Cassini begins the 20-day Rev158 on December 3 at its farthest distance from Saturn, called apoapse. At this point, Cassini is 2.38 million kilometers (1.48 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. Fifty-three ISS observations are planned for Rev158, the majority dedicated to the two satellite encounters (Titan and Dione) and Saturn storm monitoring.
ISS begins its observations for Rev158 eight hours after apoapse with a quick observation of Saturn and its faded northern hemisphere storm with another planned an hour later. "Storm Watch" observations like this one 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. Ten more are planned between December 4 and December 10, while seven are planned between December 20 and 22. Between the two storm watch observations for December 3, ISS will acquire an astrometric observation of Saturn's small, inner moons, including Anthe, Pandora (with Epimetheus passing by), Epimetheus, Calypso, and Helene.
On December 7, ISS will image Titan's sub-Saturn hemisphere from a distance of 3.26 million kilometers (2.02 million miles) in an effort to look for clouds in the moon's atmosphere. 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, Prometheus, Epimetheus, and Helene. After that, ISS will again observe Saturn (this time with Tethys transiting the giant planet). Next, ISS will observe Dione nearly pass behind Rhea's south pole in the first of three mutual events Cassini will observe on December 7. Rhea will be 1.67 million kilometers (1.04 million miles) away, while Dione will be 1.95 million kilometers (1.21 million miles) away. Later in the day, after 15 hours of low activity for Cassini, ISS will take a look at Dione as it passes in front of Titan. Dione will be 1.61 million kilometers (1.00 million miles) away, while Titan will be nearly twice as far away from Cassini at a distance of 3.16 million kilometers (1.96 million miles). Afterward, ISS will observe Tethys pass between Saturn's rings and Titan's north pole. Tethys will be 2.19 million kilometers (1.36 million miles) away, while Titan will be 3.14 million kilometers (1.95 million miles) away.
On December 9, ISS will examine Titan's Fensal-Aztlan region, again searching for cloud activity. Titan will be 2.67 million kilometers (1.66 million miles) away at the time. The next day, ISS will acquire two more observations of Titan. During the first, ISS will observe two mutual events involving Titan. During the first, Tethys will pass above Titan's north pole, while during the second Rhea will pass in front of Titan's north pole. Afterward, ISS will observe a crescent Titan, focusing on the moon's hazy atmosphere. During this observation, Titan will be 2.01 million kilometers (1.25 million miles) from Cassini.
On December 12 at 02:00 UTC, Cassini will reach periapse for Rev158 at an altitude of 135,190 kilometers (84,000 miles) from Saturn's cloud tops. During this periapse passage, Cassini will perform a non-targeted encounter of Enceladus followed by a targeted one of Dione. The Enceladus encounter occurs at 04:27 UTC with a close approach altitude of 19,906 kilometers (12,369 miles). During this observation, ISS will acquire a mosaic across Enceladus' leading hemisphere, starting with coverage over the moon's southern hemisphere.
Afterward, Cassini will begin observing Dione as the spacecraft approaches for its third targeted encounter with that moon. The first observation is by the Composite Infrared Spectrometer (CIRS) as it scans the night side of Dione using first its FP3 channel, then its FP1 channel. These observations are designed to measure the thermal inertia of the extensive fractures that cover Dione's trailing hemisphere. ISS will ride along to acquire a set of seven images during the FP1 raster scan across the crescent of Dione. Following the CIRS observation, the Radio Science Subsystem (RSS) takes over so that Cassini can perform a gravity pass of the satellite at closest approach. This allows for a better understanding of Dione's internal mass distribution. In other words, RSS is looking to determine if Dione is differentiated, like Enceladus, or more homogeneous, like Tethys, in its distribution of high density rocky material and low density icy material. Given its much higher silicate content compared to Tethys, which has a density consistent with little to no rocky material, it is thought that Dione is differentiated into an icy mantle surrounding a rocky core. Closest approach occurs at 09:39 UTC at an altitude of 98.8 kilometers (61.4 miles) during the RSS gravity observation. Following the RSS pass, ISS will acquire a 14-frame mosaic of Dione's anti-Saturn hemisphere including a WAC color sequence with Dione near Saturn's bright limb. After the mosaic, ISS will image a series of mutual events as various moons pass behind Dione including Mimas (both ingress and egress), followed by Pandora, Prometheus, and Epimetheus. Being slower moving, Epimetheus will be in frame when the two smaller moons, Pandora and Prometheus, pass behind Dione. Finishing up, ISS will acquire a pair of 2x2 mosaics of Dione.
A day after periapse, Cassini encounters Titan on December 13 at 20:11 UTC for the 80th time. This is the last of six Titan flybys planned for 2011 with the next encounter scheduled for January 2. T79 is a relatively high-altitude flyby with a close-approach distance of 3,585 kilometers (2,227 miles). This flyby will allow for imaging of Belet and Adiri 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 variety of nadir-pointing, temperature map scans as well as limb scans measuring aerosols over Titan's north pole and mid-southern latitudes. At closest approach, control of spacecraft pointing will switch to the Visual and Infrared Mapping Spectrometer (VIMS). The instrument will acquire high-resolution data covering central Belet, a large dune field straddling Titan's equator between Senkyo and Adiri, and Ching-Tu, a sand-poor, narrow dune field to the southwest of Adiri. Afterward, VIMS will acquire mosaics of Adiri, where changes were seen following last year's Arrow Storm, and Belet. Finally, VIMS and CIRS will finish up the encounter with a global VIMS mosaic and a mid-infrared temperature map of Titan's day side. Over the next two days, ISS will acquire a series of images of Titan designed to monitor clouds in Titan's atmosphere, if any exist at the time of the encounter. On December 15, these observations start out at a distance of 636,000 kilometers (395,000 miles) from Titan and continue until Titan is 1.09 million kilometers (0.68 million miles) away. On December 16, the 15-hour observation starts out at a distance of 1.38 million kilometers (0.86 million miles) and ends when Cassini is 1.69 million kilometers (1.05 million miles) from Titan. On both cases, Cassini's view of Titan will be centered on the dark Belet dune field.
On December 20, ISS will acquire an astrometric observation of Saturn's small, inner moons, including Janus, Polydeuces (with several other moons in the same view), Pandora, Atlas (with Mimas making a cameo appearance), Calypso and Helene. On December 21, ISS will image Titan from a distance of 3.84 million kilometers (2.39 million miles) in an attempt to monitor clouds across the moon's sub-Saturn hemisphere. After a Saturn storm watch observation, ISS will acquire another astrometric observation, this time observing Pallene, Helene, Prometheus, Epimetheus (with Mimas sneaking into frame), and Polydeuces. Finally on December 23, ISS will acquire a lengthy, 15-hour observation of Suttungr. This observation will be taken from a distance of 17.0 million kilometers (10.6 million miles), and when combined with additional observations of this irregular satellite, is designed to measure Suttungr'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. This observation can also be used to measure the rotation period of this small moon.
On December 23, Cassini will reach apoapse on this orbit, bringing it to a close and starting Rev159. Rev159 includes a distant, targeted flyby of Titan.
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