NASA's Van Allen Probes have made a surprising and unusual discovery. They've found a particle accelerator in the heart of Earth's radiation belts, speeding up tiny particles to more than 99 percent the speed of light.

The radiation belts around Earth were first discovered when the U.S. launched its very first satellites into space. That's when scientists quickly realized how dangerous these belts were. In fact, most satellite orbits have to duck below the radiation belts or circle outside of them in order to keep instruments from being exposed to dangerous radiation.

"Until the 1990s, we thought that the Van Allen belts were pretty well-behaved and changed slowly," said Geoff Reeves, the first author of the new paper, in a news release. "With more and more measurements, however, we realized how quickly and unpredictably the radiation belts changed. They are basically never in equilibrium, but in a constant state of change."

Scientists always knew there was something in space responsible for speeding up the particles in these radiation belts. Until now, though, they weren't sure what that was. In order to understand these belts a bit better, the researchers developed the Van Allen Probes to fly straight through this intense area of space. There, the instruments were meant to determine which of two processes accelerated the particles: radial acceleration or local acceleration.

Radial acceleration involves transporting particles perpendicular to the magnetic fields that surround Earth, from areas of low magnetic strength far from Earth to areas of high magnetic strength nearer Earth. Particle speeds in this scenario will speed up when the magnetic field strength increases. In contrast, the local acceleration theory dictates that particles gain energy from a local energy source--similar to the way hot ocean water spawns a hurricane above it.

So what did the researchers find? Using data from the probes, they discovered that there was a rapid energy increase of high-energy electrons in the radiation belts during the past October. The observations showed an increase in energy that started right in the middle of the radiation belts and gradually spread both inward and outward. This implied that there was a local acceleration source.

"In this particular case, all of the acceleration took place in about 12 hours," said Reeves. "With previous measurements, a satellite might have only been able to fly through such an even once, and not get a chance to witness the changes actually happening. With the Van Allen Probes we have two satellites and so can observe how things change and where those changes start."

The findings could lead to better predictions in the complex chain of events that intensify the radiation belts to the point that they disable satellites. In addition, they show a little bit more about how our universe works, which could lead to a better understanding of how to create spacecraft that can endure the harsh realities of space.

The findings are published in the journal Science.