[For trouble viewing the images/movies on this page, go here]
Cassini begins the 17-day Rev154 on September 22 at its farthest distance from Saturn, called apoapse. At this point, Cassini is 2.37 million kilometers (1.47 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, to image the rings edge-on, and to look at Saturn's cloud tops without the rings obscuring the view. Thirty-six ISS observations are planned for Rev154, the majority designed to monitor cloud systems in Saturn's atmosphere. During this orbit, Cassini will also encounter the icy moon Enceladus.
ISS begins its observations for Rev154 the day after apoapse with a Saturn storm watch observation. Four more such observations are planned between September 24 and 30, with another 17 planned between October 3 and 10. These 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. These images can be used to track the progress of the northern hemisphere storm that began in early December 2010. On September 26, ISS will monitor Titan for a little more than five hours, searching for clouds across Titan's Shangri-La region. Twenty-one sets of CB3 and MT1 images are planned with each four-image set taken 15 minutes apart. If clouds are visible during this time, these frames can be used to track cloud movement and evolution. This observation will be taken from a distance of 875,000 kilometers (544,000 miles). This observation will be followed up by a compositional mapping sequence by the Composite Infrared Spectrometer (CIRS). However, no ISS ride-along images are planned during the transit of Saturn's rings by Titan. On September 27, ISS will acquire a 15-hour light curve observation of the outer, irregular satellite, Skoll. This observation will be used to pin down the orbital period of that small moon, as Cassini ISS has done for several other outer satellites such as Albiorix, Siarnaq, Ymir, Bebhionn, and Kiviuq. The next day, Cassini will perform an orbit trim maneuver (#292) to keep the spacecraft on track for the Enceladus encounter.
On October 1 at 16:53 UTC, Cassini will reach periapse for Rev154 at a distance of 135,850 kilometers (84,410 miles). During this periapse, Cassini will perform a targeted encounter with Enceladus. First though, ISS will acquire a short, rotation movie of Dione. The image sequence will last six hours and mostly cover the moon's trailing hemisphere and its wispy streaks. The range to Dione will remain fairly constant around 500,000 kilometers (310,000 miles). At 13:52 UTC, Cassini will perform a targeted encounter with Enceladus (E14). This is the first of three encounters planned during October and November. Cassini will pass the icy moon at a distance of 99 kilometers (62 miles). During the encounter, ISS will take three observations of the satellite. The first is taken on approach when only a thin crescent is visible. This observation is designed to monitor Enceladus' south polar plume. Composed of water ice and very fine particles, the plume is more visible at high phase angles than when the satellite appears more fully illuminated. As a bonus, ISS will also image Epimetheus as it passes behind Enceladus' north pole. This observation ends at 12:00 UTC. Around closest approach, Cassini will acquire in situ data as it passes directly over the south pole. Both the Ion and Neutral Mass Spectrometer (INMS) and the Radio and Plasma Wave Science (RPWS) instruments will measure the composition of material coming out of Enceladus' south polar vents (the Cassini Plasma Spectrometer would also take data, if it was working). The second ISS observation is a ride along observation with CIRS. CIRS will measure the temperature of Enceladus' equatorial region while the moon is in eclipse. ISS will image the moon as it leaves Saturn's shadow and will continue to take a few frames of the moon's northern hemisphere. After a few temperature scans by CIRS, ISS will acquire its third observation, a full-disk color observation of the moon's leading hemisphere taken from a distance of 125,000 kilometers (75,000 miles). Shortly afterward, ISS will acquire another Dione color observation. This time Cassini will cover the moon's leading hemisphere from distance ranging from 157,000 kilometers (98,000 miles) to 280,000 kilometers (174,000 miles).
On October 3, ISS will image the L4 Lagrange point region of the moon Titan, about 60 degrees ahead of the giant satellite in its orbit. This type of Lagrange point has been found to host Trojan moons before in the Saturn system. Cassini discovered the L5 Trojan moon of Dione now named Polydeuces in 2004. Another, Telesto, shares the same orbit as Tethys but lies 60 degrees ahead of it. This satellite search observation could detect objects as small as 80 meters near Titan's L4 point. Another L4 satellite search will be taken that day, looking for moons sharing Enceladus' orbit. On October 4, ISS will again image Titan's sub-Saturn hemisphere from a distance of 2.63 million kilometers (1.64 million miles). Saturn's rings will obscure portions of Titan's northern hemisphere. Also on October 4, ISS will image Rhea as it passes in front of Titan. Titan will be 2.82 million kilometers (1.75 million miles) from Cassini, while Rhea will be 1.15 million kilometers (0.71 million miles) away. Finally, on October 4, 9 and 10, ISS will acquire three astrometric observations of Saturn's small, inner moons.
On October 10, Cassini will reach apoapse on this orbit, bringing it to a close and starting Rev155. The next orbit will see a targeted encounter of Enceladus and non-targeted observations of Dione, though there will be few observations for the first week of the orbit thanks to solar conjunction.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC). Rhea and Enceladus basemaps by Steve Albers.