The drifting chunk of rock was large, but its mass was small enough that it would have been destroyed if it had drifted into the warp on its own. It was likely a piece of a larger asteroid which had been struck and damaged by another drifting object within the warp. It was also old, its small cloud of debris indicated that it had probably drifting for years. The only reason that navigation computers noted it at all was because it would pass within 50km of the gathered ships. Since the object wasn’t large enough to do serious damage to the structure of the Morpheus even if it hit, and the other ships could move out of the way, the object was not logged as a concern.