Let's be realistic here, Craig. 28 antiprotons is hardly a nanogram!!!. A proton/antiproton only weighs 1.67 x 10^-24g. Even a million of them would be 9 orders of magnitude too light.
The 15ng supply max they say Earth has is squillions of them!!!. That might be enough, but they're awfully spread out. They would be very hard to collect. Plus, it apparently takes several years to replenish the supply of them when they're exhausted. Unless you want to smash up a few large asteroids and build a ring system around the planet!!!!
