300 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1963 



The second major problem in the design of the space telescope is 

 that produced by its thermal environment while in orbit. Sunlight 

 intermittently illuminates one side or the other of the space craft. 

 This variable heating on one side coupled with the intense cold of 

 outer space on the other produces a large and changing temperature 

 gradient between the outer skin of the space craft and the telescope. 

 It will be necessary to keep the thermal gradient small in the optical 

 system if high resolution performance is to be obtained. The problem 

 of computing what the thermal gradients will be before the space 

 craft is in orbit is a difficult mathematical problem, and one that must 

 be done with accuracy before the engineers can design a structure to 

 meet the requirements. 



The third problem area, one common to all complicated mecha- 

 nisms, is that of lifetime. Even when the probability of failure is 

 very small for any one component of a system, when several hundred 

 thousand components must function correctly the probability of fail- 

 ure of the system becomes large enough to present trouble. In the 

 case of the orbiting astronomical observatories, the design lifetime is 

 to be one year — an exceedingly difficult specification to design with 

 confidence in the results. The environment of the hyper-vacuum of 

 space causes many problems that we have no counterparts on earth. 

 Lubricants evaporate, even gross metal, like magnesium, evaporates 

 at such a high rate as to weaken structures. Moving parts tend to 

 weld together since all the surface contaminants that contribute to 

 low friction on earth evaporate. Primary cosmic rays can ruin the 

 best high voltage insulation at a single impact, not to mention the 

 gradual destruction of electronic components by the energetic particles 

 in the Van Allen radiation belts. The list of problems rapidly ex- 

 tends as one looks closer into the actual design of a space telescope, 

 yet the rewards of a new view of the universe draw astronomers 

 onward. 



