Green nanotechnology promises high-tech solutions to global ecological challenges. Focused on environmental risks and economic growth in the here and now, the promotional discourse of green nanotechnology firmly rejects science fiction. However, the extent to which green nanotechnology forecloses science fiction actually impedes its own green aspirations. Drawing on the novels of Kim Stanley Robinson—particularly the Mars trilogy and the Science in the Capital trilogy—this essay proposes that the successful confluence of environmental politics with molecular science (green nanopolitics) depends on practicing science fiction in the real world, cultivating a green futurity already in the present.