Physicist James A. Van Allen and scientists waited tensely for confirmation that the first U.S. satellite, Explorer I, had reached orbit. It was January 1958 and the launch came just months after the Soviets' launch of the first Sputnik satellite.
Van Allen, who died Wednesday at age 91, had designed instruments on board Explorer I that would discover belts of radiation now known as the Van Allen Belts. His experiments taught scientists to look at space not as a vacuum but as a place pulsating with energy, waiting to be explored.

When the signal from Explorer I finally came, "it was exhilarating," Van Allen told The Associated Press in an interview 35 years later. "We were really on top of the world, professionally speaking."

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.

The discovery of the belts spawned a whole new field of research known as magnetospheric physics.

"Many of my generation really went into space science because of that discovery," said Edward Stone, professor of physics at the California Institute of Technology. "It revealed a whole new area of science that was just waiting to be discovered."

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.

"I love to work and I love this subject," he said in 1993. As for quitting, he said, "not as long as I'm able I won't."

Van Allen was born Sept. 7, 1914, in Mount Pleasant, Iowa. As an undergraduate at Iowa Wesleyan College, he helped prepare research instruments for one of Admiral Richard E. Byrd's Antarctic expeditions. He got his master's and Ph.D. from the University of Iowa.

After serving in the Naval Reserve during World War II, he was a researcher at Johns Hopkins University Applied Physics Laboratory in Laurel, Md., supervising tests of captured German V-2 rockets and developing similar rockets to probe the upper atmosphere.

One of the highlights of this early research was the 1953 discovery of electrons believed to be the driving force behind the northern and southern lights.

His projects included the Pioneer 10 and 11 flights, which studied Jupiter's radiation belts in 1973 and 1974 and Saturn's radiation belts in 1979. He continued to monitor data from the Pioneer 10 for decades as it became the most remote manmade object, billions of miles away.

Van Allen was named to the National Academy of Sciences in 1959. In 1987, he received the National Medal of Science, the nation's highest honor for scientific achievement.

Two years later, he received the Crafoord Prize, awarded by the Royal Swedish Academy of Sciences in Stockholm each year since 1982 for scientific research in areas not recognized by the Nobel Prizes.