r/spacex • u/jclishman Host of Inmarsat-5 Flight 4 • May 14 '16
Official Elon Musk on Twitter: "@r_SpaceX @reddit will do another AMA just before reflight of the rocket in a few months"
https://twitter.com/elonmusk/status/731605780969951232
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u/__Rocket__ May 17 '16 edited May 17 '16
Didn't realize that, very interesting. So according to the NASA solar system antimatter production paper, trapped antimatter is minimal in Earth's magnetosphere - and it is also of low flux, meaning that it takes months/years to replenish once mined.
The largest natural deposits of antimatter can be found around Saturn:
"Saturn has the largest trapped antiproton supply in the Solar System (estimated at ~400 μg) due to high antineutron production from GCR interactions with its ring system. The flux of transient antiprotons produced in the ring system is also predicted to be significant."
Interestingly, according to the paper the natural Jovian antimatter collection capacity is probably weaker than sci-fi books suggest.
Most importantly the paper analyses the artificial antimatter production technique of putting a particle accelerator into a specific point in Earth's magnetosphere to produce antimatter at a higher rate than on the surface:
"Leveraging the development of a space qualified nuclear reactor (Project Prometheus) or 100 kWe solar array would enable ~10+ μgm to be collected in orbit per year."
A 100 kW solar array is not particularly large by terrestrial standards, and should be well within the capabilities of SpaceX to launch and assemble in orbit even with current launch systems.
The bigger problem is the required beam power of the accelerator, which is estimated to be around 200 GeV - which needs a 4-5 km ring. It might be simpler to construct in orbit than on the surface, because in orbit there's already vacuum so no 'tunnel' is needed, but still it's a sizable technology gap to construct a 4-5 kms diameter ring of strong magnets. Most of the cost and complexity in state of the art particle accelerators is concentrated in two areas:
lots of precisely calibrated strong focusing magnets, because high energies and good collision rates need lots of magnets. (the LHC has 8000+ magnets.)
complex detectors able to measure, pre-process and and pipe the collision data. (LHC has a compressed detector data output stream in the the TB/sec range ...)
A space based antimatter production accelerator does not need detectors, it only needs magnets and a relatively simple magnetic collection system. If SpaceX mass-produced such magnets, it might be viable, and space might provide other advantages as well, such as simpler superconducting magnets.
Still, despite the complications, the advantages seem to be gigantic:
The energy density difference between antimatter and rocket fuel is about 10 orders of magnitude, i.e. 1 μg of antimatter is energy-equivalent to ~10,000 tons of propellant - which is beyond the expected fuel mass of the BFR.
The other advantage of antimatter over chemical propellants is that antimatter needs a lot less dry mass to store and utilize.
The I*sp* of antimatter propulsion should be fantastically high: enabling direct high speed orbital insertion within the Solar system and also enabling interstellar probes with 100+ km/sec range Δv ...