JAMES VAN ALLEN was a path-breaking astrophysicist whose pioneering contributions in the late 1950s to the space age made him a household name in the US and beyond.

He became famous for his discovery that the Earth is surrounded by intense regions of energetic particles — regions that have since become known as the Van Allen radiation belts. The discovery spawned the entirely new field of scientific research known as magnetospheric physics.

Van Allen made the momentous discovery by installing tiny Geiger counters among the instruments he put on board Explorer I, the first US Earth satellite sent into space as part of the American programme for the International Geophysical Year 1957-58.

The Explorer I satellite, which weighed only 14kg, was launched from Cape Canaveral (now Cape Kennedy) in Florida on January 31, 1958, shortly after the first Sputnik launches by the Soviet Union. The satellite was taken into space by the Jupiter C rocket, a direct descendant of the A4 (V2) rocket that the Germans used to bombard London during the war.

The scientific instruments carried by Explorer I were designed and built by Van Allen, and it was his Geiger counter that provided the first direct evidence that intense regions of energetic particles surround the Earth. The Geiger counters were able to detect and measure particles or other radiation passing into it.

The Van Allen radiation belt is a torus of energetic charged particles which are trapped by the Earth’s magnetic field. Theories about a radiation belt, which predate the space age, were confirmed by Explorer I, and the regions of trapped radiation were mapped out by the Explorer IV and Pioneer III space missions. This further research showed that the Van Allen belt in fact consists of two belts around the Earth, an inner radiation belt and a bigger outer radiation belt. The inner one mainly contains protons while the outer mainly contains electrons.

The big outer belt extends from an altitude of between about 6,250 miles and 40,625 miles and has its greatest intensity between 9,062 miles and 11,875 miles above the Earth.

There is debate about which craft first discovered the outer belt — the US Explorer I or the Soviet Sputnik II. Similar radiation belts exist around other planets, but only the ones surrounding the Earth are called Van Allen belts.

It is thought that the Van Allen belts are created by the collision of the Earth’s magnetic field with the solar wind (a continuous outward flow of particles, mainly protons and electrons, from the Sun into interplanetary space). Particles from the solar wind then become trapped within the Earth’s magnetic field.

Electronic circuits, solar cells and sensors can be damaged by the radiation in the Van Allen belts. Electronics carried on satellites must, therefore, be hardened against radiation damage to operate reliably. Exposure of astronauts to the radiation in the belts could damage their health. However, the Apollo astronauts who travelled to the Moon spent so little time in the belts that any radiation they received was harmless.

Nasa deliberately minimised the amount of radiation they received by timing Apollo launches and using lunar transfer orbits that only skirted the edge of the belt over the Equator.

James Alfred Van Allen was born in 1914 at Mount Pleasant, Iowa, to Alfred Morris, a lawyer, and Alma Olney Van Allen. Between 1918 and 1927 he went to public elementary schools in Mount Pleasant, and between 1927 and 1931 to the public high school, Mount Pleasant. In 1931 he enrolled at the Iowa Wesleyan College, Mount Pleasant, Iowa, receiving his BSc in physics in 1935. He then went to the University of Iowa where he received his MSc in physics 1936 and his PhD in physics in 1939.

Between 1939 and 1942 Van Allen did research at the Carnegie Institution, Washington, and at the Applied Physics Laboratory at Johns Hopkins University, Baltimore, working to develop a rugged vacuum tube (valve) and proximity fuses for weapons used in the war, particularly anti-aircraft projectiles used by the US Navy.

In the autumn of 1942 he was commissioned as an ordinance and gunnery officer and combat observer officer in the US Navy and posted to the Pacific to field-test and complete operational requirements for the proximity fuses.

After the war he returned to the Applied Physics Laboratory and worked on a variety of high-altitude experiments, pioneering the use of V2 and Aerobee rockets for this work. The Aerobee was a small rocket, 8m long, used for high atmospheric and cosmic radiation research.

In 1951 he was appointed professor and head of the Department of Physics (later the Department of Physics and Astronomy) at the University of Iowa, a post he held until his retirement in 1985.

He then became Carver Professor of Physics Emeritus at the University of Iowa, and continued his research in the areas of space and magnetospheric physics.

Between 1949 and 1962 he was the leader of a number of scientific expeditions to study cosmic rays and the Earth’s magnetic field, using American ships, in the Central Pacific, the Gulf of Alaska, the Arctic, the Atlantic, Central Pacific, South Pacific and Antarctic areas.

He pioneered the use of balloons, Aerobee rockets and the combination of the two, for the measurement of the intensity of cosmic rays at high altitudes. It was this work that led to his involvement in Explorer 1 and the discovery of the belts that bear his name.

Van Allen had been a staunch advocate of unmanned space exploration since the space age began. After Explorer 1, he continued to explore planetary physics throughout the solar system. Van Allen was the principal scientific investigator in a total of 24 space missions, including flights to Mars, Venus, Jupiter and Saturn.

His discoveries in space prompted Time magazine to put him on its cover on May 4, 1959.

Van Allen published more than 280 research papers in scientific journals and research monographs. He edited the book Scientific Uses of Earth Satellites (1958); co-authored Pioneer — First to Jupiter, Saturn and Beyond (1980); wrote Origins of Magnetospheric Physics (1983); wrote 924 Elementary Problems and Answers in Solar System (1993); and edited Cosmic Rays, The Sun and Geomagnetism: The Works of Scott E. Forbush (1993).

He served on many important panels, commissions and advisory boards. He received numerous awards, among them: the Crafoord Prize of the Royal Swedish Academy of Science; the Commander of the Order du Mérite pour la Recherche et L’Invention; the David and Florence Guggenheim International Astronautics Award; the US Navy Distinguished Public Service Award; the Distinguished Civilian Service medal (US Army); the Nasa Medal for Exceptional Scientific Achievement; the Gold Medal of the Royal Astronomical Society; the US National Science Board Vannevar Bush Award; and the US National Medal of Science. Thirteen universities awarded him honorary doctor of science degrees.

Van Allen was an excellent teacher of undergraduates and an inspiring supervisor of graduate research students doing advanced degrees in physics and astronomy at the University of Iowa. He supervised about 80 of them, and many are now leaders in the community of space physics.

He is survived by his wife, Abigail, and their three daughters and two sons.

Professor James Van Allen, astrophysicist, was born on September 7, 1914. He died on August 9, 2006, aged 91.