What Are the Van Allen Belts? Earth's Trapped Radiation

The Van Allen belts are two doughnut-shaped regions around Earth where the planet's magnetic field holds energetic protons and electrons in place. The inner belt, mostly protons, reaches from roughly 1,000 to 12,000 km above the surface. The outer belt, mostly electrons, reaches from roughly 13,000 to 60,000 km. They were found in 1958 by accident, when a Geiger counter on the first American satellite stopped counting because it was being flooded with particles. Spacecraft cross them routinely, and the dose depends on how fast and where.

The Geiger counter that read zero

On the night of January 31, 1958 (February 1 in Greenwich time), a Juno I rocket lifted Explorer 1 from Cape Canaveral. The satellite was small, built by the Jet Propulsion Laboratory under William Pickering, and its main scientific instrument was a Geiger-Müller tube designed by George Ludwig in James Van Allen's cosmic-ray group at the University of Iowa. The job was to count cosmic rays above the atmosphere.

The orbit was elongated, with a lowest point of 358 km and a highest of 2,550 km. Most of the time the counter reported about 30 counts per second. Sometimes it reported nothing at all. Every one of the zero readings came from altitudes above about 2,000 km over South America, while passes near 500 km looked normal.

A counter that reads zero looks broken. Van Allen's group read it the other way around. A Geiger tube that is hit by far too many particles at once cannot register them separately, so it saturates and goes quiet. The silence meant that at high altitude the instrument was being drowned in radiation, far more than cosmic rays alone could supply. Explorer 3, which carried a tape recorder that Explorer 1 had lacked, supplied the confirming data. Soviet detectors on Sputnik 2, built by Sergei Vernov's group, had flown months earlier and recorded fluctuating counts, but the team attributed them to solar particles and missed the belts.

The belt that Van Allen's team described became one of the best-known results of the International Geophysical Year. Explorer 1 itself kept transmitting until its batteries died on May 23, 1958, having lasted 111 days of a planned 120, and it stayed in orbit until March 31, 1970.

A Geiger counter that read zero was the first evidence that Earth is wrapped in a radiation belt: the instrument had been saturated, not broken.

How a magnetic field traps a charged particle

To understand why particles pile up in a doughnut, start with what a magnetic field does to something carrying a charge. It does not speed the particle up or slow it down. It bends its path sideways. A proton or electron in a magnetic field therefore corkscrews around the field line, spiraling like a bead threaded onto an invisible wire. This is called gyration.

Earth's field behaves roughly like a bar magnet, with lines that leave one hemisphere, arch out over the equator, and re-enter the other hemisphere. Near the equator the field is weaker; near the poles, where the lines crowd together, it is stronger. That difference is what produces the trap.

The bounce

As a spiraling particle travels toward a pole, the field strengthens. The particle's forward motion along the field line slows, stops, and then reverses, sending it back toward the equator. It has hit a magnetic mirror. When it reaches the opposite hemisphere it does the same thing there. The result is a particle bouncing back and forth between hemispheres, with each reversal point called a mirror point.

An analogy that helps: imagine a marble rolling inside a long trough whose ends curve upward. The marble rolls down, climbs the far end, runs out of speed, and rolls back. The magnetic mirror works in an analogous way, except that the "hill" is a strengthening magnetic field and the marble is a charged particle.

The drift

There is a third motion. Because the field is stronger closer to Earth, the spiral is slightly tighter on the Earth-ward side of each loop, and the particle gradually slides sideways around the planet. Electrons drift eastward and protons drift westward. Over time a particle that began at a single point ends up spread into a ring, and together the rings fill a doughnut-shaped region around the planet.

Put together, gyration, bounce, and drift keep a particle captive for as long as nothing disturbs it. Three nested motions, each with its own pace, trace out a surface around Earth like a thread wound on a spool. A particle escapes only when something breaks the pattern: a collision with the atmosphere, a gust from the solar wind, or a wave that nudges it out of its path.

Two belts and a gap between them

The classic picture is two belts. They differ in their particles, their stability, and their behavior, and it helps to see the differences side by side.

