Robert Murdock put the letter aside and clenched his fists. Only brief hours remained to him, and the small Kansas town of Thayerville was an impossible distance across space. He knew he would never reach it alive. The lines of an ancient poem by Robert Frost whispered through his mind: But I have promises to keep, And miles to go before I sleep He had promised his parents that he would come home--and he meant to keep that promise. The doctors had shown him that it was impossible. They had charted his death; they had told him when his heart would stop beating, when his breathing would cease. Death, for Robert Murdock, was a certainty. His alien disease was incurable. But they had listened to his plan. They had listened, and agreed. Now, with less than a half-hour of life remaining, Murdock was walking down one of the ship's long corridors, his boot-heels ringing on the narrow metal walkway. He was ready, at last, to keep his promise. Murdock paused before a wall storage locker, twisted a small dial. A door slid smoothly back. He looked up at the tall man standing motionless in the darkness. Reaching forward, Murdock made a quick adjustment. The tall man stepped down into the corridor, and the light flashed in his deep-set eyes, almost hidden behind thrusting ledges of bone. The man's face was hard and square-featured. "My name is Robert Murdock," said the tall figure in the neat patrol uniform. "I am 41 years of age, a rocket pilot going home to Earth." He paused. "And I am sound of mind and body." Murdock nodded slowly. "Indeed you are," he said. "How much longer do you have, sir?" "Another ten minutes. Perhaps a few seconds beyond that," replied Murdock. "I--I'm sorry," said the tall figure. Murdock smiled. He knew that a machine, however perfect, could not experience the emotion of sorrow, but it eased him to hear the words. You will be fine, he thought. You will serve well in my place and my parents will never suspect that their son has not come home to them. "It must all be perfect," said Murdock. "Of course," said the machine. "When the month I am to spend with them is over they'll see me board a rocket for space--and they'll understand that I cannot return to them for another twenty years. They will accept the fact that a spaceman must return to the stars, that he cannot leave the service before he is 60. Let me assure you, sir, it will all go well." Yes, Murdock told himself, it _will_ go well; every detail has been considered. My voice is his voice, my habits his own. The tapes I have pre-recorded will continue to reach them at specified intervals until their death. They will never know I'm gone. "Are you ready now, sir?" the tall figure asked gently. Murdock drew in his breath. "Yes," he said, "I'm ready now." And they began to walk down the long corridor. * * * * *