In the novel return from the stars by s. lem, which is concerned with the problems a returning twin in the twin paradox situation might face, there is the following passage "her eyes were shining and attentive: i was thirty then. the expedition i was a pilot on the expedition to fomalhaut. that’s twenty-three light years away. we flew there and back in a hundred and twenty years ship time. four days ago we returned . . the prometheus—my ship — remained on luna. i came from there today. that’s all." assuming that all accelerations are instantaneous and the velocity of the prometheus was constant in between, with what speed did it travel from the earth to fomalhaut