Suppose you are planning a trip in which a spacecraft is to travel at a constant velocity for exactly eight months, as measured by a clock on board the spacecraft, and then return home at the same speed. Upon return, the people on earth will have aged exactly seventy-five years. According to special relativity, how fast must you travel? (Express your answer to at least 5 significant figures as a multiple of c -- for example, 0.95585c.)