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M. Cuk, J.A. Burns (Cornell U.), J.N. Cuzzi, S. Sugita (NASA Ames Res. Ctr.)
To evaluate Cassini's prospects for detecting meteoroid impacts on Saturn's rings, we assume an isotropic cometary flux striking the rings after being focused by Saturn's gravity for individual incoming directions (Cuzzi and Durisen 1990). The most energetic impacts occur in Saturn's shadow where Saturn's and the rings orbital velocities align. To estimate how the light flash's intensity depends on a meteoroid's velocity we combine laboratory results (Burchell et al. 1996) with modelling (Artemieva et al. 2000) developed for Leonid meteoroids hitting the Moon. For Cassini at ~13 Saturn radii looking near synchronous orbit, impacts of 10-kg meteoroids should be visible as 6th to 9th magnitude flashes lasting < 0.03 s; they should appear every ~2\times 104 s in the WAC field of view (cf. Showalter 1998). Less luminous flashes coming from smaller bodies appear a few times more frequently. The NAC sees fewer 10-kg events (~4\times 105 s) but more small impacts.
Meteoroid impacts may be associated with Saturn's spokes (Goertz and Morfill 1983). The formation rate of spokes is consistent with the impact rate for 10-kg meteoroids (10-4 per second per radian; Cuzzi and Estrada 1998). This meteoroid size is also sufficient to produce the neutral gas that, after photoionization, drifts to account for the spoke event (Morfill and Goertz 1983). Very large boulders are prominent in the central B ring since the nominal size distribution for ring particles starts at about 10 kg. Thus most of a typical meteoroid's collisional kinetic energy will be dissipated.
Since it may be problematic to distinguish such infrequent signals from cosmic ray hits on the CCD, we will asses strategies to recognize such events. For example, after the emergence of the target area from the shadow, Cassini will trace back spoke locations to search for the flashes of the initiating impact. Alternatively we will look for streaks (``Shooting stars'') crossing diaphanous rings.
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