A (yet-to-be-built) spacecraft starts from the Earth moving at constant speed to the (yet-to-be-discovered) planet Retah, which is 26.4 lt h away from Earth. It takes 52.8 h (according to an Earth observer) for the spacecraft to reach this planet. [A lt-h is a light-hour.] Assuming that the clocks are synchronized at the beginning of the journey, what is the time elapsed in the spacecraft's proper frame of reference for this one-way journey? Answer in units of h.