An invisible shield, some 7,200 miles (11,600 km) above Earth, has been found blocking 'killer electrons' from bombarding our planet.

These electrons can whip around the planet at near-light speed and have been known to threaten astronauts, fry satellites and damage space systems.

If they hit Earth on a large scale, they could knock out power grids, radically change the planet's climate and drive up rates of cancer.

The shield was discovered in the Van Allen radiation belts - two doughnut-shaped rings above Earth that are filled with high-energy electrons and protons

But while scientists know this mysterious shield exists, they are baffled as to how it formed and how exactly it works.

'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,' said Professor Daniel Baker from the University of Colorado Boulder.

The shield was discovered in the Van Allen radiation belts - two doughnut-shaped rings above Earth that are filled with high-energy electrons and protons.

Held in place by Earth's magnetic field, the Van Allen radiation belts swell and shrink in response to incoming energy disturbances from the sun.

They were discovered in 1958 and are comprised of an inner and outer belt extending up to 25,000 miles (40,000km) above Earth's surface.

Held in place by Earth's magnetic field, the Van Allen radiation belts swell and shrink in response to incoming energy disturbances from the sun

'Somewhat like the shields created by force fields on Star Trek (pictured) that were used to repel alien weapons, we are seeing an invisible shield blocking these electrons,' said Professor Daniel Baker

WHAT ARE THE VAN ALLEN BELTS? The Van Allen radiation belts are two layers of charged particles orbiting around Earth. They are kept in place by the planet's magnetic field. The belts are comprised of an inner and outer belt extending up to 25,000 miles (40,000km) above Earth's surface. The radiation levels across the belts differ depending on where in the belt they are. The belts were discovered in 1958 by scientist James Van Allen from the University of Iowa while aboard the first U.S space mission. One theory suggested the particles were accelerated as they entered the belt and were trapped inside. The second suggested that particles were created inside the belt and accelerated by a force from within. Nasa sent probes up to track the behaviour of the belts and these satellites have finally proved that electric fields inside the belts tear electrons from atoms and charge them at super speeds. Advertisement

Last year, Professor Baker led a team that used the twin Van Allen Probes launched in 2012 to discover a third, transient 'storage ring'.

This ring is located between the inner and outer Van Allen radiation belts and seems to come and go with the intensity of space weather.

This ring appears to block the ultrafast electrons from breaching the shield and moving deeper towards Earth's atmosphere.

'It's almost like these electrons are running into a glass wall in space,' said Professor Baker.

'It's an extremely puzzling phenomenon.'

The team originally thought the highly charged electrons, which are looping around Earth at more than 100,000 miles per second, would slowly drift downward into the upper atmosphere.

But the impenetrable barrier seen by the twin Van Allen belt spacecraft stops the electrons before they get that far, said Professor Baker.

The group looked at a number of scenarios that could create and maintain such a barrier.

The team wondered if it might have to do with Earth's magnetic field lines.

These lines trap and control protons and electrons, bouncing them between Earth's poles like beads on a string.

They also looked at whether radio signals from human transmitters on Earth could be scattering the charged electrons at the barrier, preventing their downward motion.

Killer electrons whip around the planet at near-light speed and have been known to threaten astronauts, fry satellites and degrade space systems during intense solar storms

But neither explanation held scientific weight, Professor Baker said.

'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,' he added.

'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.'

Another scenario is that the giant cloud of cold, electrically charged gas called the plasmasphere, which begins about 600 miles (960km) above, is scattering the electrons at the boundary.

WE'RE LIVING IN A WORLD SURROUNDED BY PARTICLE HURRICANES In March, scientists looking at charged particles in space discovered particle hurricanes surrounding Earth. These highly-charged particles are part of the Van Allen belts. Since the belts were first discovered in 1958, astronomers have been trying to trace the source of the super-acceleration. The acceleration is so high that satellites have to protect their sensitive components with shielding if their orbit spends significant time in the belt. Data from Nasa's twin space probes suggests that these the particles may be supercharged by local acceleration, similar to how hurricanes are driven by the evaporation of warm ocean water on Earth. This theory explains why the belts change shape - because the particles numbers swell and drop in different conditions. Space scientists believe these findings are significant because they could also be used to explain the high-energy radiation belts that circle Saturn and Jupiter, as well as around sun-like stars. Advertisement

It could be doing this with low frequency, electromagnetic waves that create a plasmapheric 'hiss'.

The hiss sounds like white noise when played over a speaker, Professor Baker said.

While the plasmaspheric hiss may play a role in the puzzling space barrier, Professor Baker believes there is more to the story.

'I think the key here is to keep observing the region in exquisite detail, which we can do because of the powerful instruments on the Van Allen probes,' he said.