This is for a story plot. Suppose for a moment that you actually had a way of boosting a slug of metal, say around the mass of your average bus, all the way up to 2/3c, and send it heading for Earth. 1) Would there be any way at all of detecting the "incoming round"? Even if this thing was making its way in from all the way out past Pluto and there were plenty of telescopes of various kinds everywhere from Phobos to Europa? How likely would anyone be to spot something that small moving that fast? 2) What would you detect? Some kind of X-Ray emissions from the slug smashing stray molecules and occasional pebbles into plasma occurring along a statistically unlikely straight line? 3) Could its path even be calculated, or would relativistic effects start to render any such calculations pretty much an indeterminate moot point? 4) Even if you could move something about the size of the ISS into its path, say as far out as the orbit of the Moon, wouldn't a slug like that pass clean through something like the ISS with just about zip in the way of lost momentum? I.e., short of pushing something the mass of the Moon itself into the slug's path, you ain't stoppin it, or even appreciably slowing it down?