342 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 64 



months. The period from July 1957 through December 1958 had been 

 selected to coincide with maximum activity in the sun. Since solar 

 phenomena involving ultraviolet and corpuscular radiation cannot be 

 observed on the ground because the atmosphere cuts off most or all 

 of their effects, the satellites were to carry instrumentation that would 

 measure these and other astrophysical events and telemeter the data 

 to ground stations. 



The initial purpose of the Observatory's program for the optical 

 tracking of satellites was primarily surveillance — ^that is, to keep the 

 object in view as it went around the earth, particularly if its radio 

 transmitter failed for one reason or another. In fact, the transmitters 

 in several of the first satellites did fail, so that the optical system was 

 often the only means of tracking. 



The Smithsonian Astrophysical Observatory planned also and more 

 importantly to make secondary use of these satellites. It was to track 

 them as passive objects and analyze the resultant data to derive new 

 knowledge about the earth and its atmosphere. 



The satellites could, of course, be tracked by other means — radio, 

 radar, and doppler measurements in particular. At the time, however, 

 none of these was nearly so accurate as the optical techniques de- 

 veloped by the Observatory. Optical tracking was based on astro- 

 nomical methods that had been refined over a long period of time and 

 were well understood by scientists. The other methods were relatively 

 new, and until actually employed in the tracking of a satellite were not 

 wholly predictable. These techniques were quickly refined following 

 the launching of Sputnik I. 



The first American discoveries from satellites were made almost 

 entirely with Moonwatch observations of Satellites 1957 Alpha and 

 Beta. For Satellite 1958 Alpha the observations were primarily 

 Baker-Nmm. And for Vanguard I, the observations were mainly 

 Minitrack, because the satellite was too faint except for occasional ob- 

 servations by the Baker-Nmm cameras. All of these observations 

 were used for research purposes and it was Vanguard I from which 

 the most important early determinations concerning the structure and 

 variation of the upper atmosphere were derived. These facts serve 

 to emphasize once again the close and necessary cooperation that 

 existed among the projects of the IGY and that continues today 

 among the various programs of the U.S. space effort. 



The first satellite research of the Observatory concerned the upper 

 atmosphere. The atmosphere had already been explored by balloons 

 and probed by rockets to a height of about 200 kms., and approximate 

 profiles of temperature, density, and composition drawn for that re- 

 gion. What scientists now wished to do was to refine that picture and 

 to extend it to the boundary of the interplanetary medium. They 

 had realized from the first, of course, that passive satellites could be 



