January 31, 1958: The U.S. launches its first satellite, sparking discovery of the Van Allen radiation belts

In early 1958, two scientists at the University of Iowa — James Van Allen and George Ludwig, one of Van Allen’s graduate students — were puzzling over an unusual set of results.

They had just received early data from a Geiger sensor aboard Explorer 1, the first U.S. satellite, in orbit around Earth. The sensor was supposed to detect ionizing radiation, but the duo couldn’t make sense of the results. Sometimes the data aligned with predictions, but other times, the instrument seemed to detect nothing at all. Was their apparatus faulty?

The detector was Explorer 1’s primary payload, selected “by virtue of preparedness and good fortune,” Van Allen wrote in a 1990 article for Annual Review of Earth and Planetary Sciences. He was one of few space physicists at the time, well-versed in high-altitude experiments that relied on balloons, sounding rockets, and combined “rockoons” to probe auroras and cosmic rays.

Van Allen had proposed satellite-based cosmic ray experiments in the mid-1950s, but the U.S. didn’t have a program for launching satellites yet. That changed in 1957 during the International Geophysical Year, a worldwide collaborative effort to study the Earth and sun. More than 60 countries participated, including the Soviet Union and the U.S., despite the geopolitical tension of the Cold War.

When the Soviet Union’s satellite Sputnik went into orbit on Oct. 4, 1957, the U.S. was caught off guard and began racing to launch its own satellite. Van Allen was ready.

On Jan. 31, 1958, Explorer 1 became the first U.S. satellite in space.

Onboard was Van Allen’s experimental instrument, consisting of a single Geiger-Müller tube that generated electrical pulses in response to ionizing radiation; a scaling circuit to optimize data collection; and a telemetry system that transmitted data — but only when the satellite was close to one of 16 ground-based receiving stations.

Those gaps in transmission made the puzzling results even more difficult to interpret. Van Allen and Ludwig modified the design for Explorer 3, adding a magnetic tape recorder that could store data for a complete orbit. The satellite launched in March. Again, the sensor gave reasonable radiation event counts for altitudes of 200 or 300 miles, but gave very low or zero readings at altitudes of 500 to 600 miles.

When Carl McIlwain, another graduate student in the group, returned from a scientific expedition and saw the data, he offered an idea — "something that we all knew but had temporarily forgotten,” Van Allen told Scientific American readers in 1959. “A sufficiently high level of radiation can jam the counter and send the apparent rate to zero.”

A lab experiment confirmed that at more 35,000 counts per second, an identical sensor gave a reading of zero. In Van Allen’s words, “We had discovered an enormously high level of radiation, not a lack of it.” Each new data tape supported this interpretation.

“At altitudes below about 400 km (240 mi) we confirmed earlier measurements of cosmic ray intensity,” Van Allen wrote in a 1981 article in Air & Space. But as the satellites swung out to higher altitudes, he said, “we encountered an enormous increase in intensity, hundreds of times what we expected.”

The radiation was so intense, Van Allen realized, that prolonged space travel through the area would devastate humans and scientific instruments. These became key considerations in the planning of future space missions.

Explorer 4, Pioneers 1 and 3, and Sputnik III, all of which launched later in 1958, shed light on the dynamic structure and dimensions of the high-radiation region. Within a few years, the discovery had coalesced into a two-belt system, described by Van Allen in Air & Space as two distinct radiation belts encircling Earth like a donut, with the Earth occupying the center hole.

The inner belt is dominated by energetic protons thought to be produced by interactions between cosmic rays and particles in the Earth’s atmosphere. The more variable outer belt is dominated by lower-energy electrons thought to originate in the sun and magnetic storms.

Although the discovery was largely unexpected, some scientific groundwork had been laid nearly 50 years earlier. Norwegian mathematician and physicist Carl Størmer had demonstrated that the Earth’s magnetic field could trap incoming charged particles — given the right energy and direction — and even worked out their spiraling orbits.

As solar wind, cosmic rays, and geomagnetic storms approach the Earth’s magnetic field, charged particles experience the Lorentz force and begin spiraling around the Earth’s curved magnetic field lines. Because magnetic field strength increases near the poles, some particles slow in the direction parallel to the field line, reverse course, and spiral back toward the other pole. Particles can be trapped in a cycle of spiraling back-and-forth between poles for hours to years, leading to a buildup of ionizing radiation.

Størmer’s theoretical work gave the researchers a strong framework for understanding their experimental observations early on. In May 1958, they reported the existence of the high-radiation region at the APS Spring Meeting in Washington, D.C., positing that charged particles trapped by the Earth’s magnetic field were the source.

The discovery caught the attention of the scientific community and the public, making headlines around the country. In July, President Dwight Eisenhower signed the National Aeronautics and Space Act of 1958, establishing a civilian space agency.

“Space exploration was transformed from being an arcane field with only a handful of participants to an activity of high national visibility,” Van Allen recalled in the Annual Review article. The following year, astrophysicist Thomas Gold coined the term “magnetosphere,” and scientists began writing papers on “magnetosphere physics.”

Over the years, space-based experiments continued uncovering the story of particles trapped in what we now call the Van Allen radiation belts, including many led by Van Allen’s group at the University of Iowa. Radiation belts have also been discovered around other planets.

In 2012, NASA sent two robotic probes into the Van Allen belts to explore the origins of their high-energy charged particles, how they respond to solar variations, and how they evolve in the environment. Among other discoveries, the probes detected a temporary third radiation belt, highlighting the dynamic nature of these systems.

“Space science is not a professional discipline in the usual sense,” Van Allen wrote in the Annual Review article. “Rather, it is a loosely defined mixture of all of these fields plus an exotic and expensive operational style.”