268 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1960 



radio astronomy discoveries. In this article we shall omit the fasci- 

 nating story of radar studies of the ionosphere, of meteor trails and 

 aurorae, and concentrate on extraterrestrial objects, such as the moon, 

 planets, sun, and the tenuous contents of interplanetary space. 



Radar came dramatically to the attention of astronomers in 1946, 

 when war-developed equipment proved capable of bouncing an echo 

 off the moon, 240,000 miles away. For the next 12 years, the story of 

 radar astronomy was that of moon echoes. Over that entire period, 

 rapid improvements were being made in radar technology, yet these 

 were still not sufficient to permit detection of the next most distant 

 target, Venus. 



Then, within a year of the March 1959 announcement of successful 

 contact with Venus, by a group imder Robert Price at Lincoln Labora- 

 tory, there came the news that V. R. Eshleman's team at Stanford Uni- 

 versity had detected solar echoes (February 1960). And there is talk 

 that radar contact with Mars may be attempted during its opposition 

 in December 1960, or at the February 1963 opposition. Wliy so much 

 sudden activity after a 12-year interval when only the moon was 

 observable ? 



THE EFFECT OF DISTANCE 



The reasons are clear when one appreciates the important role that 

 distance plays in a radar detection. Venus at inferior conjmiction is 

 some 100 times more distant than the moon, whereas Mars at closest 

 opposition is only 1% times as distant as Venus, and the sun 314 times. 

 In a one-way transmission, the energy received is proportional to the 

 inverse square of the distance; however, with radar the energy must 

 not only reach the target but be propagated back again, suffering 

 another inverse-square attenuation. The result is a received signal 

 energy proportional to the inverse fourth power of distance. Venus 

 has a diameter a little over 3I/2 times that of the moon, and thus 

 roughly 10 times the reflecting area. But its 100-fold greater distance 

 means that the energy returned is 10/(100)* or 10"' that from the 

 moon, if for both bodies the power reflected is proportional to area. 



Making the same calculation for each of the planets, and plotting 

 their detectability relative to the moon's, we get the pattern of points 

 in figure 1. Several satellites and minor planets are also included. 

 Clearly, after we have bridged the gap of 10^ in detectability from the 

 moon to Venus, there are many radar targets in close succession. 



Another increase in lO'' in radar performance beyond that needed 

 to detect Venus would encompass all the planets except Pluto — pro- 

 vided detectability depended only upon the diameter of the body and 

 its distance from earth. Unfortunately, matters are not this simple. 

 Discre])ancies of several orders of magnitude from the numbei's given 

 in figure 1 are possible, owing to different reflectivities of the planets' 