Property	Inner belt	Outer belt
Altitude (approximate)	About 1,000 to 12,000 km	About 13,000 to 60,000 km
Main particles	Protons above 100 MeV, plus electrons of hundreds of keV	Mostly electrons of 0.1 to 10 MeV
Stability	Fairly stable over years	Highly variable, changes within a day
Peak intensity	Close to Earth	Around 4 to 5 Earth radii
Main source	Decay of neutrons made by cosmic rays hitting the atmosphere	Particles from the solar wind and magnetotail, energized by waves

Altitude ranges are approximate. A NASA gallery page gives the inner belt as roughly 1,000 to 8,000 miles above the equator and the outer belt as roughly 12,000 to 25,000 miles, so exact boundaries depend on the definition.

MeV stands for million electron-volts, a unit of particle energy. A proton at 100 MeV carries far more energy than one in the solar wind, which is why belt protons are hard to shield against.

The slot

Between the two belts sits a relatively empty gap, called the slot or sometimes the safe zone, at roughly 2 to 4 Earth radii. Electrons there do not last. Their lifetime is about one or two days, compared with more than 100 days in the inner belt. The gap exists because very low frequency radio waves scatter particles out of their orbits and into the atmosphere.

Why the belts are not centered on Earth

Earth's magnetic axis is tilted and offset from the planet's center. The result is that the inner belt dips closest to the surface over the South Atlantic, a region called the South Atlantic Anomaly. There, the inner boundary can drop to roughly 200 km, low enough that satellites and the International Space Station, which orbit in that range, pass through thin parts of the belt on every orbit. The ISS explained covers the station's orbit and how crews manage radiation.

Where the particles come from

The two belts are fed by different processes, which explains why they behave differently.

The inner belt's protons come largely from a nuclear process high in the atmosphere. Cosmic rays, energetic particles from outside the solar system, slam into atmospheric nuclei and knock out neutrons. Neutrons have no charge, so the magnetic field cannot hold them, and some fly outward into the belt region. A free neutron is unstable and decays into a proton, an electron, and a neutrino within minutes. If that happens inside the belt, the new proton is trapped. This is a slow, steady supply, which is why the inner belt changes little from year to year. Lower-energy protons are thought to diffuse inward during geomagnetic storms.

The outer belt is far more restless. Electrons are injected from the magnetic tail of the magnetosphere after geomagnetic storms and then boosted to higher energies by plasma waves. They are lost to the atmosphere, to the magnetopause where the field meets the solar wind, and by diffusing outward. The balance of gain and loss can swing the outer belt's intensity up or down dramatically in a day.

The solar wind is the ultimate source of most trapped particles. For the connection between the belts and the glow overhead at high latitudes, see aurora and the northern lights, where the same magnetosphere dumps particles into the upper atmosphere.

Starfish Prime and the belts people built

The belts are natural, but humans have added to them. In 1958, the United States ran Project Argus, a set of high-altitude nuclear detonations designed to test an idea from physicist Nicholas Christofilos: that electrons released by an explosion would be captured by the magnetic field and spread into a shell around the planet. The experiments temporarily raised the number of electrons in the belts.

The larger event came on July 9, 1962. A thermonuclear warhead of about 1.4 megatons, launched from Johnston Atoll in the Pacific, detonated at roughly 400 km altitude. The explosion, named Starfish Prime, released a flood of electrons into the geomagnetic field. By one estimate the number was on the order of 10²⁹. These electrons raised the intensity of the inner belt by several orders of magnitude.

The consequences reached the sky and the ground. About 1,450 km away in Hawaii, the electromagnetic pulse knocked out roughly 300 streetlights, set off burglar alarms, and damaged a telephone microwave link that carried service from Kauai to the other islands. In the following months, six or more satellites failed. Telstar 1 and Ariel 1 were among those disabled, as were TRAAC and Transit 4B, and the cause was the artificial radiation that the explosion had added.

Event	Date	What it showed
Explorer 1	January 31, 1958 (US time)	Counter saturation revealed trapped radiation
Project Argus	1958	Explosion electrons could be trapped and spread by the field
Starfish Prime	July 9, 1962	One 1.4-megaton burst intensified the inner belt by orders of magnitude and disabled several satellites
Van Allen Probes launch	August 30, 2012	Two spacecraft sent to sample the belts directly

The belts proved vulnerable to human interference, and the effects lasted months to years.

