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Cassini finishes up 2011 and starts its new year with the 24-day Rev159, which begins on December 23 at its farthest distance from Saturn. 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, 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. Seventy-eight ISS observations are planned for Rev159, the vast majority dedicated to the encounter with Titan and Saturn storm monitoring.
ISS begins its observations for Rev159 seven hours after apoapse on December 24 with a quick observation of Saturn and its faded northern hemisphere storm with another planned an hour later. 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. Sixteen more are planned between December 24 and January 1, while thirty-one are planned between January 6 and 15. Between the two storm watch observations for December 24, ISS will acquire an astrometric observation of Saturn's small, inner moons, including Prometheus, Atlas, Anthe, Methone, Calypso, Polydeuces, and Epimetheus. 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.
On December 25, 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. ISS will take a look at Titan again on December 26, this time from a distance of 3.71 million kilometers (2.30 million miles). Following a Saturn storm watch observation tacked on to the end of the Titan observation, ISS will acquire another astrometric observation, this time covering Telesto, Pallene, Janus, Helene, Pandora, Methone, and Calypso, after which ISS will again observe Saturn (this time with Tethys transiting the giant planet). Cassini takes another "Titan Monitoring Campaign" observation on December 30 from a distance of 1.73 million kilometers (1.07 million miles). This observation is designed to study Titan's upper haze layers rather than the moon's surface because the phase angle will be too high to study the surface effectively with ISS. After another Saturn storm watch observation, ISS will acquire another astrometric observation, this time looking at Anthe, Pallene, and Epimetheus.
Two days before periapse, Cassini encounters Titan on January 2 at 15:14 UTC for the 81st time. This is the first of nine Titan flybys planned for 2012 with the next encounter scheduled for January 30. This encounter, called T80, is a high-altitude flyby with a close-approach distance of 29,416 kilometers (18,278 miles). This flyby will allow for imaging of the Senkyo and southern trailing hemisphere regions of Titan outbound from the encounter. The Composite Infrared Spectrometer (CIRS) will be the primary pointing instrument during the inbound leg of this flyby while Titan is visible as a narrow crescent. The instrument will perform a Titan-staring, compositional measurement followed by two temperature map scans and a limb scan measuring aerosols over the night side of the northern hemisphere.
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 is T81 coming up at the end of the month. The two optical-remote sensing instruments will focus their attention on the southern trailing hemisphere, taking wide-angle-camera images and VIMS cubes of features like Polaznik Macula. Outbound, RADAR will perform radiometry scans across Titan's southern trailing hemisphere, filling a gap in the instrument's dielectric constant map of Titan. Next, VIMS will acquire stellar occultation observations of Titan's atmosphere using CW Leonis, a near-infrared bright star and R Leonis, a red hypergiant star. Both are in the constellation Leo. Finally, VIMS will finish up the encounter with a global VIMS mosaic.
On January 4 at 13:03 UTC, Cassini will reach periapse for Rev159 at an altitude of 206,330 kilometers (128,210 miles) from Saturn's cloud tops. During this periapse passage, Cassini will acquire a series of observations of Saturn's atmosphere designed to study the dynamics of weather patterns across the planet's northern hemisphere. First up, though, ISS will observe the crescent of Enceladus in order to understand the current state of the south polar plume. By taking observations of the plume at different times, researchers hope to test the hypothesis that the vents for the plume may be opened and closed by tidal forces. These different observations occur when Enceladus is at different points of its orbit, when the pull of Saturn's gravity can produce expanding and contracting forces across the moon's surface. These forces switch from causing expansion to causing contraction depending on orbital phase. Afterward, CIRS will scan along Saturn's limb just south of the ring shadow in order to measure the thermal structure of the planet's stratosphere. Next, the Ultraviolet Imaging Spectrometer (UVIS) will observe an occultation of the Trapezium Cluster in the constellation Orionis by Saturn in order to measure the structure of Saturn's upper haze layers. VIMS will then acquire a high resolution map of Saturn's northern hemisphere, taking VIMS cubes as Saturn rotates underneath the spacecraft. Finally, 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.
On January 6, ISS will acquire a lengthy, eight-hour observation of Ymir, the second largest of Saturn's distant, irregular moons. This observation will be taken from a distance of 18 million kilometers (11.2 million miles), and when combined with additional observations of this irregular satellite, is designed to measure Ymir's brightness at different phase angles. This can provide information about the nature of its surface (whether it is rough or smooth for example) even if Cassini never approaches within millions of kilometers of it. In this case, Ymir will be at a very low phase (nearly full), though it will still appear as nothing more than a bright point of light in narrow-angle-camera images. The next day, ISS will conduct another emission angle study of Saturn's atmosphere. On January 8, ISS will take a look at Titan's sub-Saturn hemisphere from a distance of 3.05 million kilometers (1.89 million miles). Following the Titan observation and another set of Saturn storm watch images, ISS will ride along with a 22-hour CIRS observation of Saturn, allowing ISS to map the planet's day side across two Saturn days.
On January 9, ISS will observe Titan's Fensal-Aztlan region, this time from a distance of 3.29 million kilometers (2.05 million miles). Afterward, ISS will begin the first of four Saturn wind measuring observations that will be acquired over the next day and a half. Saturn's winds across different latitude zones are measured by tracking cloud features from one day to the next. These four sequences will cover a period of three Saturn days. Between January 11 and 15, ISS will image Titan each day, searching for clouds across Titan's leading and anti-Saturn hemisphere. During these observations, the closest Cassini comes to Titan is 1.89 million kilometers (1.18 million miles) on January 15. On January 11, ISS will perform a lengthy Saturn storm watch observation acquiring a set of images at the beginning and end of a three-hour sequence, rather than the usual time span of only two minutes.
On January 16, Cassini will reach apoapse on this orbit, bringing it to a close and starting Rev160. Rev160 includes a distant, targeted flyby of Titan similar to the one in this orbit.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC). Enceladus and Dione basemaps by Steve Albers.