Good comment, sets lots of context right. > Maybe an asteroid or comet that got flung by a high-gravity body (black hole? neutron star?) or supernova debris co

Good comment, sets lots of context right. > Maybe an asteroid or comet that got flung by a high-gravity body (black hole? neutron star?) or supernova debris could do that? I don't think a black hole can fling any particle with speed orders of magnitude higher than the initial speed of either the black hole or the particle.