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Cassiniís journey at Saturn continues with Rev 42, its 43rd orbit of the ringed planet, as Cassini observes Titanís trailing hemisphere, Saturnís atmosphere and ring system, and several of Saturnís icy satellites.
Cassini begins Rev42, on March 31 at its farthest distance from Saturn, called apoapsis. The first week of Rev42 is filled with observations of Saturnís small satellites (to refine their orbits), the F-ring, and the Saturn upper atmosphere. On April 6, the Ultraviolet Imaging Spectrometer (UVIS) will obtain observations of Saturnís aurora borealis. The Imaging Science Subsystem (ISS) cameras will be riding along on that observation with the goal of creating a movie of Saturnís aurora using the wide angle cameraís ultraviolet filters. On April 5, the Visual and Infrared Spectrometer (VIMS) will study a different type of glow, examining methane fluorescence in Saturnís upper atmosphere, and ISS will ride along during those observations.
On April 8, Cassiniís cameras will obtain a radial scan of the Saturnian ring system. These observations are designed to mosaic the rings from the faint, inner D-ring out to the F-ring, and to study the ringsí high resolution structure. The geometry of this observation can be seen at right. Shortly after this observation, Cassini will observe Dione from a distance of 270,000 km (168,000 mi). These images will show the south polar region as well as the southern portion of the anti-Saturnian hemisphere (the side that always faces away from Saturn). These will be the highest resolution observations thus far obtained showing the same region seen up-close by Voyager 1. (Even higher resolution observations of that region, along Dioneís leading hemisphere, are planned during Rev43 in late April.) The next day, April 9, Cassini will turn its cameras back to the ring system and the small ring-region moons, particularly Atlas.
Cassini encounters Titan for the 29th time on April 10, with a closest approach distance of only 990 km (615 mi). Like the last few encounters with Titan, this flyby (known as T28) will allow for imaging of the northern portion of Titanís trailing hemisphere following closest approach. The Cassini cameras will take two mosaics of this region. These mosaics (and a lower resolution context mosaic) focus on an area centered at 45ļ North Latitude, 240ļ West Longitude, north of the dark region named Belet. The T28 observations will allow for a follow-up on discoveries made in T25, T26, and T27 images, such as a set of lineaments north of Belet Ö features which were also observed by the RADAR instrument during T21 (December 12, 2006). The discovery of northern mid-latitude clouds in late February suggests that these observations may also allow for cloud tracking. During closest approach, the Cassini RADAR instrument will obtain a Synthetic Aperture Radar (SAR) swath covering similar terrain as the swath obtained in February, during the T25 encounter (see PIA09182). This RADAR swath will cover, like the T25 swath, the northern portion of a Caspian Sea-sized dark region discovered by ISS in February. Shortly after closest approach, VIMS will obtain several high-resolution observations of the northern half of the trailing hemisphere, including the dark lineaments mentioned above.
The last 5 days of Rev42 continue the ring and small satellite observations that characterized the first week of this orbit. Several observations are planned for imaging Saturnís small satellites and refining the orbits of these little worlds. Cassiniís cameras will be pointed at Hyperion on April 12, with the goal of better understanding Hyperionís photometric and rotational properties. Unlike most satellites in the solar system, Hyperion doesnít have a stable rotational axis. Observations such as these allow for monitoring of Hyperionís evolving rotational state.
On April 13, Cassini will observe Titan from a distance of 1.23 million kilometers (767,000 miles), allowing for images of Titanís north polar region (on the trailing side) at image scales better than 7.3 kilometers (4.5 miles) per pixel. This observation was designed to study photometry of Titanís atmosphere and surface, but it will also allow for continued monitoring of the large, Caspian Sea-sized dark region discovered in late February.
On April 15, Cassini will acquire distant images of Iapetusí trailing hemisphere, including a triplet of large craters east of the dark region known as Cassini Regio.
Cassini begins the following orbit 44, Rev43, on April 16, during which it will encounter Titan for the 30th time. Also in Rev 43, ISS will obtain new images of Dione, Iapetus, Saturnís rings, and best of all, the south polar jets of Enceladus.