In the near-space just outside of Earth’s atmosphere, there’s some nasty stuff – particularly, so-called “killer electrons” that have been known to threaten astronauts and fry satellites as they whip through space at nearly the speed of light. If this is the case, how is Earth immune to these deadly forces? According to researchers at the University of Colorado Boulder, Earth is shrouded in an invisible shield that’s remarkably sci-fi in nature.

The shield was discovered in the Van Allen radiation belts, two rings above Earth held in place by Earth’s magnetic field. Discovered in 1958, it’s regarded as one of the first significant discoveries of the space age. At the edges of the belt, which contain the aforementioned electrons as well as other atomic particles, something strange happens – particles just… stop.

“It’s almost like theses electrons are running into a glass wall in space,” said Professor Daniel Baker, the study’s lead author. “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.”

The researchers have put forth a number of theories explaining the barrier, but so far none of them have stood up to much scrutiny. Interference from Earth’s magnetic fields were considered, as were man-made radio waves. Another theory involves something called the plasmasphere, a ball of cold gas that begins 600 miles above Earth and stretches out into the radiation belts. It’s possible that the plasma somehow scatters the electrons at the barrier, but even that explanation is incomplete.

“Nature abhors strong gradients and generally finds ways to smooth them out, so we would expect some of the relativistic electrons to move inward and some outward,” said Baker. “It’s not obvious how the slow, gradual processes that should be involved in motion of these particles can conspire to create such a sharp, persistent boundary at this location in space.”

Ultimately, what’s most important now and in the future is careful, sustained observation, the researchers say.