updated 6/11/2007 5:56:01 PM ET 2007-06-11T21:56:01

Homer J. Stewart, an early pioneer of rocket research who helped develop the first successful U.S. satellite, Explorer I, has died. He was 91.

Stewart, an emeritus professor of aeronautics at the California Institute of Technology, died May 26 at his home in Altadena, the university said in a statement.

Stewart came to Caltech in 1936, but in the late 1950s took a leave of absence to advise on the preparation of Explorer I.

Following the launch of the Soviet satellite Sputnik I in October 1957, there was a frenzied effort by the United States to launch a satellite of its own.

Explorer I was sent into orbit in January 1958, upping the ante in the space race, played out against the backdrop of the Cold War, in which the Americans and Soviets engaged in a decades-long quest to achieve supremacy in space.

In that year, Stewart became director of planning and evaluation for the National Aeronautics and Space Administration.

He later joined Wernher von Braun, the German rocket pioneer, in Washington to testify before a Senate panel about the lagging performance of the United States in aerospace and missile development.

The panel was told that Russian missile guidance system had become accurate enough to hit an American city from 5,000 miles away and Russian space and missile technologies were a full 12 to 20 months ahead of their American counterparts.

The senators urged stepping up the pace of the "national approach" to defense and aerospace technology.

Stewart also helped in preparations for the 1959 lunar flyby mission of Pioneer IV, the first U.S. space probe to escape Earth's gravity, and he also made recommendations for planning what would become the Apollo missions to the moon.

A native of Dubuque, Iowa, Stewart received his bachelor's degree at the University of Minnesota in 1936 and a doctorate in aeronautics at Caltech in 1940. He was an early researcher at the Jet Propulsion Laboratory, the federally financed aerospace research facility managed by Caltech. He retired from the university in 1980.

Stewart is survived by two daughters, a son and two grandchildren.

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