252 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1957 



possible. Thus, in the meteoric problem, we find relationships that 

 involve the mass, the density, and the luminous efficiency, but we 

 cannot determine any one of these quantities separately. Knowledge 

 of any one, on the other hand, would lead us immediately to accurate 

 determinations of the other two quantities for observed meteors. 



Since there is every reason to believe that the energy available for 

 light production cannot exceed the original kinetic energy of the 

 body, an upper limit to the density of the meteoroid and a lower 

 limit to its mass can be approximated. The writer found (Whipple, 

 1955a) from such calculations that the densities of meteoric bodies 

 must be of the order of unity, the density of water, or less. 



Recently, Allan F. Cook and the writer have developed a technique 

 (see Whipple, 1955c) for measuring the masses of meteors. We meas- 

 ure the motions in persistent meteor trains, the faint light left along 

 the trails of fast bright meteors after the body has passed. Photo- 

 graphs of such trains, made by opening and moving the Super- 

 Schmidt meteor cameras at 2-second intervals after bright meteors 

 had passed, make it possible to measure winds in the high atmosphere 

 (see pi. 5, fig. 1). In one case of a multiple-photographed double- 

 station train, it was possible to measure the forward or coasting 

 momentum of the meteoric gases and trapped air masses. This first 

 result indicates that the density of a meteor is as low as 0.05 

 gm/cm 3 or y 2 o the density of water. 



If a body is much less dense than water but is still made of ordinary 

 earthy materials, one would expect it to be exceedingly porous and, 

 therefore, exceedingly fragile. McCrosky (1955) , who has been study- 

 ing the fragmentation problem in photographic meteors, finds that 

 among the faint meteors some 20 percent become luminous almost 

 instantly instead of increasing their light gradually as the well- 

 behaved meteor does. He concludes that these bodies must become 

 visible because of sudden fragmentation of the entire meteoric mass. 

 He finds indeed that this fragmentation occurs at a nearly constant 

 pressure introduced by the resistance of the atmosphere, a pressure 

 of only one-third of a pound per square inch. Many of the meteoric 

 masses are so fragile that a block a foot or two in height would crush 

 at the bottom under its own weight, at normal gravity. 



Thus we have evidence that meteoric bodies from comets are ex- 

 tremely fragile, of low density, and, therefore, very porous. This 

 conclusion is to be expected from the writer's hypothesis (Whipple, 

 1953) concerning the nature of the comets from which this debris 

 has been ejected. According to this theory, the nucleus of a comet 

 is a conglomerate of interstellar or interplanetary dust formed from 

 gases at a temperature of only a few degrees absolute, perhaps when 

 the sun and planets were formed. Cometary activity is then the result 



