METEORS; — WHIPPLE 253 



of solar heating that vaporizes ices at the surface of the cometary 

 nucleus. These ices include ordinary ice from water, solid ammonia, 

 possibly even solid methane, and other compounds of carbon, nitrogen, 

 and oxygen with hydrogen. 



The remaining meteoritic material, made of the heavier, less volatile 

 compounds in the original dust, must remain very loosely cemented. 

 Most of this material is fragmented into extremely fine particles by 

 the cometary ejection process, but a small amount of it holds together 

 sufficiently well to form the cometary streams of meteors and the 

 sporadic meteors from comets. 



One would expect, on the basis of typical cosmic abundances, that 

 the initial cometary nucleus might be about the density of water and 

 that the final density of the meteoritic material might be the order 

 of one-third the density of water. On the other hand, it is very likely 

 that the initial dust in space consists of extremely porous masses, com- 

 parable to low-density smoke particles observed from artificial sources. 

 Hence the cometary nucleus itself can be of very low mean density, 

 and the final meteoritic fragments even more porous and rare. It 

 is not certain whether we shall be able to recover such fragile frag- 

 ments on the surface of the earth, because of their violent interaction 

 with the earth's atmosphere. Tiny ones may come through without 

 being seriously damaged. 



While the photographic method of studying meteors was being 

 perfected, a radically different and powerful technique came into 

 use. Chamanlal and Venkataraman (1941), of India, heard whistles 

 from continuous-wave radio transmitters, audible simultaneously 

 with the occurrence of bright meteors. Pierce (1938) at Harvard 

 and Hey (see Hey and Stewart, 1947) at Cambridge, England, work- 

 ing with a pulse transmitter and receiver on the same frequency, 

 observed transient echoes from meteors. A number of investigators 

 rapidly developed methods for detecting the ionization, or electron 

 columns, produced as meteoric bodies plunge through the earth's 

 atmosphere. The methods fundamentally depend upon the fact that 

 the electromagnetically vibrating waves from radio transmitters set 

 the individual electrons into synchronous vibration. The electrons, 

 because of this induced vibration, act as independent transmitters 

 and send out radio waves of the same frequency. Thus a column 

 of electrons effectively reflects a radio wave as the electrons along 

 the column resonate in phase with the initial radiation. The reflec- 

 tion is much like that of light from a shiny cylinder. 



Without becoming involved in the complexity of electronic tech- 

 niques we can understand, qualitatively, one of the most useful meth- 

 ods of tracking meteors by radio, and of determining meteoric veloci- 

 ties. In figure 9 we see that as the ionization trail of the meteor 



