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Cassini continues its extended tour with Rev 76, the spacecraft's 77th orbit around Saturn and the second full orbit of the Cassini Equinox Mission. During this orbit, Cassini's observations focus on Saturn's atmosphere as well as several of the icy satellites.
Cassini begins Rev76 on July 10 at its farthest distance from Saturn, called apoapsis. At this point, the spacecraft is 1.25 million km (778,000 mi) from Saturn. Cassini's first observations of the orbit involve Saturn's atmosphere. On July 11, 12, and 13, the Ultraviolet Imaging Spectrometer (UVIS) performs several scans of the north polar region of Saturn, looking at the planet's aurora. ISS will ride along with the last of these observations on July 13, acquiring images of Saturn's high northern latitudes. On July 11 and 12, the Composite Infrared Mapping Spectrometer (CIRS) acquires two far-infrared temperature maps of Saturn's northern hemisphere, focusing on how the temperature of Saturn's upper atmosphere varies with latitude. On July 14 Cassini reaches periapse, its closest point to Saturn on Rev76. At that point, Cassini will be 162,033 km (100,683 mi) from Saturn's cloud tops. Near periapse, Cassini will quickly pass high over the north polar region of Saturn before descending below the ring plane 22 minutes before closest approach to the planet. Cassini's passage through the ring plane will occur with Saturn in-between the Sun and the spacecraft. To take advantage of this geometry, Cassini will perform several observations of Saturn's moons as they enter eclipse. Prior to periapse, these observations include images of Enceladus (from a distance of 461,000 km or 286,000 mi), Janus (from 261,000 km or 162,000 mi), and Rhea (from 508,000 km or 316,000 mi). When Cassini passes in and out of Saturn's shadow, UVIS and the Visual and Infrared Mapping Spectrometer (VIMS) will measure the brightness of the Sun as it passes behind the haze layers of Saturn's upper atmosphere. These occultation measurements can be used to monitor the altitude and density of the layers, as well as temperature variations with altitude. Shortly after periapse, Cassini will observe Mimas as it enter eclipse from a distance of 254,000 km (158,000 mi).
Cassini will also perform its closest encounters to date with the small inner moons Prometheus (31,418 km/19,560 mi) and Daphnis (42,603 km/26,472 mi) shortly before closest approach. However, because these moons are in the shadow of Saturn during their respective closest approaches, no observations are planned during these encounters. One of the most interesting discoveries at Saturn by Cassini has been of the South Polar Vortex. This colossal storm resembles a terrestrial hurricane, with towering clouds swirling around a central clearing (see PIA08332). However, at 8,000 km (5,000 mi) across, the vortex dwarfs terrestrial cyclones, such as the currently-raging Hurricane Bertha. On July 14 and 15, Cassini's narrow-angle camera will focus on this storm in an attempt to follow-up on its discovery before seasonal darkness comes to the south pole. The first observation, on July 14, is a high-resolution (2 km/1.2 mi per pixel) time-lapse movie observation of the center of the storm. This observation is designed to measure wind-speeds at different parts of the vortex's center; scientists would like to know, among other things, if the winds at the center of the storm are relatively calm, as in terrestrial hurricanes). This observation will also use multiple filters to look at the storm's vertical structure. The second observation, on July 15, will have a lower pixel scale (5 km/3.1 mi per pixel), but with a lower emission angle, the resolution over some hazier areas in the region may be improved. For comparison, the best previous observations of this storm had pixel scales near 17 km (11 mi) per pixel.
In the last three days of Rev76, Cassini will continue to focus on Saturn's atmosphere. On July 15, UVIS and ISS perform an observation of Saturn's southern aurora. On July 16, CIRS will acquire another far-infrared temperature map of Saturn's atmosphere.
Cassini begins Rev77 on July 17. The coming orbit includes distant observations of Dione and numerous sequences involving Saturn's rings.
Image products created in Celestia. Rhea basemap by Steve Albers.