Pedro E. Zadunaisky | Noted astronomer, 91

Pedro E. Zadunaisky, 91, an Argentine astronomer and mathematician whose calculations helped determine the orbit of Saturn's outermost moon as well as Halley's comet, died Wednesday in Buenos Aires.

Mr. Zadunaisky was a pioneer in celestial mechanics, applying mathematical models to determine how gravity and other forces alter the orbits of other objects in the solar system. He also was a senior astronomer at the Smithsonian Astrophysical Observatory. In the 1960s, he researched the orbits of celestial bodies at NASA's Goddard Space Flight Center, calculating the orbits of the first U.S. Earth satellite as well as other satellites during the U.S. space race against the Soviet Union.

His research was published in more than 40 publications worldwide, including the textbook A Guide to Celestial Mechanics, edited by the Harvard Smithsonian Center for Astrophysics.

- AP