Cassini continues its exploration of the Saturn system with the 17-day Rev165, which begins on April 23 at its farthest distance from the planet. This is also called the orbit's apoapse. At this point, Cassini is 2.37 million kilometers (1.48 million miles) from Saturn's cloud tops. The spacecraft is nearing the end of the first equatorial phase of the Cassini Solstice Mission, a phase which lasts until the Titan flyby in Rev166. During this phase, the spacecraft's orbits lie within the equatorial plane of the planet, providing opportunities to encounter Saturn's numerous moons, image the rings edge-on, and look at Saturn's cloud tops without the rings obscuring the view. Twenty-three ISS observations are planned for Rev165, the vast majority dedicated to Saturn storm monitoring and to encounters with Enceladus and Dione.
ISS begins its observations for Rev165 the day after apoapse on April 24 with a quick observation of Saturn using the wide-angle camera (WAC). This observation is one of the "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. Six more such observations are planned between April 28 and 30. Also on April 24, ISS will acquire a 14-hour light curve of the outer irregular satellite Erriapus. Cassini will be making its closest pass of the small, 10-kilometer-wide (6-mile-wide) satellite, at a distance of 6.31 million kilometers (3.92 million miles). This is still too far away to resolve the satellite as anything more than a point of light near the Orion Nebula. On April 25, the visual and infrared mapping spectrometer (VIMS) will attempt to observe the transit of a Jupiter-sized planet orbiting the star HD 189733 in the constellation Vulpecula. ISS will acquire images of the starfield surrounding the star every 3 minutes.
On April 28, ISS will take a look at Titan from a distance of 2.87 million kilometers (1.78 million miles). The observation is an effort to look for clouds in the moon's atmosphere as part of the "Titan Monitoring Campaign" (TMC). This observation of a gibbous Titan is designed to monitor clouds over the moon's Senkyo and eastern Aztlan dune fields. ISS also will be taking shorter-wavelength images to study changes in Titan's upper haze layers. Later that day, ISS will acquire an astrometric observation of Saturn's small, inner moons, including Helene, Telesto and Prometheus. Astrometric observations are used to improve our understanding of the orbits of these small satellites, which can be influenced by Saturn's larger icy satellites. On April 30, ISS will take another TMC observation of Titan that will allow for monitoring of cloud features across the Senkyo dune field from a distance of 2.49 million kilometers (1.55 million miles). On May 1, ISS will acquire a nine-hour rotational light curve of the second largest of Saturn's small, irregular, outer satellites, Ymir. The camera system will acquire another light curve observation of Ymir on May 3.
On May 2 at 12:34 UTC, Cassini will reach periapse for Rev165 at an altitude of 135,460 kilometers (84,170 miles) from Saturn. ISS observations during the periapse period will be taken during a targeted encounter of Enceladus and later during a close, non-targeted encounter of Dione.
Cassini will fly by Enceladus (E19) at an altitude of 73.6 kilometers (45.7 miles) at 09:31 UTC on May 2. ISS will image the icy satellite's south polar plume from distances ranging from 416,000 kilometers (259,000 miles) down to 106,000 kilometers (66,000 miles) while the satellite is just a thin crescent. In between, the radio science sub-system (RSS) will perform that team's first of two gravity experiments of the encounter. Next, the composite infrared spectrometer (CIRS) will acquire a series of mid-infrared scans across the night side of Enceladus, with its highest resolution data over the south polar terrain (found in earlier flybys of Enceladus to be a thermal hotspot). During the two hours surrounding closest approach, RSS will be prime, using Cassini's radio signal back to Earth to measure variations in Enceladus' gravity field. Particularly, the RSS team is looking for a mass concentration (or "mascon") at the south pole, indicative of a diapir, or sub-surface intrusion of material, that would explain the geologic activity in the region. Afterward, CIRS will obtain a map of Enceladus sun-lit side using its FP3 channel while ISS takes several images during one of the slow scans across the leading hemisphere.
At 20:13 UTC, Cassini will fly by another of Saturn's icy satellites, Dione, at a close-approach distance of 8,057 kilometers (5,006 miles). First, CIRS will acquire several scans of Dione's night side while ISS will capture a five-frame mosaic covering the sunlit crescent, centered along 70 degrees west longitude. From the time of closest approach until two hours afterward, ISS will acquire six mosaics. The first is a three-frame mosaic taken around closest approach. This mosaic will cover a fracture named Latium Chasma at resolutions near 53 meters (175 feet) per pixel. The next mosaic is a 24-frame mosaic covering much of Dione's northern anti-Saturn hemisphere, focusing particularly on Janiculum Dorsa and the region to the west of this ridge, as far as the ancient impact basin, Latinus. A three-frame mosaic will then be taken of Janiculum Dorsa for stereo imaging. Next, ISS will acquire a six-frame mosaic along the western limb, covering parts of Dione's wispy streak fractures. ISS will then image Dione's eastern limb, acquiring a set of images over Erulus crater. ISS will acquire a nine-frame mosaic across all of the visible terrain, centered over the moon's anti-Saturn hemisphere. Finally, ISS will capture a WAC movie as Dione crosses the face of Saturn.
The rest of ISS' observations for Rev165 are dedicated to a non-targeted encounter of Titan on May 6, when ISS will come within 700,000 kilometers (435,000 miles) of Saturn's largest moon. The first observation is a two-hour TMC sequence on May 5, taken from a distance of 745,630 kilometers (463,310 miles). This observation will cover western and central Xanadu. On May 6, during the distant encounter, ISS will acquire a 14-hour, 45-minute observation of Titan. This observation is broken up into several parts. First, ISS will acquire a four-frame mosaic covering the anti-Saturn hemisphere of Titan from a distance of 709,950 kilometers (441,140 miles). Afterward, ISS will acquire a series of color filters to characterize both the surface and its upper haze layers followed by sets of surface images four and eight hours later for cloud tracking. ISS will then acquire a clear-filter, WAC movie of Titan passing in front of the bright limb of Saturn. The first four frames will be separated by five minutes while the next 24 will be separated by 17 to 60 seconds. In the middle of this movie, ISS will acquire a set of red, green, and blue images with both the narrow-angle camera (NAC) and WAC just as Titan passes over Saturn's bright limb from a distance of 759,160 kilometers (471,720 miles). Next, ISS will acquire a four-frame mosaic of Titan with Saturn in the background. This mosaic is designed to map the anti-Saturn hemisphere of Titan as well as study the upper haze layers of the moon while they are silhouetted by the planet. Finally, ISS will acquire a set of red, green, and blue filter WACs of Titan with Saturn in the background.
On May 7, ISS will perform a TMC observation of Titan from a distance of 897,800 kilometers (557,900 miles). This will allow for monitoring of cloud features across the Adiri region of the moon. On May 11, ISS will acquire another TMC observation of Titan from a distance of 2.57 million kilometers (1.60 million miles), covering the boundary between Senkyo and Belet.
On May 11, Cassini will reach apoapse on this orbit, bringing it to a close and starting Rev166. Rev166 includes a targeted flyby of Titan. The flyby will bring to a close the first equatorial leg of the Cassini Solstice Mission and start the first inclined phase.
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