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Cassini continues its series of seven-day-long orbits with Rev87, the spacecraft's 88th orbit around the Ringed Planet. Cassini begins Rev87 on September 28 at its farthest distance from Saturn, called apoapsis. At this point, the spacecraft is 1.22 million km (761,000 mi) from Saturn. Like most of the orbits during 2008, Cassini is now orbiting Saturn at a high-inclination, providing opportunities to view the ring system from high above the ring plane. Such an orbit provides opportunities to study Saturn's rings and the polar regions of the planet and its satellites. The orbit's first observation by ISS, however, is a calibration observation designed to understand changes in the sensitivity of the CCDs as the mission progresses. Images will be acquired of two open star clusters, Messier 48 and the Pleiades, and the recorded brightnesses of the stars in the images will be compared with reference data acquired by Earth-based astronomers.
On September 30, ISS will acquire images of several of Saturn's small, inner satellites as part of the ongoing orbit determination campaign. Immediately after that observation, Cassini will turn its cameras to Titan in order to look for clouds over that moon's northern leading hemisphere. Cassini will also observe the F ring, looking for short-term changes within the ring due to interactions between ring particles and nearby moons and clumps. On October 1, ISS and the Ultraviolet Imaging Spectrometer (UVIS) will observe a stellar occultation of the star Alpha Crucis as it passes behind the ring system and Saturn's atmosphere.
On October 2, Cassini reaches periapse, its closest point to Saturn on Rev87. At that point, the spacecraft will be 237,537 km (147,599 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 38 minutes before closest approach. Shortly before periapse, Cassini will perform a non-targeted encounter with Tethys, passing over its northern sub-Saturn hemisphere at a distance of 78,835 km (48,985 mi). Unfortunately, no imaging of Tethys is planned for this encounter.
In the hours leading up to periapse, Cassini ISS will again observe the F-ring to look for short-term changes. After periapse, when Cassini is high above the south polar region of Saturn, the spacecraft will observe aurorae and cloud features around the planet's south pole. The next day, October 3, Cassini will acquire 34 images of the outer A ring, including the Keeler and Encke Gaps, which are carved out by Daphnis and Pan, respectively. On October 4, Cassini ISS will focus on Saturn's various satellites. Two observations are planned of Saturn's various small inner satellites as part of the Imaging Team's moon orbit determination campaign. Cassini will observe Mimas' leading hemisphere from a distance of 1.11 million km (690,000 mi). This observation will help imaging scientists better understand the surface properties of Mimas. Finally, Cassini will turn its cameras back to Titan to once again monitor clouds on the moon's trailing hemisphere.
On October 4, Cassini will focus on Saturn's second largest moon, Rhea. The first observation, covering Rhea's leading hemisphere, is designed to study Rhea's surface properties. The second, acquired as Cassini crosses Saturn's ring plane, is designed to look for the possible rings that orbit Rhea, though the low phase angle of this particular observation may make it difficult. Finally, Cassini will look at another set of faint rings, this time Saturn's E and G rings.
Cassini begins Rev88 on October 6. During Rev88, Cassini will perform its sixth close encounter with Enceladus.
Image products created in Celestia. Saturn basemap by Björn Jónsson. All dates in Coordinated Universal Time (UTC).