290 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1963 



reflecting over such a large area, would introduce so much distortion 

 that the signals would be unintelligible. However, the conclusion 

 reached from the study of the short-period fluctuations, that only 

 the central part of the lunar hemisphere was effective, entirely al- 

 tered this situation, and it seemed at least possible that if one modu- 

 lated the radio waves going out from the radio telescope with speech 

 instead of with the pulses, then one might at least be able to get back 

 intelligible speech reflected from the moon. This proved to be the case, 

 and it is now possible to converse intelligibly between any two points 

 of the world, from which it is mutually visible, by using the moon as 

 a reflecting surface. 



Radio telescopes have been used to measure the radio emissions from 

 several of the planets in the region of centimeter wavelengths. This 

 is the thermal emission appropriate to the temperature of the body, 

 and useful comparisons with the temperatures derived by optical 

 studies are being made. More surprising is the detection of large 

 sporadic outbursts on long wavelengths from Jupiter. The energies 

 involved in the generation of these radio waves must be enormous. 

 There is some evidence that the events occur on the surface of the 

 planet rather than in its atmosphere. Should this be the case, the 

 forces at work must be equivalent to the energies involved in several 

 hydrogen bombs, or in giant volcanic eruptions like the explosion of 

 Krakatoa. 



The extension of the lunar radar experiments to the nearer planets 

 presented a major challenge. The moon is 240,000 miles distant and 

 the return journey of the radio waves from earth takes 214 seconds. 

 At close approach Venus is nearly 30 million miles away and the radar 

 signal would take over 5 minutes on the journey there and back to 

 earth. In terms of sensitivity of apparatus it is 10 million times more 

 difficult to achieve success here than with the lunar echo. However, 

 a beginning has been made. An American team with a transmitter 

 of very great power on an 80-foot radio telescope, and a team at Jod- 

 rell Bank using a smaller transmitter on the 250-foot radio telescope, 

 have both achieved initial success in these Venus experiments. Even 

 with these preliminary results a direct measurement of the distance of 

 the planet has been made and the range of uncertainty about the value 

 of the solar parallax has been significantly reduced. It is hoped that 

 in the near future further extension of this work will enable the rate 

 of rotation of the planet to be measured. It is likely, too, that the 

 experiments will give some guidance on the nature of the surface of 

 the planet. 



At the moment no one can be sure whether the first determination 

 of the rotation period of Venus will come from these radio-astronom- 

 ical studies or from instruments carried in a space probe, which either 

 ofbits or make a close approach to the planet. There are, however. 



