The Van Allen belts are two doughnut-shaped zones around Earth where its magnetic field traps fast charged particles that can damage satellites and harm astronauts.
Earth's magnetic field holds two huge rings of high-energy particles around the planet, shaped like doughnuts. These are the Van Allen radiation belts, named after James Van Allen, who led the design of the instrument that found them and the study of its data.
The United States launched its first satellite, Explorer 1, from Cape Canaveral on 31 January 1958. It carried a Geiger counter. High above Earth the counter recorded far more radiation than scientists expected from cosmic rays — and above about 900 miles (about 1,450 km) the counts stopped altogether. That was the first observation of the radiation belts.
Explorer 1 was America's first satellite, following the Soviet Union's Sputnik 1 and 2. NASA describes the belts' radiation as a kind of space “weather” that affects how well and how reliably technology works in orbit.
Measured above the equator, NASA places the inner belt roughly 1,000–8,000 miles up (about 1,600–12,900 km) and the outer belt roughly 12,000–25,000 miles up (about 19,300–40,200 km).
The belts change. NASA's twin Van Allen Probes, launched on 30 August 2012, found a temporary third belt within days of launch. It lasted about a month and appeared again during strong solar activity. The mission was designed for two years but collected data for almost seven.
A single fast particle can upset small, sensitive instruments and electronics, and the radiation is a risk to unprotected astronauts. Orbit choice matters. Geostationary satellites sit 35,786 km up — inside NASA's outer-belt range — for their whole working lives. At the end of 2024 ISRO had 31 operational satellites there, so they have to be built to cope with that radiation for years.
Compare satellites listed in geostationary orbit with those in low Earth orbit: the geostationary group spends its whole life inside the outer belt's range. Browse tracked satellites
Belt positions are often given in Earth radii (R), measured from Earth's centre. Using the equatorial radius from NASA's Earth Fact Sheet, R = 6,378 km:
So a geostationary satellite sits well inside the outer belt's range. Its speed follows from v = √(GM/r). With GM = 398,600 km³/s², v = √(398,600 / 42,164) = √9.45 ≈ 3.07 km/s, matching ESA's figure of about 3 km/s.
A satellite heading to geostationary orbit usually first enters a transfer orbit that runs from a few hundred kilometres up to near 35,786 km. On every lap it crosses the inner belt and dips into and out of the outer belt, so the time spent in transfer orbit adds to the radiation dose its electronics must survive.
Explorer 1's counter fell silent above about 900 miles (1,448 km) — which, in Earth radii, is r ≈ 7,826 km ≈ 1.23 R, right at the inner belt's lower edge. The counter was not broken; it was overwhelmed.
Explorer 1, launched by the USA on 31 January 1958. Its Geiger counter recorded far more radiation than expected and then stopped counting at high altitude.
The inner belt is closer to Earth and mostly protons, made when cosmic rays hit the atmosphere. The outer belt is farther out and mostly electrons from the Sun trapped by Earth's magnetic field.
At 35,786 km they sit within the outer belt's range for their whole lives, so their electronics are exposed to trapped energetic particles for years.