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Cassini continues its series of week-long orbits with Rev73, the spacecraftís 74th orbit around the Ringed Planet. This short, penultimate orbit of Cassiniís primary mission lends itself to a more dedicated set of observations--imaging Saturnís atmosphere, rings, and several of its moons, including Dione and Enceladus. Cassini begins Rev73 on June 19 at its farthest distance from Saturn, called apoapsis. At this point, the spacecraft is 1.27 million km (786,000 mi) from Saturn. On June 20, Cassini will observe Saturnís northern hemisphere over a period of 6.5 hours. The observation is designed to study cloud dynamics in Saturnís atmosphere by acquiring a time-lapse movie of cloud features with both the narrow-angle and wide-angle cameras. On June 21, Cassini will observe Saturnís icy moon Tethys from a distance of 770,000 km (478,000 mi). While images will be acquired of Tethysí northern leading hemisphere (including the large impact basin Odysseus), the observation is designed to allow the Composite Infrared Spectrometer (CIRS) and Ultraviolet Imaging Spectrometer (UVIS) instruments to scan and stare at that moon. On June 23, Cassini reaches periapse, its closest point to Saturn on Rev73. At that point, the spacecraft will be 163,000 km (101,000 mi) from Saturnís cloud tops. Near periapse, Cassini will quickly pass high over the north polar region of Saturn before descending below the ring plane 25 minutes before periapse. This will allow Cassini to observe both of Saturnís polar regions within a day of each other. The north polar region will be observed late in the day (UTC) on June 22. This sequence will allow UVIS and ISS to look for auroral emissions on the night-side of Saturn. A day later, on June 23, ISS will acquire several wide-angle camera mosaics of the south polar region. Prior observations of this area revealed a vortex centered on Saturnís south pole (see PIA08332). Shortly afterward, ISS will acquire a seven-frame mosaic of Saturnís limb with the rings in the background. Similar views have revealed refraction of Cassiniís view of the rings as the light reflecting off the rings passes through the atmosphere (see PIA07521).
Also on June 23, Cassini ISS will acquire images of theta Tau, a binary star system in the constellation Taurus. The different colors of the two stars will assist in calibrating both camerasí photometric responses. On June 24, Cassini ISS will acquire several image sequences of Saturnís icy moon Dione. These images will be acquired at distances ranging from 820,000 to 1.08 million km (510,000 to 673,000 mi). Cassini will be almost directly between the Sun and Dione at the time of these observations, allowing the spacecraft to view the satellite at very low phase angles, which is important for constraining Dioneís fine-scale surface properties.
On June 25, Cassini ISS will acquire two additional calibration observations. The first is another in a sequence of photometric response calibrations--this time observing the bright star Vega in the constellation Lyra. The second is a set of wide-angle camera calibration lamp observations where Cassini will try to point the camera to a part of the sky with no bright stars. The only other sequence for June 25 is a distant observation of Enceladus.
Cassini begins the final orbit of the primary mission, Rev74, on June 26. Rev74 includes distant observations of Janus, Enceladus, Tethys, Dione, Mimas, and Saturnís rings.
Image products created in Celestia. Rhea basemap by Steve Albers.