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ANNUAL REPORT SMITHSONIAN INSTITUTION, 195 7 



diameter. Micrometeorites are heated as they strike the earth's upper 

 atmosphere, at an altitude from 130 to 100 km., but because of the 

 small ratio of their mass to surface area, they can radiate away the 

 heat of impact fast enough to prevent heating above the melting 

 point of ordinary materials such as iron or silicates. Thus many 

 of these particles, as has been shown by Opik (1956) and the writer 

 (Whipple, 1950, 1951), can reach the surface of the earth without 

 being greatly damaged. The larger particles in coming through 

 the atmosphere may be melted and fused into small globules by this 

 process without losing much of their mass. 



EARTH'S 



MORNING 

 METEOR 



18.5 

 mi/sec 



EVENING 

 METEOR 



Figure 1. — Schematic diagram showing how meteorites collide with the earth. 



Larger particles, perhaps the order of a thousandth of a gram or 

 greater, produce enough light by friction with the earth's atmos- 

 phere to be visible as telescopic meteors and produce enough electrons 

 to give radar echoes as radio meteors. Both the telescopic and radio 

 techniques can, of course, be used to observe much brighter meteors, 

 and the lower limit of their sensitivity is well below that of the 

 naked eye. Meteors visible to the naked eye fall in the category of 

 visual meteors; today the extremely sensitive Super-Schmidt meteor 

 cameras in New Mexico can photograph nearly to the limit of naked- 

 eye visibility. 



On certain days of the year meteors occur in showers, when the 

 earth happens to cross a stream of meteoric particles in space. All 



