METEORlS — WHIPPLE 257 



The writer (Whipple, 1955b, 1955c) has recently shown that a 

 few tons of cometary dust injected into the solar system each second 

 would be adequate to maintain the zodiacal light indefinitely. The 

 particles are continuously lost by collisions among themselves, by the 

 gravitational effects of the planets, particularly Jupiter, by the inter- 

 stellar wind produced by the sun's motion through the interstellar 

 gas, by the action of sunlight according to the Poynting-Robertson 

 effect, and by corpuscular radiation from the sun in the form of out- 

 going hydrogen protons. Opik (1956) has shown that the extended 

 solar corona also adds drag to these little particles. The three latter 

 effects cause the particles in the zodiacal cloud to spiral slowly in to- 

 ward the sun, the rate depending upon the size of the particle. Pio- 

 trowsky (1953) has also shown that the grinding of the asteroids 

 may produce sufficient material to maintain the zodiacal cloud. At 

 the moment it is not possible to distinguish certainly between these 

 two hypotheses but other evidence suggests that the cometary source 

 is much the more important. Observational and theoretical advances 

 should settle the question definitively within the next few years. 



Now we can bring together, to complete this discussion, radio me- 

 teors, photographic meteors, the zodiacal light, micrometeorites, 

 comets, and corpuscular radiation from the sun. If meteoric densi- 

 ties are typically as low as our single measure suggests, the discrepancy 

 van de Hulst (1947) found between the total influx of zodiacal parti- 

 cles and meteoritic masses disappears. He measured the integrated 

 dimensions of the interplanetary matter rather than its mass ; hence 

 a low density would reduce his estimate many times. Furthermore, 

 the low density would increase the older estimate of the total 

 meteoritic masses, and hence remove the discrepancy entirely. 



The continual bombardment of meteoric debris of course constitutes 

 a real hazard to rockets, artificial satellites, and space vehicles, which 

 may be subjected to erosion or puncture. Elaborate ballistics ex- 

 periments and careful calculations have shown, however, that optical 

 surfaces exposed to space should not be affected functionally in less 

 than about a year, and that for the presently planned small satellites, 

 the rate of puncture will be, on the average, only about once 

 in five days. 



Probably about 2,000 tons of meteoritic debris fall on the earth 

 from interplanetary space each day. This large mass, however, is 

 still quite negligible compared to that of the earth. In five million 

 years the total accumulation would add up to only an inch over the 

 entire surface. 



Although some of our discussion has led beyond the borderline of 

 scientific certainty, I have attempted to distinguish clearly between 



