Isso pode ser "E milhas a percorrer antes que eu durma"(1958) (também conhecido como" Mas tenho promessas de cumprir ... ") por William F. Nolan, que foi publicado pela primeira vez em Ficção científica de infinito, August 1958 e está disponível no Internet Archive.
Esta história foi proposta anteriormente como resposta a essa outra pergunta (a partir do qual as informações acima são extraídas), mas não foram aceitas. No entanto, parece uma combinação muito boa.
O protagonista é um astronauta que contrai uma doença incurável (toda em negrito abaixo é minha):
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.
O plano é ter um robô substituto para ele em sua viagem de volta:
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.
Nos parágrafos finais, está fortemente implícito que os pais elaboraram um plano semelhante para o benefício de seu filho:
"Well," said a man at the fringe of the crowd, "there they go."
His companion sighed and shook his head. "I still don't think it's
right somehow. It just doesn't seem right to me."
"It's what they wanted, isn't it?" asked the other. "It's what they
wrote in their wills. They vowed their son would never come home to
death. In another month he'll be gone anyway. Back for another twenty
years. Why ruin it all for him?" The man paused, shading his eyes
against the sun. "And they are perfect, aren't they? He'll never
know."