Exploring tlie Solar System by Radar^ 



By Paul E. Green, Jr., and Gordon H. Pettengill 



Lincoln Laboratory^ 

 Massachusetts Institute of Technology 



[With 3 plates] 



Ask the average person about exploitation of the sohxr system, and 

 he will probably give you an image of giant rockets firing complicated 

 instruments into space. Or perhaps he will remind you of the richly 

 detailed picture built up over tlie centuries from optical studies and 

 added to during the last few years by infrared and radio observations. 

 It may not occur to him, however, that radar teclmiques are begin- 

 ning to play an important role, too. 



Radar is, in a sense, simply two-way radio. Some sort of signal 

 is emitted by a directive antenna on the earth, travels to the object 

 being studied, is reflected in many directions, and a tiny remnant of 

 it eventually arrives back at the earth to be collected by the same 

 antemia. Since we know exactly what the transmitted signal is, we 

 can compare the returned echo with what was transmitted, so as to 

 test something about the target body, perhaps something that would 

 be difficult to isolate and study in any other way. 



One of the simplest examples of such a test is the measurement of 

 distance. If the experimenter knows the speed at which energy 

 travels, he can determine the target's distance just by measuring the 

 elapsed time between transmission and reception — a much more direct 

 and usually more accurate method than the optical use of trigono- 

 metric parallax. (Time measurements to one part in a billion are 

 common with today's electronic equipment.) But, as we hope to 

 show here, many more things than this have been done, and still 

 more will assuredly be done in the next few years. 



The first radars were not militaiy devices at all, but instruments 

 used to probe the structure of the ionosphere by vertical soundings. 

 These date back to 1920, 6 years before K. G. Jansky made his first 



iRppriiitcd by permission from Sky and Telescope, vol. 20, No. 2, July 1960. 

 => Operated with support from the U.S. Army, Navy, and Air Force. 



267 



