The magnificent, horrible blast broke into a million gobs and sparks of molten metal--from what had once been a space ship's hull. Superheated gas from ignited rocket fuel shot out. Scattered far and wide, the white-hot fragments of the wreck continued on their way, following the original direction of the once bold X911 toward Io. Their speed increased gradually, as the gravity of the Forbidden Moon pulled them. The larger chunks, falling at meteoric speed, would bury themselves deep in the cold Ionian deserts. The secret of Io had claimed another victim, one who might have been victorious. But Io's mystery was still unviolated. Evan Harwich had seen other ships, disabled and unmaneuverable for some reason beforehand, go to their ends like this; but he was still not used to the spectacle, and to the unholy wonder it provoked in him. Dazzled and almost blinded, he guided his patrol boat shakily away from the Forbidden Moon. There was cold sweat in his thick, black hair, under his leather helmet; and cold sweat too on his narrow, bristly cheeks. His movements of the controls were a trifle vague and fumbling with emotion, making his patrol boat waver a little in its course. For perhaps the millionth time Harwich wondered: "What makes Io so dangerous? Dammit all, those scientists who claim that there is a deadly shell of unseen energy completely enveloping the Forbidden Moon, must be right! There isn't anything else that could explain the continual destruction of all rocket craft that come within that five-thousand-mile limit!" Evan Harwich was ready to accept this much as fact. But beyond this, there was still a vast, unguessable question mark. Was this shell of energy a natural phenomenon; or was it something planned, made, intended for a purpose? If the latter guess was right, who could have created such a gigantic screen of force? What kind of beings? What kind of science? Io was an almost dead world, Harwich knew. Very cold. Very little water and air. Astronomers had taken photographs of its terrain through powerful telescopes, from the other moons of Jupiter. Very little could be seen on those photographs but deserts and grey hills, and curious formations which might be the magnificent ruins left by an extinct race. Evan Harwich was far from a weakling; but cold chills were playing over his big body as he groped to understand the unknown. His vision was clearing somewhat, after having been so dazzled by the incandescent blast that had accompanied the destruction of the X911 a moment ago. In the feeble sunlight, so far out here in the void, Harwich saw a second rocket, leaving the scene of the disaster along with himself. Evidently someone else had witnessed that weird demonstration of Io's destructive might, too! Squinting through a pair of binoculars, Harwich read the obviously ancient craft's number. Then he snapped on his radio again.