Van Allen radiation belt

• 1. Introduction

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  The Van Allen radiation belt is a zone of charged particles held in place by Earth's own magnetic field, and for most of human history, no one knew it was there. James Van Allen changed that in 1958, when he published the article that first described these invisible shells wrapping around our planet. What he found was not a curiosity. It was a force that would shape the future of satellites, space exploration, and even Cold War military strategy. How does Earth's magnetic field trap particles from the sun and hold them for days, months, or more than a hundred days at a stretch? What happens when a spacecraft flies through? And why did the United States once detonate nuclear bombs inside this zone? Those are the questions this documentary will answer.

• 2. A Zone of Trapped Energy

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  Earth has two main radiation belts, and they stretch from an altitude of roughly 640 to 58,000 kilometers above the surface. Most of the particles that fill them arrive from the solar wind, while others come from cosmic rays striking the upper atmosphere. The magnetic field catches these particles and sets them into a complex motion: they spiral along magnetic field lines, bounce back and forth between the poles, and drift slowly around the planet. Electrons drift eastward; protons drift westward. The inner belt sits from about 1,000 to 12,000 kilometers up, and it holds high concentrations of energetic protons, some with energies exceeding 100 MeV. The outer belt begins at roughly 3 Earth radii and extends to about 10 Earth radii, shaped almost like a donut and reaching its greatest intensity around 4 to 5 Earth radii out. Between them sits a gap, sometimes called the safe zone or safe slot, which is why medium Earth orbits cluster there. The Sun does not have radiation belts of its own, because it lacks a stable, global dipole field of the kind that makes long-term trapping possible.

• 3. Discovery and Early Theory

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  Kristian Birkeland, Carl Størmer, Nicholas Christofilos, and Enrico Medi had already explored the idea of trapped charged particles as far back as 1895, building the theoretical foundation that made the discovery possible. The confirmation came from satellites. The second Soviet satellite, Sputnik 2, carried detectors designed by Sergei Vernov; the American satellites Explorer 1 and Explorer 3 followed in early 1958. Together they confirmed the belt's existence. Explorer 4, Pioneer 3, and Luna 1 then produced the first maps of the trapped radiation. Van Allen, working at the University of Iowa, published the article that named and described the belts that same year. The name stuck, and it has applied specifically to Earth's belts ever since, even as similar structures have been found elsewhere in the solar system.

• 4. Project Argus

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  In 1958, the United States detonated low-yield nuclear bombs at an altitude of 300 miles, deliberately adding electrons to the radiation belts. The tests went by the name Project Argus, and they were designed to evaluate what became known as the Christofilos effect: the idea that nuclear explosions in space could release enough electrons, trapped in Earth's magnetic field, to disable the warheads on intercontinental ballistic missiles. The project was eventually shut down for two reasons. The first was the treaty banning atmospheric testing. The second was a more practical concern: the extra radiation generated by the tests might have blocked the Apollo moon missions. Project Argus is a rare instance where the structure of the radiation belts was not just observed but actively, deliberately altered.

• 5. Radiation, Satellites, and Astronauts

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  Satellites passing through the Van Allen belts face real danger. Solar cells, integrated circuits, and sensors can all be damaged by the radiation. Miniaturization has made the problem worse, because smaller circuits carry a total electric charge now comparable to the charge of incoming ions. The Chandra Space Telescope has its sensors turned off whenever it passes through the belts. The INTEGRAL space telescope was placed into an orbit specifically designed to avoid time inside them. Astronauts traveling to the Moon on the Apollo missions passed through the belts as well. Mission planners minimized exposure by routing spacecraft at high speed through the thinner upper portions of the outer belt, bypassing the inner belt entirely. The one exception was Apollo 14, whose trajectory took it through the heart of the trapped radiation. Proton fluxes inside the belts can reach 100,000 particles per square centimeter per second for protons above 20 MeV. Electrons above 1.5 MeV reach fluxes of up to a million particles per square centimeter per second.

• 6. The Transient Third Belt

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  On the 28th of February 2013, NASA's Van Allen Probe team announced the discovery of a third radiation belt. It consisted of high-energy ultrarelativistic charged particles and was triggered by a coronal mass ejection from the Sun. Unlike the familiar inner and outer belts, this third belt split the outer belt on its outer side and persisted for about four weeks before merging back into the outer belt. Its unusual staying power has been traced to the extreme energy of its particles. They were too energetic to scatter into the atmosphere through interactions with atmospheric waves at low latitudes. Without that scattering mechanism, the particles simply stayed trapped until an external shock wave from the Sun finally destroyed the belt. The Van Allen Probes, which launched on the 30th of August 2012 and operated until their fuel ran out in 2019, made this detection possible.

• 7. Antimatter and Removal

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  In 2011, the Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics experiment, known as PAMELA, confirmed that the Van Allen belts confine antiprotons. Passing through the South Atlantic Anomaly, it detected antiproton levels far above what normal particle decays would predict. The measured energy of those antiprotons ranged from 60 to 750 MeV. Estimates suggest only about 10 micrograms of antiprotons exist across the entire belt. The extreme energy released when antimatter annihilates matter has prompted proposals to collect these antiprotons for spacecraft propulsion, though the concept depends on developing antimatter collectors and containers that do not yet exist. Separate proposals aim at draining the belts entirely. One concept, called HiVOLT, or High Voltage Orbiting Long Tether, was proposed by Russian physicist V. V. Danilov and refined by Robert P. Hoyt and Robert L. Forward. A second approach involves beaming very-low-frequency radio waves from the ground. Draining the belts around other planets has also been raised: before any mission to Europa, which orbits inside Jupiter's own radiation belt, researchers have suggested that clearing the local belt could reduce the hazard to spacecraft and instruments.