A belt of antimatter has been discovered circling the Earth, which in future could be used to fuel voyages that race at breakneck speeds to other planets in the Solar System.

Antimatter has properties that are opposite those of normal matter – for example the positive charge on a proton is negative in an antiproton. When antimatter and normal matter come into contact, they annihilate spectacularly, releasing energy. The Italian-run PAMELA (Payload Antimatter Matter Exploration and Light Nuclei Astrophysics) satellite, launched in 2006, has found thousands of times more antiprotons than expected in a region of the innermost Van Allen radiation belt called the South Atlantic Anomaly. The anomaly appears to be a concentrated region of a much larger antimatter belt, and is the point at which the innermost radiation belt is nearest the Earth’s surface (an altitude of about 500 kilometres) and Earth’s magnetic field lines, which confine the belts, are at their weakest. 

James Bickford, the senior member of the technical staff at Draper Laboratory in Cambridge, Massachusetts, USA, has calculated that Earth’s antimatter belt contains 160 nanograms of antiprotons. This in itself doesn’t sound much – pure annihilation of this antimatter would produce just ten kilowatt-hours of energy – but it dwarfs the amount of antimatter that we can create in particle accelerators on Earth. (As an example, the Fermi National Accelerator Laboratory in Illinois, USA, would take an entire year, running up costs of millions of dollars, to create just one nanogram of antiprotons if the lab was used exclusively for that purpose.)

The antiprotons are produced via Earth’s interaction with incoming cosmic rays from beyond the Solar System. Cosmic rays are charged particles moving at close to the speed of light, ejected from phenomena such as supernovae and their remnants. When they encounter Earth’s atmosphere they collide with atmospheric molecules and decay via pair production into antiprotons and antineutrons. Because of their charge, the antiprotons are trapped on the magnetic field lines in which they form; those that form deeper into the atmosphere quickly annihilate with a particle of normal matter. However, antineutrons with no charge can escape back into space where they decay into antiprotons and become trapped in Earth’s magnetic field at much greater altitudes, where they can survive for years.

PAMELA discovered an over-density of antiprotons within the 60–750 MeV energy range contained within the South Atlantic Anomaly, but this may only be the tip of the iceberg. 

“PAMELA’s orbit is limited to altitudes between 350–600 kilometres and the antiproton radiation belt is expected to extend up to thousands of kilometres,” says Alessandro Bruno, a co-author on a paper describing the results that will appear in Astrophysical Journal Letters. “Some of these particles are produced in the confinement region of the magnetosphere and become trapped, especially in the exosphere where the density is low enough to allow antiprotons to be gathered, since losses due to annihilation or ionisation are significantly reduced.”

What practical use is 160 nanograms of antimatter spread hundreds to thousands of kilometres above our heads? Dreams of science fiction have depicted spaceships running on antimatter reactions, but Bickford, as part of a study for NASA’s Institute for Advanced Concepts (see here for more), has looked at how antiprotons could instead instigate nuclear fission reactions to produce the energy to propel a spaceship. For example, 30 nanograms of antiprotons collected from the radiation belts around Earth would be sufficient for a nuclear powered spacecraft to reach Mars in just 45 days, compared to the nine months that it will take NASA’s Curiosity rover after it blasts off this November. The trick, however, is catching the antimatter in the first place. 

Bickford envisages something called a plasma magnet. It would be installed on the space vehicle, which would have to orbit Earth, fuelling up as it passes through the antimatter belts (alternatively, a craft could dock with an orbiting fuel depot that does the same thing). An electric current running through four giant 100-metre loops, arranged perpendicular to one another, would create a rotating magnetic field that induces another electric current in a surrounding plasma (ionised gas) that creates a second, stronger magnetic field that traps and stores the antiprotons. “When you want to turn the engine on and annihilate the antiprotons, you would have them collide with a dense target near the high strength region of the magnetic field,” says Bickford. This induces a fission reaction of the atoms within the target, generating energy that can be used to power the ship. “Under the correct conditions [the magnetic field gradient] will act like a nozzle and propel the vehicle forward.”

In his NASA report, Bickford speculates that missions not just to Mars but Jupiter (10 micrograms of antiprotons would be sufficient to fuel a 100 ton payload on a one-year round trip to the giant planet), or fast missions to the ‘heliopause’ at the edge of the Solar System (only just reach by the Voyager spacecraft after three decades) or to the Sun’s gravity focus point (550 times further from the Sun than the Earth, from where distant light magnified by the Sun’s gravity in a gravitational lens focuses would create a giant natural telescope) would be feasible. Although there isn’t enough antimatter around the Earth to power all these missions (it replenishes at a rate of two nanograms per year) it could fuel some prototype spacecraft, while other planets could also be mined for their antimatter – Bickford’s report states that Saturn is the most copious producer of antiprotons, with 240 micrograms per year. Antimatter could ultimately be used to fuel flights to nearby stars such as Alpha Centauri, and could be of use to starship designs such as Project Icarus, which is a long-term joint venture by the British Interplanetary Society and the Tau Zero Foundation. However, it’s going to take a long time before we are in a position to harness the power of the antimatter belts.

There are so many issues involved in antimatter as a potential propulsion technology that right now solar sails, laser and microwave beaming and nuclear rate ahead of it in the feasibility arena,” says journalist Paul Gilster, who runs the Centauri Dreams website that is affiliated with the Tau Zero Foundation, of which he is a director. “This is not to say that it’s not an extremely promising idea, given how much energy is unlocked when matter and antimatter annihilate each other, but the antimatter belt near Earth provides nowhere near enough antimatter for an Icarus-style mission. Here we are talking about just enough antimatter to ignite a fission or fusion reaction for a mission within the Solar System.”

Bickford agrees. “There is a significant amount of overhead associated with doing this for the first precursor interstellar missions,” he says. “In contrast, there are still many missions within our Solar System that are much easier to complete that would not need the level of infrastructure development that we’re talking about here. I suspect people will focus on these options first.”