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J. E. Colwell, L. W. Esposito (LASP, Univ. of Colo.)
The first ring occultation to be observed by Cassini is of the star Xi Ceti on October 6-7, 2004, from a distance of 6.22 million km. The Ultraviolet Imaging Spectrograph (UVIS) includes a High Speed Photometer with a bandpass of ~110-190 nm that has an integration period of 8 msec for this observation. The Fresnel zone at this distance is ~40 m, and the speed of the occultation gives a radial sampling interval of 5-8 m. The occultation covers the C ring, Cassini Division, A ring, and F ring. The star is relatively faint so the spacecraft downlink was scheduled to occur while it was occulted by the optically thick B ring. The predicted count rate of 1000 counts/second for this star translates into a maximum measurable optical depth of 0.53 with a 1 second integration for a radial resolution of better than 1 km. The occultation is particularly well-suited for the low to intermediate optical depth regions of the C and F rings and the Cassini Division, and will have lower effective resolution in the higher optical depth regions of the A ring. This is the first of over 60 planned stellar ring occultations to be observed by UVIS during the four year nominal mission of Cassini. We will apply the technique of Showalter and Nicholson (Icarus, Vol. 87, 285-306, 1990) to study the size distribution of large (meter-sized and larger) particles in rings through an analysis of the statistical properties of the observed star intensity. Occultations at different distances from the rings provide complementary information on the largest particles because of the dependence of the Fresnel zone on distance. The Xi Ceti occultation is by far the most distant that Cassini will observe. We will present our initial findings on ring structure and particle sizes.
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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.