Antimatter exists in small doses as in, nanogram doses. Those doses are good for reducing critical-mass in nuclear reactions. In space such could get us past the various bans on enriched-metal nuclear work in space (nobody wants Little Boy in equatorial space). How about... antimatter-on-hydrogen?
Last week Casey Handmer roused himself to post - and yesterday, Gerrit Bruhaug. I may as well get in on it.
Antimatter, mostly antihydrogen but also antihelium and even more exotic stuff, comes out of particle accelerators one nanogram at a time. In theory we could also get it from space. Directly, antihelium at least is so rare it's not worth the bother. Nah: orbital spacemen could get much more antihydrogen from our Van Allens.
Handmer's maths aim more for the ISP, than for thrust. rlelder noted in the comments that for thrust, we should be looking at a pulse method, like ol' Boom Boom or that fusion afterburner. For getting off of Earth we can't use either: we'll still need the (chemical) Super Heavy Booster. I'll add that the SHB's use of methane seems better than hydrogen just for storage, although a small-scale nuclear reactor at a hurricane-/tsunami safe distance up Rio might offer JIT hydrogen to Boca and, whilst we're at it, to Tamaulipan steelworks.
Once already in a midrange Earth orbit, Handmer and Bruhaug are saying we don't need to be goosing fissile matter, which would require too much paperwork up there anyway. We could just spike the antiprotons against regular protons, also farmable by the nanogram from the solar wind. The economics and the safety-requirements of collecting fuel in high Earth orbit make nanogram-farming more attractive than (somehow) hurling stuff up from Earth, and perhaps more-so bringing kilotonnes of radioactive-whatever from some space rock toward Earth.
I expect antimatter-driven cargos could be larger than cargos sent by solar-sail. They would really come into their own, past our solar-system's snow line, the "solar" part delivering diminishing-returns by inverse-square AU.
So now let's get to thermodynamics (boo!). Bruhaug notes that the heat from these monsters exceeds what even those refractory metals can withstand; you know... the rare metals like iridium we are hoping to mine from those outer asteroids (titanium can be had from our moon). Interstellar ships will need ten megameters of radiator space: compare Earth equatorial radius 6.38 megameters.
We're just going from planet-to-asteroid presently. But it seems that our refactory-metal imports may need, in fact, to be functioning radiators, themselves. Not stored in a "cargo bay".
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