Walker, Halliday & Resnick — Relativity: Problem 27

27 The premise of the Planet of the Apes movies and book is that hibernating astronauts travel far into Earth’s future, to a time when human civilization has been replaced by an ape civilization. Considering only special relativity, determine how far into Earth’s future the astronauts would travel if they slept for 120.0 y while traveling relative to Earth with a speed of 0.999 90c, first outward from Earth and then back again.