Dust Impacts Detected by the Cassini Radio and Plasma Wave Instrument Near Saturn's Ring Plane on July 1, 2004

During the inbound and outbound passes of the Cassini spacecraft through Saturn's ring plane on July 1, 2004, the Radio and Plasma Wave Science (RPWS) instrument detected many small particles striking the spacecraft. When a small particle strikes the spacecraft at a high velocity it is instantly vaporized, producing a small cloud of plasma that expands outward from the impact site. As the plasma cloud sweeps over the RPWS electric field antennas it produces a voltage pulse, the amplitude of which is believed to be proportional to the mass of the impacting particle. Two types of measurements are made: waveform measurements from the x-axis dipole antenna, and spectrum measurements from the w-axis monopole antenna. The waveform measurements provide a determination of the impact rate and the relative mass distribution, and the spectrum measurements provide a determination of the rms particle mass. The impact rate at both ring plane crossings provides a good fit to the sum of two Gaussians, with a peak impact rate of about 1000 per second (the exact value depends on the voltage threshold used), and a north-south thickness (at half the peak rate) of about 300 km. The mass distribution depends on the distance from the ring plane, varying from about m-2 near the ring plane, at z=0 ±100 km, to m-4 well away from the ring plane, at z=500 ±100 km, where z is the north-south distance from the ring plane. We are still working on estimates of the rms mass and the typical size of the dust grains, and will present preliminary results on these estimates.