The energetic electrons had a half-life of only a few days, but a later report said some remained in the belts for about five years. Nobody would repeat the experiment today, and nuclear testing in space was banned by treaty the following year. The episode did leave behind a useful accidental experiment, since it showed how long particles persist in the trap.

What the Van Allen Probes learned

For decades the belts were studied by passing spacecraft, mostly one at a time, which made it hard to tell whether a change in particle counts was a change in time or in place. NASA's answer was a pair of identical spacecraft. The Van Allen Probes, built and operated by the Johns Hopkins Applied Physics Laboratory, launched together on an Atlas V on August 30, 2012. They flew in a highly elliptical orbit with a lowest point of 618 km and a highest of 30,414 km, which carried them through both belts on every orbit. Each carried five instrument suites covering particles, waves, and fields.

The mission was planned for two years and ran for about seven. Probe B was deactivated on July 19, 2019, and Probe A on October 18, 2019. Both ran out of propellant, which they needed to keep their solar panels pointed at the Sun. Radiation damage did not end the mission, even though the probes spent their lives inside the thing they were studying.

The third belt

Months after launch, the probes found something the two-belt picture did not predict. Baker and colleagues reported in Science in 2013 that a distinct, third ring of electrons with energies above 2 MeV had formed on September 2, 2012. It sat between about 3.0 and 3.5 Earth radii and persisted largely unchanged for more than four weeks. Then a powerful interplanetary shock wave arrived and virtually annihilated it. The textbook figure of two belts had been a long-term average, and the system was more dynamic than that.

An invisible wall

A later study, led by Baker and published in Nature in 2014, found a sharp inner edge to the outer belt. Electrons with energies above about 5 MeV cannot penetrate below it, even though the region beyond the edge is full of them. The authors found that it is not a physical boundary in Earth's magnetic field, and proposed that very slow inward diffusion combined with weak, persistent wave scattering deep inside the plasmasphere creates the barrier. It matters for spacecraft design, because the boundary is sharp enough that orbit choice relative to it changes how many of the most energetic electrons a satellite meets.

A new belt after a superstorm

The belts continue to surprise. After the powerful solar storms of May 2024, NASA's CIRBE CubeSat (the Colorado Inner Radiation Belt Experiment) found evidence of new belts. According to Li and colleagues in the Journal of Geophysical Research: Space Physics (2025), an electron belt with energies of 1.3 to 5 MeV had formed at L of roughly 2.5 to 3.5, a region normally free of relativistic electrons, and a distinct belt of 6.8 to 20 MeV protons formed near L = 2. The electron belt stayed intact until a magnetic storm on June 28, 2024, and the proton belt appeared even more stable.

These events happened near the peak of solar cycle 25, a period covered in Solar Maximum 2025 explained. The Sun's activity does not just light up the sky. It reshapes the radiation environment of near-Earth space.

Crossing the belts without cooking yourself

The belts raise an obvious question for anyone planning to leave low Earth orbit: how dangerous are they to cross? The answer depends on speed, trajectory, and shielding, and the record offers real data.

Apollo

Apollo flights left Earth orbit and passed through the belts on their way to the Moon. NASA's account in Working on the Moon reports crew-averaged skin doses for the nine lunar missions, based on Bailey's figures in NASA SP-368. They ranged from 0.16 rad on Apollo 8 to 1.14 rad on Apollo 14, and the average across the nine missions was 0.46 rad. One rad is 10 milligray, so these correspond to about 1.6, 11.4, and 4.6 mGy.

NASA's account says the majority of each crew's dose came from passage through the belts. Outside them, the crews were exposed mainly to galactic cosmic rays, at rates of about 1 to 3 millirad per hour on the way to and from the Moon. Apollo 14 had the largest dose in Bailey's figures, which NASA attributes to high doses during belt passages. NASA's own mission report gives lower numbers for that flight than Bailey's total, a discrepancy the NASA history page itself notes.

