The hurried rat-a-tat of knuckles hammered on the cabin door. Commander Bob Curtis roused himself from his doze, got up from his chair, stretched himself to his full, lanky height and yawned. That would be Nelson, his navigating officer. Nelson always knocked that way—like a man in an external state of jitters over nothing at all. Curtis didn't hurry. It pleased him to let Nelson wait. He moved slowly to the door, paused there, and flung a backward glance at the man in the cabin with him—Zukor A