286 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1963 



nucleus as the comet approaches the sun. One comet which has 

 been of considerable contemporary interest in the study of meteors 

 during the last decade is the Giacobini-Zinner Comet. In October 

 1946 the earth crossed the orbit of the comet only a few days from 

 the position of the nucleus. For a few hours between midnight and 

 6 a.m. on October 10 thousands of meteors could be seen in the sky, 

 but before this and afterward the meteor rate was of the usual 

 sporadic value of a few per hour. This was a clear and spectacular 

 demonstration of the close relationship between meteors and comets. 



The systematic study of meteors has been severely handicapped by 

 the difficulty of making observations, as the sky is so frequently 

 either obscured by cloud or made light by moonlight. Radio astron- 

 omy has given us new methods of investigating these meteors which 

 overcome the difficulty of cloud, moonlight, or daylight. When the 

 meteoric particle evaporates in the high atmosphere it leaves behind 

 a trail of ionized particles as well as the luminous trail by which we 

 see it. The electrons in this ionized trail are efficient scatterers of 

 radio waves, A beam of radio waves transmitted from a radio tele- 

 scope is scattered by the trail and the returned signal can be detected 

 by the receiving part of the telescope equipment as a transient echo. 

 If the recording equipment consists of a cathode ray tube with a 

 suitable time base, it is possible to observe the diffraction pattern 

 which is formed as the ionized trail crosses the perpendicular from 

 the receiver to the trail. This is the radio analog of the diffraction 

 of light at a straight edge — ^the rhytlmiic variations in brightness as 

 the shadow merges into the light. In the radio case, since the range 

 can be measured and the wavelength is known, the precise velocity 

 of the meteor can be determined. If these observations are made from 

 three spaced receiving stations using one transmitter, the exact orbit in 

 space of a single meteoric particle can be obtained. 



The relative infrequency of the meteors seen by a single observer 

 gives a false impression of the vast numbers which the earth encoimters 

 in its journey through space. The number entering the earth's atmos- 

 phere which are big enough to produce a trail sufficiently bright to be 

 seen in a small telescope is about 8,000 million every day. These are 

 small grains of dust weighing only about a ten-thousandth of a gram. 

 Using radio techniques one can detect particles of even smaller size, 

 and the numbers increase by about 2^ times for every fainter magni- 

 tude. The particles detected by the most sensitive radio-meteor 

 equipment available today are probably being swept up by the earth 

 at the rate of about a million million per day. The numbers seem 

 to increase endlessly as the size goes down, but when the radius of the 

 particles is less than about a millimeter then these particles are too 

 small to burn up. For these the ratio of the surface area to the mass 

 is so large that the energy of interaction when the particles begin to 



