Relativity, Twin Paradox, Time Dilation, Lenghts Contraction, Lorentz Transformation.
Earth, the spaceship must reach the planet. In the frame of the Earth only the spaceship is in motion (the frame of the spaceship is empty) and the Earth-planet distance is greater than observed in the spaceship, it is not the contracted distance d/γ. In the frame of the Earth S it is t > t_1 and