The distance from Planet X to a nearby star is 12 light-years (a light-year is the distance light travels in a year) measured in the rest frame of Planet X. The relative velocity between Planet X and the star is negligible.
(a) How fast must a spaceship travel from Planet X to the star in order to reach the star in 7years according to a clock fixed on the spaceship?
(b) How long will the trip take according to a clock fixed on Planet X?
(c) What is the distance from Planet X to the nearby star, according to an astronaut on the spaceship?