Iowa City, Iowa ? Physicist James A. Van Allen, a leader in space exploration who discovered the radiation belts surrounding the Earth that now bear his name, died Wednesday. He was 91.

The University of Iowa, where he taught for years, announced the death in a statement on its Web site.

In a career that stretched over more than a half-century, Van Allen designed scientific instruments for dozens of research flights, first with small rockets and balloons, and eventually with space probes that traveled to distant planets and beyond.

Van Allen gained global attention in the late 1950s when instruments he designed and placed aboard the first U.S. satellite, Explorer I, discovered the bands of intense radiation that surround the earth, now known as the Van Allen Belts.

The bands spawned a whole new field of research known as magnetospheric physics, an area of study that now involves more than 1,000 investigators in more than 20 countries.

The discovery also propelled the United States in its space exploration race with the Soviet Union and prompted Time magazine to put Van Allen on the cover of its May 4, 1959, issue.

“He was one of the most important people in the entire area of space science,” said Thomas Zurbuchen, professor of space science and aerospace engineering at the University of Michigan.

Physicist James Van Allen is shown at one of 10 radio-telescope antennas across the globe that make up the Very Long Baseline in this February 1994 file photo in North Liberty, Iowa. Van Allen, a leader in space exploration who discovered the radiation belts surrounding the Earth that now bear his name, died Wednesday. He was 91.

“What he did for our understanding was really a crucial step into bringing humanity into space,” he said. “What he’s done has shown that space is an immensely interesting place.”

The folksy, pipe-smoking scientist, called “Van” by friends, retired from full-time teaching in 1985. But he continued to write, oversee research, counsel students and monitor data gathered by satellites. He worked in a large, cluttered corner office on the seventh floor of the physics and astronomy building that bears his name.

Though he was an early advocate of a concerted national space program, Van Allen was a strong critic of most manned space projects, once dismissing the U.S. proposal for a manned space station “speculative and … poorly founded.”

Explorer 1, which weighed just 31 pounds, was launched Jan. 31, 1958, during an emotional time just after the Sputnik launches by the Soviet Union created new Cold War fears. The instruments that Van Allen developed for the mission were tiny Geiger counters to measure radiation.

The success of the flight created nationwide celebration. Equally exciting for the scientists was the discovery of the radiation belts, a discovery that happened slowly over the next weeks and months as they pieced together data coming from the satellite.

“We had discovered a whole new phenomenon which had not been known or predicted before,” Van Allen said. “We were really on top of the world, professionally speaking.” Later in 1958, another scientist proposed naming the belts for Van Allen.

His later projects included the Pioneer 10 and 11 flights, which studied the radiation belts of Jupiter in 1973 and 1974 and the radiation belts of Saturn in 1979.

In 1987, President Reagan presented Van Allen with the National Medal of Science, the nation’s highest honor for scientific achievement.