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Cassini continues its exploration of the Saturn system with the 22-day Rev 220, which begins on August 7th at its farthest distance from the planet. This is also called the orbit's apoapse. At this point, Cassini is 2.67 million kilometers (1.66 million miles) from Saturn's cloud tops. Rev 220 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 final targeted encounter of the mid-sized, icy moon Dione, as well as "Voyager-class" non-targeted encounters with Enceladus and Tethys. Thirty-seven ISS observations are planned for Rev 220 with the majority focused on Saturn's atmosphere and icy satellites.
For ISS's first observations of Rev 220, on August 12, the camera system will acquire three quick observations of Saturn using the Wide-Angle Camera (WAC). These observation are part of a series of "Storm Watch" observation sequences 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. Five more storm watch observations will be acquired between August 14 and 16. Between the second and third storm watch observation, ISS will observe a crescent Titan from a distance of 1.62 million kilometers (1.00 million miles). Cassini will observe a crescent Titan two more times on August 14 and August 15. After the third storm watch observation of Saturn, 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. Two more astrometric observations will be acquired on August 14 and 15.
On August 13, ISS will ride along with the Ultraviolet Imaging Spectrometer (UVIS) to perform a calibration observation by taking slews of the bright star, Spica (Alpha Virginis). On August 15, ISS will observe Tethys as it passes in front of more distant Hyperion. Tethys will be 1.23 million kilometers (0.76 million miles) away during the occultation. Immediately afterward, ISS will acquire an observation of the faint and dusty E ring, which is produced by jets of icy dust erupting from Enceladus's south polar region.
On August 18 at 01:24 UTC, Cassini will reach periapse for Rev 220 at an altitude of 223,510 kilometers (138,880 miles) from Saturn's cloud tops, between the orbits of Enceladus and Tethys. At 18:33 UTC on August 17, Cassini will perform its fifth and final targeted encounter with the icy moon Dione, approaching it at a distance of 474 kilometers (295 miles) from its surface. Inbound during D5, the Composite Infrared Spectrometer (CIRS) will perform several scans across Dione's anti-Saturn hemisphere. ISS will ride-along to acquire clear filter snapshots that will cover most of the visible disk, including some with Saturn in the background.
During the five hours surrounding closest approach, the Radio Science Subsystem (RSS) will control pointing, acquiring gravity science. The high-gain antenna will transmit a signal to Earth, which will be analyzed to see how it changes as Cassini passes by Dione. Dione has the second highest density of Saturn's mid-sized icy satellites, implying a larger amount of rocky material in its interior, unlike Tethys, which is composed almost entirely of water ice. Researchers will be looking to understand how Dione's rock fraction is distributed in its interior: concentrated into a rocky core like Enceladus or undifferentiated like Rhea. Right at closest approach, the ISS and RSS fields-of-view will cross Dione, allowing for very high resolution imaging. The closest image, acquired at closest approach, will have a resolution of 2.8 meters (9 feet) per pixel, though there will likely be significant smear due to the spacecraft's rapid motion with respect to Dione. Outbound, after the RSS experiment, ISS will ride-along with CIRS to image a crescent Dione.
After the Dione encounter, Cassini will perform a pair of non-targeted encounters with Saturn's icy satellites. First, Cassini will pass by Tethys at a distance of 41,892 kilometers (26,031 miles). ISS will ride along with CIRS to acquire a mosaic of Tethys leading hemisphere, which will include imaging of Tethys's large impact basin, Odysseus. Afterward, Cassini will flyby Enceladus at a distance of 53,164 kilometers (33,035 miles). ISS will acquire a pair of observation during this encounter. First, it will perform a quick observation of the anti-Saturn side of the northern hemisphere of Enceladus. While this side of Enceladus has been seen before, the phase angle will be better for imaging the numerous tectonic and impact structure in this region. The second observation will be used to image Enceladus's south polar plume. Both will be acquired in the hours after closest approach. Between the two Enceladus observations, and then again after the second one, ISS will observe the distant outer moon Bestla from a distance of 12 million kilometers (7.48 million miles). While it is only 7 kilometers (4 miles) across and will only appear as a point of light at that distance, lightcurves created from measuring its brightness change from image to image can be used to estimates its shape, rotation period, and the orientation of its rotational axis.
On August 19, ISS will observe a crescent Titan from a distance of 1.71 million kilometers (1.06 million miles). ISS will perform four similar observations between August 21 and August 26. The closest of these will be acquired on August 24 at a distance of 1.37 million kilometers (850,000 miles). These observations will be used to monitor changes in Titan's upper haze layers. On August 20, the camera system will acquire a pair of observations of Saturn's atmosphere. Saturn will be visible as a thin crescent at the time. The first is a set of limb observations, useful for understanding the upper haze layers in Saturn's atmosphere. During the second, ISS will ride along with UVIS as it scans across the crescent. Between, August 21 and 24, ISS will acquire three observations of Saturn's south polar aurorae. These will be used to look for changes in the intensity of the aurorae. For example, ISS observed a dramatic increase in its brightness in May. On August 26, ISS will observe the distant, outer moon Hati from a distance of 9.93 million kilometers (6.17 million miles). Immediately afterward, ISS will perform a dark current calibration of the Wide-Angle Camera. The shutter will be closed, so "real" features or stars will not be visible.
On August 28, Cassini will reach apoapse, bringing Rev 220 to a close and starting up the next orbit, Rev 221, which will include a non-targeted flyby of Dione.
Image products created in Celestia. Dione, Tethys, and Enceladus maps by Paul Schenk. All dates in Coordinated Universal Time (UTC).