Luigi Jacchia, a former Smithsonian Astrophysical Observatory (SAO) physicist and a distinguished researcher, whose analysis of density fluctuations in the Earth's upper atmosphere was one of the first scientific triumphs of the space age, died 8 May 1996 after a long illness. He was 85.
Jacchia's discovery of diurnal variations in atmospheric density was based on painstaking analyses of satellite orbital data provided by SAO's world-wide network of Baker-Nunn cameras. The result, often compared to the discovery of the Van Allen Belts as one of the major milestones in space physics, was recognized by the Regents of the Smithsonian Institution in 1980 with the award of the Hodgkins Medal.
A man of extraordinary charm, wit, and erudition, who spoke over a dozen languages, Jacchia was born in Trieste, Italy, and received his doctorate from the University of Bologna in 1932. He taught at Bologna until 1939, when he left Europe to become a Research Associate of the Harvard College Observatory. During World War II, he applied his linguistic skills to war work as scientific consultant to the Office of War Information's Foreign Language Broadcasting and Monitoring Service.
After the war he returned to Harvard; and, in 1956, was asked by director Fred Whipple to join the newly invigorated SAO, whose headquarters had moved from Washington, DC to Harvard, with a change in focus from solar research to space science. As one of SAO's first Cambridge-based staff members, Jacchia quickly became involved in planning for the International Geophysical Year (1957-58), developing programs to obtain the composition and density of the upper atmosphere from observations of satellite motion. In the early Sputnik era, he found slight changes in satellite orbital periods in a cycle of roughly 27 days, approximately the period of the Sun's rotation. Comparing satellite motion with solar activity, Jacchia found a correlation between orbital fluctuations and the heating of the upper atmosphere by sunspots and flares.
Jacchia continued his analysis of satellite drag data over longer periods to produce atmospheric models that became international standards. Working primarily with Jack Slowey of SAO, Jacchia refined those models to include seasonal and diurnal factors and developed formulae for determining the effects of solar activity on the orbital lifetimes of satellites. Indeed, Jacchia's description of the dynamic relationship between the Sun and the earth's atmosphere was dramatically demonstrated in 1979 by the unexpectedly rapid orbital decay (and reentry) of Skylab due to unusually high solar activity.
In addition to the AAS and other societies, Jacchia was a member of IAU Commission 22 (Meteors); Committee on Space Research, Working Groups 1 and 2; International Union of Geodesy and Geophysics, International Association of Geomagnetism and Aeronomy, Commission 8; and the U.S. Committee for the Extension of the Standard Atmosphere. The author of more 200 papers, ranging from atmospheric physics to studies of comets and meteors, Jacchia is perhaps best known for his description of Earth's near-miss with disaster. In the summer of 1974, while vacationing in the American Northwest, he watched a giant meteor, bright enough to be seen and photographed in broad daylight, enter the Earth's atmosphere and, like a flat stone skipping over a pond, sail right back into space. Appropriately, the man who as much as any other had helped define the atmosphere was also an eyewitness to how, on that particular day, it saved us from catastrophe. Jacchia never married.
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