Exercise 6.9.8

In point of view of Earth, the astronaut should travel $2\times 99∕0.99=200$ years. So it will come back in the year 2200. However, let $T_v$ and $T_{-v}$ be the transformation from the space on Earth to the space of the astronaut in the tour forward and the tour backward respectively. We may calculate that

Hence the astronaut spent $14.1$ years to travel to that star measured by himself. Similarly, we may compute that

Hence he spent $14.1$ years to travel back in his mind. Combining these two, he spent $28.2$ years. Hence he would be return to Earth at age $48.2$.