Robson Hind cleared his throat. "We can change accelerators in two hours," he declared. With a quick reassumption of authority he began to order his crew into action. It took nearer three hours than two to change accelerators despite Hind's shouted orders. At last the job was completed. Hind made a final check, floated over to the control panel and started the fuel feed. With a confident smile he threw in the accelerator switch. The meter needles climbed, soared past the red lines without pausing, and just in time to prevent a second blowback, Hind cut the power. "_There's metal in the field!_" His voice was high and unsteady. * * * * * Everyone knew what that meant. The slightest trace of magnetic material would distort the delicately balanced cylinder of force that contained and directed the Hoskins blast, making it suicidal to operate. Calmly Captain Reno voiced the thought in every mind. "It must be cleared. From the outside." Several of the men swore under their breaths. Interplanetary space was constantly bombarded, with an intensity inverse to the prevailing gravitation, by something called Sigma radiation. Man had never encountered it until leaving Earth, and little was known of it except that short exposure killed test animals and left their bodies unpredictably altered. Inside the ship it was safe enough, for the sleek hull was charged with a Kendall power-shield, impervious to nearly any Sigma concentration. But the shielding devices in the emergency spacesuits were small and had never been space-tested in a region of nearly equalized gravitations. The man who emerged from the airlock would be flipping a coin with a particularly unpleasant form of death. Many pairs of eyes turned toward Robson Hind. He was jet chief. "I'm assigned, not expendable," he protested hastily. "If there were more trouble later...." His face was pasty. Assigned. That was the key word. Barry Barr felt a lump tightening in his stomach as the eyes shifted to him. He had some training in Hoskins drivers. He knew alloys and power tools. And he was riding Four unassigned after that broken ankle had made him miss Three. He was the logical man. "For the safety of the ship." That phrase, taken from the ancient Earthbound code of the sea, had occurred repeatedly in the indoctrination manual at Training Base. He remembered it, and remembered further the contingent plans regarding assigned and unassigned personnel. For a moment he stood indecisively, the nervous, unhumorous smile quirking across his angular face making him look more like an untried boy than a structural engineer who had fought his way up through some of the toughest tropical construction camps of Earth. His lean body, built more for quick, neatly coordinated action than brute power, balanced handily in the zero gravity as he ran one hand through his sandy hair in a gesture of uncertainty.