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Cassini continues its exploration of the Saturn system with the 14-day Rev 224, which begins on October 21 at its farthest distance from the planet. This is also called the orbit's apoapse. At this point, Cassini is 2.00 million kilometers (1.24 million miles) from Saturn's cloud tops. Rev 224 occurs during the second Equatorial phase of Cassini's extended-extended mission. During this 10-month phase, Cassini will orbit within the orbital plane of Saturn's rings, allowing for frequent encounters with Saturn's icy satellites. This orbit includes the second of three targeted flybys with Enceladus planned for October and December 2015. Twenty-six ISS observations are planned for Rev 224 with the majority focused on Enceladus, Saturn's distant moons, and Saturn's atmosphere.
For ISS's first observations of Rev 224, occurring on October 24, the Narrow-Angle Camera (NAC) will monitor clouds across Titan's sub-Saturn hemisphere from a distance of 2.80 million kilometers (1.74 million miles). Immediately afterwards, the camera system will acquire a quick observation of Saturn using the Wide-Angle Camera (WAC). This observation is part of a series of "Storm Watch' observation sequences designed to take advantage of short, two-minute segments when the spacecraft turns to point the optical remote sensing (ORS) instruments back at Saturn, as a waypoint between other experiments' observations. These sequences include blue, clear, two methane band, and one full-frame, continuum band filter images. Five more storm watch observations will be acquired between October 24 and 27. ISS will observe Titan on October 26, again looking for clouds across its Fensal-Aztlan region, from a distance of 1.96 million kilometers (1.22 million miles). The camera system will observe Titan again on October 27, this time from a distance of 1.62 million kilometers (1.00 million miles).
On October 25 and 26, ISS will acquire a pair of observations of the distant, outer moon Skathi. By measuring the brightness of Skathi in the 220 images planned for the observations, researchers hope to calculate how long its day is, where its north pole points, and estimate its shape. The first observation will be acquired from a distance of 14.6 million kilometers (9.05 million miles). At only 8 kilometers (5 miles) across, Skathi will appear as a faint point of light in these images. After the Titan observation on October 26, ISS will acquire an astrometric observation of Saturn's small, inner moons. Astrometric observations are used to improve our understanding of the orbits of these small satellites, which can be influenced by Saturn's larger icy moons. Another astrometric observation will be acquired on October 27. Another way to improve our understanding of the orbits of Saturn's moons is through mutual events, where one moon passes in front of or near another moon, as seen from Cassini. ISS will observe one such mutual event on October 27, observing Janus pass in front of Tethys' north polar region. Janus will be 954,160 kilometers (592,890 miles) from Cassini, while Tethys will be 1.28 million kilometers (796,230 miles) away.
On October 28 at 11:56 UTC, Cassini will reach periapse for Rev 224 at an altitude of 114,880 kilometers (71,380 miles) from Saturn's cloud tops, between the G ring and the orbit of Mimas. Early on October 28, ISS will acquire five-hour observation of the small, distant, outer moon Bestla. Bestla will be 7.87 million kilometers (4.89 million miles) away. A follow-up observation of Bestla will also be acquired on October 29.
At 15:23 UTC on October 28, Cassini will perform a targeted flyby of Enceladus for the twenty-first time. The close approach altitude is 49 kilometers (30 miles) over the moon's high southern latitudes. This is the second of three Enceladus flybys planned for 2015, with the next and final encounter of the Cassini mission coming on December 19. This is also the last very close flyby of Enceladus for the Cassini mission, as the next encounter will have a close approach altitude that is 100 times greater. ISS will start out the encounter with an imaging observation. This observation will include a pair of stares at Enceladus to acquire multi-spectral data with the Narrow-Angle Camera (NAC), a pair of scans for the Composite Infrared Spectrometer (CIRS), and a four-frame mosaic. The mosaic will be acquired from a distance of 63,640 kilometers (39,540 miles) and has a resolution of 380 meters (1,250 feet) per pixel. The mosaic will cover the sub-Saturn and trailing hemispheres of Enceladus, areas that have not been observed at this scale since the first Enceladus flyby in February 2005 during Rev 003. Afterward, the Ion and Neutral Mass Spectrometer (INMS) will be prime, including during closest approach. INMS will directly measure the composition of gas in the jets of Enceladus's south polar plume during closest approach. The Cosmic Dust Analyzer (CDA) will perform similar measurements of the dust in the plume. ISS will acquire five Wide-Angle-Camera (WAC) images during closest approach while riding along with INMS, though images may be pointed off Enceladus or pointed on the dark side of the moon's day/night terminator. After a CDA observation, ISS will acquire an observation of Enceladus's south polar plume designed, along with a companion observation on October 29, to see how activity varies depending on the moon's position along its orbit.
On October 30 and 31, ISS will ride along with a pair of Ultraviolet Imaging Spectrometer (UVIS) observation to image aurorae over Saturn's south polar region. On October 31, the imaging system will look at Titan's haze layers from a distance of 1.42 million kilometers (0.88 million miles). ISS will perform a similar observation on November 1 from a distance of 2.10 million kilometers (1.30 million miles). On November 4, ISS will acquire a pair of observation of two of Saturn's distant, outer moons, Hati and Ijiraq. Hati will be 12.5 million kilometers (7.77 million miles away), while Ijiraq will be 10.5 million kilometers (6.52 million miles) away. Like the observation of Bestla and Skathi earlier this orbit, these sequences will be used to estimate the orientations of Ijiraq and Hati's rotation axes and their shapes. These moons are too far away and too small for surface features to be visible during these observations.
On November 4, Cassini will reach apoapse, bringing Rev 224 to a close and starting up the next orbit, Rev 225, which will include a targeted flyby of Titan and a very close, non-targeted encounter with Tethys.
Image products created in Celestia. Enceladus map by Paul Schenk. All dates in Coordinated Universal Time (UTC).