1. Grün E., et al., Space Sci. Rev.60, 317 (1992).
2. E. Grün et al. Geophys. Res. Lett. in press.
3. The physical cross section of the Galileo DDS is 1000 cm 2 . However because the detector is at the base of the cylindrical detector housing the effective cross section is always smaller than the physical cross section and depends on the relative velocity of dust grains and the detector and the detector's orientation. For the purposes of calculating the detected number density of large grains we used the physical cross section because the effective cross section depends on the unknown orbit parameters of the detected grains. Figure 1 shows the effective cross section (in square centimeters) of the DDS for two different assumed orbits: circular prograde and circular retrograde. Because the effective cross section of the DDS is much higher for the retrograde orbit model than the prograde model it is probable that the detected grains were on retrograde orbits. The probability that any one grain is on a retrograde rather than prograde orbit can be estimated from the ratio of effective cross sections for the two orbit models. Thus the detection at day 253 and a rotation angle of 80° is approximately 800 times more likely to be of a grain on a retrograde rather than a prograde orbit.
4. Colwell J. E., Horányi M., J. Geophys. Res.101, 2169 (1996).
5. Sittler E. C., Strobel D. F., ibid92, 5741 (1987).