Results of Pioneer 10 and 11 Meteoroid Experiments: Interplanetary and near‐Saturn

Abstract

The meteoroid penetration detectors on Pioneer 10 (channel 0) recorded 95 penetrations through the 25‐µm stainless steel test material while the spacecraft was between 1 and 18 AU. The spatial density of 10−9g meteoroids is found to be essentially constant between 1 and 18 AU. The meteoroid penetration detectors on Pioneer 11 recorded 87 penetrations (55 on channel 0 and 32 on channel 1) through the 50‐µm stainless steel test material while the spacecraft was between 1 and 9 AU. It is found that the meteoroids between 4 and 5 AU are not in direct circular or near‐circular orbits near the ecliptic plane. The Pioneer 11 data obtained between 4 and 5 AU are best explained by the meteoroids being in randomly inclined orbits of high eccentricity. If meteoroids are in these cometlike orbits, the great increase in penetration flux previously measured near Jupiter with the Pioneer 10 experiment cannot be attributed to gravitational focusing unless the size distribution of meteoroids changes substantially between 1 and 5 AU. At Saturn encounter, the penetration flux increased by about three orders of magnitude, probably as the result of impacts from ring particles. Saturn's ring E is estimated to be 1800 km thick with an optical thickness greater than 10−8.

The Student Dust Counter: Status report at 23 AU

“…Beginning at about 3 AU from the Sun, the Pioneer 10 and 11 observations of particles larger than about 10 μm in radius showed a constant spatial density (number of particles per unit volume) out to 18 AU (Humes, 1980). At this distance, the argon and nitrogen gas mixture in the Pioneer pressurized detector cells froze and no further measurements could be made.…”

“…It is designed to map the spatial and size distribution of interplanetary dust particles in order to verify the existence of the predicted structures in our interplanetary dust disk. Table 1 lists the five spacecraft that have carried dust detectors beyond the asteroid belt: Pioneers 10 and 11 (Humes, 1980), Galileo (Grün et al, 1992a), Ulysses (Grün et al, 1992b), and Cassini (Srama et al, 2004). SDC provides the first dust measurements beyond 18 AU, where the Pioneer sensors stopped working.…”

Dust Measurements in the Outer Solar System

“…Pioneer 11 crossed three times the region between 3.7 and 4.9 AU at different impact configurations with respect to the meteoroid population (outbound, and after Jupiter fly-by inbound and outbound again). Humes, 1980). The threshold sensitivities of the Pioneer 10 and 11 instruments are 8 · 10~1 0 and 6 · 10 -9 g, respectively, at an assumed impact speed of 20 km/s.…”

“…Both spacecraft carried several detectors which measured different aspects of interplanetary dust. Only the meteoroid penetration detectors recorded significant concentrations of meteoroids outside 3 AU (Humes, 1980). These detectors are simple but highly reliable detectors which record the puncture of a pressurized cell by a meteoroid impact.…”

“…Extensive model calculations by Humes (1980) showed that the spatial density of 6 · 10~9 to 8 · 10~1 0 g meteoroids is essentially constant out to 18 AU. The multiple flux measurements with Pioneer 11 inside Jupiter's orbit allowed Humes to compare them with different inclination-and eccentricity-distributions of solar system dust.…”