concreting in space

Rig an engine on and take them closer to hearth orbit, you can start mine them on the way anyway, Yeah, one thing that I always liked about the mass driver as a reaction engine for asteroid retrieval is that if you're mining and refining on the way in, you can use the less-valuable parts of what you're separating out as reaction mass. I hate to see people propose things like nuclear steam rockets, because there you're throwing away one of the most valuable parts of the material you're bringing back.