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From 1939 he has been a Research Associate, Harvard College Observatory.  Between 1941 and 1945, he participated in foreign language broadcasting and monitoring, utilizing his outstanding linguistic ability for the N.B.C. Word Wide Radio Foundation.  He has been a Scientific Consultant to the Office of War Information; Staff Member of the Division of Industrial Cooperation at the Massachusetts Institute of Technology; and Research Associate in Electrical Engineering, M.I.T.

Dr. Jacchia was appointed as a Physicist at SAO in 1956, joining the first group of scientists at SAO, along with Professor F.L. Whipple, when it moved to Cambridge in 1955.

Dr. Jacchia is an authority on the earth's upper atmosphere.  When the United States initiated its space program, it became immediately apparent that a key uncertainty was the extent to which the tenuous gases in the earth's upper atmosphere would exert drag on orbiting artificial satellites.  As it turned out, this is the most important effect limiting the lifetime of such satellites, and therefore it has an enormous impact on the U.S. space program.  Atmospheric drag is strongly affected by solar activity because the high-energy emissions associated with such activity are absorbed in the upper atmosphere, heating it and causing it to expand to higher altitudes where it causes drag on satellites.

The effect of solar activity on satellite drag was observed by Jacchia in 1959 by analysis of satellite orbits.  Since that time he has developed the effect into a finely-honed tool to explore the dynamic response of the upper atmosphere to solar radiation, as he has described in seventy (70) papers since that time.  The importance of such effects was vividly demonstrated in 1979, when the orbit of Skylab decayed much more quickly than expected because of an unusual degree of solar activity.  Data from Dr. Jacchia's analyses have been widely used in constructing models of the earth's upper atmosphere.