Antiproton ring found around Earth
by Hazel Muir /  04 August 2011

Antiprotons appear to ring the Earth, confined by the planet’s magnetic field lines. The antimatter, which may persist for minutes or hours before annihilating with normal matter, could in theory be used to fuel ultra-efficient rockets of the future. Charged particles called cosmic rays constantly rain in from space, creating a spray of new particles – including antiparticles – when they collide with particles in the atmosphere. Many of these become trapped inside the Van Allen radiation belts, two doughnut-shaped zones around the planet where charged particles spiral around the Earth’s magnetic field lines. Satellites had already discovered positrons – the antimatter partners of electrons – in the radiation belts. Now a spacecraft has detected antiprotons, which are nearly 2000 times as massive.

Heavier particles take wider paths when they spiral around the planet’s magnetic lines, and weaker magnetic field lines also lead to wider spirals. So relatively heavy antiprotons travelling around the weak field lines in the outer radiation belt were expected to take loops so big they would quickly get pulled into the lower atmosphere, where they would annihilate with normal matter. The inner belt was thought to have fields strong enough to trap antiprotons, and indeed that is where they have been found. Piergiorgio Picozza from the University of Rome Tor Vergata, Italy, and colleagues detected the antiprotons using PAMELA, a cosmic-ray detector attached to a Russian Earth-observation satellite. The spacecraft flies through the Earth’s inner radiation belt over the south Atlantic.

Between July 2006 and December 2008, PAMELA detected 28 antiprotons trapped in spiralling orbits around the magnetic field lines sprouting from the Earth’s south pole (Astrophysical Journal Letters, DOI: 10.1088/2041-8205/737/2/l29). PAMELA samples only a small part of the inner radiation belt, but antiprotons are probably trapped throughout it. “We are talking about of billions of particles,” says team member Francesco Cafagna from the University of Bari in Italy. “I find it very interesting to note that the Earth’s magnetic field works a little bit like the magnetic traps that we are using in the lab,” says Rolf Landua at the CERN particle physics laboratory near Geneva, Switzerland. There, researchers have been trying to trap antimatter for ever longer periods to compare its behaviour with that of normal matter.

Alessandro Bruno, another team member from Bari, says antimatter in the Earth’s radiation belts might one day be useful for fuelling spacecraft. Future rockets could be powered by the reaction between matter and antimatter, a reaction that produces energy even more efficiently than nuclear fusion in the sun’s core. “This is the most abundant source of antiprotons near the Earth,” says Bruno. “Who knows, one day a spacecraft could launch then refuel in the inner radiation belt before travelling further.” Millions or billions of times as many antiprotons probably ring the giant planets.

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The Earth has a ring of antimatter encircling it, says new data from the Pamela space satellite, a multinational project aimed at studying the planet’s magnetosphere. The discovery has fueled speculation that the antimatter might someday be harvested to power advanced spacecraft. The antimatter particles are antiprotons—mirror opposites of regular protons (antimatter has similar properties to regular matter, just with opposite charge and spin). The antimatter is a few hundred miles up, in a ring near one of Earth’s Van Allen belts, which collect charged particles created or trapped by the planet’s magnetic field.

It was as challenge for the Pamela satellite to find the antiprotons in the Van Allen belt, a zone where high-energy particles are common. But the effect of antiprotons is unmistakable when it collides with normal matter, since both annihilate each other instantly, producing a measurable amount of energy and certain charged particles. Pamela discovered the number of annihilations taking place in the Van Allen belt was about 1,000 times higher than in typical space. Now that we know there’s a potential jackpot of antimatter in orbit, what good is it? Considering researchers spend incredible sums of money and energy creating antimatter here on Earth, the idea of simply collecting it is extremely tempting. However, there are serious challenges standing in the way. Even if we could create some kind of craft or tool capable of harvesting the antiprotons in orbit, there still isn’t a way to reliably contain them. The longest time that scientists have been able to contain antimatter is about 17 minutes.

Still, that 17 minutes is much longer than previous times, and if advances keep pace, it’s possible the technology for harvesting and containing antimatter could become available in the future. Since, in an annihilation, the entire mass of both the antimatter and matter gets converted to energy, it’s the most efficient fuel in the universe, and many have speculated that antimatter will eventually fuel advanced spacecraft. “One of the main uses of antimatter would be a starship,” scientist Michio Kaku told PCMag after an antimatter discovery last fall. “Because you want concentrated energy. And you can’t get more concentrated than antimatter.” Both NASA and DARPA have loose plans for antimatter-fueled starships. While those projects are a long way off, the Pamela discovery provides a glimmer of hope that they’ll someday become realized.

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