The Sealed Sky refers to a phenomonon surrounding Aios that prevents travel to any bodies beyond the planet, even to its moons. Roughly spherical, the apparently magical barrier will gently push away any objects that approach its interior at a shallow angle, while objects that meet the barrier at a perpendicular angle are often crushed by the repulsive force. Objects travelling particularly fast directly away from Aios are vapourised upon contact wtih the barrier.
While its exact nature is unknown, the Sealed Sky originates with the Envoys, who established it with the utterance, "None under the moon shall free the prisoner, nor pass beyond his cell." The prisoner in question is believed to be the Titan Sorathiel, whose destruction was unseen and its ultimate fate left a mystery after the destruction of Uris.