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The motion of matter particles (shown in a laboratory simulation above) and antimatter particles or antiparticles in the Earth's radiation belts are identical, but in opposite directions. (NASA)

A belt of antimatter particles wraps around Earth, despite the huge amount of matter nearby that would normally annihilate them, scientists have discovered.

Antiprotons trapped by the Earth's magnetic field were detected by the PAMELA satellite in a radiation belt several hundred kilometres above the Earth's surface, according to an article published in the August issue of The Astrophysical Journal Letters. The PAMELA satellite, launched in 2006, is designed to study charged particles in cosmic radiation, including antimatter.

Antimatter is made up of "antiparticles" that have the same mass as corresponding particles of matter, but an opposite charge. For example, the antimatter counterpart of a positively charged proton is a negatively charged antiproton.

The trouble with antimatter

Antimatter is produced in equal quantities with matter when energy is converted into mass — this happens in particle colliders and is believed to have happened during the Big Bang at the beginning of the universe. That's why physicists are puzzled about why there is no longer a significant amount of antimatter in the universe.

But it's very difficult to study antimatter in order to answer such questions because antimatter and matter are both annihilated the moment they encounter each other, producing pure energy.

In nature, very little antimatter exists and it rarely stays in existence for very long. That's because the moment antimatter touches matter, which makes up most of the universe, both the matter and antimatter are annihilated, producing pure energy — an efficient process that powers spacecraft in Star Trek and other works of science fiction.

In the laboratory, electromagnetic fields can be used to keep charged antiparticles away from other particles that would otherwise annihilate them.

The newly discovered antiproton belt survives because the Earth's geomagnetic field acts as a "bottle to stably hold particles, which get accumulated," said Alessandro Bruno, a physicist at the University of Bari in Italy, who co-authored the study.

The geomagnetic field causes them to spiral around certain corridors (the geomagnetic field lines), bouncing back and forth between the magnetic poles as they drift around the Earth, Bruno added in an email to CBC News.

The antiprotons detected by PAMELA are formed by nuclear reactions that result when cosmic rays from space interact with particles in the Earth's atmosphere.

Similar to Van Allen radiation belts

The antiproton belt is analogous to the Van Allen radiation belts, discovered in 1958, which are produced the same way and behave similarly.

"The motion of particles and antiparticles are identical, except for the fact that they spiral and drift in the opposite direction," Bruno said.

There are at least tens of thousands for every antiproton trapped around the Earth. Nevertheless, the results show that the level of trapped antiprotons in the belt in the South Atlantic Anomaly — an area above the Atlantic Ocean off the coast of Brazil — is currently about 1000 times higher than in nearby interplanetary space "constituting the most abundant source of antiprotons near the Earth," the paper said. The South Atlantic Anomaly is a region where the flow of high-energy particles is particularly heavy.

Particle levels in the antiproton belt result from a balance between the antiprotons created by nuclear reactions and those that are destroyed. Destruction happens when antiprotons are annihilated by contacting matter or are ionized (altered by a charged particle). Because of all the matter in the Earth's atmosphere, annihilation happens more at low altitudes.

"Above altitudes of several hundreds of kilometres, the loss rate is significantly lower, allowing a relatively large supply of antiprotons to be produced," Bruno said.

The discovery of the antiproton belt confirms previous theoretical predictions.

Bruno told BBC News that it might one day really provide energy to spacecraft.

The study was led by Piergiorgio Picozza, a professor with the INFN or Italian Institute for Nuclear Physics. Dozens of other scientists from Italy, Russia, Sweden and Germany contributed to the study, which was funded mainly by INFN and the Italian Space Agency.