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Zadunaisky's Math Determined Halley's Comet Orbit

Pedro Elias Zadunaisky, an Argentine astronomer and mathematician whose calculations helped determine the orbit of Saturn's outermost moon, Phoebe, as well as Halley's Comet, died Wednesday. He was 91.

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.

Zadunaisky also was a Senior Astronomer at the Smithsonian Astrophysical Observatory and in the 1960s researched the orbits of celestial bodies at NASA's Goddard Space Flight Center, calculating the orbits of the first U.S. Earth satellite, Explorer I, as well as other satellites during the U.S. space race against Russia.

Born in the Argentine city of Rosario on Dec. 10, 1917, Zadunaisky earned a civil engineering degree at the National University of Rosario, then pursued applied mathematics and specialized in celestial mechanics. He earned three Guggenheim fellowships for research at Columbia University in 1957, Princeton University in 1958 and at the University of Texas at Austin in 1977.

Zadunaisky left Argentina in 1966 along with many other professors and scientists after a crackdown on university protests against a military coup known as "the night of the long clubs."

Zadunaisky soon returned, and eventually taught at both the University of Buenos Aires and the National University of La Plata. He also directed astrodynamical investigations at Argentina's National Commission on Space Activity.

In 2000, an asteroid discovered by a team of his former students was named 4617 Zadunaisky in his honor.

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