True to Harwich's predictions, the rearrangement of the Energy Barrage Penetrator for far greater power than the original had possessed, did not take really a lot of time. Within forty hours after the patrol pilot's arrival at the lab, the task of installing the Arnold apparatus in the old space ship, RQ257, was complete. The tests of the Penetrator had been made, and judged as successful as anyone could have hoped for. The space ship stood ready there in the laboratory room, a slender, copper helix wrapped around its hull. "All set, eh?" George Bayley boomed jovially. "Got your emergency supply-packs loaded aboard, too, eh? But you won't need them, boys," he added seriously. "You've got everything in your favor. And in five hours you'll be back here with Clara and me, at the lab with a dandy story to tell." Bayley seemed honest and sincere, now. Evan Harwich almost felt sheepish about the matter. Maybe he'd misjudged the big, bearish printer. Anyway, he watched his every move, during the assembly and installation of the Penetrator. Paul Arnold was whistling a little tune of confidence and exultation. Harwich's pulses beat happily, his thoughts on the enigma of the Forbidden Moon, that now must yield to the new Energy Barrage Penetrator. Superscience there on Io! Unutterable wonders! Who could guess beforehand what the Forbidden Moon's vast screen of force was meant to bar from intrusion? But maybe they would soon know! Only Clara Arnold showed worry. There was a slight shadow in her amber eyes, when she took Harwich's hand. "I suppose this is only a preliminary test flight to Io and back," she said. "Not much dangerous exploration. But please be careful," she pleaded. "Please be careful, Evan." The spaceman muttered a word of thanks. Evan. His first name. To have Clara Arnold use it like that might have given a new meaning to life. His heart was suddenly pounding very hard, before he remembered that diamond on her left hand. She was promised to George Bayley. The girl and the printer retreated from the laboratory chamber, waving a farewell. The space ship was sealed. The great exit doors in the ceiling of the lab opened wide, and the air rushed out. In another moment the RQ257 was shooting skyward. In the night, among the welter of stars, huge Jupiter and his many satellites shone down on the Ganymedean deserts. The nose of the ship swung unerringly toward Io. * * * * * The RQ257, wrapped in its protecting halo of blue fire from the Penetrator, struck the Forbidden Moon's tremendous, invisible envelope of energy, squarely. There was a snarling sound in the ship's interior. White sparks lanced through cold space beyond the windows of the cabin, as two opposed forces fought each other. But the RQ257 bored on steadily. "We're going to make it, Paul!" Harwich shouted through the reeking, dinning cabin. "Of course we are!" young Arnold yelled back at him. "How could we fail!"