The idea of putting a Star Trek-like force field around the entire Earth seems like the fodder for a fairly silly science fiction epic out of the 1930s, but according to space scientists, such a barrier already exists. 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.

The curious thing is, the belts are better behaved than originally thought. Instead of drifting down toward Earth's atmosphere, the inner edge of the outer belt has a very sharp edge at 7,200 mi from Earth, leaving a space between the inner and outer belts, and in this this gap there are no ultrafast electrons. This cutoff is so pronounced that scientists studying it compare it to a glass wall that the electrons can't penetrate.

The barrier was discovered using 20 months of data from NASA's pair of Van Allen probes, which were launched in August 2012 and, as the name suggests, work in tandem to study the belts.

A team of scientists led by the University of Colorado Boulder looked through the Van Allen probes' data in hopes of finding the cause of the barrier. After eliminating human activity, such as radio transmissions, and the Earth's magnetic field, they found the most likely candidate in the form of the plasmasphere, which is a cloud of cool charged particles that extends from the upper regions of the Earth's atmosphere from 600 mi (965 km) up to near the edge of the outer Van Allen belt.

According to the team, the ultrafast electrons interact with the plasmasphere and are repelled. Although the electrons' energy is so high that they could easily penetrate the cloud, the electrons instead move around the Earth in huge circles at 100,000 mps (160,000 km/s), grazing the plasmasphere. The allows the plasmasphere to nudge the electrons away in much the same manner as a relatively flimsy traffic barrier deflecting a speeding car back onto the road when the car could have crashed through the barrier if it had hit it dead on. This interaction is called the plasmaspheric “hiss,” because it makes a sound over radio receivers that can be compared to static.

Another piece of evidence for the barrier is when it fails under extreme conditions, such as a massive solar flare, which can erode the plasmasphere and push the electrons through and into the gap. However, such a breach would only last a short time, which scientists say demonstrates the strength of the "hard, fast" barrier.

The team's results were published in the journal Nature.

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