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All good things must come to an end, and Cassini completes the final orbit of its primary mission with Rev74, its 75th orbit around Saturn. July 1st marks the beginning of Cassiniís two-year extended mission, dubbed the Cassini Equinox Mission.
Cassiniís observations during this orbit focus on Saturnís icy moons and ring system. The spacecraft begins Rev74 on June 26 at its farthest distance from Saturn, called apoapsis. At this point, Cassini is 1.25 million km (779,000 mi) from Saturn. ISS does not perform imaging, except for the occasional navigation image, until June 29, when the narrow-angle camera will observe Dione, Tethys, and Rhea. These observations occur in quick succession. First, Tethys will be observed at a distance of 691,000 km (429,000 mi). Prominent in those images will be the large impact basin Odysseus. Next, Dione will be observed from a distance of 807,000 km (502,000 mi) with the bright, fractured terrain of its trailing hemisphere being featured. Finally, Cassini will observe the northern trailing hemisphere of Rhea from a distance of 786,000 km (488,000 mi). On June 30, Cassini reaches periapse, its closest point to Saturn on Rev74. 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 planet's north polar region before descending below the ring plane 25 minutes before closest approach to Saturn. On the day of periapse, Cassini will observe several of Saturnís icy moons, including Enceladus, Janus, and Mimas. The Composite Infrared Spectrometer (CIRS) will be the driver of these observations, scanning the surfaces of the moons, measuring their surface temperatures, and watching how these change as the moons enter eclipse behind Saturn.
As it approaches the ring plane, Cassini performs a non-targeted encounter with Enceladus at a distance of 84,255 km (52,353 mi). Two observations are planned for this encounter, though unfortunately none are planned during closest approach. The first occurs one to two hours before the encounter, when Cassini is high above the moonís northern sub-Saturn hemisphere at a distance of 169,000 km (105,000 mi). This will allow Cassini to observe much of the same terrain seen during the flyby in March of this year. The second observation sequence occurs an hour after closest approach, when Cassini is 120,000 km (75,000 mi) above the southern sub-Saturn hemisphere of Enceladus. This second observation will allow for Cassiniís first detailed observations of the fracture systems in this area, such as Isbanir Fossa and the ridge belts that surround the south polar region. CIRS will observe the start of the Enceladus eclipse. Cassini has a very good reason for not observing Enceladus during its closest approach to that satellite: the spacecraft will be encountering another moon at the same time. Cassini will perform its closest-yet flyby of the co-orbital moon Janus at a distance of 30,975 km (19,246 mi). While only a crescent of the irregularly-shaped satellite will be visible, this flyby will provide our best opportunity to observe the fine-scale surface details on this moon. However, this flyby may not be well suited for examining one of the interesting mysteries of this moon--the dark spots that dot its surface (see PIA07529). Following its second set of observations of Enceladus, Cassini turns its cameras and spectrometers to Mimas. Mimas will be 193,000 km (120,000 mi) from Cassini during the sequence. The southern sub-Saturn hemisphere will be visible.
As Cassini dives below the ring plane, the spacecraft turns its attention to the ring system. Right after the Mimas observation, Cassini will observe the outer edge of the A ring, looking at the fine-scale details of the ring margin. Following this observation, Cassini ISS will acquire an enormous azimuthal scan of the inner C ring and the Colombo Gap. With all these observations onboard, Cassini will then point its high-gain antenna at Earth to downlink the data it has gathered during a busy periapse period.
Following the downlink, the Cassini primary mission comes to an end at midnight UTC on July 1, four years after the Saturn orbit insertion burn. With one mission ending, another begins, as the Cassini Equinox Mission gets underway. The first observations by ISS in the extended mission include a spoke formation time-lapse movie sequence and three images of Anthe.
Cassini begins the first full orbit of the extended mission, Rev75, on July 3. Rev75 includes distant observations of Enceladus, Dione, and Saturnís rings.
Image products created in Celestia. Mimas and Enceladus basemap by Steve Albers.