SECRETARY'S REPORT 77 



both important to the whole subject of meteors and probably helpful 

 in the study of other objects and events in interplanetary space. 



Kobert E. Briggs completed his study of the space distribution of 

 interplanetary dust particles. He found that the light scattered by 

 the computed distribution is in good agreement with observed zodia- 

 cal light, particularly along the ecliptic, and that most of the particles 

 must have diameters of the order of 1 micron and fairly rough surfaces 

 of low reflectivity. He is now turning his attention to the velocity 

 distribution of interplanetary dust particles. The results of this 

 study should be valuable for current and future research on the nature 

 of interplanetary space, and perhaps provide new estimates on the 

 significance of particle collisions and breakup. 



Dr. Paul W. Hodge and Dr. Frances Wright continue their study of 

 the rate of accretion of meteoritic matter by the earth, especially 

 that in the form of dust particles collected by jet aircraft at altitudes 

 ranging from 30,000 to 90,000 feet. They plan to analyze material 

 gathered at even greater heights on the X-15. In addition, they have 

 examined dust from very old ice deeply embedded in the Greenland 

 and Arctic icecaps. 



The director's study of the erosion and puncturing of bodies in 

 free space continues. The preliminary results obtained have been re- 

 ported at three scientific meetings and two publications in press. In 

 brief, he has found that space erosion increases from a few angstrom 

 units per year for irons to 10 times this rate for stones and nearly 

 a thousand times greater rate for fragile material from comets. The 

 correlation of erosion rate with brittleness or compressive strength 

 indicates cratering by impacts with dust particles in space. A mean 

 space density of about 10"-" gm/cm ^ is required, consistent with 

 measures of scattered simlight. Near the earth a liigher rate appears 

 to prevail, possibly from moon dust, as he has previously suggested. 

 The most dangerous place with respect to meteoritic puncture is prob- 

 ably the moon's surface. The project has importance for fundamental 

 research on the interplanetary mediima and for space engineering 

 problems. 



Dr. Fred A. Franklin has completed his dynamical and photometric 

 studies of the rings of Saturn. He has obtained for rings A and B 

 the values of their optical and physical thicknesses, their masses, and 

 the fraction of their volume occupied by particles. He now also has 

 a value for the average radius of the individual particles and a 

 measure of the roughness of their surfaces. The derived thicknesses 

 of the rings are surprisingly small, measured in inches. The rings are 

 found to be an enduring feature of the solar system. 



Syace studies. — Two major projects for Orbiting Astronomical Ob- 

 servatories have made satisfactoiy progress. The director and Dr. 



