SATELLITE-TRACKING PROGRAM — HATES 343 



used for the determination of atmospheric density and temperature. 

 They would thus be able to obtain corrections to the profiles that had 

 been more or less guesswork before the first satellites were launched. 

 What they had not realized was that there were such large variations 

 of the atmospheric density related to phenomena outside the earth 

 and that the satellites, simply through the irregularities of their mo- 

 tions, could monitor those variations. 

 Dr. Jacchia has described the motion of a satellite in orbit : 



In a first approximation, then, we can say that the satellite describes an 

 elliptical orbit, but the plane of this ellipse slowly rotates, and the major axis 

 of the ellipse rotates in this plane. Moreover, we shall find small periodic 

 deviations from the elliptic motion in the course of one revolution. The motions 

 of the orbital plane and of the major axis are progressive and slow when com- 

 pared to the orbital motion ; they are called secular perturbations, a term taken 

 from the theory of planetary motions, in which the period of such perturbations 

 amounts to many centuries. All the other gravitational perturbations are much 

 smaller and of an oscillatory character, and are called periodic perturbations. 



Atmospheric density causes a "drag" on the motion of the satellites. 

 Continuing with Dr. Jacchia's description : 



This atmospheric drag has seemingly paradoxical effects. While a gun pro- 

 jectile is decelerated by drag in the course of its trajectory, the same drag accel- 

 erates a satellite in its orbit. The reason for this paradox is that drag causes 

 the satellite to lose energy and to fall to smaller orbits in which the period of 

 revolution is shorter. Although the kinetic energy of the satellite increases, 

 the total energy involved in the course of one revolution decreases. . . . 



Much information about the upper atmosphere can therefore be derived by 

 analyzing the motion of satellites. The rate at which the satellite's period de- 

 creases with time — the so-called orbital acceleration — yields a value for the 

 atmospheric density at perigee height. True, to have an accurate determination 

 of density we must first know how the density varies with atmospheric height 

 (the local "scale height"). Then we must have an exact knowledge of the drag 

 mechanism, and we must make sure that no drag other than atmospheric drag 

 operates on the satellites. And finally we must know the exact physical char- 

 acteristics of the satellite (if the satellite is a sphere, the problem is relatively 

 simple ; not quite so simple if it is a cylinder or an irregular body ) . 



At a meeting at the Observatory in 1957, scientists adopted a model 

 atmosphere based on the latest results from rocket and balloon explor- 

 ations. Virtually all research to that date consistently underestimated 

 atmospheric densities above 100 km. Before any satellites were 

 launched. Dr. Theodore E. Sterne of the Observatory's staff worked 

 out a theory of orbital variations due to drag. However, he and other 

 scientists prayerfully hoped that the drag would be so small that in 

 fact it could be taken into account by empirical corrections in orbit 

 computations; that is, they expected that once the satellite was up, 

 they could then best determine corrections for atmospheric drag to be 

 included in the computations. 



The first efforts to derive the orbit of Sputnik I, launched October 4, 

 195Y, from early observations by Moonwatch teams convinced sci- 



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