Actually, asteroid the size of a small moon at the very least (i.e. enough to cast significant shadow) drifting unexpectedly into position where it'd obstruct hyperlane is ludicrously unlikely to the point of the possibility being negligible. Any such celestial bodies within a system would be accounted for in the charts and odds of meeting one in the void in between can't even be called astronomical. Foul play aside, it's immensely more likely that it's the ship itself that strayed into the shadow of one due to faulty nav data or jump execution.