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Cassini continues its exploration of the Saturn system with the 22-day Rev 221, which begins on August 28 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 221 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 a non-targeted encounter with Dione. Twenty-seven ISS observations are planned for Rev 221 with the majority focused on Saturn's atmosphere.
For ISS's first observations of Rev 221, on August 29, the Narrow-Angle Camera (NAC) will perform a dark current calibration. Dark current is the background electrical current within the CCD chip in the NAC. These calibration observations (a similar one was performed for the WAC during Rev 220) are used to update calibration software so that they can take into account changes in background current as the mission progresses. The shutter will be closed, so "real" features or stars will not be visible.
Between September 1 and September 7, ISS will focus its attention on Saturn's atmosphere. While Cassini orbits within the ring plane, its instruments can get an unobscured view of both the northern and southern hemispheres of Saturn. On September 1 and September 5, ISS will ride along with the Ultraviolet Imaging Spectrometer (UVIS) as it scans across Saturn. On September 4, ISS will acquire a pair of observations designed to use cloud features at various latitudes to measure wind speeds. Winds on Saturn can reach as high as 400 meters per second (900 miles per hour) near the equator. Similar observations will be acquired on September 6 and 7, including an equatorial-mapping, ride-along observation, with the Visual and Infrared Spectrometer (VIMS). In addition to these Saturn observations, ISS will observe Titan on September 5 and 6. The closer of the two observations will be taken on September 6 from a distance of 2.53 million kilometers (1.57 million miles). Both observations will allow for cloud tracking across Titan's sub-Saturn hemisphere.
On September 8 at 21:41 UTC, Cassini will reach periapse for Rev 221 at an altitude of 223,030 kilometers (138,580 miles) from Saturn's cloud tops, between the orbits of Enceladus and Tethys. A few hours earlier, Cassini will perform a non-targeted encounter with Dione at a distance of 41,945 kilometers (26,063 miles). During this encounter, ISS will acquire several mosaics that will cover the leading and anti-Saturn hemispheres from distances ranging from 120,000 kilometers (74,600 miles) to 41,945 kilometers (26,063 miles). These mosaics will also include a pair of footprints showing an occultation of Enceladus by Dione. Another footprint will show Polydeuces appearing to pass above Dione's north pole.
After a Composite Infrared Spectrometer (CIRS) observation of Dione, ISS will focus its attention on Enceladus. ISS will acquire a pair of observation focusing on monitoring the icy plume emanating from Enceladus's south polar region. Between those two sequences, ISS will observe a crescent Enceladus from a distance of 128,300 kilometers (79,700 miles). The leading hemisphere grooved terrain will be visible near the terminator. After the three Enceladus observations, ISS will ride-along with CIRS to acquire a few images of a crescent Dione from a distance of 430,000 kilometers (267,000 miles).
Between September 9 and September 11, ISS will acquire three observations of Enceladus's south polar plume, monitoring how its brightness changes with time. Between the first two Enceladus observations, ISS will observe the distant outer moon Greip from a distance of 12.1 million kilometers (7.51 million miles). While it is only 7 kilometers (4 miles) across and so would only appear as a point of light at that distance, lightcurves created from measuring its brightness change from image to image and can be used to estimate its shape, rotation period, and the orientation of its rotational axis. There is some uncertainty in its position, so this observation will consist of a series of two-frame mosaics to ensure that the satellite is imaged.
On September 11, the camera system will acquire a pair of 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. Four more will be acquired between September 12 and September 19. On September 11, ISS will also ride along with UVIS to observe Saturn's south polar aurorae. This observation 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 September 13 and 14, ISS will observe the distant, outer moon Bergelmir from a distance of 20.1 million kilometers (12.5 million miles) for a day and a half. Like the earlier observation of Greip, this observation will be used to measure Bergelmir's rotational period. Finally, on September 17, ISS will acquire a photometric calibration observation of the star Vega.
On September 19, Cassini will reach apoapse, bringing Rev 221 to a close and starting up the next orbit, Rev 222, which will include a targeted flyby of Titan and a non-targeted encounter of Dione.
Image products created in Celestia. Dione and Enceladus maps by Paul Schenk. All dates in Coordinated Universal Time (UTC).