damaging the spacecraft (those with r ีฟ 0.13 mm or m ีฟ 10 ฯช5 g). Previous work by Divine (1989) evaluated the An evolutionary model of the G Ring incorporating theoretical results from R. M. Canup and L. W. Esposito (1995, Icarus vulnerability of components of the spacecraft to impact by 113, 331-352) yields a complete particle size distribution that particles at 7-15 km/sec, corresponding to relative velociis consistent with existing observations. Results from numerical ties for Cassini ring plane crossings in low to highly inclined modeling demonstrate that a G Ring origin from the disruption orbits. Ring particles larger than 0.13-1.0 mm in radius of a 1.5-3 km progenitor satellite can match all known properare capable of puncturing critical spacecraft subsystems, ties of the ring. In addition, we estimate the population of the most sensitive of which range in cross-sectional area unseen macroscopic material from both observational upper from 0.05 to 3.17 m 2 (Tan and Tsuyuki 1989). Divine conlimits and our theoretical model for the region surrounding sidered various models for the size distribution of particles the G Ring, where the Cassini spacecraft will likely make its in the micrometer to centimeter size range for orbital radii innermost passes in the saturnian system. For models that fit greater than 2.37R S (exterior to the A Ring) which were all available data, the probability of Cassini striking a hazardconsistent with the observational upper limits and charged ous ring particle is less than 1%. ยฉ 1997 Academic Press particle absorption data from Pioneer 11. These models yield a wide range of expectation values for the number of critically injuring impacts experienced by Cassini per I. BACKGROUND ring plane crossing, from 10 ฯช8 to 0.4 (Divine's Table 6), all consistent with the existing observations. The G Ring of Saturn, first observed by Voyager 1, is a narrow and tenuous dust ring located between 2.75 and
In this work we adopt an alternative approach. We consider an evolutionary model of the G Ring constrained 2.88R S with a mean optical depth of about 10 ฯช6 . A recent reanalysis of Voyager photometric images of the G Ring to match the existing observational data. In this model, the origin of the ring is associated with the catastrophic by Showalter and Cuzzi (1993) reveals a ring whose optical depth is dominated by extremely small, short-lived dust fragmentation of a progenitor satellite by a meteoroid or cometary impact (see, e.g., Colwell and Esposito 1992, particles (r ี 0.03 ศm) described by a steep power-law size distribution. Charged particle absorption data from 1993, Showalter and Cuzzi 1993). The largest remaining fragments from the initial disruption become G Ring Pioneer 11 constrains the total surface area of large bodies in the ring to 10-40 km 2 (e.g., Van Allen 1983) within a parent bodies, which act as sources for the observed dust ring. Our model tracks the ring particle population region about 1000 km in width.
Interest in the G Ring has recently grown due to both as it evolves due to (1) accretion onto parent bodies, (2) collisional release as parent bodies collide, (3) produc-the first Earth-based observations of the ring (obtained in the summer of 1995 during the Saturn ring plane crossing) tion due to ejection by meteoroid impact into the parent bodies, and (4) removal due to both plasma drag and and the upcoming Cassini mission to Saturn. The Cassini spacecraft will make its inner-most crossings of Saturn's catastrophic fragmentation. Predicted ring particle populations are consistent with Voyager photometry (Sho-equatorial plane in the region surrounding the G Ring, between the outer edge of the A Ring (a ฯญ 2.27R S ) and walter and Cuzzi 1993), Voyager dust impact detections during the G Ring crossing (PWS/PRA instruments, the orbit of Mimas (a ฯญ 3.1R S ). While the orbital tour will avoid areas where ring material has already been detected, Gurnett et al. 1983), and parent body surface-area constraints from Pioneer 11 charged particle absorption data the observational upper limits on the optical depth of material in the ''empty'' regions are consistent with a significant (Van Allen 1983).
We predict the steady-state population of material in population of macroscopic particles capable of critically 28