A newly discovered belt of antimatter circling Earth could be an astronaut's best friend. The belt, which consists of antiprotons trapped by Earth's magnetic field several hundred kilometers above the planet's surface, may ultimately become a key source of fuel for missions venturing beyond the solar system.
Researchers analyzing data from the PAMELA  (Payload for Antimatter/Matter Exploration and Light-nuclei Astrophysics) satellite, a joint mission among scientists from Italy, Germany, Russia, and Sweden, reported  the findings 26 July on arXiv.org and in the 20 August Astrophysical Journal Letters.
"The results are credible and are consistent with models that previously predicted [the belt's] existence," says James Bickford, senior member of the technical staff at the Draper Laboratory in Cambridge, Massachusetts, who was not part of the study. Bickford's own research predicts that other planets, including Jupiter, Saturn, Neptune, and Uranus, should have similar antiproton belts. Saturn may produce the greatest number of antiprotons because of interactions between cosmic rays—energetic charged particles from space—and the planet's icy rings.
Antiprotons are particles with the same mass as more familiar protons but with opposite charge. To hunt for them, the PAMELA team analyzed data the craft gathered from July 2006 to December 2008 during the first 850 days of its science mission. The researchers, who include Alessandro Bruno of the University of Bari in Italy and the INFN in Bari, focused on a region known as the South Atlantic Anomaly, where Earth's inner Van Allen radiation belt comes closest to the planet's surface and the density of particles encountered by the craft would likely be the highest. In its low Earth orbit, PAMELA regularly plows through this region.
The team identified 28 antiprotons with kinetic energies between 60 million and 750 million electron volts. That number is far higher than scientists would expect to see shooting toward Earth from distant reaches of the galaxy. The antiparticles appear to form a thin belt around Earth , gyrating around Earth's magnetic field lines and bouncing back and forth between the planet's north and south magnetic poles, the team notes. The belt, which extends from a few hundred to about 2000 kilometers above Earth, "constitutes the most abundant source of antiprotons in Earth's vicinity," Bruno says.
Most of the antiprotons come from antineutrons, which are generated when energetic cosmic rays strike the upper atmosphere tens of kilometers above Earth's surface, Bruno notes. The antineutrons escape the atmosphere and then decay into antiprotons at much higher altitudes. Pairs of protons and antiprotons are also directly produced in the cosmic ray collisions with the atmosphere. The antiprotons tend to congregate several hundred kilometers above Earth, where ordinary matter is so scarce that they are unlikely to meet up with their particle counterparts—protons—and destroy each other on contact.
Changes in the energy of particles in the belt could help verify and calibrate models that attempt to quantify the effect on Earth's environment of solar flares and other outbursts from the sun, Bickford notes.
More provocatively, antiprotons in the belt might one day be harvested to fuel missions that would travel far beyond the solar system, Bickford says. The best concepts for antimatter propulsion require that the antiprotons be collected and isolated. At the right moment, the antiprotons would mix with protons and annihilate them to produce highly energetic charged particles to propel the craft. The number of antiprotons needed, however, would exceed the amount in the newly discovered belt or even the much larger belt that may circle Saturn, Bickford says.
But Bickford says some researchers have proposed that a smaller supply of antiprotons, comparable to the population of the belt, could be used as a catalyst to trigger nuclear reactions that would release charged particles. (The antiprotons in this scheme would be gathered into a vast fuel tank, a nearly weightless cloud hundreds of meters long surrounding the craft.)
Traveling at nearly the speed of light, the charged particles produced by the nuclear reactions would fly out of the back end of the craft, propelling it beyond the solar system. In contrast, Bickford notes, a spacecraft using chemical propellant to journey well past the fringes of the solar system at high speed would need a fuel supply more massive than the observable universe.
"Antiprotons are incredibly expensive and difficult to produce and store on Earth," and even the world's most powerful atom smashers could produce and trap only nanograms of the stuff each year, Bickford adds. That's why the antiprotons in the belt are nearly a trillion times more valuable than diamonds, he says.