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Cassini continues its exploration of the Saturn system with the 24-day Rev160, which begins on January 16 at its farthest distance from the planet. This point is also called the orbit's apoapse. At this point, Cassini will be 2.90 million kilometers (1.80 million miles) from Saturn's cloud tops. The spacecraft is in the middle of the first equatorial phase of the Cassini Solstice Mission, which lasts until May 2012. During this phase, the spacecraft's orbits lie within the equatorial plane of the planet, providing opportunities to encounter Saturn's numerous moons, image the rings edge-on, and look at Saturn's cloud tops without the rings obscuring the view. Sixty ISS observations are planned for Rev160, the vast majority dedicated to an encounter with Titan and Saturn storm monitoring.
ISS begins its observations for Rev160 a day after apoapse on January 17 with a quick observation of Saturn and its faded northern hemisphere storm. Three more such observations are planned that day. These "Storm Watch" observation sequences are designed to take advantage of short, two-minute segments when the spacecraft turns the optical remote sensing (ORS) instruments back to Saturn as a waypoint between other experiments' observations. These sequences include blue, clear, two methane band, and one full-frame, continuum band filter images. Twenty-two more are planned between January 18 and 25, while seven are planned between February 6 and 8. Between the first two storm watch observations on January 17, ISS will image Titan's Fensal-Aztlan region from a distance of 3.94 million kilometers (2.45 million miles) in an effort to look for clouds in the moon's atmosphere as part of the "Titan Monitoring Campaign". ISS will take a look at Titan again on January 18, this time from a distance of 2.23 million kilometers (1.39 million miles). After the Titan observation on January 17, ISS will acquire a pair of Saturn cloud tracking observations. Both last for two hours using sets of images taken 10 minutes apart in order to measure wind speeds in Saturn's atmosphere by tracking clouds. Four more of these will be taken on January 19 and 20, though each of these later Saturn wind observations will last five hours rather than two. Cassini takes another "Titan Monitoring Campaign" observation on January 20 from a distance of 2.87 million kilometers (1.78 million miles), looking for clouds across the moon's trailing hemisphere.
On January 21, ISS will observe different latitudes of Saturn's atmosphere at low, moderate, and high emission angles to study again Saturn's upper haze layers and their effects on our ability to observe lower-altitude cloud structures. A similar observation will be taken on January 26. On January 22, ISS will image Titan's Senkyo, Garotman Terra, Tollan Terra, and Yalaing Terra regions from a distance of 3.20 million kilometers (1.99 million miles) in an effort to look for clouds in the moon's atmosphere. This observation also will be useful to see if the remnant bright spot in Yalaing Terra from the late 2010 "Arrow Storm" remains. A similar observation taken from a distance of 3.06 million kilometers (1.90 million miles) will be taken on January 24 and will cover the sub-Saturn hemisphere. On January 23, the Composite Infrared Spectrometer (CIRS) will observe Saturn for 22 hours while ISS rides along, taking a set of images ever couple of hours. Afterward, ISS will observe Siarnaq for 12 hours in order to pin down the orientation of this small, outer satellite's north pole axis.
On January 28 at 18:30 UTC, Cassini will reach periapse, its closest point to Saturn, for Rev160 at an altitude of 206,310 kilometers (128,200 miles). The only ISS observation planned for this period is a ride along observation with the Visual and Infrared Mapping Spectrometer (VIMS). ISS will acquire a series of narrow-angle-camera images of Saturn's night side to look for lightning.
Two days after periapse, Cassini encounters Titan on January 30 at 13:40 UTC for the 82nd time. This is the second of nine Titan flybys planned for 2012 with the next encounter scheduled for February 19. T81 is a high-altitude flyby with a close-approach distance of 31,131 kilometers (19,343 miles). This flyby will allow for imaging of Adiri and the southern anti-Saturn hemisphere regions of Titan outbound from the encounter. Before the encounter, CIRS and the Ultraviolet Imaging Spectrometer (UVIS) will acquire spectral scans of Titan's night side. First, CIRS will scan across Titan using its far-infrared channel. Next, UVIS will acquire an extreme and far-ultraviolet scan followed by a stare at the bright limb of Titan.
At closest approach, control of spacecraft pointing will switch to ISS with the Visual and Infrared Mapping Spectrometer (VIMS) riding along. This flyby is one of only two flybys during the Solstice Mission where the camera is the "prime" instrument during closest approach of a Titan encounter. The other prime flyby for ISS was T80 earlier this month. The two optical remote sensing instruments will focus their attention on Ontario Lacus, a large hydrocarbon lake near Titan's south pole, during a five-hour observation. Scientists will be interested too see whether lake levels have dropped further since the last time the lake was observed during the T51 flyby in March 2009. This is the last opportunity for these instruments to view Ontario Lacus before the end of the mission because this area is moving into darkness as the seasons progress toward southern winter and northern summer. Afterward, CIRS and VIMS will repeat their observations from the inbound leg of the flyby, this time covering Titan's day side. ISS will ride along with the inbound and outbound CIRS observations. Between February 1 and 4, ISS will acquire three cloud monitoring sequences. Unlike the regular Titan Monitoring Campaign observations, these are much lengthier and are designed not only to monitor for the presence of clouds, but also to track them so their speeds can be measured. These observations will be centered over Belet, eastern Belet, and Hetpet Regio, respectively. A shorter, Titan Monitoring Campain sequence will be taken on February 6 from a distance of 3.58 million kilometers (2.22 million miles) centered on the Senkyo region on Titan.
After another Saturn storm watch sequence, ISS will acquire an astrometric observation of Saturn's small, inner moons, including Janus and Polydeuces. Astrometric observations are used to improve our understanding of the orbits of these small satellites, which can be influenced by Saturn's larger icy satellites. A second astrometric sequence will be taken on February 7, this time covering Epimetheus, Telesto, and Prometheus. Afterward, ISS will search for Trojan satellites near the L5 point on Titan's orbit. Similar co-orbital moons have been found for Tethys and Dione. Finally on February 7, ISS will image Titan once again for 11 hours, tracking clouds across Titan's sub-Saturn hemisphere.
On February 9, Cassini will reach apoapse on this orbit, bringing it to a close and starting Rev161. Rev161 includes a targeted flyby of Titan.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).