In every case, the crews were beyond the belts within about ten hours of launch, and did not re-enter them until the last ten hours before splashdown. The lesson was that the path mattered. A fast, carefully chosen trajectory crossed the belts in hours, and a lingering orbit within them would have been much worse. The mission side of this story, including what the crews brought back, is in the Apollo Program's science legacy.

Apollo mission	Crew-averaged skin dose (rad)	Equivalent (mGy)
Apollo 8 (lowest)	0.16	1.6
Average of nine lunar missions	0.46	4.6
Apollo 14 (highest)	1.14	11.4

Source: NASA, Working on the Moon, citing Bailey. The Apollo 14 mission report gives lower values; see text.

Artemis I

The next generation of crewed Moon missions can be measured with better instruments. Artemis I, an uncrewed Orion flight that launched on November 16, 2022, lasted over 25 days and reached a maximum distance of 434,522 km from Earth. Two mannequin torsos, Helga and Zohar, carried radiation sensors through the flight. Zohar wore an AstroRad protective vest and Helga did not. The mission produced the first continuous measurements of radiation between Earth and the Moon.

The German Aerospace Center (DLR) reported the early findings, which were published in Nature. During the crossing of the inner belt, the dose rate differed by a factor of four between the most and the least shielded areas of the capsule. A 90-degree turn of the spacecraft at the end of the belt crossing cut the radiation dose inside the capsule by about 50 percent. A more heavily shielded storm shelter could limit the dose from a large solar particle event to about 150 millisieverts, which is below the level where acute radiation sickness is expected. The researchers concluded that exposure on future Artemis missions lasting days to weeks is unlikely to exceed NASA's current astronaut limits, assuming similar conditions. The programme itself is outlined in the Artemis Program explained.

Satellites

Hardware is at risk too. Radiation can damage solar cells, integrated circuits, and sensors. Miniaturized digital electronics are more vulnerable because the charge they use to store a bit can be comparable to the charge of an incoming ion, so a single particle can flip a bit. Spacecraft that spend significant time inside the belts need radiation-hardened electronics. Some missions adapt in other ways: the Chandra X-ray Observatory turns off its sensors when it crosses the belts, and the INTEGRAL observatory's orbit was designed to avoid them. How satellites choose their orbits, belts included, is covered in how satellites orbit Earth.

Things people get wrong about the belts

"The belts are a shield." The nickname "Earth's radiation shield" appears often, and half of it is right. The magnetic field that creates the belts does help deflect the solar wind and protect the atmosphere. But the belts themselves are not the shield. They are the radiation that has been trapped. A shield would be on the outside. The belts are the stuff that got caught.

"Astronauts cannot pass through them." They can, and have. The belts are a hazard to be minimized, not a wall. Apollo crews crossed them within hours and received modest doses, and Artemis I measured the crossing directly. The more serious radiation concerns for deep-space travel come from solar particle events and long exposure to galactic cosmic rays, a theme that comes up in how astronauts survive in space.

"The belts are two fixed doughnuts." They are not. The outer belt can change by large factors within a day, the Van Allen Probes saw a third belt appear and vanish, and storms in 2024 built new ones. The picture of two stable rings is an average.

"The belts exist everywhere." Any planet with a strong enough magnetic field and a supply of charged particles can have them, and Jupiter's are far more intense than Earth's. A planet without a global magnetic field has no stable belts. That difference ties into why Mars looks the way it does, which is the subject of why Mars lost most of its atmosphere.

"Starfish Prime made the belts." It made an extra, artificial population of electrons. The natural belts were discovered four years earlier by Explorer 1.

Why the belts matter beyond astronauts

The belts are not only a hazard map. They are one of the few places where physicists can watch plasma physics at work in a natural laboratory. The waves that scatter particles into the atmosphere, the injections from the tail, and the sharp edge at 5 MeV all test models that also apply to fusion experiments, other planets, and distant stars.

They also drive practical forecasting. Satellite operators care about the outer belt because its electrons can penetrate spacecraft surfaces and build up charge that discharges damagingly. Space weather forecasters watch the belts the way terrestrial forecasters watch a river, looking for a rise after a storm upstream. And mission planners use the belt maps to choose orbits. A satellite that must linger at a given altitude may pick one that avoids the worst belt regions, while another may simply be built tough enough to ride through.

