Discovered by a pair of NASA space probes, the natural shield protects the Earth and near-Earth satellites from so-called "killer electrons" with a precision that cuts it off like a wall of glass.

The barrier was found within the Van Allen Belts, which are two distinct zones of radiation that are shaped like a pair of distended donuts that grow, shrink, shift, and even split or merge under the impact of radiation from the Sun. Formed by charged particles captured by the Earth's magnetic field, they were discovered by America's first space probe, Explorer I in 1958. The inner belt spans from 400 to 6,000 mi (650 to 9,500 km) above the Earth and the outer belt is between 7,200 and 36,000 mi (12,00 to 58,000 km).

The belts are of more than academic importance. The radiation from the particles are a serious hazard to satellites and astronauts, and scientists pay close attention to the belts' activity because a sudden expansion could end up damaging satellites in low Earth orbit and even pose a hazard to manned space missions.

The most dangerous of these charged particles are the ultrafast, ultrarelativistic, or "killer" electrons. These are moving so fast that they pack a massive punch and damage electronic circuits and living tissue.