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Though Earth has a relatively small antiproton supply in terms of very high delta-v missions, the supply is likely large enough to enable a bootstrap mission where a spacecraft first stops in Earth orbit for a partial ‘fuel up’ and then travels to Saturn to extract most of the fuel for an interstellar precursor mission.
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Earth has a minimal (0.25 - 15 ng) trapped supply of antiprotons. This is replenished over a period of several years. The low level is due to inefficiencies in backscattering albedo antineutrons from the atmosphere. Significant fluxes of positrons may exist, though a quasi-static supply < 1 μg is most likely.
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.
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Source: Extraction of Antiparticles Concentrated in Planetary Magnetic Fields, (April 2006). James Bickford Principal Investigator, NIAC
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Over a period of 850 days, between July 2006 and December 2008, sensors onboard PAMELA detected 28 antiprotons
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