METEORSi — WHIPPLE 247 



in the atmosphere. We can determine the path of the meteor with 

 an error of only a few feet, its velocity with an error of less than one 

 part in a thousand, and measure its deceleration, caused by the re- 

 sistance of the atmosphere, to a significant accuracy at several points 

 along the longer trails. Dr. L. G. Jacchia, who has been in charge 

 of the reduction and analysis of the data, finds that the faster meteors 

 enter the atmosphere at an altitude of about 75 miles, and generally 

 die out by an altitude of 50 miles. Some of the slowest meteors are 

 first photographed well below 50 miles altitude and the largest of 

 these has been followed down to an altitude of about 25 miles. The 

 faster meteors are scarcely slowed down at all by the resistance of 

 the atmosphere, but their surface rapidly disintegrates under the 

 heat or friction of the atmosphere. When the meteor disappears 

 practically nothing remains of its original mass, although the final 

 particle is still moving at only a slightly reduced velocity. Some of 

 the very slowest meteors move at speeds of only 7 to 8 miles per 

 second; in one case only could we trace the meteor's speed down to 

 about 5 miles per second. 



In considering the large amount of light and heat generated by 

 these small bodies as they pass through the earth's atmosphere, we 

 must remember that their original kinetic energy corresponds to many 

 times that of an equal mass of a high explosive such as TNT. 

 Hence the energy of friction is adequate to remove and destroy the 

 body before the remaining nucleus can be much slowed down by 

 atmospheric resistance. 



Among some 500 photographic meteors that have now been analyzed 

 for velocities and orbits, we find no certain cases of meteors moving 

 in hyperbolic orbits. That is, there are no meteors that certainly 

 originated from interstellar space. If they exist, they must constitute 

 not more than 1 percent of the total number of photographic meteors 

 observed. Furthermore, the writer has shown that at least 90 per- 

 cent of the photographic meteors pursue orbits similar to those of 

 comets of both long and short period. If any average naked-eye 

 meteors come from a broken planet the number does not exceed 10 

 percent of the total number observed and probably is less than 1 per- 

 cent. Figure 5 shows the distribution of comet and meteor orbits ar- 

 ranged according to an arbitrary criterion, K, introduced by the 

 writer (Whipple, 1954) . The quantity K is defined as follows : 



if=log 1 o( r ^)-l (1) 



where q' is the aphelion distance in astronomical units and e is the 

 orbital eccentricity. The logarithmic quantity is the inverse square 

 of the aphelion velocity. 



Out of 1,600 known asteroids only 3 give positive values of the K 

 criterion while some 13 of the shorter period comets give negative 



