilometers (1.22 million miles) away. Enceladus will be 2.22 million kilometers (1.38 million miles) away. Also, ISS will acquire astrometric observations of several of Saturn's small satellites including Methone, Telesto, Pallene, Epimetheus, and Janus. Over the next two days, Cassini will take three more such observations covering Anthe, Polydeuces, Telesto, Calypso, Pallene, and Helene.
On February 1, ISS will image a transit of Janus by Epimetheus when Cassini is 2.16 million kilometers (1.34 million miles) from Epimetheus. Janus and Epimetheus are co-orbital moons, and their gravitational interaction causes them to swap orbits every four years. Unlike most mutual events, when the satellites only appear to be close to one another from Cassini's perspective but are actually hundreds of thousands kilometers or miles from each other, in this case the two co-orbital satellites actually will be only 11,600 kilometers (7,200 miles) from one another at the time of the transit observation. This observation comes during the latter stages of the orbital swap between Janus and Epimetheus, with Epimetheus now closer on average to Saturn than Janus. The two will switch back on January 21, 2014. The mutual event observation Cassini will take on February 1 will help improve our estimate of the masses of both bodies by refining our knowledge of the interaction between these moons during the orbital swap. Later on February 1, ISS will observe Janus, appearing as a dark silhouette in the shadow of Saturn and transiting the south pole of Enceladus.
On February 2, ISS will observe the small outer satellite Erriapus (Saturn XXVIII) in order to better understand the physical properties of this distant moon, 9.95 million kilometers (6.19 million miles) from Cassini. At this distance, the 10-kilometer-wide (6-mile-wide) satellite will look like a faint star in Cassini images. Even so, variations in the satellite's apparent brightness over the 13-hour observation can be used to help determine Erriapus's orbital period. Cassini can also observe Erriapus and Saturn's other outer satellites at phase angles not possible from Earth (in this case, a sun-moon-spacecraft angle of 72 degrees). Comparing its brightness at this moderate phase angle to terrestrial observations at much lower phase angles can be used to obtain information on the satellite's surface properties, e.g. whether the surface is dusty or rocky. This long observation will be combined with two shorter observations acquired during Rev124 and on February 4 to improve the signal-to-noise ratio of the lightcurve plot, and to search for possible secondary moons.
Cassini reaches apoapse on February 4, bringing Rev125 to an end and starting Rev126. Rev126 includes the closest flyby to date of Saturn's innermost large moon, Mimas, proving an opportunity to image its large impact basin, Hershel, at high resolution. In addition, Cassini will fly by the small Trojan moon, Calypso, proving the highest resolution images to date of that moon.
Image products created in Celestia. Dione basemap by Steve Albers. All dates in Coordinated Universal Time (UTC).