James A. Van Allen, the physicist who made the first major scientific discovery of the early space age, the Earth-circling radiation belts that bear his name, and sent spacecraft instruments to observe the outer reaches of the solar system, died yesterday in Iowa City. He was 91.

The cause was heart failure, family members said. Dr. Van Allen was a longtime professor of physics and astronomy at the University of Iowa, and, with the discovery of the Van Allen belts of intense radiation surrounding Earth, he became a leading figure in the new field of magnetospheric physics, which grew in importance as spacecraft began exploring the planets.

A legendary lecturer and an inspiration to several generations of budding physicists and astronomers, Dr. Van Allen continued to show up at his office-laboratory until a month or so before he died.