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Cassiniís journey at Saturn continues with Rev 43, its 44th orbit of the ringed planet, as Cassini observes Titanís trailing hemisphere, Saturnís atmosphere and ring system, and several of Saturnís icy satellites.
Cassini begins Rev43, on April 16 at its farthest distance from Saturn, called apoapsis. At this point, Cassini is 2.1 million km (1.3 million mi) from Saturn. The first week of Rev43 is filled with observations of Saturnís small satellites (to refine their orbits), the F-ring, and Saturnís outermost large satellite, Iapetus. Three observation sequences are planned for Iapetus (not including a similar sequence at the end of Rev42) between April 16 and April 21. These observations are designed to observe Iapetusí trailing hemisphere, a region of bright, icy, cratered terrain, which contrasts greatly with the much darker leading hemisphere. While these observations are distant when compared to the targeted flyby coming up in September of this year, the images taken during Rev43 will provide important information about the topography of the large impact basins in this region. The closest observations in Rev43 will be taken from a distance of 2.3 million km (1.4 million mi), compared to 1,400 km (870 mi) in September.
On April 24, Cassini reaches its closest point to Saturn during this orbit Ė called periapse Ė at a distance of 344,000 km (214,000 mi). During this period, Cassini will observe Saturnís illuminated ring system as well as two of Saturnís large inner moons: Dione and Enceladus. Cassini will observe Enceladus on April 24. During this observation, Enceladus will be visible only as a crescent, like the image at right (PIA07758). While this geometry is not suitable for observing the surface, it is perfect for observing Enceladusí south polar jets that erupt from vents along the moonís south polar ďtiger stripes.Ē This plume has been observed on multiple occasions over the past 2 years. However this will be the best opportunity to observe the individual jets that form the larger plume since the image at right was taken in late November 2005.
Observations such as these will be helpful in understanding the nature of the south polar plume and how the jets erupt. For example, one theory that has been proposed is that the individual jets, which erupt from the prominent fractures crossing the south polar region, are tidally controlled: i.e., they erupt from cracks that open and close due to variations of Saturnís gravitational pull on Enceladus as it orbits Saturn in its eccentric orbit. Determining the locations of the jets observed during this observation and comparing those to the source vents identified from the previous Cassini images of the jets would help prove or disprove this theory.
Shortly after the Enceladus observation, Cassini will turn its sights on Dione and observe this moonís leading hemisphere, best seen previously by Voyager 1. This region includes a number of fracture systems, such as Latium Chasma. These fratures appear similar to the bright fractures associated with ďwispy terrainĒ on the other side of Dione, but do not have the bright canyon walls that make the ďwispy terrainĒ so bright. This region also contains a number of impact basins, such as Aeneas and Dido, suggesting that the area has been resurfaced less than the area seen during the Dione targeted encounter in October 2005.
Cassini encounters Titan for the 30th time on April 26, with a closest approach distance of only 980 km (609 mi). Like the last few encounters with Titan, this flyby (known as T29) will allow for imaging of the northern portion of Titanís trailing hemisphere following closest approach. The cameras will observe the surface starting around 6 hours after closest approach, with ISSí main mosaic from this encounter beginning around 12 hours after closest approach (geometry shown below at right). Cassiniís RADAR instrument will be in the driverís seat for much of the close approach period, obtaining a Synthetic Aperture Radar (SAR) swath covering terrain similar to the swath obtained in February, during the T25 encounter (see PIA09182). This SAR swath will start in the eastern part of the dark region named Fensal, and continue north through Elpis Macula, finally ending up near the north pole, just north of the dark, lake-like features seen by ISS and RADAR during T25.
The last 5 days of Rev43 continue the ring and small satellite observations that characterized the first week of this orbit. However, Cassiniís orbit is starting to show signs of the ď180-degree transferĒ the spacecraft is performing to bring its apoapsis in line with the daylight side of Saturn. Rather than seeing a crescent Saturn as Cassini nears apoapsis, during Rev43, it will observe a half-phase Saturn. Several observations are planned for imaging Saturnís small satellites and refining the orbits of these little worlds.
On April 29, 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, and cloud features in Titanís northern hemisphere.
Cassini begins the following orbit, its 45th (Rev44), on May 2, during which it will encounter Titan for the 31st time. In Rev 44, ISS will obtain new images of Epimetheus and Saturnís atmosphere and ring system, as well as Cassiniís first images of the distant Saturnian moons Skathi, Kiviuq, Mundilfari, Ijiraq, and Bestla. Donít get too excited: the pixel scale at Bestla will be 146 km (90 mi) per pixel compared to its diameter of 7 km (4 mi). However, the observations will help with calculating the orbits of these small moons.
Images (except Enceladus image) created in Celestia. Iapetus map by Steve Albers.