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Cassini continues its exploration of the Saturn system with the 24-day Rev161, which begins on February 9 at its farthest distance from the planet. This is also called the orbit's apoapse. At this point, Cassini is 2.84 million kilometers (1.77 million miles) from Saturn's cloud tops. The spacecraft is in the middle of the first equatorial phase of the Cassini Solstice Mission, a phase 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, to image the rings edge-on, and to look at Saturn's cloud tops without the rings obscuring the view. Forty-two ISS observations are planned for Rev161, the vast majority dedicated to an encounter with Titan and Saturn storm monitoring.
ISS begins its observations for Rev161 two days after apoapse on February 11 with a quick observation of Saturn and its faded northern hemisphere storm with a second such observation planned later 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. Eleven more such sequences are planned between February 12 and 18, while four are planned for February 24 and 25. Between the first two storm watch observations on February 11, ISS will acquire an astrometric observation of Saturn's small, inner moons, including Epimetheus, Prometheus, Methone, 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. After another Saturn Storm Watch observation, ISS will observe Titan for 11 hours as part of the "Titan Exploration at Apoapse" (TEA) campaign. These lengthy observations are designed to monitor clouds on the satellite as well as to track the clouds' motions. This TEA observation will cover the Fensal-Aztlan region of Titan from a distance of 3.82 million kilometers (2.38 million miles).
On February 14, ISS will take a look at Titan, this time covering eastern Xanadu and northwestern Fensal from a distance of 2.89 million kilometers (1.80 million miles). The observation is an effort to look for clouds in the moon's atmosphere as part of the "Titan Monitoring Campaign" (TMC). These TMC observations are shorter than TEA observations and are simply used to see if clouds are present rather than tracking the clouds' movements. After another Saturn storm watch observation, ISS will acquire an astrometric observation, looking at Janus, Anthe, Telesto, Polydeuces, and Prometheus. On February 16, ISS will observe a crescent Titan in order to examine the moon's upper haze layers, such as the north and south polar hoods. This observation will be taken from a distance of 1.59 million kilometers (0.99 million miles). The next day, the wide-angle camera will be used to examine the vertical structure of the E ring. Researchers will be examining the location of the ring with respect to the planet's equatorial plane to study the effects of solar radiation pressure and Saturn's magnetic field on the eccentricity of the fine particles in the E ring.
Two days before periapse, Cassini encounters Titan on February 19 at 08:43 UTC for the 83rd time. This is the third of nine Titan flybys planned for 2012, and is the last of the first equatorial phase of the mission. The next flyby is scheduled for May 22. T82 is a relatively high-altitude flyby with a close-approach distance of 3,803 kilometers (2,363 miles). This flyby will allow for imaging of Titan's Senkyo region outbound from the encounter and a crescent Titan inbound. For nearly the entire encounter, the Composite Infrared Spectrometer (CIRS) will be the primary pointing instrument, with ISS riding along. Before closest approach, CIRS will acquire a series of nadir and limb temperature and compositional mapping scans using both their far- and mid-infrared channels. ISS also will have a short observation ten hours before encountering Titan. The camera will run through a series of methane, continuum-band, and ultraviolet filters in order to study the moon's upper haze layers.
At closest approach, CIRS will acquire a pair of limb temperature scans using their far-infrared channel. These scans will focus primarily on atmosphere above Titan's mid-southern latitudes. CIRS also will look at the composition of aerosols in the moon's haze layers. For the rest of the encounter, CIRS will acquire another set of nadir and limb temperature and compositional mapping scans, this time covering Titan's sunlit side.
On February 21 at 05:23 UTC, Cassini will reach periapse for Rev161 at an altitude of 134,680 kilometers (83,690 miles) above Saturn's cloud tops. Both of ISS's periapse observations will be taken the day before. First, the narrow-angle camera will ride along with the Ultraviolet Imaging Spectrometer (UVIS) to monitor Saturn's aurora australis. Next, ISS will observe Enceladus' plume, monitoring activity at the south polar jets just five weeks before the next flyby of the moon.
On February 24, ISS will image Titan's sub-Saturn hemisphere from a distance of 2.88 million kilometers (1.79 million miles). ISS will take a look at Titan again on February 29, this time from a distance of 2.81 million kilometers (1.75 million miles) and covering eastern Xanadu and the region around Menrva.
On March 1, Cassini will reach apoapse on this orbit, bringing it to a close and starting Rev162. Rev162 includes a close, non-targeted flyby of Enceladus.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).