METEORS — WHIPPLE 255 



in strength with increasing distance from the reflection point. For 

 a specified wavelength of the radio waves, such a curve yields the 

 angular velocity of the meteor at a point where it passes perpendicular 

 to the line of radio sight. 



Ordinary radar techniques with pulses measure the time required 

 for the radio signal to travel from the transmitter to the trail and 

 back to the receiver again, and hence the distance to the reflection 

 point. The angular velocity coupled with the distance then deter- 

 mines the true spatial velocity of the growing ion column, and there- 

 fore of the meteoric body in its trajectory. This method and similar 

 related methods for measuring meteor velocities were developed 

 chiefly by scientists in England. (See Davies and Ellyett, 1949; 

 Manning, Villard, and Peterson, 1949; Hey, Parsons, and Stewart, 

 1947; McKinley, 1951 ; and Lovell, 1954.) 



From more than 10,000 measurements of meteoric velocities, Mc- 

 Kinley (1951) concludes that the velocities determined even from 

 very faint radio meteors, somewhat below naked-eye visibility, do 

 not indicate a statistically significant number of hyperbolic veloci- 

 ties, beyond the parabolic limit of 72 km./sec. Similarly, Almond, 

 Davies, and Lovell (1953) at Manchester, England, come to the same 

 conclusion from a more detailed analysis of fewer meteors, observed 

 from radiants near the apex of the earth's motion in the early morning 

 hours, and from the antapex direction in the early evening hours. At 

 present, no clear evidence for the existence of any hyperbolic meteors 

 has been found by radio-meteor astronomers. The general uncer- 

 tainties in the methods of observation, however, permit the possibility 

 that as many as one-half of 1 percent of the total might come from 

 outer space. 



The radio technique is capable of detecting meteors whose lumi- 

 nosity is 100 times fainter than that of meteors we can detect visually. 

 The radio and photographic results are in full agreement and indicate 

 that at least 99.5 percent of all the observed meteors are certainly 

 members of the solar system. 



A method of determining the radiant points of meteor streams by 

 means of radio echoes was developed by Clegg (1948). Later Aspi- 

 nall, Clegg, and Hawkins (1951) carried out a continuous survey 

 of stream radiants using twin antenna beams. Radio echoes were 

 obtained in turn in each antenna, as the earth rotated, and the time 

 of appearance of long-range echoes gave the time of transit and 

 declination of the radiant. 



An extremly important property of the radio technique was first 

 demonstrated when Hey and Stewart (1947) discovered extremely 

 dense meteor streams in the daylight hours, particularly in May, 

 June, and July. Thus the radio technique has the enormous advan- 



