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Cassini gets 2010 started with the 16-day-long Rev124, the spacecraft's 125th orbit around the Ringed Planet. Cassini begins Rev124 on January 3 at its farthest distance from Saturn, called apoapse. At this point, Cassini is 2.2 million kilometers (1.37 million miles) from Saturn's cloud tops.
Cassini ISS starts its observations for Rev124 a few hours after apoapse by imaging Saturn's largest moon, Titan, from a distance of 2.89 million kilometers (1.79 million miles). This observation will provide an opportunity to monitor clouds in Titan's troposphere on the satellite's sub-Saturn hemisphere. On January 4, ISS will observe the small outer satellite Erriapus (Saturn XXVIII) in order to better understand the physical properties of this distant moon, 8.1 million kilometers (5.03 million miles) from Cassini. At this distance, the 10-kilometer- (6-mile-) wide satellite will look like a faint star in Cassini images. Even so, variations in the satellite's apparent brightness over the three-hour observation can be used to help determine Erriapus' orbital period. Cassini can also observe Erriapus and Saturn's other outer satellites at phase angles not possible from Earth (72 degrees in this case). 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. Two days later, on January 6, ISS will take a 13-hour movie of spokes on the unlit face of Saturn's B ring. On January 7, ISS will acquire a low-resolution, 15-hour movie of the ringlets within the Encke Gap in the outer A ring. The wide-angle camera will also be used to acquire another movie of spokes on the B ring. On January 10, ISS will acquire two observations of Saturn's ring system, including a 2.5-hour movie of the outer edge of the A-ring and an imaging sequence that bounces between 2- and 3-frame mosaics of the D ring and frames in the Roche Division between the A and F rings.
On January 11, Cassini will reach the periapse of Rev124, its closest point to Saturn in the orbit. At periapse, the spacecraft will be 120,620 kilometers (74,950 miles) above Saturn's cloud tops. The only ISS observations planned for the period around periapse are support imaging for Composite Infrared Spectrometer (CIRS) radial scans of the sunlit face of the rings and a ride along observation with a Visual and Infrared Mapping Spectrometer (VIMS) radial scan of the rings. The second observation will provide an opportunity to take color, narrow-angle-camera images of different portions of the rings.
Cassini encounters Titan on January 12 at 23:11 UTC for the 66th time. This is also the third of four Titan encounters spaced 16 Earth days apart that have been used to raise the inclination of the spacecraft's orbit to 20 degrees then back down to equatorial orbit. This brief inclination increase is designed to set up an ansa-to-ansa radio occultation of Saturn's rings during Rev125. The close approach distance for the encounter (known as T65) is 1,072 kilometers (666 miles). This flyby will allow for imaging of the anti-Saturn hemisphere of Titan outbound to the encounter. However, for much of the encounter, outside of the four hours surrounding closest approach, the CIRS team will control spacecraft pointing, or be "prime." Another exception includes a five-hour period between 9 and 14 hours before closest approach, when VIMS will acquire several image cubes over a narrow crescent Titan. VIMS will be monitoring clouds over the trailing hemisphere as well as continuing that team's observations of a diminishing ethane cloud over the north polar region. CIRS will measure the composition of Titan's atmosphere on the limb of the moon over its south polar region using mid-infrared spectroscopy as well as perform two scans across the face of Titan over its northern hemisphere.
During closest approach, the Ion and Neutral Mass Spectrometer (INMS) will have prime pointing during the encounter, allowing the instrument to gather high resolution spectral data of Titan's upper atmosphere. During T65, INMS will sample Titan's southern polar atmosphere. The INMS team will compare data acquired during the encounter to data acquired last year when Cassini repeated flew over the south polar region. In particular, the team will be looking for seasonal changes in the upper reaches of Titan's atmosphere. The RADAR team will use the opportunity to look for additional changes at Ontatio Lacus through both Synthetic Aperture Radar (SAR) imaging and altimetry. RADAR will acquire two short SAR swaths across the dark region Mezzoramia -- possibly a dry, south polar twin of Kraken Mare -- and across Ontario Lacus. RADAR will then take a short altimetry swath across Ontario, providing an opportunity for RADAR to study the physical properties of the liquid that fills the lake as well as look for lake level changes. In the two hours before and after closest approach, the Cassini Plasma Spectrometer (CAPS) will use its prime coverage time to examine the interactions between Titan and Saturn's magnetosphere. During the 20 hours following CAPS post-encounter coverage, CIRS will acquire temperature and compositional maps of Titan's atmosphere by measuring Titan's spectra in the mid- and far-infrared.
On January 14, 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 12 frames (each consisting of five to six images) will be taken from distances ranging from 693,000 to 936,000 kilometers (430,000 and 580,000 miles). Similar to imaging taken after the Titan encounter during the previous orbit, these frames will be centered north of Titan's equator to ensure coverage of clouds over Titan's northern mid-latitudes. However, this observation might cause some clouds in the southern hemisphere to be missed, particularly in the earlier frames. These frames will be compared to two more cloud monitoring observations to be acquired on January 15 and 16 from 1.2 and 1.7 million kilometers (0.75 and 1.05 million miles) away, allowing for more extended cloud tracking. On January 15, ISS will acquire an astrometric observation known as a SATELLORB of several of Saturn's small satellites including Prometheus, Pandora, Atlas, Methone, and Pallene. Astrometric observations are used to help provide better orbital calculations for some of these small rocks, which can be effected by gravitational interactions with the larger icy moons.
On January 16, ISS will image a transit of Janus by Epimetheus, when Cassini is 2.12 million kilometers (1.32 million miles) from Epimetheus. 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 will be 11,600 kilometers (7,200 miles) from one another at the time of the transit observation. Over the next few days, Janus and Epimetheus will make repeated passes of each other, coming as close as 10,000 kilometers (6,200 miles) on January 21. The gravitational interactions between the two moons during these encounters will cause the two to swap orbits, 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 January 16 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.
Cassini reaches apoapse on January 19, bringing Rev124 to an end and starting Rev125. Rev125 includes a flyby of Saturn's largest moon, Titan (T66) and non-targeted encounters of Enceladus and Dione.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).