Ponder how difficult current antimatter work is. We produce the stuff in our particle accelerators and rely on extracting antiparticles from collision debris. One in about 105 proton collisions actually produces an antiproton that can be collected. This is why we see figures like $62.5 trillion per gram (some estimates are even higher) for antiproton production costs. Not only that, but once we have created antimatter, we have to store it in a vacuum in magnetic/electric fields to keep it from any contact with normal matter.
All these are problems with using antimatter for propulsion. After all, it’s one thing to store tiny amounts of antimatter in bulky Earth-based traps, and quite another to scale storage up to protect the antimatter from annihilation for a period of months or years, not to mention the need to transport it into orbit for uses in space. But as James Bickford (Draper Laboratory, Cambridge MA) and team point out, antimatter creation and storage in space seems more straightforward. The interactions between high-energy galactic cosmic rays (GCR) and matter in the interstellar medium both produce and trap such antiparticles. Can we adapt this principle to space technologies?
Bickford has been studying for some time now a method of capturing and storing antimatter in a magnetic funnel, a tiny magnetosphere that would be generated around a spacecraft. In an e-mail to Centauri Dreams, the physicist elaborates:
“I believe you can store antiprotons (or positrons) in the magnetic scoop which I’ve proposed for capturing antiparticles produced naturally in the environment. During the collection process, the antiparticles can be transferred to closed field lines and stably trapped in the mini-magnetosphere that surrounds the spacecraft. Most of the issues traditionally associated with antimatter storage are not relevant in such a system. As a bonus, the field also acts as a radiation shield.”
It’s a shrewd insight, and one that Bickford has been developing in a recently completed project for NASA’s Institute for Advanced Concepts. Bickford considers the magnetosphere surrounding the Earth as a prime area for study, and in his work analyzes how antiparticles are produced and confined due to the nuclear reactions between those high-energy cosmic rays and elements of the atmosphere. His work proceeds with a look at the total supply of antiprotons that should surround not just the Earth but other bodies in the Solar System.
These investigations find that the Earth has a small trapped supply of antiprotons in the range of 0.25 to 15 nanograms which is steadily replenished. Saturn, on the other hand, should by the Bickford model generate perhaps 400 micrograms of antiprotons due to the interactions of GCRs and the ring system. So here’s just one creative concept: collect antiprotons near the Earth to propel a bootstrap mission, with the spacecraft proceeding on to Saturn for the bulk of its fuel. Find a way to produce still more antimatter and even more exotic missions become forseeable, about which more in a moment.
But let’s look first at the operation of that magnetic funnel, which would collect antiprotons in regions of high intensity local production. The technique would use high temperature superconducting loops to collect antiprotons, and would rely on a magnetic bottle formed from the same superconducting loops to store the particles. From a paper on this work that Bickford intends to present at this summer’s New Trends in Astrodynamics conference in Princeton:
We have proposed the use of a magnetic scoop to concentrate the antiparticles from the space environment. The concentrated flux can then be transferred to the mini-magnetosphere that forms around the scoop to store the antiparticles for long periods of time. A magnetic scoop placed in a low inclination orbit can be designed to intercept nearly the entire antiproton supply trapped in a planet’s radiation belt. The scoop can also be optimized to operate in deep space where it can trap portions of the background flux.
Particles and antiparticles at various energies can coexist in the same device since the large trapped volume (km3 or more) and natural vacuum afforded by the space environment minimizes losses.
Add to this another factor, that the natural supply of antiprotons could theoretically be augmented by a particle accelerator placed in orbit, removing the need for bulky ground storage and transport into Earth orbit. The large storage volumes available would allow the generating system to be placed within the antimatter trap, an efficient way to trap nearly all the antiprotons produced. Bickford figures a 100 kWe generator could produce roughly 10 micrograms per year; a 1 GWe source would allow 100 milligrams in the same period, a level, Bickford notes, so far above what is currently possible that it is “…sufficient to enable the first interstellar missions to nearby stars.”
Centauri Dreams‘ take: These ideas are remarkably productive, and should push antimatter research into new directions. We’ve examined the benefits of magnetic sail technology on many occasions, wedding it to solar wind propulsion and in some thinking to particle beams. Here is a way to use the natural properties of a magnetic scoop to both produce and house antimatter in workable amounts, and with reasonable hopes for success with technologies that will be coming into their own in the not so distant future (the biggest issue may be with superconductor performance, which will have to be improved significantly).
For more, see Bickford’s NIAC report. The preprint of the upcoming presentation is Bickford, Schmitt, Spjeldvik et al., “Natural Sources of Antiparticles in the Solar System and the Feasibility of Extraction for High Delta-V Space Propulsion,” as yet unpublished. We need further work on this persuasive and provocative concept.