Nasa's Van Allen Space Probe Finds Mysterious 'Radiation Belt' Surrounding Earth (PICTURES)
An enormous ring of radiation surrounded planet Earth for more than four weeks last year before being annihilated by a powerful interplanetary shock-wave.
And nobody noticed.
Scientists at Nasa say that they discovered the surprise radiation 'attack' using their twin Van Allen space probes.
The Van Allen craft are currently flying around the planet to detect high-energetic charges particles, and their find shows how much more we have to understand about how the universe really works.
It is already known that zones of radiation exist around the Earth. The first - the Van Allen belts - were found in 1958.
They are made up on a inner zone of high-energy electrons and positive ions, which remains stable for decades.
The outer zone is mostly made of electrons, and its intensity can wax and wane in a matter of hours, depending on the strength of the solar wind and other particles from the sun.
But when Nasa launched the Van Allen craft in the summer of 2012, they were surprised to find evidence of a third belt made of super-high-energy electrons about 11,900 to 13,900 miles from the surface.
The belt of radiation formed on 2 September and lasted for roughly a month, Nasa said.
It was blown into space by a shock-wave caused by a sudden increase in solar winds.
Daniel Baker at the University of Colorado said the results were so surprising he assumed at first that there had been a mistake.
"But I soon realized the lab had built such wonderful instruments that there wasn't anything wrong with them, so what we saw must be true," he told Space.com.
The radiation belts pose no danger to humans on Earth, but can affect satellites and other space equipment.