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 meteoroid penetration detectors on the Pioneer 10 splicecraft recorded 67 meteoroid penetrations through the 25-•tm stainless steel test material while the spacecraft was between 1.0 and 5.1 AU. Ten of these penetrations occurred during the encounter with Jupiter.• The cumulative spatial density of meteorolds with masses greater than 2 X 10 -9 g his been calculated from these data for interplanetary space and for the near-Jupiter space. The spatial density is found to be essentially constant in interplanetary space between I and 5 AU, approximately l0 -9 m -a, and 1-2 orders of magnitude greater near Jupiter. There was no increase in the spatial density of meteorolds in the asteroid belt and hence no evidence that there is a significant asteroidal component of 2 X l0 -9 g meteorolds. It is uncertain whether the meteoroids detected near Jupiter were in orbit about Jupiter or were gravitationally focused toward the planet from solar orbits. Prior to thePioneer 10 mission, no measurements had been made of the concentration of meteoroids in interplanetary space beyond 1.6 A U or near Jupiter. It was generally believed that the meteoroid population had two components, a cometary component and an asteroidal component. Southworth [1967] estimated the distribution of cometary meteoroids:in the solar system from the orbits of approximately 13,000 radar meteors. This result was largely dependent on extrapolation because only meteoroids that cross 1 AU can be observed as radar meteors. He concluded that the spatial density decreased monotonically outward from the sun, varying as R-•'5-R -•"ø between the orbits of earth and Jupiter. More recently, Southworth and Sekanina [1973] estimated the distribution of meteoroids in the solar system from the orbits of approximately 20,000 radar meteors observed in 1969.Southworth and Sekanina reached the new and surprising conclusion from these data that the spatial density was at a minimum at 0.7 AU and at a maximum in the asteroid belt, between 2 and 3 A U. They were uncertain about the fraction of the sample that was cometary in origin and the fraction that was asteroidal. The mass distribution of small asteroidal meteoroids was estimated by Kessler [ 1970] based on the mass distribution of the large observed asteroids and the intensity of the counterglow.Kessler concluded that the spatial density of asteroidal meteoroids is a maximum at 2.5 AU having a value of 3 x l0 -9 m -8 for 2 x 10 -• g meteoroids. This is a factor of 4.4 greater than the spatial density that he calculated for 2 X 10 -9g meteoroids at 1 AU. Calculations by Whipple [1971] placed the likely upper limit on the spatial density of asteroidal meteoroids at the order of 5-10 times that near the earth's orbit. The concentration of meteoroids in interplanetary spacebetween 1.0 and 5.1 AU and near Jupiter has now been measured with the pressurized cell penetratioh detectors on Pioneer 10. These detectors have a sensitivity of approximately 2 X 10 -* g. Meteoro M detectors of the pressurized cell type have been used p...

For many years it has been suggested that lava tubes on the Moon could provide an ideal location for a manned lunar base, by providing shelter from various natural hazards, such as cosmic radiation, meteorites, micrometeoroids, and impact crater ejecta, and also providing a natural environmental control, with a nearly constant temperature, unlike that of the lunar surface showing extreme variation in its diurnal cycle. An analysis of radiation safety issues on lunar lava tubes has been performed by considering radiation from galactic cosmic rays (GCR) and Solar Particle Events (SPE) interacting with the lunar surface, modeled as a regolith layer and rock. The chemical composition has been chosen as typical of the lunar regions where the largest number of lava tube candidates are found. Particles have been transported all through the regolith and the rock, and received particles flux and doses have been calculated. The radiation safety of lunar lava tubes environments has been demonstrated.

The concentration of meteoroids of mass ~ 10(-8) gram in interplanetary space, in the asteroid belt, and near Jupiter has been measured. The data confirm the Pioneer 10 observation that the asteroid belt is not highly populated with small meteoroids, suggest that the high concentration of small particles around Jupiter is the result of gravitational focusing, and provide an indication of the mass distribution of meteoroids in interplanetary space.

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