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Cassini finishes 2008 with Rev98, the spacecraftís 99th orbit around the Ringed Planet. Thanks to the Titan encounter that occurs just prior to the start of Rev98, Cassiniís orbits around Saturn are starting to increase in size. This rev, or orbit, lasts 9.5 days.
Cassini begins Rev98 on December 21 at its farthest distance from Saturn, called apoapse. At this point, Cassini is 1.18 million km (738,000 mi) from Saturn. The spacecraft is in a high-inclination orbit here, as it was for most of 2008, providing opportunities to study the rings and the polar regions of Saturn and its satellites. Rev98 starts only four minutes after closest approach to Titan during the T49 encounter while Cassini is turning to acquire its second of two RADAR SAR swaths. The details of the observations taken during this flyby are covered in the Rev97 Looking Ahead article.
Once Cassini is finished beaming back data from the T49 flyby on December 22, the Composite Infrared Spectrometer (CIRS) will make a temperature map of the unlit side of the ring system from a position 30 degrees above the ring plane. ISS will ride along with the CIRS observation, looking for spokes in the B ring. The next day, on December 23, ISS will observe several of Saturnís small satellites, including Janus, Epimetheus, and Prometheus.
On December 24, two stellar occultations are planned. The Ultraviolet Imaging Spectrometer (UVIS) will focus on two B-type stars, Delta Centauri (Ma Wei) and Beta Crucis (Mimosa), as they pass behind Saturn and its rings. These occultations are used by Cassiniís four remote-sensing instruments to measure the opacity of the rings system and Saturnís upper atmospheric haze layers. ISS will look at Prometheus during two observations designed to monitor its orbital motion. On December 25, UVIS will observe another stellar occultation, this time using the B-type star Alpha Arae (Tchou or Choo) to acquire high-resolution opacity data from Saturnís A and B rings. Immediately following, ISS will make a 5-hour movie of the 1.47 Rs ringlet in the outer C ring. Following this movie, CIRS will acquire another temperature map of the ring system -- this time while Cassini is 20 degrees above the ring plane.
On December 26, Cassini reaches periapse, its closest point to Saturn on Rev98. At that point, Cassini will be 548,000 km (341,000 mi) from Saturnís center. Shortly after periapse, ISS will acquire a distant view of Enceladus to observe the plume erupting from that moonís South pole. Two days later, on December 28, ISS will obtain a two-frame, wide-angle camera mosaic over Saturnís southern hemisphere. ISS also will take a look at Titanís sub-Saturn hemisphere from a distance of 2.08 million km (1.29 million mi).
On December 29 and 30, ISS will perform two observations of Saturnís southern hemisphere, both will be two-frame mosaics similar to the Dec. 28 observation. Also on December 30, ISS will image several of Saturnís moons, including Titan, Pan, Janus, Prometheus, Polydeuces, and Calypso. Finally, UVIS will acquire an extreme and far ultraviolet observation of Saturn while ISS obtains a wide-angle camera polarimetry and photometry sequence using numerous filters.
Cassini reaches apoapse on December 31, beginning Rev99.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).