"Shields up, planet Earth!" Scientists say they have identified an invisible shield in space, like something out of "Star Trek," that protects our planet from potentially harmful radiation.

That radiation is in the form of electrons in the outermost band of the Van Allen radiation belts that move at almost the speed of light and can damage satellites or even put astronauts in danger.

However, researchers at the University of Colorado Boulder and MIT, along with others, have discovered that these so-called ultrarelativistic electrons, despite their intense energy level, can get no closer to the Earth's surface than about 6,800 miles above us -- where something is stopping them cold.

"It's almost like these electrons are running into a glass wall in space," says study leader Daniel Baker, director of CU-Boulder's Laboratory for Atmospheric and Space Physics. "Somewhat like the shields created by force fields on Star Trek that were used to repel alien weapons, we are seeing an invisible shield blocking these electrons. It's an extremely puzzling phenomenon."

What's holding the high-energy radiation away from the Earth isn't our planet's magnetic field, the researchers have determined, but rather a phenomenon dubbed "plasmaspheric hiss" -- low-frequency electromagnetic waves detected in the Earth's upper atmosphere that, if played through a speaker, sound like white noise or static.

The scientists, writing in the journal Nature, say they believe the waves protect us by deflecting incoming high-energy electrons, pushing them to collide with neutral gas atoms in the upper atmosphere and eventually disappear.

This natural, apparently impenetrable barrier to particle motion -- the plasmasphere -- appears to be extremely rigid and forms "an extremely sharp" boundary at the very inner edge of the radiation belt, they say.

"It's a very unusual, extraordinary, and pronounced phenomenon," says John Foster, associate director of MIT's Haystack Observatory. "What this tells us is if you parked a satellite or an orbiting space station with humans just inside this impenetrable barrier, you would expect them to have much longer lifetimes. That's a good thing to know."

The researchers' study analyzed data from twin NASA spacecraft, the Van Allen Probes, orbiting inside the harsh radiation belts to record the behavior of high-energy electrons in the region.

Almost 2 years of data gathered by the probes revealed the barrier keeping ultrarelativistic electrons away from our planet, the scientists said.

The Van Allen Probes, specially designed and built to withstand the extreme conditions in which they operate, have gathered the most detailed data yet on the behavior of the Earth's radiation belt.

"It's like looking at the phenomenon with new eyes, with a new set of instrumentation, which give us the detail to say, 'Yes, there is this hard, fast boundary,'" Foster says.

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