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Cassiniís journey at Saturn continues with Rev 64, its 65th orbit of the ringed planet. Cassiniís orbit around Saturn is really starting to tighten--this orbit lasts a scant ten days. As such, Cassiniís slate of observations is more tightly focused, with sequences involving Rhea, Tethys, and Saturnís atmosphere, rings, and small satellites. Cassini begins Rev64, on April 6 at its farthest distance from Saturn, called apoapsis. At this point, Cassini is 1.48 million km (917,000 mi) from Saturn. The high inclination of this orbit allows for detailed study of Saturnís ring system from high above the ringplane. Several observations of the rings are planned from this perspective. On April 6, 7, and 10, three narrow-angle camera observations are planned covering the Cassini Division, various parts of the A and B rings, and the F ring, respectively. On April 6 and 9, two wide-angle camera mosaics are planned of the entire ring system (except the E ring), including what should be an incredible, 22-frame, 5-color mosaic from a distance of 930,000 km (580,000 mi). The graphic at left provides a nice preview of what this mosaic should look like, but it must be kept in mind that ISS will be imaging the dark side of rings, so the B ring will actually appear dark and the C ring bright. On April 10, the Ultraviolet Imaging Spectrometer (UVIS) will observe a stellar occultation, looking at variations in the ring system by staring at the blue-white giant star Beta Centauri and watching it flicker as the rings pass in front of it from Cassiniís perspective. ISS will ride along with UVIS, observing scattering effects and the dark side of the rings. Before periapse, Cassini will also perform observations of Saturnís atmosphere and small satellites. On April 8, Cassini will acquire a two-frame mosaic of Saturnís northern hemisphere, examining clouds and haze in this area, which is slowly emerging from northern winter. Between April 6 and 8, Cassini performs observations of several of Saturnís small satellites. The observations are designed to study the orbits of these objects and how they might evolve over short periods due to perturbations from other satellites in the system. On April 8, ISS will observe the small outer satellite Ymir for over nine hours. The goal of this observation is not to observe surface features (the small satellite will be impossibly far away for that), but to obtain a rotational light curve by acquiring 314 images, taking one every couple of minutes. Variations in the brightness of the satellite will allow scientists to determine the length of day on this moon (its rotation period).
On April 11, Cassini reaches periapse, its closest point to Saturn on Rev64. At that point, Cassini will be 259,000 km (161,000 mi) from Saturnís cloud tops. Four observations are planned for April 11 and 12 of the icy satellite Rhea and Saturnís ring system.
On April 12, Cassini will observe Rhea from a distance of 346,000 km (215,000 mi)--close enough for Rhea to nearly fill the narrow-angle field of view. During this observation, ISS will take a look at the south polar region of Rhea, as well as its southern anti-Saturn hemisphere.
Cassini observations of the rings during periapse include high-resolution imaging of the outer B-ring, a 260-frame azimuthal scan of the Encke Gap in the outer A-ring, and a stare at the B-ring to look for spokes.
Following periapse, Cassini will perform several observations of Saturn and its satellites. On April 15, ISS and the Visual and Infrared Mapping Spectrometer (VIMS) will observe Saturnís northern hemisphere. On April 14, distant observations of Tethys (to study the photometry of the surface) and Saturnís small satellites (for orbit determination) are planned.
Cassini begins the following orbit, Rev65, on April 16.
Image products created in Celestia. Rhea map created by Steve Albers using Cassini ISS images.