Radiation Environment Notice: The following post contains accurate descriptions of Earth’s magnetic field doing an enormous amount of unpaid work. Readers are advised that the charged particle environment beyond low Earth orbit has not been adjusted for comfort, convenience, or the scheduling requirements of crewed spaceflight.

A Discovery Nobody Was Specifically Looking For

On the 31st of January 1958, the United States launched Explorer 1 — its first successful satellite, a pencil-shaped cylinder the size of a large thermos flask — into low Earth orbit. On board was a Geiger counter, designed by physicist James Van Allen of the University of Iowa, intended to measure cosmic radiation in space.

The instrument worked. Then it stopped reporting. Then it worked again. Then it stopped.

This was not a malfunction. The Geiger counter was so saturated with radiation that it was registering nothing at all — the particle equivalent of a microphone placed directly inside a jet engine. Van Allen correctly interpreted the silence as the loudest possible signal: there was so much radiation up there that the instrument had simply given up.

What Explorer 1 had stumbled into were the Van Allen radiation belts — two vast, doughnut-shaped regions of trapped charged particles, held in place by Earth’s own magnetic field, circling the planet at altitude, present the entire time, and entirely undocumented until a thermos flask full of instruments found them by accident.

Earth had been running this radiation infrastructure for approximately four billion years without informing anyone. Van Allen filed the first incident report in 1958. The planet has not responded to it.

What the Doughnuts Actually Are

The belts exist because of Earth’s magnetosphere — the region of space dominated by our planet’s magnetic field, generated by the movement of molten iron in the outer core. This field deflects the continuous stream of charged particles flowing outward from the Sun, and in doing so, funnels some of them into stable, looping orbits around the planet.

The result is two distinct zones:

The inner belt sits roughly 700 to 6,000 kilometres above Earth’s surface. It contains primarily high-energy protons — the kind that can penetrate spacecraft walls and damage electronics. It is relatively stable and persistent.

The outer belt extends from roughly 13,000 to 60,000 kilometres out and is composed mainly of high-energy electrons. It is considerably more dynamic — expanding and contracting in response to solar activity, sometimes dramatically. During solar storms, the outer belt can swell to several times its usual size in a matter of hours.

Between them sits a relatively calmer gap, sometimes called the safe zone — which is the kind of name that sounds reassuring until you look at the actual particle counts and reconsider your definition of safe.

The outer belt can swell to several times its normal size during solar storms. This is not an upgrade. It is the opposite of an upgrade.

The South Atlantic Anomaly: Earth’s Weak Spot

The planet’s magnetic field is not — as one might hope — uniformly distributed. Over the South Atlantic Ocean and part of South America, the inner Van Allen belt dips unusually close to Earth’s surface, to altitudes as low as 200 kilometres. This region is called the South Atlantic Anomaly, and it is where satellites and spacecraft experience dramatically elevated radiation even in low Earth orbit.

The International Space Station passes through it on every orbit. Astronauts report occasional light flashes — radiation interacting directly with the optic nerve — as the station crosses the anomaly. Electronics on board are more likely to glitch. NASA monitors the region continuously.

The anomaly exists because Earth’s magnetic poles are not perfectly aligned with the geographic poles, and because the magnetic field itself is slowly drifting and weakening over geological timescales. Over the past two centuries, the field has weakened by roughly nine percent. The anomaly is growing.

The South Atlantic Anomaly is, in the most technical sense available, a known issue. A patch has not been scheduled.

The Apollo Problem Nobody Talks About Enough

To reach the Moon, you must pass through the Van Allen belts. Twice. This was not a minor consideration for the Apollo programme — mission trajectories were specifically designed to transit the belts as quickly as possible, through the thinner regions at higher latitudes, minimising exposure time.

Apollo astronauts received estimated total mission doses of between roughly 2 and 11 millisieverts — low enough to be considered acceptable, high enough to confirm that the belts were doing exactly what the instruments suggested. Several astronauts reported the same phenomenon ISS crews later documented: light flashes during the transit, the visual system registering particles that the eye itself had not technically seen.

The Artemis programme faces the same transit problem. Artemis I carried dedicated radiation instruments through the belts in 2022. Artemis II will carry humans through them for the first time since Apollo 17 in 1972, with real-time dosimetry running throughout.

Why This Matters More Than It Used To

For missions staying in low Earth orbit — the Space Station, most commercial flights — the belts are a background consideration, manageable and well-characterised. For missions heading beyond them, the calculus changes considerably.

On the lunar surface, there are no belts to worry about. There is also no global magnetic field and no meaningful atmosphere. The protection the belts represent is absent, and in its place is the full, unmediated radiation environment of deep space — galactic cosmic rays arriving from every direction, solar particle events with hours of warning if you’re fortunate, and a statistical accumulation that becomes a mission-limiting factor on any stay measured in weeks rather than days.

The belts, in other words, are both a hazard in transit and a reminder of what their absence looks like.

Earth’s magnetosphere deflects roughly the same amount of solar wind energy every second as a small nuclear power station produces. It asks for nothing. It files no invoices. It has been doing this for approximately four billion years and has not once requested a performance review.

Further Reading

The full picture — galactic cosmic rays, solar particle events, what we’re doing about all of it aboard the ISS and the Artemis programme, and why the most advanced radiation shielding strategy currently available is, in certain respects, a cave — is explored in “Is Space Trying to Kill Us? (Radiation)”, Season 3, Episode 26 of The Multiverse Employee Handbook.

James Van Allen’s Geiger counter found the answer by going silent. The universe, as ever, communicates primarily through what it declines to say.

🎧 Listen to Season 3, Episode 26