The Sun's own environment is a good companion topic. Parker Solar Probe, touching the Sun, is the other side of the same story: a spacecraft built to survive the source of the particles that, after a long trip, end up trapped around Earth.

FAQ

Q: What are the Van Allen belts made of? A: They are made of charged particles, mostly protons and electrons, trapped by Earth's magnetic field. The inner belt holds protons above 100 MeV and electrons of hundreds of keV. The outer belt holds mostly electrons of 0.1 to 10 MeV. Most of the particles originate in the solar wind, though the inner belt's protons come largely from neutron decay.

Q: Who discovered the Van Allen belts? A: James Van Allen's group at the University of Iowa, using a Geiger counter on Explorer 1, launched on January 31, 1958 (US time). Explorer 3 confirmed the finding. Soviet detectors on Sputnik 2, designed by Sergei Vernov, had registered the radiation in 1957, but the team misread it as solar particles.

Q: How far above Earth are the Van Allen belts? A: The inner belt extends from roughly 1,000 to 12,000 km above the surface, and the outer belt from roughly 13,000 to 60,000 km. Over the South Atlantic Anomaly the inner belt dips as low as about 200 km. Different sources define the edges slightly differently.

Q: Can astronauts safely cross the Van Allen belts? A: Yes, when the crossing is quick. Apollo crews were beyond the belts within about ten hours of launch, and their crew-averaged skin doses ranged from 0.16 to 1.14 rad. Artemis I measured the crossing and found that spacecraft orientation and shielding changed the dose significantly.

Q: Did Starfish Prime create a new Van Allen belt? A: It created an artificial population of trapped electrons that intensified the inner belt by several orders of magnitude, which disabled several satellites, including Telstar 1. Most of the electrons decayed within days, though a later report said some persisted for about five years.

Q: Do other planets have radiation belts? A: Planets with strong global magnetic fields, such as Jupiter, do. Jupiter's belts are far more intense than Earth's. A planet without a global field has no stable trapping region, which is one reason Mars, with only patchy crustal fields, lacks the belts Earth has.

The takeaway

The Van Allen belts turned out to be what happens when a magnetic field meets a stream of charged particles: the field catches some, and they bounce and drift in patterns that last from days to years. Explorer 1 found them because a counter went silent, and decades of spacecraft since have shown that the picture of two steady rings is only an average. The belts swell, split, and sometimes grow a third ring. They are a manageable hazard for crews who pass through quickly and a genuine engineering problem for satellites that stay. What makes them worth understanding is that they are a working example of a plasma physics system, one that people have already disturbed once with a bomb and now monitor with a fleet of instruments. For the surrounding topics, the complete space guide is the best starting map.

Sources

• Explorer 1, Wikipedia: launch, orbit, instrument, the zero-count anomaly, Van Allen's interpretation, end of mission.
• Van Allen radiation belt, Wikipedia: altitudes, particle energies, trapping physics, slot, satellite effects, Project Argus.
• NASA, Van Allen radiation belts image gallery (February 28, 2013): two donut-shaped regions and mile-based altitude ranges.
• Van Allen Probes, Wikipedia: launch, orbit, instruments, end of mission.
• Baker et al. 2013, A Long-Lived Relativistic Electron Storage Ring Embedded in Earth's Outer Van Allen Belt (UNH repository): the third belt, September 2, 2012.
• Starfish Prime, Wikipedia: yield, altitude, satellites lost, Hawaii effects.
• NASA, Working on the Moon: Radiation: Apollo crew skin doses and belt-passage timing.
• DLR, Orion radiation protection tested: initial findings from Artemis I: Artemis I dose results.
• Li et al., Journal of Geophysical Research: Space Physics (NSF PAR record): CIRBE observations of the new electron and proton belts after the May 2024 storm.
• Baker et al. 2014, An impenetrable barrier to ultrarelativistic electrons in the Van Allen radiation belts, Nature 515, 531: the sharp inner edge of the outer belt.