170 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 64 



pheric size of the meteorite and of the proton flux to which it has been 

 subjected. 



Cometary science. — Comets and their associated phenomena are inti- 

 mately related to other aspects of the solar system, such as zodiacal 

 dust, micrometeorites, and evolution of the planets. Cometary in- 

 vestigations hence constitute a vital link in the Observatory^ 's overall 

 research program. 



A study by Dr. Wliipple of the secular variation in the absolute 

 brightnesses of comets leads to the possibility that several of the known 

 periodic comets may disappear within the coming decade. The ob- 

 servational rediscoveries of the periodic comets suggest that these cal- 

 culations are more pessimistic with regard to the lifetimes of comets 

 than is justified; nevertheless, the predictions should serve a useful 

 purpose in stimulating search for the rediscovery of old comets and in 

 clarifying the question concerning the actual decay processes whereby 

 comets do, indeed, cease to be visible. 



Dr. "Whipple, in the study mentioned earlier, has also discussed the 

 evidence that a thin belt of comets probably remains in a plane not far 

 from the mean plane of the planets, but outside the orbit of Neptune. 

 Such a belt of comets can accoimt for disturbances of Neptune's 

 motion. 



The utilization of the Baker-Nunn cameras for comet observations 

 has been expanded.^^- ^ Using photographs thus obtained, the Ob- 

 servatory is pursuing three objectives: determination and understand- 

 ing of the motion of comet tails ; photometry of comets ; and time-lapse 

 motion pictures to docmnent the changes in a comet and its tail with 

 time. 



The research on tail motions is guided by Daniel Malaise," who has 

 previously observed that the direction of the tail of a comet may 

 oscillate significantly about the line directed through the comet away 

 from the sun. The explanation of this phenomenon is not clear, but 

 it may be related to some characteristic of the solar wind. Baker- 

 Nunn photographs are an excellent source of the observational data 

 required to pursue tliis topic. Data obtained during the past year 

 are being analyzed. 



The photometric investigations are the responsibility of Dr. 

 Southworth. For this purpose, defocusing lenses have been sent to 

 a number of the stations. An miresolved question in cometary 

 astronomy is whether comet magnitudes do indeed change in correla- 

 tion wdth solar activity, as some investigators have reported. A study 

 of this question is one of several investigations based on the photo- 

 metric data. 



See footnotes, p. 177. 



