Robert Forward used to talk about antimatter factories in space, installations that would draw their power from the Sun. He would point out that at a distance of 1 AU, our star delivers a gigawatt of energy for each square kilometer of collector. And being Robert Forward, he thought big: Build a collector array one hundred kilometers on a side to produce a power input of ten terawatts, enough to drive several antimatter factories at full power and produce a gram of antimatter each day.
Forward saw the antimatter problem as a matter of scaling and cost (and he often talked about ‘small problems of engineering’). As we’ve seen in the last few days, James Bickford (Draper Laboratory) is more than aware of both these issues, but unlike Forward, he’s keen on mining naturally occurring sources of antimatter right here in the Solar System. Forward’s huge factories may some day be built, but for now, let’s talk about how to get our early antimatter missions into the realm of possibility by learning how to cull the needed antimatter, if only in the minute quantities that would kick a payload up to 100 kilometers per second. Along the way, we establish the expertise to go further.
What Bickford has in mind is using a plasma magnet to create a magnetic scoop that can influence the trajectory of a charged particle over large distances. In an equatorial orbit around Earth, such a scoop could trap antiparticles occurring in the planet’s radiation belt as they, obeying the Lorentz force, bounce back and forth between their mirror points in the Northern and Southern hemispheres. The antiproton belt our spacecraft is tapping is analogous to the protons in the Van Allen radiation belt, and provides a close to home source of antimatter before we look out into the Solar System, where we find further stores especially around Saturn.
We’re still talking about large structures to collect this material, but nothing like Forward’s huge collector arrays (or, for that matter, his vast Fresnel lens in the outer Solar System that would enable a laser-beamed lightsail mission). In his Phase II NIAC report, available here on Centauri Dreams, Bickford talks about using high temperature superconductors to form two pairs of RF coils with a radius of 100 meters and a weight of some 7000 kilograms combined. 200 kW is needed to operate the system, achievable through nuclear or solar power. What happens is straightforward: The magnetic field driven by the RF coils concentrates the incoming antiprotons and then traps them.
Comparing this concept with what we do today, Bickford finds that space harvesting of antimatter is five orders of magnitude more cost effective than the Earth-based alternative. Working out Earth’s antiproton flux in relation to this scheme, he talks about collecting 25 nanograms per day, with up to 110 nanograms stored in the central region between the coils. And note: If you’ve looked at Bickford’s earlier Phase I report, be aware that his plasma magnet supplants that report’s single coil loop. The Phase II paper goes through all of the options that led to this result.
And here’s a spectacular mission concept from the report:
The baseline concept of operations calls for a magnetic scoop to be placed in a low-inclination orbit, which cuts through the heart of the inner radiation belt where most antiprotons are trapped. Placing the vehicle in an orbit with an apogee of 3500 km and a perigee of 1500 km will enable it to intersect nearly the entire flux of the Earth’s antiproton belt. The baseline mission calls for a fraction of the total supply to be trapped over a period of days to weeks and then used to propel the vehicle to Saturn or other solar system body where there is a more plentiful supply. The vehicle then fully fills its antiproton trap and propels itself on a mission outside of our solar system.
We’re talking realistic missions into the Kuiper Belt and perhaps to the Sun’s gravitational focus, where so many interesting things happen to enhance the image of objects examined from there (bear in mind that the Sun’s gravity focus, unlike an optical lens, allows incoming radiation to stay on the focal axis at distances greater than 550 AU — just get to that distance and a new category of observation becomes possible). In any case, missions of greater speed and with even more ambitious targets are possible. As Bickford notes: “Future enhanced systems would be able to collect from the GCR [galactic cosmic ray] flux en route to further supplement the fuel supply.”
For that matter, how about creating an antimatter ‘fuel depot’ that could be used to support closer-in missions to Mars and the asteroids? All seem feasible provided the collection system can be made to function. But the idea is compelling: Bickford says the plasma magnet influences the trajectory of charged particles near the spacecraft, which then follow the field lines and are concentrated in the throat of the collection system. The momentum of the particles is then braked by RF and/or electrostatic methods so that the antimatter becomes trapped in the inner coil region. Long-term storage seems possible given the paucity of protons in the region between the coils, hence little loss by annihilation.
We’ll talk again soon about the technologies needed to make this come together, and discuss Bickford’s ideas on future research possibilities. As happens so often in examining space technologies, we can sketch out the shape of deeply promising concepts that rely on the development of a true space-based infrastructure to come to fruition. The major point remains: Building that infrastructure will open up possibilities denied to us now, among them the opportunity to experiment with propulsion methods that may one day power true interstellar missions.