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Earth’s Dirty Secret: Our Magnetic Field Traps Antimatter

August 9, 2011


Satellite confirms the existence of antimatter belts surrounding our planet, opens hopes for fuel use

The proton is a familiar figure for those who have taken high school physics. With a +1 charge it is a key constituent to most of the matter of the universe. But nature holds an outlandish vanishing twin — the antiproton. This exotic antimatter particle carries a -1 charge.

© VTM Physics Blog
Fifty-six years after their first laboratory observation, a treasure trove of antiprotons — a component of antimatter (right) — has been discovered within the Earth’s magnetic field.

Now astrophysicists have discovered a treasure trove of antimatter hidden in the Earth’s magnetic field, which could hold the key to grand insights and new space travel possibilities.

I. What is Antimatter?

The antiproton was first predicted by luminary physicist Paul Dirac in his 1933 Nobel Prize lecture [PDF]. It would take physicists over two decades to prove Professor Dirac right. In 1955 Emilio Segrè and Owen Chamberlain, research professors at the University of California, Berkley, created [PDF] an antiproton in the lab. The discovery earned the pair a Nobel Prize in Physics.

In coming decades, physicists would fill in more pieces of the puzzle of antimatter. Antiprotons, it was theorized, were the results of high-energy collisions between proton nuclear radiation and the nuclei of atoms. The collision trades the energy of the colliding particle in order to create the particle pair.

Typically, the particles combine back together. When protons and antiprotons come in close proximity with each other, they eliminate each other, converting to radiation (energy). But occasionally the antiproton survives, by separating from its sister particle and avoiding protons. Such events fill the reaches of space with low levels of antimatter.

As antiprotons are charged, it has been shown that they can be trapped by strong magnetic fields — key to preventing them from destruction in he lab. That knowledge leads to an intriguing question that has occupied the minds of many — the Earth has a relatively large and “strong” magnetic field (peak of ~60 microteslas), so could it trap antimatter?

II. Antimatter — Closer to Home Than we Thought

Physicists believe they, at last, have the answer to that question, and it’s a confirmation that antimatter is indeed trapped by the Earth’s magnetic field.

Charged Van Allen belts hold alpha particles — energetic helium nuclei — and energized protons in place above the atmosphere. Similarly, bands of antiprotons hold each other via repulsion. However, that trapping effect can’t protect the antiprotons from the matter found in the atmosphere. When the antiproton comes in contact with that matter, it annihilates it.

The reactions are how the PAMELA (Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics) instrument detected the belts. The reactions produce energy and charged particles, that add to similar trapped particles emitted by the sun.

PAMELA was built by Russia, Italy, Germany, and Sweden and launched aboard Russia’s RESURS-DK1 satellite in 2006. The 470 kg, 335-watt instrument cost around $32M USD to build.

The observations by PAMELA fulfilled a key part of its mission objective and were a critical success. It was no easy task — looking for antiprotons was akin to looking for a needle in a haystack, as energetic traditional matter is much more abundant inside the magnetic field.

The bands were found to be located several hundred kilometers from the Earth’s surface in the region overlapped by low earth orbit (LEO). The antiproton deposits spotted by PAMELA were three orders of magnitude (>1,000 times) higher than that typically seen in this region of space, evidence of a trapping effect.

The study, which has a long authors list, is in print [abstract] as an early release of the journal Astrophysical Journal Letters.

In an interview with the BBC, University of Bari (Italy) professor Alessandro Bruno, the paper’s eleventh author remarks, “[The band is] the most abundant source of antiprotons near the Earth… Trapped antiprotons can be lost in the interactions with atmospheric constituents, especially at low altitudes where the annihilation becomes the main loss mechanism. Above altitudes of several hundred kilometres, the loss rate is significantly lower, allowing a large supply of antiprotons to be produced.”

III. Antiproton Mining?

While the discovery is fascinating from a pure physics standpoint, it also represents a rare resource discovery. Here on Earth, antiprotons can only be produced in small quantities using >20 GeV nuclear collisions at advanced physics facilities like Fermilab. Given the energy used to create and trap them, these particles are extremely valuable.

The respectively bountiful supply in orbit could be, in theory, harvested at a sustainable rate. The harvested particles could be used for a variety of purposes.

The Earth’s magnetic field traps energetic particles from the sun. Particles are held in belts, by repulsive forces.

One application could be in pure physics — determining greater insight into the nature of antimatter. Another potential application could be in special cancer treatments. Antiprotons have shown great promise in treating malignancies, much like alpha particle ion therapy.

Lastly, a space ship could harvest a stock of the antiparticles as fuel to travel to the Earth’s neighbors, like Mars or Jupiter. The U.S. National Aeronautics and Space Administration has already cooked up [report; PDF] antimatter engine plans. Such designs could become a reality in coming decades.

In short, this year may mark the beginning of a new kind of resource race — a race that takes mankind to the shallow depths of space to gain this scarce kind of particle, one that is literally quite different from almost everything we know in the world around us. To the antimatter miner pioneers of generations to come: happy hunting!

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