James A. Van Allen, a pioneering scientist in space exploration and a physics professor for 35 years at the University of Iowa, died this morning at the age of 91. A news release on the university’s Web site did not state a cause of death.

Mr. Van Allen is most noted for his discovery of bands of intense radiation that encircle the Earth, trapped by its magnetic field. The existence of the bands, later named in his honor, was confirmed by Explorer I and other early satellite missions, in which Mr. Van Allen was involved. He later found similar radiation belts around other planets, and some of his graduate students have continued to play key roles in NASA missions.

He was an outspoken critic of the manned space program and how it drained resources that would otherwise have flowed to robotic missions that he felt were much more cost-effective means of space exploration.